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FUNDAMENTALS AND APPLICATIONS OF HIGH-FIELD ASYMMETRIC WAVEFORM ION MOBILITY SPECTROMETRY FOR THE ANALYSIS OF EXPLOSIVES
By
ALEX CHING-HONG WU
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2009
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© 2009 Alex Ching-Hong Wu
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To my parents and my beloved wife, Rosalind
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ACKNOWLEDGMENTS
I would like to extend my thanks and appreciations to those who have contributed to my
achievement, and for their support, encouragement and guidance. My deepest thanks and
gratitude are addressed to my advisor, Dr. Richard Yost, for his consistent support and direction.
He has provided me a great example of how to be a successful scientist and a decent person, and
he has given me the academic and personal freedom to pursue the projects I enjoyed. I truly
appreciated all the insightful and thoughtful guidance I received from him.
A special thanks to Dr. David Powell for all of his help with the research about DPIS and
for sharing his knowledge about mass spectrometry. Dr. Ben Smith is acknowledged as graduate
advisor for his advice and guidance, but also as a scientist for answering questions related to my
work. Sincere gratitude is extended to the other members of my committee, Dr. Ronald
Castellano and Dr. Joseph Delfino, for their intellectual conversations throughout my research.
The members of the Yost groups, past and present, are thanked for their help, suggestions,
and friendship. I give special thanks to Mike Napolitano, Marilyn Prieto, Erick Molina, Leonard
Rorrer, Rich Reich, Dan Magparangalan, and Dr. Dodge Baluya for their nice and warm
friendship and critical reading of this dissertation. I would also like to acknowledge Dr. Jennifer
Bryant, Dr. Rachelle Landgraf, Dave Pirman, and Kyle Lunsford for their valuable contributions
to my research. President Yu-Ih Hou, Director-General Cho-Chiun Wang, and Commissioner
Mao-Sui Huang are acknowledged for giving me this opportunity and supporting my study here
in the United States.
Last, but not least, I would like to thank my parents for their love and support. They have
always given me lots of encouragement when I needed it and have been proud of me for
whatever I had accomplished. I am grateful to my wife, Rosalind, and my son, Adam, both of
whom have sacrificed a great deal of time for me over the past few years. I would especially like
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to thank Rosalind for her unfailing support and encouragement that made the completion of this
dissertation possible. My precious Adam is acknowledged as my motivation to succeed.
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TABLE OF CONTENTS page
ACKNOWLEDGMENTS ...............................................................................................................4
LIST OF TABLES...........................................................................................................................8
LIST OF FIGURES .........................................................................................................................9
CHAPTER
1 INTRODUCTION AND INSTRUMENTATION .................................................................15
Background.............................................................................................................................15 High-Field Asymmetric Waveform Ion Mobility Spectroscopy ............................................19 Quadrupole Ion Trap Mass Spectrometry...............................................................................22
Quadrupole Ion Trap Theory...........................................................................................22 Ion Motion in the Ion Trap ..............................................................................................24 Finnigan LCQ..................................................................................................................25
Atmospheric Pressure Ionization............................................................................................27 Atmospheric Pressure Chemical Ionization (APCI)........................................................27 Distribution Plasma Ionization Source (DPIS)................................................................28
FAIMS/MS .............................................................................................................................29 Overview of Dissertation........................................................................................................29
2 PROPERTIES AND CHARACTERIZATION OF EXPLOSIVES BY ATMOSPHERIC PRESSURE IONIZATION (API)-MASS SPECTROMETRY .............................................39
Introduction.............................................................................................................................39 Experimental...........................................................................................................................40
Atmospheric Pressure Chemical Ionization (APCI)........................................................40 Distributed Plasma Ionization Source (DPIS).................................................................41
Results and Discussion ...........................................................................................................42 Reactant Ions ...................................................................................................................42 Ionization Chemistry .......................................................................................................45 Nitroaromatic Compounds ..............................................................................................46
TNT ..........................................................................................................................46 TNB..........................................................................................................................47 Tetryl ........................................................................................................................48 DNT..........................................................................................................................48 DNB .........................................................................................................................49
Nitramines .......................................................................................................................50 RDX .........................................................................................................................50 HMX.........................................................................................................................51
Nitrate Esters ...................................................................................................................52 NG ............................................................................................................................52 PETN........................................................................................................................53
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Conclusions.............................................................................................................................54
3 FUNDAMENTALS OF HIGH-FIELD ASYMMETRIC WAVEFORM ION MOBILITY SPECTROMETRY (FAIMS) FOR THE ANALYSIS OF EXPLOSIVES.......72
Introduction.............................................................................................................................72 Experimental...........................................................................................................................74 Results and Discussion ...........................................................................................................76
Effects of CV Scan Rate..................................................................................................76 Effect of Curtain Gas Flow Rate .....................................................................................77 Effects of DV...................................................................................................................78
CV value...................................................................................................................79 Signal intensity.........................................................................................................80 Peak width ................................................................................................................81
Effects of Carrier Gas Composition ................................................................................82 TNT in different carrier gas compositions ...............................................................84 TNT in O2 and mixture of N2/O2..............................................................................84 Explosives in mixture of N2/He ...............................................................................85
Effects of Electrode Temperature....................................................................................88 Conclusions.............................................................................................................................91
4 PERFORMANCE OF APCI-FAIMS-MS FOR ANALYSIS OF EXPLOSIVES ...............113
Introduction...........................................................................................................................113 Experimental.........................................................................................................................113 Results and Discussion .........................................................................................................114
Repeatability of CV Values...........................................................................................114 Separation ......................................................................................................................116
Resolving power.....................................................................................................117 Separation and resolution between isomeric explosives ........................................117 Separation and resolution of explosive mixtures ...................................................119
Quantitation ...................................................................................................................120 Reproducibility.......................................................................................................120 Limit of detection and linear dynamic range .........................................................121
Conclusion ............................................................................................................................124
5 CONCLUSIONS AND FUTURE WORK ...........................................................................140
Conclusions...........................................................................................................................140 Future Work..........................................................................................................................143
Ionization Source...........................................................................................................144 FAIMS...........................................................................................................................144 Mass Spectrometer ........................................................................................................145
LIST OF REFERENCES.............................................................................................................146
BIOGRAPHICAL SKETCH .......................................................................................................153
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LIST OF TABLES
Table page 2-1 Gas-phase acidity values for reactant ions. .............................................................................61
2-2 Mass spectral data of nitroaromatic compounds analyzed by APCI-MS................................62
2-3 Mass spectral data of nitroaromatic compounds analyzed by DPIS-MS in the closed configuration. .....................................................................................................................63
2-4 Mass spectral data of nitroaromatic compounds analyzed by DPIS-MS in the open configuration. .....................................................................................................................64
2-5 Mass spectral data of nitramines analyzed by APCI-MS. .......................................................65
2-6 Mass spectral data of nitramines analyzed by DPIS-MS in the closed configuration.............66
2-7 Mass spectral data of nitramines analyzed by DPIS-MS in the open configuration. ..............67
2-8 Mass spectral data of nitrate esters analyzed by APCI-MS.....................................................68
2-9 Mass spectral data of nitrate esters analyzed by DPIS-MS in the closed configuration. ........69
2-10 Mass spectral data of nitrate esters analyzed by DPIS-MS in the open configuration..........70
2-11 The main ions of explosive compounds determined by APCI and DPIS..............................71
3-1 The main analytical characteristics of FAIMS on detecting explosives. ................................99
4-1 Repeatability of CV values from five replicate analyzes of explosive compounds. .............126
4-2 Resolving power for explosive compounds...........................................................................127
4-3 Resolution between TNT, TNB and DNT isomers. ..............................................................128
4-4 Resolution of explosive mixtures. .........................................................................................132
4-5 Reproducibility of peak areas from five replicate analyzes of explosive compounds. .........136
4-6 Linear dynamic range and limits of detection for the nitroaromatic explosives collected by full scan and SIM mode. .............................................................................................137
4-7 Linear dynamic range and limits of detection at the optimum CV for transmission of the nitro aromatic explosives for varied collection time........................................................137
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LIST OF FIGURES
Figure page 1-1 Structures of the explosives studied in this work. ...................................................................31
1-2 Hypothetical plots of the dependence of ion mobility on electric field strength for three types of ions. ......................................................................................................................32
1-3 Ion motion between two parallel plates during the application of an electric field. A simplified asymmetric waveform is applied to the upper plate. ........................................32
1-4 Polarities of CV and DV combinations required to transmit specific type of ions. ................33
1-5 LCQ quadrupole ion trap showing ion trajectory....................................................................34
1-6 Ion motion in a quadrupole ion trap mass spectrometer. For an ideal quadrupole ion trap (r0
2 = 2z02) the potential will be purely quadrupolar..........................................................35
1-7 Mathieu stability diagram for an ion trap for the regions of simultaneous stability in both the r- and z-directions. The line βz=1 intersects the qz axis at 0.908, corresponding to the low mass cut-off (LMCO) of an ion that can be stored in the trap. .............................35
1-8 Schematic of the Thermo LCQ ion trap used in these experiments. .......................................36
1-9 Thermo LCQ APCI source. .....................................................................................................37
1-10 The configuration of distributed plasma ionization source. ..................................................37
1-11 Schematic of APCI source, FAIMS cell and heated capillary interface to mass spectrometer. (not to scale) ................................................................................................38
2-1 Configuration of DPIS. (A) Schematic, (B) Actual picture. ...................................................56
2-2 Comparison of reactant ions generated by DPIS observed with air, methanol, methanol/water, and 10 ppm TNT in (A) negative mode and (B) positive mode. ............57
2-3 Schematic procedure of reactant ions formation by DPIS. .....................................................58
2-4 Three different configurations where the DPIS was placed: closed, open and fully open configuration. .....................................................................................................................58
2-5 Mass spectra of negative ions generated in air by DPIS with fully open, open and closed configuration. .....................................................................................................................59
2-6 Mass spectra of negative ions generated in air by APCI with fully open, open and closed configuration. .....................................................................................................................60
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2-7 Comparison of the reactant ions intensity as a function of the composition between oxygen and nitrogen...........................................................................................................61
2-8 Negative APCI mass spectra of nitroaromatic compounds: (A) TNT (MW = 227), (B) TNB (MW = 213), (C) Tetryl (MW = 287), (D) 2,4-DNT (MW = 182), (E) 2,6-DNT (MW = 182), (F) 3,4-DNT (MW = 182), (G) 1,3-DNB (MW = 168). ..............................62
2-9 Negative DPIS mass spectra of nitroaromatic compounds in the closed configuration: (A) TNT (MW = 227), (B) TNB (MW = 213), (C) Tetryl (MW = 287), (D) 2,4-DNT (MW = 182), (E) 2,6-DNT (MW = 182), (F) 3,4-DNT (MW = 182), (G) 1,3-DNB (MW = 168). ......................................................................................................................63
2-10 Negative DPIS mass spectra of nitroaromatic compounds in the open configuration: (A) TNT (MW = 227), (B) TNB (MW = 213), (C) Tetryl (MW = 287), (D) 2,4-DNT (MW = 182), (E) 2,6-DNT (MW = 182), (F) 3,4-DNT (MW = 182), (G) 1,3-DNB (MW = 168). ......................................................................................................................64
2-11 Negative APCI mass spectra of nitramines: (A) RDX (MW = 222), (B) RDX + Cl, (C) HMX (MW = 296), (D) HMX + Cl. ..................................................................................65
2-12 Negative DPIS mass spectra of nitramines in the closed configuration: (A) RDX (MW = 222), (B) RDX + Cl, (C) HMX (MW = 296), (D) HMX + Cl. ......................................66
2-13 Negative DPIS mass spectra of nitramines in the open configuration: (A) RDX (MW = 222), (B) RDX + Cl, (C) HMX (MW = 296), (D) HMX + Cl...........................................67
2-14 Negative APCI mass spectra of nitrate esters: (A) NG (MW = 227), (B) NG + Cl, (C) PETN (MW = 316), (D) PETN + Cl..................................................................................68
2-15 Negative DPIS mass spectra of nitrate esters in the closed configuration: (A) NG (MW = 227), (B) NG + Cl, (C) PETN (MW = 316), (D) PETN + Cl.........................................69
2-16 Negative DPIS mass spectra of nitrate esters in the open configuration: (A) NG (MW = 227), (B) NG + Cl, (C) PETN (MW = 316), (D) PETN + Cl. ...........................................70
3-1 The design of the brass capillary extender. .............................................................................93
3-2 The actual picture of the brass capillary extender. ..................................................................93
3-3 Effect of CV scan rate on CV value, peak intensity, and peak width. (DV= −4000V)...........94
3-4 Effect of curtain gas flow rate on CV for the ions of tested explosives. (DV= −4000V) .......94
3-5 Effect of curtain gas flow rate on peak width for the ions of tested explosives. .....................95
3-6 Effect of curtain gas flow rate on signal intensity for the ions of tested explosives. ..............95
3-7 SI-CV spectra for the [M]- ion (m/z 227) of TNT: variation of the DV. .................................96
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3-8 Mass spectra for the TNT: variation of the DV.......................................................................96
3-9 Graph of CV versus DV for the ions of tested explosives.......................................................97
3-10 Graph of signal intensity versus DV for the ions of tested explosives..................................97
3-11 Graph of peak width versus DV for the ions of tested explosives. .......................................98
3-12 Mass spectra of explosives acquired by APCI-MS and APCI-FAIMS-MS: (A) TNT, (B) TNB, (C) 2,4-DNT, (D) 2,6-DNT, (E) 3,4-DNT, (F) Tetryl, (G) RDX, (H) HMX, (I) PETN, (J) NG..............................................................................................................100
3-13 TIC-CV spectra for TNT in different carrier gas composition at DV of -4000V. ..............101
3-14 SI-CV spectra for the [M-H]- ion of TNT (m/z 226) in oxygen carrier gas at DV from −2500 to −4500 V in −500 V increments. .......................................................................102
3-15 Graph of (A) CV, (B) peak width, and (C) signal intensity versus DV for the [M-H]- ion of TNT in N2/O2 mixtures from 0% to 50% O2.........................................................103
3-16 Graph of CV versus carrier gas composition for the ions of tested explosives in N2/He mixtures............................................................................................................................104
3-17 Graphs of signal intensity versus carrier gas composition for the ions of tested explosives in N2/He mixtures...........................................................................................104
3-18 Graphs of peak width versus carrier gas composition for the ions of tested explosives in N2/He mixtures.................................................................................................................105
3-19 Graph of (A) CV, (B) peak width, and (C) signal intensity versus DV for the [M]- ions of TNT in N2/He mixture. Red circle shows that TNT presents an even stronger type C ion behavior in high helium content.............................................................................106
3-20 Graph of (A) CV, (B) peak width, and (C) signal intensity versus DV for the [M-NO2]- ions of tetryl in N2/He mixture. Red circle shows that Tetryl presents an even stronger type C ion behavior in high helium content.......................................................107
3-21 Calculated electric field as a function of radial distance between cylindrical FAIMS inner/outer cylinders at different temperature and DV. ...................................................108
3-22 Graph of (A) CV, (B) peak width, and (C) signal intensity versus cell temperature for the [M]- ions of TNT and 2,6-DNT. ................................................................................109
3-23 Calculated electric field as a function of radial distance between cylindrical FAIMS inner/outer cylinders at DV of −4500 V. (I: inner electrode temperature, O: outer electrode temperature, Planar: planar FAIMS cell) .........................................................110
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3-24 Graph of (A) CV, (B) peak width, and (C) signal intensity versus inner and outer electrode temperatures () for the [M]- ions of TNT. ....................................................111
3-25 Graph of (A) CV, (B) peak width, and (C) signal intensity versus inner and outer electrode temperatures () for the [M]- ions of 2,6-DNT. .............................................112
4-1 Structures of the isomeric explosives studied in this research. .............................................128
4-2 CV spectra of a solution mixture of 2,4-DNT, 2,6-DNT, and 3,4-DNT at DV of -5000 V and in the carrier gas of 20:80 helium/nitrogen. ..............................................................129
4-3 CV spectra of a solution mixture of TNT, 2,6-DNT, and 3,4-DNT at DV of -5000 V and in (A) the nitrogen carrier gas, (B) the carrier gas of 20:80 helium/nitrogen. .................130
4-4 CV spectra of a solution mixture of TNB, 2,4-DNT, and 2,6-DNT at DV of −5000 V and in the carrier gas of 10:90 helium/nitrogen......................................................................131
4-5 CV spectra of a solution mixture of TNT, RDX, and HMX at DV of 4500 V with the carrier gas of 30:70 helium/nitrogen................................................................................133
4-6 CV spectra of a solution mixture of TNT, NG, and PETN at DV of 4500 V with the nitrogen carrier gas. .........................................................................................................134
4-7 IS-CV spectrum of nitroaromatic explosives at DV of −4500 V and in the nitrogen carrier gas.........................................................................................................................135
4-8 Mass spectra for analytes containing 50 ng/mL explosives collected by APCI-MS and APCI-FAIMS-MS ranging from m/z 50 to 500: (A) TNT, (B) TNB, (C) 2,4-DNT, (D) 2,6-DNT, (E) 3,4-DNT ,(F) 1,3-DNB, (G) Tetryl.....................................................138
4-9 Mass spectra for analytes containing 10 ng/mL explosives collected by APCI-MS and APCI-FAIMS-MS ranging from m/z 150 to 300: (A) TNT, (B) TNB, (C) 2,4-DNT, (D) 2,6-DNT, (E) 3,4-DNT ,(F) 1,3-DNB, (G) Tetryl.....................................................139
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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
FUNDAMENTALS AND APPLICATIONS OF HIGH-FIELD ASYMMETRIC WAVEFORM
ION MOBILITY SPECTROMETRY FOR THE ANALYSIS OF EXPLOSIVES
By
Alex Ching-Hong Wu
August 2009 Chair: Richard A. Yost Major: Chemistry
Over the past ten years, the world has been stunned and outraged by a series of attacks on
civilian targets that used explosive devices. These attacks led to widespread demands for
identification of the perpetrators, along with calls for improved security measures to prevent such
incidents in the future. Detection techniques such as X-ray scanners, Raman spectroscopy,
Terahertz spectroscopy and ion mobility spectrometry are currently in use or under development;
however, none of these techniques are appropriate for all necessary applications. High-field
asymmetric-waveform ion mobility spectrometry (FAIMS) coupled to a mass spectrometer is an
alternative technique that provides improvements to mass spectral signal-to-noise,
orthogonal/complementary ion separation to mass spectrometry, enhanced ion and complexation
structural analysis, and potential for rapid analyte quantitation.
The primary goal of this research is to contribute to the understanding of ionization by an
atmospheric pressure ionization (API) source and ion behavior in a FAIMS cell to assist the
future development of a portable explosive detector to investigate explosives in the field. In this
work, the ionization mechanism of two API sources, atmospheric pressure chemical ionization
(APCI) and distributed plasma ionization source (DPIS), are discussed. The spectra of eleven
explosives ionized by both sources were collected and characterized. The results show that APCI
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provides a consistent and simple ionization, while DPIS presents more discrimination by various
ion fragments and is more amenable for monitoring a certain classes of explosives in the field.
Variation in FAIMS parameters, such as dispersion voltages (DV), compensation voltage (CV)
scan rate, curtain gas flow rate, carrier gas composition, and electrode temperature, was explored
for their effect on explosive ions. A systematic evaluation of the performance of API-FAIMS-
MS demonstrates sensitivity at the picogram level, short detection time (30 seconds), and
excellent resolution such that isomers of the same explosive can be successfully resolved. The
results from these studies with the laboratory procedure show promise for FAIMS to be used as an
explosive detector that presents a sensitive, selective, specific and rapid technique.
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CHAPTER 1 INTRODUCTION AND INSTRUMENTATION
Background
With ongoing worldwide terrorist activity, explosives analysis is becoming an increasingly
critical issue of public security. Despite long-term research and development into explosives
analytical techniques, the demand still remains for the development of an explosives detector
with properties that include higher sensitivity, selectivity, specificity, near-real time analysis and
portability.
In recent years, a wide variety of techniques including gas chromatography-electron
capture detection (GC-ECD),1-3 gas chromatography-mass spectrometry (GC-MS),4-8 liquid
chromatography-mass spectrometry (LC-MS),9, 10 high-performance liquid chromatography-
tandem mass spectrometry (HPLC-MS/MS), 11, 12 and high-performance liquid chromatography-
ultraviolet detection (HPLC-UV)13, 14 have been developed and applied to detect explosives
under various conditions.15
These techniques are not, however, void of certain difficulties. Analysis of explosives by
GC can be problematic because of their low vapor pressure and thermal instability. Due to its
highly polar nature, cyclotrimethylene trinitramine (RDX) often gives poor peak shapes, and
cyclotetramethylene tetranitramine (HMX) is often difficult to chromatograph. Further, while
ECD is selective for explosives containing nitro groups, it does not conclusively identify
separated analytes. HPLC overcomes some of the difficulties associated with the high
temperatures required for GC analysis, but suffers from poorer resolution. Furthermore, UV
detection gives little structural information.16
Mass spectrometry is a promising method and has been widely used when coupled with
different chromatographic approaches for explosives analysis. This promise is owed to the
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advantages of high sensitivity, fast response, and the additional selectivity available from
MS/MS and ion/molecule reactions.17 However, the need for sample preparation and
dependence on a separation device substantially restricts the capabilities of mass spectrometry
for explosive detection. Therefore, in order to overcome the limitations of MS, the ionization
sources recently introduced for explosive research, including electrospray ionization (ESI)10, 18, 19
and atmospheric pressure chemical ionization (APCI),20, 21 have emphasized operation at
atmospheric pressure. More and more attention has been invested to develop ionization sources
with the capacity for the direct ionization of explosives on solid surfaces, such as atmospheric
pressure matrix-assisted laser desorption/ionization (MALDI),22 thermal desorption mass
spectrometry,23 and secondary ion mass spectrometry (SIMS),24 and to build up an ionization
source which can be operated under ambient conditions, such as direct analysis in real time
(DART)25, 26 and desorption electrospray ionization (DESI).27, 28 A distributed plasma ionization
source (DPIS) was invented according to the same demand, but consumes less power.29 Without
the electrosprayed solvent such as DESI, and complex configuration such as DART, the DPIS
provides portable characteristics, such as small size and ease of operation, and could potentially
be coupled to portable mass spectrometers.17
Currently, ion mobility spectrometry (IMS) is used at over 10,000 airports worldwide for
screening handcarried articles.30 IMS is an alternative explosives separation device that offers
the benefit of the favorable gas-phase ionization chemistry for explosives at ambient pressure
and the satisfactory selectivity obtained by mobility analysis.31 IMS provides practical
advantages of fast, highly sensitive and specific detection, instrumentation simplicity and
comparatively low costs of operation.32 However, selectivity of IMS is not enough, resulting in
false alarms in some cases.33 Therefore, based on similar principles to IMS, high-field
17
asymmetric waveform ion mobility spectrometry (FAIMS) was developed as a new technique for
atmospheric pressure and room temperature separation of gas-phase ions.34, 35 With FAIMS, ions
are separated based on differences between their mobility in weak and strong electric fields.36
Organic explosives belong to various chemical classes, including nitrate esters, nitramines
and nitroaromatics, and have very different physical properties, which make their analysis by a
single method difficult. In addition, various amounts of by-products can be found in an
explosive, depending on the way it was manufactured. Isomers of nitroaromatic compounds,
which are typically by-products, cannot be easily analyzed on classical chromatographic methods
because of their similar behavior on typical stationary phases.37 Current technology attempts to
combine several analytical methods to achieve higher selectivity and sensitivity for explosive
detection.21 FAIMS and MS are two separate orthogonal detection methods, which separate ions
depending on their differential ion mobility and ratio of mass-to-charge. FAIMS-MS can be
expected to effectively separate isomers or isobaric compounds38 and provide specific and
deterministic detection of the full range of military, commercial, and improvised explosive
compounds by matching unknown sample vapors to a known library of mobility and/or mass
spectra signatures. Both FAIMS and MS can be downsized to a hand-held, concealed portable
detector and used as a field-deployable device for the detection of explosives.39
FAIMS is a valuable separation technique that exhibits low parts per billion (ppb) limits of
detection for continuous vapor streams and is suitable as a gas chromatographic detector owing
to its fast response and low memory40, 41. It also possesses capabilities of separating isomers and
providing additional information orthogonal to MS.38, 42, 43 The interfacing of FAIMS with MS
offers potential advantages over the use of mass spectrometry alone or with other
chromatographic methods. Such advantages include improvements to mass spectral signal-to-
18
noise, orthogonal/complementary ion separation to mass spectrometry, enhanced ion and
complexation structural analysis, and the potential for rapid analyte quantitation.44 Gaining
understanding of how this technique functions will benefit the development of a highly sensitive,
accurate, rapid and on-scene explosives detector, which may be potentially used for the fast
detection of bombs or explosives in the antiterrorism field. This system could also be
implemented in other areas, such as pharmaceutical analysis, forensic investigation,
environmental conservation, and food monitoring.
The eleven explosives (Figure 1-1) investigated in this research are the compounds most
widely used for military or terrorist attack, and can be divided into three categories:
nitroaromatic, nitramine, and nitrate esters.
The nitroaromatic compounds include 2,4,6-trinitrotoluene (TNT), MW=227.13; 1,3,5-
trinitrobenzene (TNB), MW=213.1; N-Methyl-N,2,4,6-tetranitroaniline (tetryl), MW=287.14;
1,3-Dinitrobenzene (1,3-DNB), MW=168.11; 2,4-dinitrotoluene (2,4-DNT), MW=182.13; 2,6-
dinitrotoluene (2,6-DNT), MW=182.13; and 3,4-dinitrotoluene (3,4-DNT), MW=182.13.
Among them, TNT is one of the most commonly used explosives for military and industrial
applications. Tetryl is a sensitive explosive compound used to make detonators and explosive
booster charges. TNB and 1,3-DNB are formed through photodecomposition of TNT from
sunlight and are readily detected in explosive contaminated water; and DNTs are the major by-
products from the TNT manufacturing process.
The nitramine compounds include 1,3,5-triazinehexahydro-1,3,5-trinitro (RDX),
MW=222.12; and 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), MW=296.16. RDX is
typically used as a component in mixtures with other explosives such as TNT and as a plastic
explosive. HMX is the main byproduct of RDX and used exclusively for military application.
19
The nitrate ester compounds include pentaerythritol tetranitrate (PETN), MW=316.14; and
1,2,3-propanetriol trinitrate (nitroglycerin, NG), MW=227.09. PETN is primarily used in
booster and bursting charges of small caliber ammunition, in upper charges of detonators in
some land mines and shells, and as the explosive core of detonation cords. NG, which is widely
used in industrial explosives, has been the main component in many dynamites.
High-Field Asymmetric Waveform Ion Mobility Spectroscopy
High-field asymmetric waveform ion mobility spectrometry (FAIMS), also commonly
referred to as differential mobility spectrometry (DMS), is a new technique used for atmospheric
pressure, room temperature separation of gas-phase ions.45 With FAIMS, ions are separated
based on differences between the mobilities of the ion in the presence of weak (K0) and strong
(Kh) electric fields.36 At low electric fields (e.g., 200 V/cm), the ion drift velocity is proportional
to the field strength, and the mobility (K0) which is independent of the applied field is constant.
However, at high electric fields (e.g., 10,000 V/cm) the ion velocity is no longer directly
proportional to the applied field, and the mobility (Kh) is dependent upon the applied electric
field. The mobility of a given ion under the influence of an electric field can be expressed by
Kh/K0 = 1 + α(E/N)2 + β(E/N)4 (1-1)
where K0 is the coefficient of ion mobility at zero electric field , α and β describe the dependence
of the ion mobility at a high electric field in a particular drift gas, and N is the gas number
density.46
There are three possible behaviors of changes in ion mobility with electric field strength, as
illustrated in Figure 1-2: “type A” behavior, or exponential increase in mobility proportional to
change in field strength, “type B” behavior, or exponential increase in mobility followed by
exponential decay as field strength increases, and “type C” behavior, which is exponential decay
20
in mobility as field strength increases. The change in mobility affects the direction the ion
travels toward or away from the plates of the FAIMS cell.
The change in mobility at high field appears to reflect the size of the ion, its interaction
with the bath gas, and the structural rigidity of the ion.30 These designations are not absolute but
depend on the buffer gas; ions often shift toward type C with increasing mass.47 Cations and
anions exhibit similar trends, and (in N2 or air) the transition from A to C occurs over the ~100-
350 Da range. Type B ions are found in that transition region.48 More recent experimental and
theoretical work has demonstrated that these differences in ion behavior can be ascribed to
interactions of the ion structure, collision cross-section and instrumental parameters.49 Most of
the explosives studied in this research act as type A or B ions. Figure 1-3 illustrates the ion
motion in a FAIMS cell for a positive type A ion.
Ions are transmitted past the electrode surfaces by a carrier gas that flows between the two
parallel electrodes shown in Figure1-3. The waveform, which consists of a high voltage
component, Vhigh, lasting for a shorter period of time thigh, and a low voltage component of
opposite polarity, Vlow, lasting for a longer period of time, tlow, is applied to the upper plate to
produce the required electric field. This waveform is synthesized such that the integrated
voltage-time being applied to the upper electrode during one complete cycle of the waveform is
zero (equation 1-2).50
Vlow tlow + Vhigh thigh = 0 (1-2)
If the ion mobility is the same at high and low electric fields, the ion will experience zero
net displacement towards an electrode and will be transmitted through the FAIMS device.
However, the mobilities of most ions depend on electric field strength over the range used in
these experiments, resulting in a net displacement of the ions towards one of the electrodes. To
21
select which ions should maintain a trajectory through the electrodes without striking, a direct
current (DC) potential, referred to as a compensation voltage (CV), is applied to the upper plate.
The CV scans are generated by scanning through a range of compensation voltages and
measuring the ion abundance transmitted through the FAIMS device as a function of
compensation voltage.51 The compensation voltage required depends on the ion’s ratio of Kh/K0,
dispersion voltage (DV), the temperature, the pressure, and the gas flow rates.52
The FAIMS cell used in this research has a cylindrical geometry, in which an ion focusing
region is generated in the annular space between the two concentric cylinders due to the
nonuniform electric field in the cell.53 In the FAIMS cell, both the magnitude and the polarity
(positive or negative) of the DV have an effect on the CV. The FAIMS instrument works with
both positively and negatively charged ions in one of four modes: P1, P2, N1 and N2. The letter
portion of the mode indicates the polarity of the ions. The number portion refers to FAIMS
instrument conditions. Briefly, P1 and N1 mean that positively and negatively charged ions are
optimally separated by a DV with same polarity; P2 and N2 imply positively and negatively
charged ions are best separated by a DV with opposite polarity. For example, CV spectra
collected for a positive type A ion in P1 mode display the following tendencies with increasingly
positive DV: the peak shifts to more negative CV values, the response increases substantially,
and the peak widens.52 The waveform (P1 and P2 modes for cations and N1 and N2 modes for
anions) in cylindrical FAIMS cell focuses either type A or C ions, but defocuses and eliminates
the other type of ions from the gap.50 Ions of type B are normally focused as type C ions; though
CV has the same sign as for type A ions. Thus DV and CV have opposite signs for type A
(analyzed in the P1 and N1 modes) and same signs for type C (in P2 and N2) ions as Figure 1-
4.54
22
The fundamentally optimum waveform profile is rectangular, in the sense that the
equations relating Kh/K0 to the measured signals are very simple. However, such a waveform
requires excessive electronic power consumption for typical electrode sizes. Most FAIMS
systems, therefore, employ a more practical bisinusoidal waveform. These waveforms are
described mathematically by the equation 1-3
VD(t) =f+1
[f sin(2πwct)+sin(4πwct- )]Vmaxπ2
(1-3)
where wc is the frequency, Vmax (“dispersion voltage”) is the peak amplitude, and f controls the
waveform profile. Most FAIMS systems have adopted the optimum f = 2.55 Note that research
using the rectangular waveform is underway in our laboratory.56
The advantages of FAIMS as a gas-phase, ion-processing and separation tool include: (1)
high sensitivity provided by an ion focusing mechanism; (2) the ability to separate ions at
atmospheric pressure and room temperature; (3) the ability to separate ions on a continuous basis
rather than in discrete pulses; and (4) simplicity in interfacing to a mass spectrometer.47
Quadrupole Ion Trap Mass Spectrometry
Quadrupole Ion Trap Theory
The quadrupole ion trap mass spectrometer (QITMS) was initially described by Paul and
Steinwedel in a patent filed in 1953 in Germany and was awarded a U.S. patent in 1960.57 The
quadrupole ion trap is a three-electrode device that consists of a hyperbolic ring electrode placed
between two endcap electrodes (Figure 1-5). Generally, the endcap electrodes are held at ground
potential and radio frequency (RF) and DC potentials are applied on the ring electrode. The
confinement of the ions within the trap is illustrated by the quadrupolar potential that is
represented in Figure 1-6. As can be seen from this figure, ions in the central part of the trap are
23
confined in the axial z-direction; however, in the radial r-direction ions are accelerated towards
the end caps and are not confined. Simultaneous confinement (trapping) of the ion in both
directions can be obtained by changing the polarity of the field every time the ion is approaching
the electrodes. The magnitude of the trapping potential is described by the equation 1-4.
Dz= z/mV/(4z02Ω2) (1-4)
where Dz is the depth of the quadrupolar potential, z/m is the inverse of the mass to charge ratio
of the ion, V is the amplitude of the RF voltage, z0 is the distance from the center to an end cap,
and Ω is the frequency of the RF voltage. Depending on the value of the fundamental RF
voltage, ions of different m/z are trapped inside the ion trap.58 For an ideal quadrupolar field, the
following identity is given as equation 1-5
r02 = 2z0
2 (1-5)
so that once the magnitude of r0 is given the sizes of all three electrodes and the electrode
spacings are fixed. However, it has been pointed out by Knight59 that the ratio of r02 to z0
2 is not
necessarily restricted to 2. Regardless of the value of this ratio, the size of the ion trap is
determined largely by the magnitude of r0, and most commercial ion traps, r0 is either 1.00 or
0.707cm.60
The gaseous ions, positively or negatively charged, can be stored or confined inside the
trapping potential well when appropriate potentials are applied to the electrodes of the ion trap.61
The ions with varied mass to charge ratios can be measured by changing the electric field within
the device when their trajectories become sequentially unstable. A stable ion will possess a
trajectory that allows the ion to be trapped, or contained, within the specific electric field of the
trap; however, an unstable ion will have a trajectory that increases in magnitude toward the
24
endcap electrode. Ions that are ejected through the exit endcap electrodes are focused by the
conversion dynode accelerating potential through the exit lens towards the ion detection system
and detected.
Ion Motion in the Ion Trap
The motion of ions inside the trap can be described mathematically by the solutions to the
second-order linear differential equation described originally by Mathieu.62 Solutions to the
differential equation are in terms of two reduced parameters, az and qz, which can be used to
calculate whether an ion will have a stable or unstable trajectory in the trap under the defined
conditions of the electric field. The values of az and qz depend on the dimensions of the trap and
the potentials applied according to equations 1-6 and 1-7:
( ) 222 2162
Ω+−
=−=oo
rz zrmeUaa (1-6)
( ) 222 282
Ω+−
=−=oo
rz zrmeVqq (1-7)
where the subscripts z and r represent axial and radial motion between and perpendicular to
the endcaps, respectively; U is the DC amplitude applied to the ring electrode (if any), V is the
RF potential applied to the ring electrode, e is the charge on an ion, m is the mass of an ion, r0 is
the inner radius of the ring electrode, z0 is the axial distance from the center of the device to the
nearest point on one of the endcap electrodes, and Ω is the angular drive frequency. Solutions to
these equations in the r-and z-directions are solutions of two kinds, which either represent stable
or unstable trajectories. The set of solutions can be readily represented in the form of a Mathieu
stability diagram (Figure 1-6). The coordinates of the stability region in Figure 1-6 are the
Mathieu parameters az and qz. According to the Mathieu equation one can generate a stability
25
diagram that shows the common region in (az, qz) space for which the radial and axial
components of the ion trajectory are stable simultaneously, such that the ion can be confined in
the trap. Different point in az and qz coordinates correspond to different values of βr and βz value
which relate to the secular frequency ω of the ion in the z and r directions, respectively (equation
1-8).
ωu = 0.5 βuΩ (1-8)
When the value of β approaches zero, the ion’s secular frequency approaches zero, and the
ion is not contained. When the value of β equals one, the ion’s secular frequency equals half the
frequency of the RF field and the magnitude of its oscillation increases so that the ion escapes
the trap or collides with one of the endcap surfaces. However, if β has a value between zero and
unity, the ion can be trapped by the oscillating fields and will oscillate in a periodic mode at its
secular frequencies in z and r direction.63 In Figure 1-7, the position of different ions is depicted
in the stability diagram for different RF amplitudes. When the fundamental RF voltage is
linearly increased, the ions move toward the boundary of the stability region (qz = 0.908, az = 0).
When ions of increasingly m/z reach the qz = 0.908 point, they become unstable in the axial
direction and are ejected from the trap. The simplest way to extend the mass range is to cause
the ion to become unstable at a value of qz lower than 0.908. This is achieved by applying an
auxiliary RF field across the endcaps with a frequency matching the oscillation frequency of an
ion of particular m/z in the axial direction (qz) while ramping the fundamental RF.64
Finnigan LCQ
The mass spectrometer used in this research is a commercial benchtop Thermo LCQ ion
trap, which is a three dimensional quadrupole ion trap based instrument designed for use with
26
external atmospheric pressure ionization (API) sources. Atmospheric pressure chemical
ionization (APCI) and electrospray ionization (ESI) source are the two major API sources used
with the LCQ. A distributed plasma ionization source (DPIS) was also used in this research.
The system is easily operated in either positive or negative ion mode. In addition, the mass
range of this instrument is m/z 150 to 2000 but can be extended to m/z 4000 for some
applications, as noted in the previous section. The LCQ has a maximum resolution of 10,000 in
the zoom scan mode, and 4000 in full scan mode.
For APCI and DPIS operation, the sample solution is infused by a syringe pump at a flow
rate of 20 μL/min, and vaporized by the standard LCQ APCI heated nebulizer. Ions are formed
by APCI or DPIS and the ions are guided through heated capillary, which helps desolvate the
ions. The ions then pass through a series of lenses, skimmers, and octopole ion guides before
making it into the ion trap mass analyzer. After the ions exit the heated capillary, the ions are
then gated by the tube lens and passed through a skimmer cone into the first and second RF
octopoles. The skimmer acts as a vacuum baffle between the higher and lower pressure regions.
The octopoles act as an ion guides and transmit ions efficiently through the region by focusing
the ions into a beam. Figure 1-8 exhibits a schematic of the Thermo LCQ ion trap mass
spectrometer used in these experiments.
After the ions enter the ion trap mass analyzer through the entrance endcap electrode, they
collide with helium buffer gas atoms and are slowed down and maintained near the center of the
trap. As described previously, an RF voltage is applied to the hyperbolic ring electrode. The
hyperbolic endcap electrodes are held at or near ground. The application of the RF voltage to the
ring electrode produces a 3-D quadrupolar field within the mass analyzer, trapping ions in their
stable trajectories. As the ring electrode RF voltage increases, the system produces a mass-
27
dependent instability to eject ions from the mass analyzer in the axial direction. Once negative
ions (as studied here) are ejected from the mass analyzer, they are attracted to a conversion
dynode, held at +15 kV. Positive ions ejected from the conversion dynode are accelerated into
the electron multiplier (held at ~ –3 kV) and then amplified for signal detection. Data were
processed using the instrument software (Xcalibur version 1.3).65
Atmospheric Pressure Ionization
Atmospheric Pressure Chemical Ionization (APCI)
Atmospheric pressure chemical ionization (APCI) is a soft gas-phase ionization technique
that works at atmospheric pressure. APCI is similar to CI in the type of ionizing reactions that
occur, except that it is accomplished in an ionization chamber at atmospheric pressure instead of
a low pressure environment (~ 1 Torr). In the APCI source, ionization is initiated either by low-
energy electrons from a radioactive β-emitting or, in our case, by a corona discharge. The APCI
technique is mainly applied to polar compounds with moderate molecular weight up to about
1500 Da and generally gives monocharged ions.66 This method was selected because it most
closely resembles an ionization source that is amenable in field instruments. Many of the other
ionization techniques either require vacuum or are large, complex systems containing lasers or
high-voltage; these techniques are less suitable for a man-portable field instrument.67 The APCI
ionization source used for this research (Figure 1-9) is designed for interfacing to liquid
chromatography; therefore, it introduces liquid samples. The liquid sample is nebulized by the
APCI nozzle into a fine mist of droplets, which are passed pneumatically via nitrogen sheath gas
into a heated region where they are vaporized. The nitrogen sheath gas and vaporized solvent
molecules then serve as reactant gas as the vapor is passed into a corona discharge which
produces reactant ions that ionize the analyte through a series of chemical reactions.63 Analyte
ionization can be achieved by primary CI process, with the formation of gas-phase buffer ions,
28
analyte molecules and solvent molecules, and secondary processes, in which electrons from the
corona discharge ionize nitrogen or other gases in the APCI source, leading to the eventual CI of
the analyte. These ion-molecule reactions include proton transfer, charge exchange, electrophilic
(positive ions) or nucleophilic (negative ions) addition, and anion abstraction (positive ions) or
nucleophilic displacement (negative ions). Most reactant ions are capable of participating in
more than one of the listed reactions. The high ionization efficiency of APCI is due to the short
mean free path at 760 Torr and thus the increased number of collisions between the sample
molecules and reactant ions.
Distribution Plasma Ionization Source (DPIS)
A distributed plasma ionization source (DPIS) consists of a dielectric between a relatively
large electrode and a small electrode (Fig 1-10). The small electrode is exposed to the media
where ions are to be created. Applying a time-varying (RF) potential between the electrodes
produces a glow discharge or plasma. A DC electric field applied between the DPIS and a
counter electrode (in this case, the heated capillary interface to the mass spectrometer) moves
ions of the selected polarity away from the DPIS. Reversing the polarity of the potential across
the dielectric inhibits the formation of a corona discharge.29
The DPIS source was invented to inhibit corona arc discharges caused by point-to-point
corona discharge ionization and to minimize direct streamers generated by conventional point-to-
plane corona discharge ionization source. It also minimizes corona point erosion and instability.
The positive ions that are produced are similar to those generated by 63Ni, 273Am, or a corona
discharge. The negative ions produced are similar to those yielded by point-to-point corona
discharge, except that the reaction region configuration aids in discriminating between the
formation of NO3-, CO3
-, and O2- ions.29, 68 The DPIS used in this research is a bulb design, in
29
which a neon bulb was used to generate the plasma in the bulb as a conductive surface and a
mesh electrode enclosing the bulb was used as a small electrode.
The DPIS has the potential to replace the point-to-plane corona discharge source for APCI
with the advantages that include design configuration flexibility, dimensional stability, simplicity
and ruggedness of design, and extended source lifetime. It also has the potential to become a
powerful ionization source to detect explosives in the field.
FAIMS/MS
Experiments were performed employing a FAIMS/MS system, comprising of a cylindrical
FAIMS device (Thermo Scientific, San Jose, CA) and a commercial ion trap mass spectrometer
(LCQ, Thermo Scientific). Gas-phase explosive ions was generated by APCI using a corona
discharge needle that is positioned at an angle of 45° and ~1 cm from the opening in the curtain
plate of the FAIMS device or by a DPIS source as described in the previous section. The
FAIMS is interfaced to the MS with a 9 cm long brass extender (i.d. = 0.76 mm, o.d. = 22 mm).
A schematic of the APCI-FAIMS-MS instrument that was utilized for this work is shown in
Figure 1-11.
Overview of Dissertation
This dissertation presents a detailed investigation into the fundamentals and applications of
high-field asymmetric waveform ion mobility spectrometry (FAIMS) to explosives analysis.
The ultimate goal of the research is to develop an efficient approach to the analysis of explosives
through the implementation of FAIMS as a separation device in conjunction with a quadrupole
ion trap mass spectrometer (QITMS).
Chapter 1 has presented an introduction to the fundamentals of FAIMS and the parameters
which may affect the separation of explosive compounds by FAIMS. A brief overview of ion
trap mass spectrometry was presented because a commercial bench-top ion trap mass
30
spectrometer was used in all applications. The introduction of API sources, APCI and DPIS, was
also included in this chapter.
Chapter 2 compared the performance of different types of API sources. API sources that
can be operated under atmospheric pressure and room temperature would be beneficial to
developing an on-scene explosives detection system. In addition to APCI, which has been
successfully applied for different kinds of explosives, DPIS is examined in this research as well.
The gas-phase chemistry of the ionization sources for explosive compounds is investigated and
presented in this chapter.
In Chapter 3, experimental parameters affecting the ion transmission of explosives in the
cylindrical FAIMS analyzer region are explored. The parameters explored in this research
include dispersion voltage (DV), compensation voltage (CV) scanning rate, curtain gas flow rate,
carrier gas composition, and electrode temperature. The evaluation of eleven explosives
analyzed by FAIMS is discussed.
In Chapter 4, information gained from chapter 2 and 3 are utilized in the practical
application of APCI-FAIMS-MS for explosive analysis. The motivation for this experiment is to
evaluate the analytical performance of the combination of APCI, FAIMS and MS and to
understand the limits of detection for explosives by this method. Systematic evaluation of
nitroaromatic, nitrate ester, and nitramine explosives using APCI/FAIMS/MS is covered.
Chapter 5 discusses conclusion and future work. The advantages and disadvantages of
utilizing FAIMS with mass spectrometric techniques for the analysis of explosives and the
optimized procedure are presented. The chapter summarizes the major conclusions drawn from
this work and offers suggestion for future research in this area.
31
TNTMW=227.13
PETNMW=316.14
HMXMW=296.16
RDXMW=222.12
TNBMW=213.1
NGMW=227.09
CH3
N+
O-
O
N+
O-
O
N+
O-
O N+
O-
O
N+
O-
O
N+
O-
O
N
N
NN
+
O-
O
N+
O-
O
N+
O-
O
N
N
N
N N+
O-
O
N+
O-
O
N+
O-
O
N+
O-
O
O
O
O
O
N+
O-
O
N+
O-
O
N+
O-
O
N+
O-
O
O
O
O
N+
O-
O
N+
O-
O
N+
O-
O
1,3-DNBMW=168.11
TetrylMW=287.14
N+
O-
O
N+
O-
O
N+
O- O
NN
+O
-
O CH3
N+O
-
O
N+O
-
O
2,4-DNTMW=182.13
3,4-DNTMW=182.13
2,6-DNTMW=182.13
CH3
N+
O-
O
N+
O-
O
CH3
N+
O-
ON
+
O-
O
CH3
N+O
-
O N+
O-
O
Figure 1-1. Structures of the explosives studied in this work.
32
A
C
B
Increasing Electric Field Strength
Ratio
, Kh/K
1.0
1.05
0.95
Figure 1-2. Hypothetical plots of the dependence of ion mobility on electric field strength for three types of ions. 52
thigh
tlow
DV
+4000 V
−2000 V
0 V
CV
Figure 1-3. Ion motion between two parallel plates during the application of an electric field. A simplified asymmetric waveform is applied to the upper plate.
33
+CV
-DV
-CV
+DVP2 P1
N2N1
Type A Type C
Type C Type A
Figure 1-4. Polarities of CV and DV combinations required to transmit specific type of ions.
34
Figure 1-5. LCQ quadrupole ion trap showing ion trajectory.65
r0
z0
35
Figure 1-6. Ion motion in a quadrupole ion trap mass spectrometer. For an ideal quadrupole ion trap (r0
2 = 2z02) the potential will be purely quadrupolar.58
Figure 1-7. Mathieu stability diagram for an ion trap for the regions of simultaneous stability in both the r- and z-directions. The line βz=1 intersects the qz axis at 0.908, corresponding to the low mass cut-off (LMCO) of an ion that can be stored in the trap.58
36
Heated Capillary
VACUUMATMOSPHERE
Tube Lens Octopoles
Skimmer Ion Trap
± 15 kv Dynode
Electron multiplierInteroctopole lens
Figure 1-8. Schematic of the Thermo LCQ ion trap used in these experiments.
37
Figure 1-9. Thermo LCQ APCI source.65
Large electrode Dielectric material
Small electrode
Figure 1-10. The configuration of distributed plasma ionization source.
38
APCI Probe
To MS
Brass ExtenderFAIMS Cell
Gas Flow
Inner Electrode Outer Electrode Heated CapillaryCurtain Plate
Corona Discharge Needle
Figure 1-11. Schematic of APCI source, FAIMS cell and heated capillary interface to mass spectrometer. (not to scale)
39
CHAPTER 2 PROPERTIES AND CHARACTERIZATION OF EXPLOSIVES BY ATMOSPHERIC
PRESSURE IONIZATION (API)-MASS SPECTROMETRY
Introduction
The development of highly sensitive techniques capable of trace explosives detection and
straightforward identification is increasingly desirable in the forensic community. These
techniques are also needed to perform field analysis of involatile and thermally unstable
explosive compounds with rapid response times, preferably without complicated sample
preparation. Mass spectrometry is a very powerful tool for forensic analysis, because it offers
high sensitivity, high selectivity, and a short detection interval.69 A variety of ionization sources
have been explored for use with mass spectrometry for explosives detection, including electron
ionization (EI),5, 8 chemical ionization (CI),11, 70 photoionization,71 desorption electrospray
ionization (DESI),72, 73 direct analysis in real time (DART),25 and API. However, each of these
ionizations has characteristics which limit their use in a portable explosive detector. For
example, EI and CI require reduced pressure to maintain a stable ionization, and photoionization
requires the use of an extra power supply and a discharge lamp and provides selective ionization.
DESI and DART generate ions under ambient conditions, allowing for direct detection of
samples on surfaces; however, they are still imperfect because the source needs neither
electrosprayed solvent to form desorbed ions as for DESI, a device with the complex
configuration of DART.74 Among these ionization sources, API has ability to directly sample
from the atmosphere and the potential for production of molecular ions/adducts in high
abundance.33
Two API methods, atmospheric pressure chemical ionization (APCI) and distributed
plasma ionization source (DPIS), were evaluated in this research to investigate the ionization
40
mechanisms for the detection of eleven explosive compounds. The APCI source has already
been developed and used to detect and analyze explosives under various conditions because of its
user-friendliness, high sensitivity, reliability, and its widespread availability, all of which enable
the detection in the ambient environment.75 APCI uses a corona discharge at atmospheric
pressure and is mainly applied to polar compounds with molecular weights up to about 1500 Da
and generally gives singly-charged ions. Recently, the DPIS has been developed to meet the
requirements of low detection limits, high-throughput, and portability.68 The DPIS is a type of
direct ionization technique for mass spectrometry that is based on the production of a
nonequilibrium plasma. This plasma is generated around one of the electrodes and is fairly easy
to use at atmospheric pressure to generate analyte ions. These API methods, with their different
ionization mechanisms, were selected because they are potentially amenable to field
measurements. Both ionization sources were well suited for detecting explosives. Thus, the
preference in choosing one ionization source over another is determined by availability, sample
medium and convenience of use.
Experimental
Atmospheric Pressure Chemical Ionization (APCI)
Solutions containing explosives are directly injected via the syringe pump of the LCQ into
the vaporizer at a flow rate of 20 μL/min with a maximum ion injection time of 50 ms for
automatic gain control (AGC). The discharge current was set at 5 μA, the vaporization
temperature was held at temperatures of 100, 150, 200, 250, or 300 °C, and the flow rate of the
sheath gas (N2, unless stated otherwise) was set at 20 (Thermo LCQ arbitrary units). The LCQ
software was used to tune the instrument as needed throughout the study in order to maximize
signal intensity. During sample introduction, these parameters were changed to optimize the ion
intensity of the molecular or major ion of the sample. This technique produces ions in air at
41
atmospheric pressure using a corona discharge. If another chemical ionization (CI) reagent gas
is not added, the main components in air serve as the primary CI reagent. A series of ions are
generated that undergo a variety of ion molecule reactions. These reactions include ion
formation from the trace species of interest, allowing their detection and measurement.76
In this research, TNT, TNB, tetryl, 2,4-DNT, 2,6-DNT, 3,4-DNT, DNB, RDX, HMX,
PETN, and NG were selected for analysis by APCI based on their structural classes:
nitroaromatic, nitramine, and nitrate ester. The explosive solutions, which were originally
prepared in acetonitrile at a concentration from 250 to 2000 μg/mL, were diluted in a solvent
composed of 65% methanol and 35% deionized water. All solutions were further diluted to a
concentration of about 10 μg/mL. Gas-phase explosive ions were generated by APCI. Negative
ion mode was generally chosen for detecting the molecular ion [M]-or deprotonated molecule
[M- H]-. However, addition of an organic acid or salt is necessary to form adduct ions for
nitramine and nitrate ester explosives such as RDX, HMX, PETN, and NG because of their lack
of acidic protons. In this research, approximately 0.1% carbon tetrachloride (CCl4) was used as
an additive in some solutions to form stable adducts ions with nitramine and nitrate ester
explosives.
Distributed Plasma Ionization Source (DPIS)
The DPIS used for this research was provided by Implant Science, Inc. (Figure 2-1) A
neon bulb can be made to glow by applying direct current between the leads. This glow comes
from the plasma that acts as a conductive surface inside the bulb and serves as an electrode on
one side of the glass dielectric surface. A mesh electrode is placed around the bulb to complete
the ion source. Argon bulb was also evaluated in this research and generated similar spectra to
neon bulb. However, the spectra presented in this chapter are all produced by neon bulb. In this
research, a RF voltage (1000-1675 Vp-p at 40 kHz) is applied on lead of the DPIS to create the
42
plasma in the bulb and an offset DC voltage (-30 to -250 V) is applied to the mesh in order to
bias the source and select the polarity of ions to be produced. Generally, no ions can be observed
when the RF voltage was applied under 1000 V, and the ion intensity increases gradually as the
RF voltage is raised. The neon bulb is positioned 3 mm away from inlet of the heated capillary.
All other parameters of the mass spectrometer are the same as APCI.
Results and Discussion
Reactant Ions
The major reactant ions produced by the DPIS in negative mode are m/z 62 (NO3-), m/z 60
(CO3-), and m/z 46(NO2
-). These ions occur because the negative ions produced are similar to
those yielded by point-to-point corona discharge except the configuration aids in discriminating
between the formation of NO3-, CO3
-, and O2- ions.68(Figure 2-2-A) The major reactant ions
produced by the DPIS in positive mode are m/z 37 [2 H2O + H]+, and m/z 55 [3 H2O + H]+.
Spectra with methanol and methanol/water show major reactant ions as m/z 33 [MeOH + H]+,
m/z 47 [2 MeOH - H2O + H]+, and m/z 65 [2 MeOH + H]+. (Figure 2-2-B).
The initial negative reactant ions formed by DPIS are primarily O- and O2- from oxygen.
Among them, O2- is formed via charge capture (reaction 2-1) and O- is formed by ion-molecule
reactions.77
O2 + e- → O2- (2-1)
The reactions produce O2-, O3
-, and O4- depending on the pressure and the energy of the O- and
O2-ion. Since O- and O2
- exist in the upper atmosphere, there is a great interest in the interaction
of O2- and O- with O2 in binary and three-body reaction.78
The following reaction occurs very quickly to generate a mixture of CO3-, NO3
-, NO2-, and
HCO3- from air around the DPIS source.79 The CO3
- ion is produced via two-body reactions of
43
O3- (reaction 2-2) and three-body reactions of O- (reaction 2-3) with CO2.80 Paulson81 also
reported that CO3- may form through the interaction between O2
- and CO2 (reaction 2-4).
O3- +CO2 → CO3
- + O2 (2-2)
O- + CO2 +CO2 → CO3- +CO2 (2-3)
O2- +CO2 → CO3
- + O (2-4)
Noted nitrogen monoxide (NO) is produced by DPIS and reacts with O3- and O4
- to produce
NO2- and NO3
- (reaction 2-5, 2-6). Takada, et al.69 have reported that NO is produced by the
corona discharge, and is able to react with O2- to produce NO3
-. The NO can also interact with
CO3-, and CO4
- to yield NO2- and NO3
-. However, according to the research of Ferguson et al.82,
less than 2% of the ground state of NO3- is generated from the reaction between NO and CO4
-.
The generation of NO2- can be also inferred by the ionization of atmospheric gases through
electron capture or charge transfer (reaction 2-7) mechanisms owing to the positive electron
affinity of NO2 (2.27 eV). The NO2- ion can also react with O3 to form NO3
-. However,
Ferguson83 suggested that the NO3- is very non-reactive and might be the terminal ion product of
the whole procedure. It appears that NO3- is destroyed by ion recombination processes or photo
detachment and may solvate with water or other species.84
NO + O3-→ NO2
- + O2 (2-5)
NO + O4- → NO3
- + O2 (2-6)
NO2 + O2- (or O-) → NO2
- + O2 (or O) (2-7)
The formation of reactant ions by DPIS is summarized in Figure 2-3.
The negative reactant ions generated by DPIS were investigated in this research under
different configurations including fully open, open, and closed environments. (Figure 2-4) The
DPIS bulb is placed 3 mm away from inlet of the mass spectrometer. The API source assembly
44
was removed in the fully open configuration, backed up 1 cm from the closed position in the
open configuration, and attached to the closed position in the closed configuration.
Representative spectra from each configuration are shown in Figure 2-4. The DPIS
generated the reactant ion NO3- and the cluster ion HNO3NO3
- in the fully open configuration.
In the open and closed configuration, the DPIS produced NO3-, CO3
-, and NO2-. In the fully
open configuration, NO3- is a very abundant reactant ion owing to the larger ionization area
surrounds the discharge bulb than corona discharge needle, which induce more oxygen and
nitrogen provided from the open air involving in the reaction. In the closed configuration, the
enclosed chamber was filled with carrier gas nitrogen and the formation of NO2- increases. The
main reactant ion generated by APCI, as shown in Figure 2-6, is CO3- in all three configurations.
This might be due to the smaller ionization area of the corona discharge needle, where
insufficient NO and O3 exist to convert CO3- to NO3
- or NO2-.
In order to discover how the gas composition generates different reactant ions, pure CO2,
N2, O2, gas mixtures of N2/CO2, N2/O2, O2/CO2, and air were applied to fill the enclosed
chamber as shown in Figure 2-4. (closed configuration) No reactant ion was observed with pure
CO2 or gas mixtures of N2/CO2, and O2/CO2, due to the deficiency of oxygen, which makes the
formation of the initial O2- ion impossible. The results are consistent with the reactions reported
before.78 It is almost impossible to totally eliminate N2 and O2 from the enclosed chamber;
therefore, some weak negative ions can be observed when applying O2 and N2 gas. However,
strong NO3- and HNO3NO3
- ion peaks were present in air, and a strong NO3- ion signal can be
observed in a gas mixture of N2/O2. The results support the assumption of reactant ions
produced by DPIS only when N2 and O2 are both present.
45
To study the influence of different compositions of N2 and O2, different mixtures of
oxygen in nitrogen were introduced to the chamber as shown in Figure 2-4; the results are shown
in Figure 2-7. The highest intensities of NO3- and CO3
- ions were generated at oxygen
percentages of 5% and 2% in nitrogen, respectively. Experiments show that the CO3- and NO3
-
ions intensities decrease with increasing amounts of oxygen resulting from less CO2 and N2
existed in the chamber. The CO2 and N2 are the major precursors in the formation of CO3- and
NO2-. The NO2
- then quickly converts to NO3- by reacting with O3. When the content of oxygen
in nitrogen is less than 2%, the formation of O-, O2-, and O3
- is insufficient to support the reaction
with CO2 and NO to generate CO3- and NO2
-.
Ionization Chemistry
Explosive ions of negative polarity at atmospheric pressure are formed in two steps: At the
first step, reactant ions R- are formed from ionizing radiation; at the second step, explosive ions
are formed from ion-molecule reactions of reactant ions with molecules of explosive substances.
Ions of explosive substances are formed by reactions such as electron capture (reaction 2-8),
electron transfer (reaction 2-9), proton abstraction (reaction 2-10), and adduct formation
(reaction 2-11).85
M +e- → M- (2-8)
M + R- → M- + R (2-9)
M + R- → [M-H]- + RH (2-10)
M + R- → MP- + [R-P] (2-11)
where M is a molecule of an explosive substance, H is a hydrogen atom, R- is a reactant ion, and
P is a part of the reactant ion.
Ions of nitroaromatic compounds are formed by reactions 2-8, 2-9, and 2-10. These species
are strong gaseous acids because of the electron-withdrawing properties of the NO2 functional
46
groups on the benzene ring and are responsible for the acidic character of the methyl group.85
Electrons from the ionization region or reactant ions can be easily transferred to nitroaromatic
compounds due to the high electron affinity of the NO2 functional group, which enables the
processes of reaction 2-8 and 2-9. Electron transfer readily occurs with a negative corona
discharge, where a high density of electrons is generated that is about 106 times as much as that
produced by a 63Ni source.86 In such a high electron density, reaction 2-8 is very efficient and
trace amounts of any nitroaromatic compounds result in the production of negative ions. The
general trend for proton abstraction from molecules of explosive substances depends on the
relative acidity of M and R-. Therefore, the higher the acidity of M compared to R-, the more
readily reaction 2-9 proceeds. For both sources used in this research, the ratio between M- and
[M - H]- is quantitatively controlled by the amount of O2 in the nitrogen.87 In DPIS, O2- and
NO2- are generated as reactant ions, and both possess strong gas-phase basicity, as shown in
Table 2-1. Some nitroaromatic compounds with higher acidity, such as TNT and 2,4-DNT, may
be ionized by proton abstraction (reaction 2-10).
Nitramines and nitrate esters tend to form ions by reaction 2-11. Because nitramines and
nitrate esters do not have a positive electron affinity nor sufficient gas-phase acidity to be ionized
by electron transfer or proton abstraction, adduct formation is the most efficient approach for
these compounds.88
Nitroaromatic Compounds
TNT
The negative-APCI spectrum of TNT (Figure 2-8A) shows the production of the [M]- ion
at m/z 227. Spangler and Lawless89 thoroughly studied the ion chemistry of TNT in air and
nitrogen and found that the main ion created in nitrogen at 166 was M- via electron attachment.
It is assumed that an electron capture mechanism occurs with APCI since TNT has three bulky
47
electron-withdrawing nitro groups and there is an easily captured electron produced by the
corona discharge. The spectrum also shows two low-intensity fragment ions, [M-NO]- at m/z
197 and [M-OH]- at m/z 210, which may form either during the ionization or during the
desolvation processes in the heated capillary.
The negative-DPIS spectra of TNT (Figure 2-9A, 2-10A) shows the predominant formation
of the [M-H]- ion. The formation of [M-H]- ions may involve proton transfer between the
analytes and basic reactant ions such as NO2- and O2
-. Proton transfer can occur for analytes
possessing gas-phase acidity stronger than that of O2- (353 kcal/mol) and NO2
- (333.7 kcal/mol),
which is true for TNT (315.6 kcal/mol) and 2,4-DNT (328 kcal/mol). The comparison of the
ratio between [M]- and [M-H]- reveals that the proton transfer is better in the open configuration
because of more reactant ions generated in open air. The DPIS spectra in the open configuration
also shows more intense fragment and adduct ions at m/z 197, 260 and 274, corresponding to the
ions of [M-NO]-, [M-NO+HNO3]- and [M+HNO2]-, respectively, due to more complicated
ionization reaction occurring around the DPIS source.
TNB
The negative-APCI spectrum of the byproduct TNB (Figure 2-8B) shows the major ion at
m/z 213, [M]- . It also produces a fragment ion at m/z 183, [M-NO]- and two adduct ions at m/z
239, [M+CN]-, and m/z 244, [M+CH3O]-. Methanol has been observed to form adducts with
other explosives.90 Proton abstraction from methanol produces a methylate (CH3O-) ion, which
is a strong Brønsted base (almost as strong as OH-) that readily reacts with many organic
compounds with proton affinities lower than 379 kcal/mol.63 Under APCI, the CN- ion generated
from acetonitrile has also been proven to react with TNB via nucleophilic attack on the benzene
ring forming a Meisenheimer complex.91
48
The major ions that appear in a negative-DPIS spectra of TNB are [M]- and [M-NO]-.
(Figure 2-9B, 2-10B) However, the intensity of [M-NO]- in the open configuration is even
higher than [M]-, which means more fragmentation occurs when the DPIS source was exposed to
open air. The other ions produced include the ion at m/z 259, which is an adduct ion resulting
from attachment of NO2-, and the ions at m/z 239 and 244, which are generated from the
adduction of the reactant ions, CN- from acetonitrile and CH3O-from methanol, respectively.
Tetryl
The negative-APCI spectrum of tetryl (Figure 2-8C) shows a greater abundance of the
[M*]- ion (m/z 242) and [M*-H]- ion (m/z 241) of N-methylpicramide. Because a
methanol/water solutions were used in this research, no [M]- ion was produced in the APCI mass
spectrum, but a highly abundant N-methylpicramide ion wass observed. This is due to the
increased hydrolysis effect from the presence of water.92 The [M+CN]- ion (m/z 313) is
generated because of acetonitrile.
Negative DPIS spectra of tetryl in both the open and closed configurations include an [M-
NO2]- ion at m/z 241, which is the [M*-H]- ion of N-methylpicramide, and the [M-NO]- ion at
m/z 257. (Figure 2-9C, 2-10C) The major difference between APCI and DPIS is the formation of
the [M*]- and [M*-H]- ions of N-methylpicramide. The most intense ion in DPIS is the [M*-H]-
ion at m/z 241, while the [M*]- ion at m/z 242 for APCI. That is mainly because more basic
reagent ions such as O2- and NO2
- are generated by the DPIS; then basic reactant ions can induce
proton transfer to from N-methylpicramide.
DNT
Dinitrotoluene (DNT) isomers are byproducts originating from the manufacturing process
of TNT, and their combined profile depends on the manufacturing processes (batch or
continuous and concentration of acids) as well as the extent of the purification.37 The negative-
49
APCI spectra of 2,4-DNT; 2,6-DNT; and 3,4-DNT (Figure 2-8D-F) all yield an [M]- ion as the
major ion at m/z 182 and a fragment ion of [M-NO]- at m/z 152, as confirmed by Lubman.93
Among the DNT isomers, only 2,6-DNT produces the minor methylate adduct ion [M+ CH3O]-
at m/z 213.
As shown in Figure 2-9D-F, the negative DPIS spectra of DNT isomers in the closed
configuration are similar to those from APCI. For 3,4-DNT, in addition to the molecular anion
[M]- at m/z 182, more fragment and dimer ions were observed at m/z 62, 152, and 350, ascribed
to [NO3]-, [M-NO2]-, and [2M-CH2]-, respectively. In contrast to the closed configuration, the
DPIS spectra in the open configuration show a different ion pattern for these DNT isomers. (2-
10D-F) When the source was exposed to open air, the principal ions presented in the DPIS
spectra for 2,4-DNT, 2,6-DNT, and 3,4-DNT are [M-H]- at m/z 181, [M]- at m/z 182, and [M-
CH2]- at m/z 168, respectively. However, all the spectra of the DNT isomers show the same [M-
NO]- ion at m/z 152 and [M-HNO2+NO3]- ion at m/z 197. The [M-NO]- ion is the most common
fragment ion form DNT isomers due to natural losses of NO. The [M-HNO2+NO3]- ion is the
resultant ion from the reaction between [M-HNO2]- and the reactant ion NO3-, which is
especially apparent for 2,4-DNT. This phenomenon also indicates that more NO3- is form with
DPIS in the open configuration which may be due to the larger ionization reaction area exposed
in open air.
DNB
The negative-APCI spectra (Figure 2-8G) of 1,3-dinitrobenzene (DNB) shows ions at m/z
168 [M]-, 199 [M+ CH3O]-, and a very weak ion at m/z 138 [M-NO]-. The formation of the [M]-
ion can be attributed to an electron capture mechanism since DNB possesses a positive electron
affinity (1.66eV). The formation of the [M-NO]- ion can be interpreted as the result of in-source
fragmentation of the [M]- ions.88
50
The negative-DPIS spectra of DNB (Figure 2-9G, 2-10G) are similar to APCI, with an
abundant [M]- ion peak at m/z 168 and a weak [M-NO]- ion peak at m/z 138. The ratio between
[M-NO]- and [M]- in the open configuration is higher than the closed configuration, which may
be because of a more energetic ionization reaction occurring around the DPIS bulb in open air
since more reactant ions were generated.
Nitramines
RDX and HMX do not have positive electron affinities or sufficient gas-phase acidity to be
ionized by electron capture, dissociative electron capture, or proton transfer.88 However,
chloride ion attachment can be a very specific and sensitive type of chemical ionization
technique for the detection of nitramine and nitrate ester explosives. Caldwell et al.94
demonstrated that the highest sensitivity of halide attachment was for strong acids with Hacid
values stronger than 350 kcal/mol. The chloride attachment has been proven as an efficient
approach for the detection of RDX, showing the limit of detection (LOD) in the femtomole
range. In this research, 0.1% of carbon tetrachloride was used as an additive for this purpose.
RDX
RDX is a powerful, highly energetic chemical that is widely used in various military and
civilian applications. The negative APCI spectra (Figure 2-11A) of RDX show a relatively
complicated ion pattern. No molecular ion was observed for RDX. The major ions for RDX are
[M+C2H4N3O]- at m/z 324, [2M+NO2]- at m/z 490 and [M+NO2]- at m/z 268. The complicated
ion pattern makes it difficult to identify RDX just by APCI spectra. The formation of chloride
adduct ions [M+Cl]- greatly enhances the analysis of RDX. The major ions observed in the
spectra (Figure 2-11B) are adduct ions [M+Cl]- and cluster ions [2M+Cl]-. The ion of [M +
35Cl]- (m/z 257) is characterized by the presence of its isotope [M + 37Cl]- (m/z 259), with one
third the abundance.
51
The negative DPIS spectra of RDX in the closed and the open configurations (Figure 2-
12A and 2-13A) both include ions [M+NO2]- at m/z 268, [M+NO3]- at m/z 284 and [2M+NO2]- at
m/z 490. Although DPIS ionization, like APCI, does not generate molecular ions for RDX, the
relatively simple ion pattern gives DPIS an advantage to detect RDX in the field where additives
might be difficult or impossible to apply. When carbon tetrachloride is used as an additive for
DPIS, additional [M+Cl]- and [2M+Cl]- ions are observed, but the intensity of [M+NO2]- and
[M+NO3]- ions is higher than [M+35Cl]- in the open configuration. (Figure 2-12B and 2-13B)
This observation can be attributed to more abundant reactant ions NO2- and NO3
- generated by
the DPIS in open air. The competition between NO2-, NO3
-, and Cl- attachment was determined
by the concentration of CCl4 in sample solution and the generation of NO2- and NO3
- ions. The
NO3- adduction in the open configuration for DPIS is more ready than in the closed configuration
due to the evident NO3- ion formation in open air.
HMX
The negative APCI spectrum (Figure 2-11C) of HMX includes as the base peak the [M-H]-
ion at m/z 295. Other relevant ions include those at m/z 123, 166, 203 and 342 representing the
fragment ions [NO3NO+CH3O]-, [M-C2N2O2-NO2]-, [M-HNO2NO2]-, [M-HNO2NO]-, and the
adduct ion [M+NO2]-, respectively. Notwithstanding the abundance of the depronated ion of
HMX, the numerous ions present in the spectrum might still interfere in the determination of
HMX. The use of a chloride additive substantially simplifies the spectra. (Figure 2-11D) The
main ions observed in the spectrum (Figure 2-12D) are only the adduct ion [M+Cl]- and the
[M+Cl−HNO2]- fragment ion.
The negative DPIS spectra of HMX in both configurations (Figure 2-12C and 2-13C) show
the same [M-H]- and [M+NO2]- ions, except for some weak fragment ions that can be observed
in the open configuration. The dominant formation of [M+NO2]- indicates that HMX prefers to
52
perform adduct formation rather than proton abstraction by DPIS. In the presence of chloride,
DPIS spectra (Figure 2-12D and 2-13D) show an abundance of the [M+Cl]- ion and its fragment
ion [M+Cl-HNO2]- for both configurations. The major difference between both configurations is
the more intense [M+NO2]- ion present in the open configuration, which is due to more NO2-
ions generated by DPIS in open air, which compete with Cl- for adduction formation.
Nitrate Esters
Nitrate esters have a high electron affinity, which makes them excellent candidates for
analysis in the negative-ion mode. However, in the absence of any additives, the mass spectra
are usually characterized by various adduct ions formed from the decomposition fragments of the
nitrate esters themselves, or the impurities present in the analytical system. The lack of
specificity in the mass spectra sometimes makes the unambiguous identification of these nitrate
esters difficult.75 In this research, 0.1% carbon tetrachloride was added to the solution in order to
overcome this problem.
NG
The negative APCI spectrum (Figure 2-14A) of NG, the active component in dynamite,
has NO3- as its most prominent ion at high temperature. Ewing31 proposed that the ionization of
NG occurs via loss of NO3-, which is the only ion observed at high temperature, as shown in
reaction 2-12. However, as the temperature is lowered, the adduct ion [M+NO3]- (m/z 289)
forms as shown in reaction 2-13.
M + e-→ NO3- + [M-NO3] (2-12)
NO3- + M → [M+NO3]- (2-13)
Besides NO3- and [M+NO3]-, [M+NO2]- (m/z 273) is the other intense ion in the spectrum.
Addition of carbon tetrachloride produces the base peak ion of [M+35Cl]- at m/z 262, with the
37Cl isotope peak at m/z 264, and its dimer ion of [2M-H+35Cl]- at m/z 488.(Figure 2-14B) The
53
fragment ion of NO3- at m/z 62 and the adduct with fragment ion [M+NO3]- at m/z 289 can be
also observed.
In general, the negative DPIS spectra of NG (Figure 2-15A,B and 2-16A,B) show a similar
ion pattern with the spectra generated by APCI. However, more abundant [M+NO3]- and
[M+NO2]- ions can be found in the spectra acquired by DPIS in the open configuration, which
suggests that the NO3- and NO2
- do not only come from the fragment of NG, but are also
generated by DPIS in open air.
PETN
Similarly, PETN was fragmented to NO3- and accompanied by adduction to form
[M+NO3]- in air. As shown in the APCI spectra of PETN (Figure 2-14C), prominent ions at m/z
62, 378, 315, and 362 are attributed to the NO3-, [M+NO3]-, [M-H]-, and [M+NO2]-, respectively.
A number of other weak ions were observed at lower m/z values, which are mainly fragment ions
of PETN. Addition of chloride ion gave an improved response for PETN by generating adduct
ions at m/z 351 and 353, corresponding to the 35Cl and 37Cl isotope peaks for [M+Cl]- (Figure 2-
14D). Clearly, having only an abundant [M+Cl]- ion represents a good target ion for monitoring
PETN.
The negative DPIS spectra of PETN (Figure 2-15C and 2-16C) include an intense
[M+NO2]- ion at m/z 362 and two weaker ions, NO3- at m/z 62 and [M+NO3]- at m/z 378, which
is less complicated than the spectra generated by APCI for PETN. When carbon tetrachloride
was used as an additive, the primary ion became [M+Cl]- ion in the open and the closed
configurations. (Figure 2-15D and 2-16D) Similar to NG, the difference between the open and
closed configurations by DPIS is the production of [M+NO3] - and [M+NO2] – ions in the open
configuration. In contrast, the spectrum (Figure 2-16D) acquired by DPIS in the closed
configuration is almost identical to the one produced by APCI (Figure 2-15D) for PETN.
54
Conclusions
The evaluation of the two API sources in this research are summarized in Table 2-11,
which compares the characteristic ions, absolute intensities, and relative intensities between
APCI and DPIS for analysis of explosives. Several observations should be noted, which help to
explain the difference in ionization mechanism and performance between DPIS and APCI.
The first observation is that DPIS typically gives more structural information through
increased fragmentation. That is presumably because the reaction region of DPIS, which
includes the space around the neon bulb, is far larger than the area around the corona discharge
needle tip where the ions are generated by APCI. DPIS creates more O2-, NO2
-, and NO3-
reactant ions, and they enhance proton abstraction and adduct formation reactions and have more
energetic reactions, increasing fragmentation. The generation of reactant ions also explains the
lower ionization efficiency of DPIS in the closed configuration because fewer and different
reactant ions are created. That is also why the spectra acquired by DPIS in the closed
configuration are more similar to those produced by APCI.
The second observation is that the spectra of explosive compounds produced by DPIS are
comparable to those formed by APCI; however, the formation of nitrate and nitrite adduct ions
with the explosives is more pronounced with the DPIS source. Typically, APCI produces a
complex spectrum of low intensity ions consisting of NO3-, M-, [M+NO2]-, [M+NO3]-, and other
fragment and background ions. The use of DPIS provides reactant ions of NO2- and NO3
-; the
spectra appear ‘cleaner’ even without the addition of chlorine, showing only the NO2- and NO3
-
reactant ions and the [M+NO2]- and [M+NO3]- product ions. This will greatly enhance sensitivity,
selectivity, and remove background interference, and will be a benefit for explosive investigation
in the field, where additives may not be available for use. In addition, different types of spectra
55
which either present more information about structure or more abundant molecular ions can be
obtained from DPIS by adjusting the amountt of surrounded air.
Lastly, APCI yields a higher ionization efficiency than DPIS for nitroaromatic compounds,
and for chlorine adducts of nitramines and nitrate esters. In fact, only a small portion of ions
generated by DPIS are able to be detected in this research due to the spatial obstruction of the
neon bulb, which is situated between the nebulizer and the inlet of mass spectrometer. Further
modifications of source geometry can be expected to improve the performance of DPIS.
In summary, DPIS has been shown in this research to ionize explosive compounds
efficiently, allowing for their identification. The design and low power consumption of DPIS
also make it ideal for portable applications Although the geometry of DPIS still needs to be
modified to obtain better performance, the rich ion patterns and decreased complexity of spectra
for nitramines and nitrate esters have provided an alternative choice other than corona discharge
for explosive investigation.
56
mesh
RF source voltage
DC offset voltage
Neon bulb
1cm
Figure 2-1. Configuration of DPIS. (A) Schematic, (B) Actual picture.
A B
57
20 30 40 50 60 70 80 90 10m/z
100
0
50
100
0
50
100
0
50
Rel
ativ
e A
bund
ance
100
0
50
37.0
59.154.8 74.9
64.9
33.047.1
64.9
33.047.1
64.9
33.049.9 60.142.1 47.1 73.958.9
Air
Methanol
Methanol/Water
TNT 10ppm
A
B
Figure 2-2. Comparison of reactant ions generated by DPIS observed with air, methanol, methanol/water, and 10 ppm TNT in (A) negative mode and (B) positive mode.
58
O2- O
O2+M O4-
O3-
O3CO3
-
CO4-
O
CO2
CO2
NO
NO
NO2-
NO3-
O3
NO
NO
NO2
NO2e-
e-
O2
y
Figure 2-3. Schematic procedure of reactant ions formation by DPIS.77
gas
Vaporizer
Mass Spectrometer
Heated Capillary
DPIS
Vaporizer
Heated Capillary
DPIS
Heated Capillary
DPIS
Closed Open Fully open
1 cm
Figure 2-4. Three different configurations where the DPIS was placed: closed, open and fully
open configuration.
59
2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0m /z
0
2 0
4 0
6 0
8 0
1 0 00
2 0
4 0
6 0
8 0
1 0 0
Rel
ativ
e A
bund
ance
0
2 0
4 0
6 0
8 0
1 0 01 2 4 .9 7
6 2 .1 7
6 2 .1 7
6 0 .1 71 2 4 .9 74 6 .0 7 7 8 .8 0
6 2 .1 0
6 0 .0 44 6 .0 7
9 2 .0 4
Fully open
NL:4.39 E4
Open
NL:4.72 E4
Close
NL:1.97E4
NO3-
HNO3NO3-
NO2-
CO3-
NO3NO-
NO3OH-
HNO3NO3-
Figure 2-5. Mass spectra of negative ions generated in air by DPIS with fully open, open and
closed configuration.
60
20 40 60 80 100 120 140 160 180 200m/z
0
20
40
60
80
1000
20
40
60
80
100
Rel
ativ
e A
bund
ance
0
20
40
60
80
10060.11
62.10
77.1478.8046.07 122.9132.10 91.97
60.13
61.1346.13 91.9332.0760.13
62.13
122.8759.13 77.1346.13
Fully open
Open
Close
NO3-
CO3-
NO2-O2
-
Figure 2-6. Mass spectra of negative ions generated in air by APCI with fully open, open and closed configuration.
61
0.00E+00
5.00E+04
1.00E+05
1.50E+05
2.00E+05
2.50E+05
3.00E+05
3.50E+05
4.00E+05
4.50E+05
0% 2% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50%
Inte
nsity
(cou
nts)
Gas concentration (O2 in N2, v/v)
Reactant ion of DPIS vs gas composition
125 [2NO3+H]-
124 [2NO3]-
62 [NO3]-
60 [CO3]-
46 [NO2]-
Figure 2-7. Comparison of the reactant ions intensity as a function of the composition between
oxygen and nitrogen.
Table 2-1. Gas-phase acidity values for reactant ions.95 Reactant ion m/z Hacid (kcal/mol)
O2- 31.9933 353
NO2- 46.0061 333.7
HCO3- 61.0168 334.6
Cl- 35.4527 328.1
CO3- 60.0089 ~324
NO3- 62.0049 317.8
62
100 150 200 250 300 350 400 450 500m /z
0
50
1000
50
1000
50
1000
50
100
Rel
ativ
e A
bund
ance
0
50
1000
50
1000
50
100227.07
213 .13
239 .07
242.13
182.07
182.07
152.20182.07
152.33168 .13 199.00
A
B
C
E
F
D
G
Figure 2-8. Negative APCI mass spectra of nitroaromatic compounds: (A) TNT (MW = 227), (B)
TNB (MW = 213), (C) Tetryl (MW = 287), (D) 2,4-DNT (MW = 182), (E) 2,6-DNT (MW = 182), (F) 3,4-DNT (MW = 182), (G) 1,3-DNB (MW = 168).
Table 2-2. Mass spectral data of nitroaromatic compounds analyzed by APCI-MS.
ExplosiveMolcularWeight(g/mole)
m/z (Ion Abundance)OpitmalVT ()
TNT 227.13 197(4%)[M-NO]-, 210(3%)[M-OH]-, 227(100%)[M]- 100
TNB 213.1183(3%)[M-NO]-, 213(100%)[M]-, 239(23%)[M+CN]-,244(8%)[M+CH3O]- 100
Tetryl 287.14 241(68%)[M-NO2]-, 242(100%)[M-NO2+H]-, 313(4%)[M+CN]- 130
2,4-DNT 182.13 165(3%)[M-OH]-, 182(100%)[M]- 130
2,6-DNT 182.13 152(7%)[M-NO]-, 182(100%)[M]-, 213(3%)[M+CH3O]- 130
3,4-DNT 182.13 152(5%)[M-NO]-, 182(100%)[M]- 130
1,3-DNB 168.11 138(4%)[M-NO]-, 168(100%)[M]-, 199(85%)[M+CH3O]- 130
63
A
B
C
E
F
D
G
1 0 0 2 0 0 3 0 0 4 0 0 5 0 0m /z
0
5 0
1 0 00
5 0
1 0 00
5 0
1 0 00
5 0
1 0 0
Rel
ativ
e A
bund
ance
0
5 0
1 0 00
5 0
1 0 00
5 0
1 0 0 2 2 6 .2 0
2 5 3 .0 01 9 7 .2 7
2 1 3 .1 32 3 9 .0 71 8 3 .2 7 2 5 8 .8 7
2 7 5 .8 76 2 .1 32 4 1 .1 3
2 5 6 .9 33 2 9 .0 72 2 8 .2 71 8 1 .0 7
1 8 2 .0 7
1 6 6 .2 7 1 9 7 .0 71 8 2 .0 7
1 5 2 .2 71 8 2 .0 7
3 5 0 .0 71 5 2 .2 7
6 2 .1 3 3 0 3 .0 71 9 7 .1 3 2 4 4 .0 01 2 1 .3 3 3 6 1 .0 01 6 8 .1 3
1 3 8 .2 7 1 9 4 .0 0
Figure 2-9. Negative DPIS mass spectra of nitroaromatic compounds in the closed configuration:
(A) TNT (MW = 227), (B) TNB (MW = 213), (C) Tetryl (MW = 287), (D) 2,4-DNT (MW = 182), (E) 2,6-DNT (MW = 182), (F) 3,4-DNT (MW = 182), (G) 1,3-DNB (MW = 168).
Table 2-3. Mass spectral data of nitroaromatic compounds analyzed by DPIS-MS in the closed configuration.
ExplosiveMolcularWeight(g/mole)
m/z (Ion Abundance) Opitmal
VT ()
TNT 227.13197(12%)[M-NO]
-, 226(100%)[M-H]
-, 227(39%)[M]
-, 253(37%)[M+CN]
-,
260(16%)[M-NO+HNO3]- 100
TNB 213.1 183(57%)[M-NO]-, 213(100%)[M]
-, 239(64%)[M+CN]
-, 100
Tetryl 287.14241(100%)[M-NO2]
-, 257(48%)[M-NO]-, 313(7%)[M+CN]-,
329(18%)[M+CNO]-130
2,4-DNT 182.13 166(7%)[M-NO2+NO]-, 182(100%)[M]-, 197(2%)[M-HNO2+NO3]- 100
2,6-DNT 182.13 152(6%)[M-NO]-, 182(100%)[M]- 130
3,4-DNT 182.1362(31%)[NO3]
-, 152(54%)[M-NO]-, 182(11%)[M]-, 303(27%)[2M-CH2-
HNO2]-, 350(75%)[2M-CH2]
-100
1,3-DNB 168.11 138(5%)[M-NO]-, 168(100%)[M]
- 130
64
A
B
C
E
F
D
G
1 0 0 2 0 0 3 0 0 4 0 0 5 0 0m /z
0
5 0
1 0 00
5 0
1 0 00
5 0
1 0 00
5 0
1 0 0
Rel
ativ
e A
bund
ance
0
5 0
1 0 00
5 0
1 0 00
5 0
1 0 0 2 2 6 .2 01 9 7 .2 7
2 7 3 .9 36 2 .1 3 3 0 4 .8 71 8 3 .2 71 4 0 .9 3
1 8 3 .2 72 5 8 .8 7
6 2 .1 32 4 1 .1 3
2 5 6 .9 3
3 0 3 .8 06 2 .1 3 2 2 7 .2 71 8 1 .0 71 8 1 .2 7
2 2 6 .2 01 6 7 .2 0 2 6 4 .3 36 2 .1 3 3 4 6 .0 01 8 2 .0 7
1 5 2 .2 0 1 9 7 .2 76 0 .1 31 6 8 .0 7
3 5 0 .0 71 8 2 .0 7 3 0 3 .0 71 5 2 .2 7 2 1 4 .0 0
1 6 8 .1 3
1 3 8 .2 71 8 3 .2 76 2 .1 3
Figure 2-10. Negative DPIS mass spectra of nitroaromatic compounds in the open configuration:
(A) TNT (MW = 227), (B) TNB (MW = 213), (C) Tetryl (MW = 287), (D) 2,4-DNT (MW = 182), (E) 2,6-DNT (MW = 182), (F) 3,4-DNT (MW = 182), (G) 1,3-DNB (MW = 168).
Table 2-4. Mass spectral data of nitroaromatic compounds analyzed by DPIS-MS in the open configuration.
ExplosiveMolcularWeight(g/mole)
m/z (Ion Abundance) Opitmal
VT ()
TNT 227.13197(68%)[M-NO]
-, 226(100%)[M-H]
-, 227(14%)[M]
-,
260(29%)[M-NO+HNO3]-, 274(41%)[M+HNO2]
- 100
TNB 213.1183(100%)[M-NO]
-, 213(87%)[M]
-, 244(22%)[M+CH3O]
-,
259(78%)[M+NO2]-
100
Tetryl 287.14 241(100%)[M-NO2]-, 257(67%)[M-NO]
- 130
2,4-DNT 182.13 181(100%)[M-H]-, 197(81%)[M-HNO2+NO3]
-, 226(25%)[M-H+NO2]
- 250
2,6-DNT 182.13 152(17%)[M-NO]-, 182(100%)[M]
-, 197(5%)[M-HNO2+NO3]
- 130
3,4-DNT 182.13138(13%)[M-CH2-NO]
-, 152(14%)[M-NO]
-, 168(100%)[M-CH2]
-,
182(11%)[M]-, 197(9%)[M-H+O]
-, 350(38%)[2M-CH2]
-130
1,3-DNB 168.11 138(37%)[M-NO]-, 168(100%)[M]
-, 183(11%)[M-HNO2+NO3]
- 130
65
A
B
C
D
100 150 200 250 300 350 400 450 500m/z
0
50
1000
50
1000
50
100
Rel
ativ
e A
bund
ance
0
50
100324.00 489.67
268.00387.87175.93 219.40123.27 469.67102.13 129.13 281.00 344.87220.93 439.00189.07
478.80
257.07
259.00
295.00123.27 203.07342.07
166.27219.33
398.07 499.07322.00121.20 419.00129.20 230.00 370.80331.13
284.07 333.13286.07
Figure 2-11. Negative APCI mass spectra of nitramines: (A) RDX (MW = 222), (B) RDX + Cl, (C) HMX (MW = 296), (D) HMX + Cl.
Table 2-5. Mass spectral data of nitramines analyzed by APCI-MS.
ExplosiveMolcularWeight(g/mole)
m/z (Ion Abundance) OpitmalVT ()
RDX 222.12102(21%)[C2H4N3O]-, 123(36%)[HNO3NO+CH3O]-, 176(42%)[M-NO2]
-, 268(64%)[M+NO2]-, 324(100%)[M+C2H4N3O]-, 100
RDX+Cl 257(45%)[M+35Cl]-, 259(13%)[M+37Cl]-, 479(100%)[2M+35Cl]-,481(31%)[2M+37Cl]- 100
HMX 296.16123(92%)[NO3NO+CH3O]-, 166(57%)[M-C2N2O2-NO2]
-,203(84%)[M-HNO2NO2]
-, 219(39%)[M-HNO2NO]- , 295(100%)[M-H]-, 342(68%)[M+NO2]
-
130
HMX+Cl284(40%)[M+35Cl-HNO2]
-, 286(13%)[M+37Cl-HNO2]-,
331(100%)[M+35Cl]-, 333(31%)[M+37Cl]- 130
66
A
B
C
D
100 150 200 250 300 350 400 450 500m/z
0
50
1000
50
1000
50
100
Rel
ativ
e A
bund
ance
0
50
100489.67268.00
284.0062.13
478.80
257.13268.00
62.13342.07
295.00
331.13
333.13284.07
Figure 2-12. Negative DPIS mass spectra of nitramines in the closed configuration: (A) RDX
(MW = 222), (B) RDX + Cl, (C) HMX (MW = 296), (D) HMX + Cl.
Table 2-6. Mass spectral data of nitramines analyzed by DPIS-MS in the closed configuration.
ExplosiveMolcularWeight(g/mole)
m/z (Ion Abundance)Opitmal
VT ()
RDX 222.12 62(16%)[NO3]-, 268(100%)[M+NO2]
-, 284(39%)[M+NO3]
-,
490(96%)[2M+NO2]-
100
RDX+Cl62(5%)[NO3]
-, 257(40%)[M+
35Cl]
-, 259(13%)[M+
37Cl]
-,
268(16%)[M+NO2]-, 284(6%)[M+NO3]
-, 479(100%)[2M+
35Cl]
-,
481(31%)[2M+37
Cl]-, 490(15%)[2M+NO2]
-,
100
HMX 296.16 295(36%)[M-H]-, 342(100%)[M+NO2]
- 130
HMX+Cl284(11%)[M+
35Cl-HNO2]
-, 286(3%)[M+
37Cl-HNO2]
-, 331(100%)[M+
35Cl]
-
, 333(32%)[M+37
Cl]-, 342(3%)[M+NO2]
-250
67
A
B
C
D
100 150 200 250 300 350 400 450 500m/z
0
50
1000
50
1000
50
100
Rel
ativ
e A
bund
ance
0
50
100268.00 489.67
284.00
62.13
478.80
284.00268.00
62.13
342.07
295.00
62.13 356.93123.27 264.40331.20
333.13284.07
Figure 2-13. Negative DPIS mass spectra of nitramines in the open configuration: (A) RDX
(MW = 222), (B) RDX + Cl, (C) HMX (MW = 296), (D) HMX + Cl.
Table 2-7. Mass spectral data of nitramines analyzed by DPIS-MS in the open configuration.
ExplosiveMolcularWeight(g/mole)
m/z (Ion Abundance)OpitmalVT ()
RDX 222.1262(27%)[NO3]
-, 268(100%)[M+NO2]-, 284(67%)[M+NO3]
-,490(92%)[2M+NO2]
- 100
RDX+Cl62(20%)[NO3]
-, 257(32%)[M+35Cl]-, 268(41%)[M+NO2]-,
284(54%)[M+NO3]-, 479(100%)[2M+35Cl]-, 490(45%)[2M+NO2]
- 100
HMX 296.16 295(23%)[M-H]-, 342(100%)[M+NO2]- 130
HMX+Cl284(13%)[M+35Cl-HNO2]
-, 331(100%)[M+35Cl]-, 333(33%)[M+37Cl]-,342(19%)[M+NO2]
- 250
68
100 150 200 250 300 350 400 450 500m/z
0
50
1000
50
1000
50
100
Rel
ativ
e A
bund
ance
0
50
10062.13
288.80
272.80125.00
261.87
62.13
263.80 488.33
62.13
377.93
314.87361.87219.40123.27 166.27 481.7389.20
350.87
352.80
A
B
C
D
Figure 2-14. Negative APCI mass spectra of nitrate esters: (A) NG (MW = 227), (B) NG + Cl, (C) PETN (MW = 316), (D) PETN + Cl.
Table 2-8. Mass spectral data of nitrate esters analyzed by APCI-MS.
ExplosiveMolcularWeight(g/mole)
m/z (Ion Abundance) Opitmal
VT ()
NG 227.0962(100%)[NO3]
-, 125(7%)[HNO3NO3]-, 273(11%)[M+NO2]
-,289(60%)[M+NO3]
- 100
NG+Cl62(54%)[NO3]
-, 262(100%)[M+35Cl]-, 264(30%)[M+37Cl]-,289(15%)[M+NO3]
-, 488(21%)[2M-H+35Cl]- 100
PETN 316.1462(100%)[NO3]
-, 123(18%)[NO3NO+CH3O]-, 315(31%)[M-H]-,347(14%)[M+CH3O]-, 362(20%)[M+NO2]
-, 378(77%)[M+NO3]- 130
PETN+Cl 351(100%)[M+35Cl]-, 353(32%)[M+37Cl]- 100
69
A
B
C
D
100 150 200 250 300 350 400 450 500m/z
0
50
1000
50
1000
50
100
Rel
ativ
e A
bund
ance
0
50
10062.13
288.80
272.80108.93 228.00
261.87
62.13
263.80 488.53
361.87
62.13 377.93351.07
350.87
352.87
62.13
Figure 2-15. Negative DPIS mass spectra of nitrate esters in the closed configuration: (A) NG
(MW = 227), (B) NG + Cl, (C) PETN (MW = 316), (D) PETN + Cl.
Table 2-9. Mass spectral data of nitrate esters analyzed by DPIS-MS in the closed configuration.
ExplosiveMolcularWeight(g/mole)
m/z (Ion Abundance)OpitmalVT ()
NG 227.09 62(100%)[NO3]-, 273(27%)[M+NO2]
-, 289(72%)[M+NO3]- 100
NG+Cl62(62%)[NO3]
-, 262(100%)[M+35Cl]-, 264(29%)[M+37Cl]-,289(3%)[M+NO3]
-, 488(21%)[M+35Cl]- 100
PETN 316.14 62(18%)[NO3]-, 315(4%)[M-H]-, 362(100%)[M+NO2]
-, 100
PETN+Cl 62(5%)[NO3]-, 351(100%)[M+35Cl]-,353(32%)[M+37Cl]- 130
70
A
B
C
D
100 150 200 250 300 350 400 450 500m/z
0
50
1000
50
1000
50
100
Rel
ativ
e A
bund
ance
0
50
10062.13 288.80
272.8786.20 109.0062.13
261.93
288.80
488.4786.20
361.93
377.9362.13
350.93
352.8762.13 377.87
Figure 2-16. Negative DPIS mass spectra of nitrate esters in the open configuration: (A) NG
(MW = 227), (B) NG + Cl, (C) PETN (MW = 316), (D) PETN + Cl.
Table 2-10. Mass spectral data of nitrate esters analyzed by DPIS-MS in the open configuration.
ExplosiveMolcularWeight(g/mole)
m/z (Ion Abundance)OpitmalVT ()
NG 227.09 62(100%)[NO3]-, 273(16%)[M+NO2]
-, 289(91%)[M+NO3]- 100
NG+Cl62(100%)[NO3]
-, 262(75%)[M+35Cl]-, 264(30%)[M+37Cl]-,273(9%)[M+NO2]
-, 289(51%)[M+NO3]-, 488(15%)[2M-H+35Cl]- 100
PETN 316.14 62(21%)[NO3]-, 315(4%)[M-H]-, 362(100%)[M+NO2]
-, 100
PETN+Cl62(12%)[NO3]
-, 315(3%)[M-H]-, 351(100%)[M+35Cl]-,353(33%)[M+37Cl]-, 362(22%)[M+NO2]
-, 378(11%)[M+NO3]- 130
71
Table 2-11. The main ions of explosive compounds determined by APCI and DPIS.
molecularcode MW Major ion
(m/z )Intensity(counts)
RelativePercentage
Major ion(m/z )
Intensity(counts)
Relativepercentage
Major ion(m/z )
Intensity(counts)
Relativepercentage
TNT 227.13 227[M]- 6.25E+06 100% 226[M-H]- 4.15E+05 100% 226[M-H]- 2.24E+05 100%TNB 213.1 213[M]- 1.39E+06 100% 183[M-NO]- 1.78E+05 100% 213[M]- 5.19E+04 100%Tetryl 287.14 242[M-NO2+H]- 1.79E+06 100% 241[M-NO2] 8.23E+05 100% 241[M-NO2] 1.06E+04 100%24DNT 182.13 182[M]- 2.30E+06 100% 181[M-H]- 6.87E+04 100% 182[M]- 2.02E+04 100%26DNT 182.13 182[M]- 3.57E+06 100% 182[M]- 1.61E+05 100% 182[M]- 2.46E+04 100%34DNT 182.13 182[M]- 1.42E+06 100% 168[M-CH2]- 2.27E+05 100% 182[M]- 1.07E+04 100%13DNB 168.11 168[M]- 6.24E+05 100% 168[M]- 8.21E+04 100% 168[M]- 3.87E+03 100%RDX 222.12 268[M+NO2]
- 1.84E+05 63% 268[M+NO2]- 1.41E+05 100% 268[M+NO2]
- 1.32E+05 100%RDX+Cl 257 [M+35Cl]- 6.77E+05 45% 284 [M+NO3]
- 1.15E+05 54% 257 [M+35Cl]- 1.65E+05 40%HMX 296.16 295[M-H]- 1.15E+05 100% 342 [M+NO2]
- 1.46E+05 100% 342 [M+NO2]- 2.74E+05 100%
HMX+Cl 331 [M+35Cl]- 5.86E+05 100% 331 [M+35Cl]- 1.13E+05 100% 358 [M+NO3]- 3.15E+05 100%
PETN 316.14 378[M+NO3]- 2.52E+05 77% 362[M+NO2]
- 3.86E+05 100% 362[M+NO2]- 2.20E+05 100%
PETN+Cl 351 [M+35Cl]- 1.96E+06 100% 351 [M+35Cl]- 4.54E+05 100% 351 [M+35Cl]- 9.04E+05 100%NG 227.09 289[M+NO3]
- 1.24E+05 60% 289[M+NO3]- 7.25E+04 91% 289[M+NO3]
- 3.51E+04 72%NG+Cl 262[M+35Cl]- 4.20E+05 100% 262 [M+35Cl]- 6.44E+04 75% 262 [M+35Cl]- 1.93E+05 100%
APCI DPIS(open) DPIS(closed)
72
CHAPTER 3 FUNDAMENTALS OF HIGH-FIELD ASYMMETRIC WAVEFORM ION MOBILITY
SPECTROMETRY (FAIMS) FOR THE ANALYSIS OF EXPLOSIVES
Introduction
FAIMS is a sensitive and selective technology for the detection and identification of trace
constituents in ambient air or liquid samples. This technology separates gas-phase ions based on
certain properties of ions that appear to be independent of both the low-field collision cross
section and the mass-to-charge ratio.51 The separation of ions by FAIMS is fast and, therefore,
may replace slower separation techniques, such as gas chromatography (GC), capillary
electrophoresis (CE) and high performance liquid chromatography (HPLC). Furthermore, the
signal-to-noise ratio (S/N) can be improved by passing ions that originate from atmospheric
pressure sources such as APCI and DPIS through a FAIMS device to select the ion of interest in
preference to the background ions. The improvement of S/N can lead to increased detection
limits, which, in some cases, will allow simplification of sample handling via preconcentration or
extraction.36 Therefore, FAIMS can offer an additional level of separation to simplify complex
mixtures that already provided by chromatography and the m/z separation by a mass analyzer.
The cylindrical geometry of FAIMS used in this research embodies a unique capability of
focusing ions by an electric field at atmospheric pressure.96 This gives higher sensitivity than
commercial IMS, in which ion diffusion in the drift tube causes the ion cloud to expand,
resulting in reduced transfer of ions to the mass spectrometer.53 The appearance of ion focusing
is known to depend on ion polarity, α-dependence sign (positive sign corresponds to an increase
in ion mobility with increasing field strength), and separation field polarity.97 The use of APCI-
FAIMS-MS can be expected to eliminate the need for chromatographic separation and allows for
very rapid sample processing and sensitive detection.
73
Investigations of FAIMS for explosive detection have been performed by several groups.
Buryakov et al. described the detection of explosive vapors in air using FAIMS,98 the qualitative
analysis of explosives by FAIMS99, and the analysis of explosives with multicapillary-column
gas chromatography and FAIMS.100 Eiceman et al.32 examined the separation of ions from
explosives in FAIMS by vapor-modified drift gas.
Three trends in ion mobility, as a function of electric field, have been reported.51 As
electric field strength increases, the mobility of a type A ion increases, a type C ion decreases,
and a type B ion initially increases before decreasing. These differences in ion behavior are
ascribed to interactions of the ion structure, collisional cross-section, and instrumental
parameters including dispersion voltage (DV), carrier gas composition, electrode temperature,
and others. The ion mobility at high-field strength and, hence the observed compensation
voltage, is also affected by the gas composition. This is likely due to long-range, ion-induced
dipole attractive interactions between the ion and the bath gas. The strength of the interaction
depends on the polarizability of the bath gas and the size and charge of the ion of interest.47
Changes in temperature are reflected in changes of the thermal energy of the source, which can
change the ion-induced dipole interaction’s potential well relative to the thermal energy of the
source.49 This may cause the compensation voltage (CV) to shift as the temperature changes.
The aim of this research is to study the effect of DV, CV scan rate, curtain gas flow rate,
carrier gas composition, and electrode temperature on the separation of negative ions from
eleven explosives (TNT, TNB, tetryl, 1,3-DNB, 2,4-DNT, 2,6-DNT, 4-DNT, RDX, HMX, NG,
and PETN). The other goal is to find optimal parameters of FAIMS separation and to assess
their analytical characteristic in the detection of explosives.
74
Experimental
The use of FAIMS requires the optimization of several parameters in order to obtain the
maximum benefit from the FAIMS device. For the purpose of this work, five parameters were
optimized: DV, CV scan rate, curtain gas flow rate, carrier gas composition, and electrode
temperature. The effects that these parameters have on the CV value, the signal intensity, and
the peak width were monitored for target explosive compounds.
Experiments were performed employing a FAIMS-MS system, comprising of a cylindrical
FAIMS device (Thermo Scientific, San Jose, CA) and a quadrupole ion trap mass spectrometer
(LCQ, Thermo Scientific). Gas-phase explosive ions were generated by atmospheric pressure
chemical ionization (APCI) using a corona discharge needle positioned at an angle of 45° and ~1
cm from the opening of the curtain plate of FAIMS device. The cylindrical FAIMS cell consists
of two electrodes, an inner and outer electrode. The combination of inner electrode, having an
outer radius of 6.5 mm, and outer electrode, having an inner radius of 9.0 mm, makes a gap of
2.5 mm for ion transmission. The asymmetric waveform (750 kHz) and the direct current (DC)
CV were both applied to the inner electrode of the FAIMS cell. The DV was ±2500 V to ±4500
V. The DV is measured as the magnitude of the high-voltage pulse of the asymmetric waveform.
A constant DC bias voltage of −25 V was applied to the outer cylinder of the FAIMS device and
to the inlet of the mass spectrometer. The curtain plate was held at -1000 V to assist negative
ions to transport across the desolvation region. In order to connect the Thermo FAIMS cell,
designed for Thermo TSQ mass spectrometer, onto the LCQ, a brass capillary extender (i.d. =
0.76 mm, o.d. = 22 mm) (Figure 3-1 and 3-2) was designed to serve as an interface. The setup of
the APCI-FAIMS-MS instrument is shown in Figure 1-11.
All ions of a given polarity and ion type (A or C) can be analyzed by scanning the CV,
which results in a CV spectrum, also called the total ion current-CV spectrum (TIC-CV). An
75
ion-selective CV spectrum (IS-CV) can be obtained for each individual ion by plotting the
intensity of ions of that specific m/z versus CV. The CV was scanned from -20.0 to 20.0 V at a
scan rate between 5.0 and 20.0 V/min. CV values and signal intensity were then taken from the
maximum peak height in the IS-CV. The peak width is taken from the full width at half
maximum (FWHM) of the peak. The mean of each value for three replicates is reported in these
studies.
The FAIMS carrier gases were passed through separate charcoal/molecular sieve filters
before being mixed together and introduced into the region between the curtain plate and the
orifice of the FAIMS analyzer at a flow rate ranging from 2.0 to 3.5 L/min. In this research, N2,
He, CO2, SF6, and the mixture of these gases were used to evaluate their influence on the CV,
peak width and signal intensity for the ions of interest.
To control the temperature of inner and outer electrodes, channels for the passage of heated
gas were drilled into both electrodes. The outer electrode consists of a cylinder with an inner
diameter (i.d.) of 18 mm that was bored into a solid block of stainless steel. Channels to the left
and right of the cylinder carry air in and out of the block. The inner electrode has a PEEK insert
that directs gas to the top of the electrode and then along the inner surface of the electrode to an
exhaust port. Under conditions of active heating of the electrode, the inner and outer electrode
temperatures were set between 40-90.
For the APCI source, the vaporizer temperature was set to 150 °C. The heated capillary
temperature and voltage were set to 130 °C and -25.0 V, respectively. The discharge current was
set at 5 μA and the tube lens offset was set to 30.0 V. The sheath gas (N2) was set to 20.0
(arbitrary units) and the injection flow rate of the analyte was maintained at 20.0 μL/min.
76
Eleven explosive compounds (TNT, TNB, tetryl, 1,3-DNB, 2,4-DNT, 2,6-DNT, 4-DNT,
RDX, HMX, NG, and PETN) were studied. These explosives were provided as acetonitrile
solution by Dr. Jehuda Yinon of the Weizmann Institute of Science, and were obtained from the
Analytical Laboratory of the Israeli Police Headquarters. The explosive solutions were further
diluted in a solvent containing 65% methanol and 35% deionized water to a concentration of 10
μg/mL. Approximately 0.1% of carbon tetrachloride was used as an additive in some solutions
to form stable adduct ions with nitramine and nitrate ester explosives.
Results and Discussion
Effects of CV Scan Rate
For practical applications, narrow peak width, high transmission, and short detection time
are desired for separation and detection techniques. In this experiment, CV scan rates were
explored to acquire better resolution, transmission, and minimized detection time.
The effect of the scan rate on CV was characterized. Spectra for TNT (m/z 227) were
collected at various scan rates from 2.5 to 20.0 V/sec; the CV value, peak width, and signal
intensity plotted as a function of the scan rate are shown in Figure 3-3. The results show that
scans with lower scan rates lead to narrower peak widths, higher intensity, and relatively
constant CV value. Higher resolution (narrower peaks) and transmission (signal intensity) would
be expected when using a lower scan rate because, for a given CV range, increased increments of
the RF voltage permits more ions to be transmitted at the optimum CV. However, lower scan
rates extend the detection time, which is not favorable for field applications. A compromised
scan rate at 10 V/s was applied throughout the following research, which induced a 30% increase
in peak widths, a 10% decrease in signal intensity, yet a 4-fold increase in detection times,
compared to a scan rate of 2.5 V/s.
77
Effect of Curtain Gas Flow Rate
In FAIMS, it is essential to introduce clean, dry gas into the FAIMS cell to achieve
optimum performance. This gas is referred to as the curtain gas and is introduced into the region
between the curtain plate and the orifice into the FAIMS analyzer. The majority of the gas exits
through the curtain plate to aid in desolvation of ions from the APCI source and to minimize the
entrance of droplets and neutral molecules from the solvents into the FAIMS analyzer. The
remainder of the gas is drawn into the FAIMS analyzer at a flow rate of ~0.7 L/min, which
carries the ions around both sides of the inner cylinder and through the heated capillary into the
mass spectrometer.
In this experiment, nitrogen was used as the curtain gas. The data were acquired with the
DV at 4000 V for HMX and PETN, and the DV at -4000V for the rest of explosive compounds.
The curtain gas flow rate was varied from 2.0 L/min to 3.5 L/min. Figure 3-4, 3-5, and 3-6 show
the effects that curtain gas flow rate has on the CV value, peak width, and signal intensity,
respectively.
Figure 3-4 illustrates the effect of increasing the gas flow rate from 2.0 L/min to 3.5 L/min
on the CV for explosive compounds. When increasing the curtain gas flow rate, it was observed
from the plot that the CV remained relatively constant for all compounds. This result verifies
that the gas flow rate has no effect on the CV.
Figure 3-5 shows that the narrowest peak width was achieved for nine of the ten
compounds at the minimum curtain gas flow rate of 2 L/min. This is unexpected, since an ion’s
resident time in the FAIMS cell is determined by the carrier gas flow rate passing through the
cell, and that should be constant at 0.7 L/min, set by the conductance of the heated capillary into
the vacuum of the mass spectrometer. In this case, however, the connection between the FAIMS
cell and the brass capillary extender and the extender and the heated capillary were not gas-tight.
78
At higher flow rates (and thus higher pressures), leakage at these connections will increase,
increasing the flow rate through the FAIMS cell. At lower curtain gas flow rates, the flow
through the FAIMS cell will be lower, and resident time for ions will increase. If an ion stays
longer in the FAIMS cell, the ion experiences more cycles of the waveform that is used to
resolve different compounds. Therefore, lower carrier gas flow rates enables the FAIMS to more
accurately resolve ions with adjacent CV values, which means higher resolutions or narrower
peak widths can be achieved.
Operating FAIMS at very low curtain gas flow rates affects the signal intensity by
inadequately desolvating ions as they enter the FAIMS cell through the orifice in the curtain
plate. On the contrary, if the flow rate is too high it may cause a decrease in intensity due to a
decreased number of ions entering the FAIMS cell that have to compete against the high flow
rate of gas exiting the orifice in the curtain plate.101 Figure 3-6 demonstrates that only HMX,
PETN, and TNT have an apparent drop in signal intensity with increasing curtain flow rate while
the rest of compounds stay relatively stable. HMX, PETN, and TNT are compounds with higher
molecular weights and larger cross sections that may enhance the interaction between these ions
and curtain gas. Consequently, these compounds may have a reduced number of ions entering
the FAIMS cell when higher curtain flow rate is applied.
Considering the effect caused by curtain flow rate on the CV value, peak width, and signal
intensity, the optimum performance of these ions was achieved at a gas flow rate of 2.0 L/min to
2.5 L/min.
Effects of DV
The separation of ions in FAIMS is based on the change in mobility of an ion in strong
electric fields. Three trends in ion mobility have been reported that show that as electric field
79
strength (dispersion voltage) increases the mobility of a type A ion increases, a type C ion
decreases, and a type B ion increases initially before decreasing. 51
The cylindrical geometry of the FAIMS cell used in this research has been proven to have
the ability to focus the ions as they are transmitted.53, 96 The apparently anomalous increase of
sensitivity with increasing applied asymmetric waveform voltage, and the behavior of ions with
the change of polarity of the waveform, to the conclusion that the device was focusing ions.52
Therefore, the magnitude and polarity of the dispersion voltage were both evaluated in this
research to explore their effects on explosives separation. The waveform with negative DV
yields spectra of type N1 for negative ions, whereas the reversed polarity waveform yields N2
type spectra for negative ions. In general, low mass ions (m/z is usually below 300) are type A
ions and are detected in N1 mode, whereas larger ions are type C ions and are detected in N2
mode. The compounds studied in the research produce ions of all three ions.
The DV is the maximum peak of the voltage applied and was varied in these studies from
±2500 V to ±4500 V in 500 V increments. Figure 3-7 and 3-8 illustrate the effect of increasing
DV on CV and mass spectra for TNT. As the magnitude of DV increases, two trends are
apparent: first, the peak shifts to more positive CV values; and second, an increase in signal
intensity for the selected TNT ion (m/z 227). Additionally, the separation of selected ions was
improved as the DV increases, generating more specific ion patterns and less background in the
mass spectra. Similar phenomenon can be also observed for the other explosives, as detailed in
the following sections.
CV value
Table 3-1 and Figure 3-9 present the values and plot of CV as a function of DV. Generally
for most ions, the magnitude of the CV increases as the DV increases in both DV polarities;
however, the C type ions, such as the Cl- adducts of PETN and HMX, show a greater CV shift in
80
N2 mode. Explosive ions which appear at positive CV during application of negative DV can be
also seen at negative CV when the polarity of the applied DV is reversed. However, as will be
discussed below, the signal intensities for those ions under reversed DV decreased since the
focusing action within the analyzer region was reversed to a defocusing action.
A greater CV shift was observed at higher fields because the ions can vary conformation
between low and high fields when the field increases and, hence, more energy is imparted on the
explosive ions.102 The more the conformations vary between high and low fields, the greater the
change in mobilities, and the greater CV.
Ion mobility at high-field strength is a result of the interaction between the ion and the bath
gas,36 which is strongly determined by ion size, shape, rigidity, and properties of the bath gas.103
Lighter explosive ions can be observed at higher CV values than heavier ions because ions with
smaller size normally have a greater percentage change in cross section than larger ions.
However, not all ions possessing same molecular weight show up at the identical CV value; for
instance, the three DNT isomers appear at different CV values because of the variations in how
the isomeric ions interact with the bath gas.
Signal intensity
The spatially inhomogeneous electric field in the cylindrical geometry FAIMS not only
separates ions but also focuses the ions which are at their correct CV values. Therefore, in most
cases, the transmission of ions increases with field strength using cylindrical geometry FAIMS.
But focusing is possible only for the ions with a noticeable α-dependence in a fairly high field.
Only the ions with substantial field dependence of mobility can effectively be focused.97 It has
also been reported that increasing DV improves the signal intensity for small ions more than for
large ones.104
81
Table 3-1 and Figure 3-10 present signal intensity as a function of the DV. For type A and
most type B ions, including TNT, TNB, Tetryl, 1,3-DNB, and DNT isomers, the largest signal
intensities were recorded at maximum DV in N1 mode, with relatively low signal intensity in N2
mode. In contrast, the transmission is maximized with increasing DV in N2 mode for some type
B ions and type C ions, including the Cl- adducts of RDX, HMX, PETN, and NG. The signal of
these ions substantially decreases starting at a DV of -3500 V in N1 mode because the decreased
ion mobility of type B ions appears when those ions experience higher electric field. This
situation mainly depends on a number of factors, namely, an increase in the diffusion coefficient
in a strong field, an increase in the amplitude of ion oscillation in the gap of the separation
chamber, and a decrease in the focusing efficiency with a decrease in α(E/N).85 The increasing
transmission at higher DV for most compounds is due to ion focusing under the effect of the
gradient of the alternating field with an unbalanced polarity, which decreases ion losses to the
walls of the FAIMS analyzer.
Peak width
The focusing of ions in cylindrical geometry is a principal factor that contributes to the
shape of the peaks obtained when sweeping the CV. The fundamental physics responsible for
peak shapes has been described in terms of the confining effect of ion focusing between
cylindrical FAIMS electrodes and the dispersive effects of diffusion and ion-ion repulsion.104
Guevremont et al.105 also reported that the widths of peaks in FAIMS are a function of the
applied DV as well as the radii of the electrodes. The peaks are narrow at low applied DV and
largest electrode radii. For a given peak in the CV scan, the lowest CV of ion transmission is
characterized by an optimum ion focus point located near the outer electrode. As the CV
increases to pull the ion cloud closer to the inner electrode, this focus point migrates towards the
inner electrode. The peak width is determined by the applied CV range, which allows the ion
82
cloud to migrate through the cell between the outer and inner electrodes. Ions are rapidly lost to
the walls when the focus location is inside the wall of either electrode, less rapidly if the focus
location is between the electrodes but is near the wall of either electrode, and minimally if the
focus point falls midway between the electrodes.
Figure 3-11 shows that type A and most type B ions have broader peaks as DV increases in
N1 mode; however, some heavier ions such as the Cl- adducts of PETN and HMX have narrower
peaks when higher DVs are applied in N2 mode. Ions with larger peak widths due to increased
ion focusing are more efficiently transmitted through the FAIMS device at higher DV values.36
The peak widths increase for type A ions in N1 mode with CV for maximum ion transmission
but do not depend on m/z or molecular weight.42
The mass spectra collected from APCI/MS and APCI/FAIMS/MS, as seen in Figure 3-12,
show that the major ions for both approaches give identical base peaks for most explosives at a
concentration of 10 μg/mL, except for some differences between the isomers of DNT, which, in
turn, may be helpful to discriminate these isomers from each other. Fewer fragment ions, cluster
ions, and background ions can be observed in the APCI/FAIMS/MS spectra showing that
FAIMS can discriminate against background and thereby dramatically increase the S/N, reducing
or eliminating the need for chromatographic separation.
Effects of Carrier Gas Composition
A quantitative description of the interaction between ions and the bath gas is a potential
well of a given depth in an energy diagram at a given temperature.36 The impact of this
interaction on the observed CV value depends on the depth of the well in comparison to the
thermal energy of the bath gas. The deeper the well relative to the thermal energy, the more
mobility will increase with E/N where N is the number density of the bath gas. The mobility
increases because of energy gains during collisions. If the thermal energy is similar to or greater
83
than the potential well, the ion mobility can increase less rapidly or will actually decrease with
E/N. This change in ion mobility results from energy losses during collision between the gas and
analyte ions.36, 47
Ion mobility at high-field strength is a result of the interaction between the ion and the bath
gas.36 To date, most FAIMS work has employed N2 (or air) or a He/N2 mixture;106-108 O2, CO2,
N2/CO2, He/SF6, and other compositions were also explored.38, 47, 109-111 Shvartsburg et al.111
reported spectacular non-Blanc effects in mixture of disparate gases such as He/CO2 or He/SF6
and described the solution as
1/Kmix(E) = ΣxjRj/Kj(E) (3-1)
where K mix is the mobility of an ion in a mixture of any number of gases, and each coefficient Rj
satisfies
RjKj[ ( (m+Mj) Σ ΣwiRi )-1/2] = Kj(E)EKj(E)
xiRi
Ki(E) (3-2)
where E is the strength of electric field, m and Mj are the molecular masses of the ion and the i-th
component of gas mixture, and wi terms are given by
wi = xi/[(m+Mi)Ki(E)] (3-3)
The basis of this effect is related to the widely differing molecular masses of these gases,
and the large difference in the mobility of an ion in each of the pure gases that compose these
mixtures. Resolution and sensitivity of FAIMS using binary and ternary mixtures is often better
than that with any individual component because of non-Blanc behavior of ion mobilities at high
electric fields.38, 110, 111
84
In this work, N2, He, CO2, SF6, O2, and the individual gas mixtures were used to evaluate
the influence made by different carrier gases.
TNT in different carrier gas compositions
The effect of different carrier gas composition on compensation voltage, peak width, and
signal intensity for TNT was studied. The pure N2, O2, He, CO2, or SF6 were evaluated in this
research. The mixture of O2, He, CO2, or SF6 with N2 were also studied and their content in N2
carrier gas varied from 0% to 50 %. DV was maintained at -4000V and the gas flow rate was set
at 2 L/min.
Figure 3-13 shows the TIC-CV spectra of TNT collected in different carrier gas
compositions. The results demonstrate TNT in pure N2 has the highest transmission, but poorer
resolution. Although the use of pure O2 and the N2/He and N2/O2 mixtures narrows the peak
width, it also decreases the signal intensity. TNT experiences the greatest change in mobility in
the carrier gas mixtures of N2/CO2 and N2/SF6, but wider peak width and lower intensity can be
also seen in the spectra. Gas types that may lead to reduced peak width and increasing signal
intensity are important considerations for FAIMS; therefore, further study of the effect of N2, O2,
and mixtures of N2/He and N2/O2 was performed in the following research.
TNT in O2 and mixture of N2/O2
In pure O2 or gas that includes O2 content, O2-, which possesses strong gas-phase basicity,
is generated as a reactant ion and easily abstracts protons from TNT to produce an [M-H]- ion at
m/z 226 instead of m/z 227. The [M-H]- ion was monitored in the experiment performed in O2
and mixture of N2/O2.
IS-CV spectra acquired for TNT in a carrier gas of pure O2 at different DVs are shown in
Figure 3-14. At DVs below -4000 V, the signal for TNT was very low due to high diffusional
loss to the cylindrical electrodes. However, the observed 10-fold increase in sensitivity at DV of
85
-4000 V was the result of an ion focusing mechanism inherent to a cylindrical geometry FAIMS
cell.53 Comparing to Figure 3-7 acquired for TNT in pure nitrogen, the transmission of TNT
observed in oxygen was roughly 60% lower than in nitrogen at DV of -4000 V.
The CV value, peak width, and signal intensity for TNT are given in Figure 3-15, which
shows plots collected at DVs from -2500 V to -4500 V in carrier gas compositions from 0% to
50% oxygen in nitrogen. In a mixture of N2/O2, the CV value was reported to comply with
Blanc’s law, which relates the mobility of an ion in a mixture of any number of gases (Kmix) with
abundance xi to its mobilities in individual constituents (Ki), as shown in equation 3-4.38
1/Kmix =Σxi/Ki (3-4)
In addition, most ions have similar mobilities in N2 and O2, as one might expect from
similar molecular mass, size, and other properties of these two gases.111 For each DV, the
difference between the maximal and minimal CV value for a mixture of N2/O2 is no greater than
0.5 V. The average difference of CV values is 0.367 V under the influence of all DV values,
which indicates the addition of O2 to N2 had little effect on the CV value. However, decreased
peak width can be seen as the O2 fraction changes from 0 to 50 %. The signal intensity collected
in the gas mixture with the O2 fraction above 20 % starts to drop when the DVs ramps up to -
4000 V, which suggests that the [M-H]- ion of TNT is on its way to converting into a C-type ion
in gas mixtures with higher O2 content.
Explosives in mixture of N2/He
He atoms are smaller and less polarizable than N2 molecules; therefore, an ion experiences
fewer collisions or interactions with the He atoms compared to the larger N2 molecule. With
minimal interaction with the carrier gas, the analyte ion may have more flexibility to alter
conformations at high and low field,102 which increases the change in mobility between high and
86
low field. Addition of He to N2 has been shown to increase resolution (decreased peak width),
sensitivity (signal intensity), and peak capacity (higher CV value) of FAIMS measurements.47, 108,
112 Shvartsburg et al.111 also verified that mixtures of grossly disparate gases, such as N2/He, can
exhibit large deviations from Blanc’s law that greatly expand the separation space. Because high
He content in the FAIMS cell may cause electrical breakdown in either the mass spectrometer or
the FAIMS cell due to the insufficient pumping capacity from the mass spectrometer, the He
content in N2 was varied from 0% and capped at 50%. DV was varied from ±2500 V to ±4500 V
in 500 V increments.
Figures 3-16, 3-17, and 3-18 show the effects in CV, signal intensity, and peak width of
explosive compounds from increasing the He content from 0% to 50 %. In Figure 3-16, most of
the type A and type C ions experience increased CV in N1 and N2 mode, respectively, as the He
content is increased. This magnitude of deviations is because of a large difference between the
molecular masses of He and N2 and between ion mobilities in these gases. He is lighter, smaller,
and less polarizable than N2; therefore, the chance or interaction with the analyte ion is reduced.
With fewer interactions with the bath gas, the analyte ion may have more flexibility to alter
conformations at high and low field, resulting in a higher CV to compensate the substantial
change. However, for type B ions such as TNT, tetryl, RDX, and NG, the CV decreases in N1
mode and decreases initially before increasing at higher He content in N2 mode. It is notable
that a gas composition of greater than 30% He causes a stable or decreasing CV for type A ions
in N1 mode. The trends observed in this experiment are in compliance with the previous report
that states that type C ions present even stronger type C ion behavior in He and, in N2, some ions
of type B and A switch to type C ion in N2/He mixtures with a larger He content.107 This may be
attributed to low He polarizability, making long-range, attractive interactions (that induce the A-
87
type behavior) less important than short-range repulsive forces.111, 113 This result may also verify
the hypothesis from IMS studies: all ions, except the lightest ones, are type C ions in He at any
electric field.114, 115
In Figure 3-17, the signal intensity for explosives generally decreases as the He content
increases for some type A ions and type B ions in N1 mode. However, the signal intensity falls
off for lighter ions such as DNT and DNB at a gas composition of greater than 30% helium.
Higher intensities were seen for type B and type C ion when adding more He in N2 mode,
showing an even stronger type C ion behavior. It has been reported that there is a boost in signal
intensity for type C ions with increasing helium content of up to about 60% helium.111 This
phenomenon may be ascribed to fewer interactions occurred between analytes and carrier gas
with increased He content, which may allow improved focusing.102
In Figure 3-18, the peak width for explosives generally decreases as the He content
increases for type A ions and type B ions in N1 mode, and increases for type C ions in N2 mode.
This trend shows that the peaks tend to narrow with decreased magnitude of CV and signal
intensity. The analyte ions are transmitted over a much narrower range of CV at a given DV in a
carrier gas with larger He proportion, which may be attributed to the less focusing for type A and
type B ions in N1 mode. In contrast, the reason for the broadened peaks for type B ions in higher
He content and type C ion in N2 mode is the enhanced focusing in the field.
In order to verify the conversion from type B ion to type C ion caused by He addition, the
CV value, peak width, and signal intensity of TNT ([M]-) and tetryl ([M-NO2]-) ions were
measured over a range of DV from 2500 to 4000 V and at 0% to 50% He. For both explosive
compounds, similar trends were observed as a function of both DV and He content: increased
CV value and signal intensity, and broadened peak width in N2 mode, which is pointed out by a
88
red circle in Figure 3-19 and Figure 3-20. This phenomenon is typical type C ion behavior,
which means these type B ions are switching to type C ions at both higher DV and He content.
Effects of Electrode Temperature
One of the benefits of increasing the FAIMS cell’s temperature is the elimination of the
residual water and contaminants from the cell, which results in an increase of sensitivity52
Separately controlling the temperature on the inner and outer electrodes is shown to be extremely
useful for manipulating the selectivity of FAIMS. The optimal temperature setting depends on
the shape of the CV/DV plots; therefore, the optimal temperature setting needs to be determined
experimentally.116 The effect of temperature on ion separation can be described by equations 3-5
to 3-7.116 The mobility at high field for a given ion can be described by:
Kh(E/N) = K[1+f(E/N)] (3-5)
where K is the mobility constant, E is the electric field, and N is the gas number density. The
number density can be calculated by:
N=(n/V) NA (3-6)
where NA is Avogadro’s constant and (n/V) is determined from the ideal gas law:
n/V = P/(RT) (3-7)
Here, R is 0.082 L atm / mol K, and P and T are parameters that are measured during the
experiment.
Theoretically, the increased temperature can lead to the increased effect of electric field,
thus increasing ion focusing between the electrodes. In this research, different inner and outer
89
electrode temperatures were manipulated to investigate the improvement in sensitivity and the
alteration of ion mobilities for explosive compounds.
An increased electric field (E/N) can be expected as temperature increases because N
decreases with increasing temperature. Figure 3-21 illustrates the calculated electric field in
FAIMS cell at different DVs and temperatures, and indicates that the electric field generated at a
DV of 4500 V and an electrode temperature of 70 ˚C is greater than it is at a DV of 5000 V and
electrode temperature of 30 ˚C.
Figure 3-22 presents plots of CV value, peak width, and signal intensity for the ions of
TNT and 2,6-DNT as function of electrode temperature. As the temperature increases, decreased
CV value, peak width, and signal intensity were observed. The situation for both explosives has
similarities to what was described at high applied DV for a type B ion, which shows decreased
ion mobility at high electric field. Compared to Figure 3-9 to 3-11, the conversion from type A
to type C behavior in this experiment occurs at a lower electric field than what is calculated from
the equations mentioned above. This may be attributed to effect of increased temperature
isomerization, dissociation,117 or the change in folding structure other than a simple decrease in
number density.
Independent control of the inner and outer electrode temperatures offers extra flexibility to
control the CV, peak width, separation, and sensitivity for different analytes.116 In this research,
the waveform is only applied to the inner electrode; therefore, the electric field between two
concentric cylindrical electrodes is non-uniform. Ions near the inner electrode are subjected to
stronger fields than those near the outer electrode. If the temperature of either electrode is
increased, the applied fields near the heated electrode will increase because of decreased number
density. Therefore, when a higher temperature is applied to the inner electrode or a lower
90
temperature is applied to outer electrode, the electric field gradient is steepened to enhance ion
focusing, as shown in Figure 3-23. In contrast, the electric field generated at a higher outer
electrode temperature and a lower inner electrode temperature is less steep and provides less ion
focusing.
Figure 3-24 and 3-25 demonstrate the effect of temperature gradient on CV value, peak
width, and signal intensity. The expected focusing was not observed at a higher inner electrode
temperature or a lower outer electrode temperature. When the temperature of the inner electrode
was increased, the CV for optimal transmission shifted to a smaller value, and a narrower peak
and lower signal intensity were observed. On the other hand, if the temperature of outer
electrode was increased, a smaller CV value, a broader peak, and an increased signal intensity
were observed. The magnitude of the changing CV value, peak width, and signal intensity for
the outer electrode temperature is smaller than that of the inner electrode, which means greater
effect was given by the alteration in temperature of the inner electrode than the outer electrode.
The results are unexpected because the [M]- ions of both TNT and 2,6-DNT are type B ions and
the differential ion mobility (Kh/K0) starts to decrease when the electrode temperature is raised
pass the turning point, as was shown in Figure 1-3 for type B ion. Because the electric field near
the inner electrode increases with the elevated inner electrode temperature, it is expected that the
Kh/Ko of TNT and 2,4-DNT ions near the inner electrode decreases, resulting in the lower
focusing strength toward the center of the FAIMS cell and, hence, losing more ions due to
diffusion and space-charge repulsion. However, if the temperature of outer electrode is
increased, the electric field may focus or defocus the ion depending on whether the magnitude of
electric field reaches the turning point where the ions possess the largest gap in mobility between
low and high electric fields. Therefore, the plots of peak width and signal intensity may fluctuate
91
when varied outer electrode temperatures are applied. This may also suggest that changing the
outer electrode temperature provides less alteration in CV value, peak width, and signal intensity.
Conclusions
FAIMS is a promising technology that functions well as a separation device and is
compatible with mass spectrometry. This is the first systematic evaluation of the effect of factors
such as DV, CV scan rate, curtain gas flow rate, carrier gas composition, and electrode
temperature for the analysis of explosive compounds.
Experiments showed that a CV scan rate of 10 V/s and curtain gas flow rate from 2.0-2.5
L/min were the optimal conditions for both resolution and transmission. An increase in CV
value and sensitivity was observed at higher a DV for both type A and type B ions in N1 and
type C ions in N2 modes. Although the ion focusing mechanism in the cylindrical cell improves
the sensitivity, it also decreases the resolution. Furthermore, the separation and sensitivity are
also influenced in FAIMS by changing the carrier gas composition. The mixture of helium and
nitrogen was shown to provide benefits to achieve better resolution and sensitivity. For type A
ions, the peak width was reduced and the CV was shifted to more positive value in N1 mode
with increasing helium content in carrier gas, which improved the resolution between those ions.
In addition, dramatic increases in sensitivity and peak width for type C ions in N2 mode were
obtained when the content of helium in carrier gas was increased; however, a broader peak width
and increased signal intensity were also seen for type B ions in the same mode, which implied
that the type B ion was transformed to a type C ion at higher DV and helium content. Finally, an
alteration may also be observed on CV value, peak width, and signal intensity with varied
electrode temperatures. Two type B ions, the [M]- ions of TNT and 2,6-DNT, have decreased
92
mobilities with increased temperature. That is also the reason that the magnitude of focusing
strength decreases when the temperature of inner electrode was raised.
Based on the understanding gained here, optimal separation or transmission can be
achieved by controlling different parameters. This knowledge will also be beneficial for the
further development of devices for explosives detection based on the FAIMS technique.
93
0.8mm
8mm
6mm
Heating Capillary
Mass Spectrometer
8mm
7mm
7mm
1.4mm 9.6mm
5mm
5mm
12mm 8mm
9.6mm 65mm
4mm
2mm
1.6mm
ID=0.76 mmOD=1.6 mm
15mm
1mm
5mm
Capillary
O-ring ID= 8mmOD= 11.1mmWide=1.6mm
3.5mm
3.5mm
19mm
FAIMS
Figure 3-1. The design of the brass capillary extender.
Figure 3-2. The actual picture of the brass capillary extender.
94
Scan Rate (TNT at -4000 Volts)
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
2.5 5.0 10.0 15.0 20.0
Scan Rate (Volts/min)
Vol
ts
0.0
2.0
4.0
6.0
8.0
10.0
12.0
Sign
al In
tens
ity (×
10 4 c
ount
s)
CV(Volts)
Peak Width (Volts)
Intensity of m/z227(Counts)
Figure 3-3. Effect of CV scan rate on CV value, peak intensity, and peak width. (DV= −4000V)
CV value vs carrier gas flow rate
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
2 2.5 3 3.5
Carrier Gas Flow Rate(L/min)
CV(V
olts)
TNT
TNB
2,4-DNT
2,6-DNT
3,4-DNT
Tetryl(-NO2)
RDX(+Cl)
HMX(+Cl)
PETN(+Cl)
NG(+Cl)
Figure 3-4. Effect of curtain gas flow rate on CV for the ions of tested explosives. (DV= −4000V)
95
Peak width vs carrier gas flow rate
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
2 2.5 3 3.5
Carrier Gas Flow Rate(L/min)
Peak
Wid
th(V
olts)
TNT
TNB
2,4-DNT
2,6-DNT
3,4-DNT
1,3-DNB
RDX(+Cl)
HMX(+Cl)
PETN(+Cl)
NG(+Cl)
Figure 3-5. Effect of curtain gas flow rate on peak width for the ions of tested explosives.
Intensity vs carrier gas flow rate
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
2 2.5 3 3.5Carrier Gas Flow Rate(L/min)
Sign
al in
tens
ity(×
10 4 c
ount
s)
TNT
TNB
2,4-DNT
2,6-DNT
3,4-DNT
Tetryl(-NO2)
RDX(+Cl)
HMX(+Cl)
PETN(+Cl)
NG(+Cl)
Figure 3-6. Effect of curtain gas flow rate on signal intensity for the ions of tested explosives.
96
0 .0 0 .2 0 .4 0 .6 0 .8 1 .0 1 .2 1 .4 1 .6 1 .8 2 .T i m e (m i n )
0
2
4
6
8
1 0
1 2
1 4
1 6
1 8
2 0
2 2
2 4 -5000V
-4000V
-3500V
-3000V
-2500V
-4500VTNT
-5 0 5 10 15CV(volts)
Sign
al In
tens
ity (×
105
coun
ts)
Figure 3-7. SI-CV spectra for the [M]- ion (m/z 227) of TNT: variation of the DV.
100 150 200 250 300 350 400 450 500m/z
0
50
1000
50
1000
50
1000
50
100
Rel
ativ
e A
bund
ance 0
50
1000
50
100227.07
197.27254.20 462.80167.1366.87
227.07
197.27243.20167.1353.80 415.13
227.13
197.13167.00 257.6773.80 482.93291.87 351.93136.13
227.00
197.13243.00 276.87167.20109.53 334.20 408.53
227.00
197.27 291.13167.00 480.33100.00 457.07312.47 361.40227.07
277.20197.20
291.13 318.73244.20152.13 373.40 420.87 482.40
-5000VNL:1.82E4
-2500VNL:6.18E3
-3000VNL:1.19E4
-3500VNL:1.57E4
-4000VNL:1.80E4
-4500VNL:1.88E4
Figure 3-8. Mass spectra for the TNT: variation of the DV.
97
CV value of explosives
-15.00
-10.00
-5.00
0.00
5.00
10.00
15.00
-4.5 -4 -3.5 -3 -2.5 0 2.5 3 3.5 4 4.5
DV(kV)
CV
(Vol
ts)
TNTTNB2,4-DNT2,6-DNT3,4-DNT1,3-DNBTetryl(-NO2)RDX(+Cl)HMX(+Cl)PETN(+Cl)NG(+Cl)
Figure 3-9. Graph of CV versus DV for the ions of tested explosives.
Intensity of major ion of explosives
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
-4.5 -4 -3.5 -3 -2.5 0 2.5 3 3.5 4 4.5
DV(kV)
Sign
al in
tens
ity(×
10 4 c
ount
s)
TNTTNB2,4-DNT2,6-DNT3,4-DNT1,3-DNBTetryl(-NO2)RDX(+Cl)HMX(+Cl)PETN(+Cl)NG(+Cl)
Figure 3-10. Graph of signal intensity versus DV for the ions of tested explosives.
98
Peak width of CV scan for explosives
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
-4.5 -4 -3.5 -3 -2.5 0 2.5 3 3.5 4 4.5
DV(kV)
Peak
Wid
th(V
olts
)
TNTTNB2,4-DNT2,6-DNT3,4-DNT1,3-DNBTetryl(-NO2)RDX(+Cl)HMX(+Cl)PETN(+Cl)NG(+Cl)
Figure 3-11. Graph of peak width versus DV for the ions of tested explosives.
99
Table 3-1. The main analytical characteristics of FAIMS on detecting explosives. DV(kV) -4.5 -4 -3.5 -3 -2.5 2.5 3 3.5 4 4.5CV(Volts) 5.7 4.3 2.8 1.0 0.9 -1.0 -1.6 -2.6 -3.6 -5.5Peak Width(Volts) 1.3 1.3 1.1 1.0 1.1 1.2 1.4 1.6 1.7 1.8Intensity of m/z 227(counts) 1.58E+05 1.37E+05 1.08E+05 6.10E+04 5.85E+04 1.18E+04 1.05E+04 5.65E+03 5.13E+03 3.65E+03CV(Volts) 7.3 5.5 3.9 2.5 1.3 -1.3 -2.1 -3.4 -5.2 -6.8Peak Width(Volts) 1.2 1.2 1.1 1.0 1.0 1.1 1.2 1.4 1.3 0.9Intensity of m/z 213(counts) 1.60E+05 1.37E+05 9.92E+04 4.09E+04 1.68E+04 5.31E+03 5.09E+03 2.20E+03 1.98E+03 1.44E+03CV(Volts) 9.6 7.3 5.3 3.2 1.7 -1.6 -2.5 -4.3 -6.6 -9.2Peak Width(Volts) 2.1 1.8 1.4 1.3 1.3 1.8 2.1 2.0 2.3 1.7Intensity of m/z 181(counts) 8.25E+04 7.64E+04 5.45E+04 2.74E+04 1.80E+04 5.74E+03 3.18E+03 2.08E+03 1.08E+03 7.15E+02CV(Volts) 10.3 7.7 5.3 3.3 1.7 -1.8 -2.9 -4.6 -7.2 -9.5Peak Width(Volts) 1.9 1.8 1.5 1.2 1.2 1.3 1.4 2.1 2.0 1.9Intensity of m/z 182(counts) 1.08E+05 8.67E+04 5.34E+04 2.90E+04 1.34E+04 8.21E+03 5.89E+03 1.93E+03 8.35E+02 8.85E+02CV(Volts) 8.0 5.8 3.8 2.3 1.3 -1.4 -1.9 -3.6 -5.4 -7.4Peak Width(Volts) 1.6 1.5 1.3 1.4 1.4 1.4 1.5 2.1 1.8 2.4Intensity of m/z 182(counts) 7.41E+04 5.93E+04 4.20E+04 1.96E+04 1.11E+04 8.06E+03 4.35E+03 2.79E+03 1.50E+03 5.67E+02CV(Volts) 10.6 7.9 5.6 3.4 2.0 -1.9 -2.8 -4.8 -7.4 -10.0Peak Width(Volts) 1.8 1.7 1.4 1.3 1.2 1.0 1.0 1.0 1.3 1.7Intensity of m/z 168(counts) 9.16E+04 6.88E+04 4.81E+04 2.63E+04 1.41E+04 5.85E+03 2.87E+03 2.44E+03 1.04E+03 8.31E+02CV(Volts) 2.7 2.2 1.5 0.9 0.5 -0.5 -0.8 -1.2 -1.8 -2.5Peak Width(Volts) 1.3 1.4 1.2 1.2 1.1 1.4 1.4 1.5 1.5 1.4Intensity of m/z 241(counts) 1.06E+05 9.51E+04 8.25E+04 4.97E+04 3.03E+04 3.58E+04 3.62E+04 3.68E+04 3.56E+04 4.18E+04CV(Volts) 2.5 2.2 2.0 1.4 1.0 0.4 0.4 0.4 0.3 0.2Peak Width(Volts) 3.2 3.5 3.1 3.3 3.6 2.0 2.3 2.4 2.4 2.5
Intensity of m/z 257(counts) 1.45E+05 1.46E+05 1.59E+05 1.22E+05 9.12E+04 3.32E+04 3.54E+04 3.89E+04 5.40E+04 7.86E+04CV(Volts) -0.4 -0.4 0.1 0.7 0.7 0.5 0.7 1.1 1.1 2.3Peak Width(Volts) 3.3 3.9 3.1 3.3 3.1 3.1 2.5 2.4 2.1 2.2Intensity of m/z 331(counts) 1.03E+05 1.35E+05 1.70E+05 1.62E+05 1.41E+05 1.61E+05 1.80E+05 1.92E+05 2.20E+05 2.83E+05CV(Volts) 0.0 0.3 0.5 0.5 0.4 0.8 1.1 1.1 1.5 2.0Peak Width(Volts) 3.3 3.1 2.9 2.7 3.1 3.1 3.1 2.8 2.8 2.5Intensity of m/z 351(counts) 1.75E+05 2.04E+05 2.21E+05 1.91E+05 1.48E+05 6.84E+04 1.84E+05 2.43E+05 2.67E+05 3.13E+05CV(Volts) 2.2 1.9 1.5 1.4 0.8 0.5 0.1 0.3 0.3 0.3Peak Width(Volts) 3.1 2.7 2.7 2.9 2.9 1.7 1.6 1.9 1.7 1.6Intensity of m/z 262(counts) 1.25E+05 1.75E+05 2.08E+05 1.76E+05 1.46E+05 2.44E+04 1.09E+05 1.33E+05 1.64E+05 1.85E+05
TNT
TNB
2,4-DNT
2,6-DNT
HMX(+Cl)
PETN(+Cl)
NG(+Cl)
3,4-DNT
1,3-DNB
Tetryl(-NO2)
RDX(+Cl)
100
0
50
1000
50
1000
50
1000
50
1000
50
100227.07
197.20 260.00213.13
239.07183.27 258.80182.07
165.13 212.93182.07
212.93152.2075.07182.07
152.27
100 150 200 250 300 350 400 450 500m/z
0
50
1000
50
1000
50
1000
50
1000
50
100241.13
313.00256.93 329.00225.07256.87
478.53258.87209.87330.93291.6766.80331.00
332.9362.00 283.87 357.9398.47 167.80
350.80
352.8062.13 306.0082.07 238.87
261.80
488.40263.7362.13443.40217.00 398.5386.13
100 150 200 250 300 350 400 450 500m/z
0
50
1000
50
1000
50
1000
50
1000
50
100241.07
256.93256.78
259.09
330.91
333.08128.96 314.74202.88 284.22
350.93
352.75315.0962.04 355.13256.85
261.96
263.9962.18
A
G
F
E
C
D
B
J
I
H
APCI-MS APCI-FAIMS-MS
0
50
1000
50
1000
50
1000
50
1000
50
100 227.07
210.07167.00213.13
183.33181.20
89.07182.00
152.13
182.00
168.07138.27 271.53212.00
Figure 3-12. Mass spectra of explosives acquired by APCI-MS and APCI-FAIMS-MS: (A) TNT,
(B) TNB, (C) 2,4-DNT, (D) 2,6-DNT, (E) 3,4-DNT, (F) Tetryl, (G) RDX, (H) HMX, (I) PETN, (J) NG.
101
1 .0 1 .5 2 .0 2 .5 3 .0 3 .5Tim e (m in)
0
50
1000
50
1000
50
1000
50
100
Rel
ativ
e A
bund
ance
0
50
1000
50
100N2m/z 227 NL:1.85E5
O2m/z 226 NL:1.17E4
60% N2/ 40% O2m/z 226 NL:3.50E4
60% N2/ 40% Hem/z 227 NL:4.02E4
60% N2/ 40% CO2m/z 227 NL:2.32E4
60% N2/ 40% SF6m/z 227 NL:2.21E4
-5 0 5 10CV (Volts)
Figure 3-13. TIC-CV spectra for TNT in different carrier gas composition at DV of -4000V.
102
0 .0 0 .2 0 .4 0 .6 0 .8 1 .0 1 .2 1 .4 1 .6 1 .8T im e (m in)
0
1 0
2 0
3 0
4 0
5 0
6 0
7 0
8 0
9 0
1 0 0
-4500V
-3000V
-2500V
-4000V
-3500V
TNT in Oxygen
-5 0 5 10 15CV(volts)
Figure 3-14. SI-CV spectra for the [M-H]- ion of TNT (m/z 226) in oxygen carrier gas at DV from −2500 to −4500 V in −500 V increments.
103
Intensity vs DV in N2/O2 mixture (TNT)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
-2500 -3000 -3500 -4000 -4500 -5000
DV(Volts)
Sign
al In
tens
ity (×
10 4 c
ount
s) 0%10%20%30%40%50%
CV vs DV in N2/O2 mixture (TNT)
0
1
2
3
4
5
6
7
-2500 -3000 -3500 -4000 -4500 -5000
DV (Volts)
CV
(Vol
ts)
0%10%20%30%40%50%
Peak Width vs DV in N2/O2 mixture (TNT)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
-2500 -3000 -3500 -4000 -4500 -5000
DV(Volts)
Peak
Wid
th (V
olts
) 0%10%20%30%40%50%
C
A
B
% O2
% O2
% O2
Figure 3-15. Graph of (A) CV, (B) peak width, and (C) signal intensity versus DV for the [M-H]-
ion of TNT in N2/O2 mixtures from 0% to 50% O2.
104
CV vs Carrier gas composition
-6.0
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
50% 40% 30% 20% 10% 0% 0% 10% 20% 30% 40% 50%
%He (V/V) He/N2
CV
(Vol
ts)
TNTTNB2,4-DNT2,6-DNT3,4-DNT1,3-DNBTetryl(-NO2)RDX(+Cl)HMX(+Cl)PETN(+Cl)NG(+Cl)
DV -4000V DV 4000V Figure 3-16. Graph of CV versus carrier gas composition for the ions of tested explosives in
N2/He mixtures.
Intensity vs Carrier gas composition
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
50% 40% 30% 20% 10% 0% 0% 10% 20% 30% 40% 50%
%He (V/V) He/N2
Sign
al in
tens
ity (×
10 4 c
ount
s)
TNTTNB2,4-DNT2,6-DNT3,4-DNT1,3-DNBTetryl(-NO2)RDX(+Cl)HMX(+Cl)PETN(+Cl)NG(+Cl)
DV -4000V DV 4000V
0.0
5.0
10.0
15.0
20.0
25.0
50% 40% 30% 20% 10% 0%
%He (V/V) He/N2
Sign
al in
tens
ity (×
104 cou
nts)
Figure 3-17. Graphs of signal intensity versus carrier gas composition for the ions of tested explosives in N2/He mixtures.
105
Peak width vs Car r ier gas composit ion
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
50% 40% 30% 20% 10% 0% 0% 10% 20% 30% 40% 50%
%He (V/V) He/N2
Peak
wid
th (
Volt
s)
TNT
TNB
2,4- DNT
2,6- DNT
3,4- DNT
1,3- DNB
Tet r yl(- NO2)
RDX(+Cl)
HMX(+Cl)
PETN(+Cl)
NG(+Cl)
DV -4000V DV 4000V
Figure 3-18. Graphs of peak width versus carrier gas composition for the ions of tested
explosives in N2/He mixtures.
106
Intensity vs DV at varied carrier gas composition TNT(m/z 227 [M]-)
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
-4.5 -4 -3.5 -3 -2.5 0 2.5 3 3.5 4 4.5
DV(kV)
Sign
al in
tens
ity (×
10 4 c
ount
s)
0%10%20%30%40%50%
CV vs DV at varied carrier gas composition TNT(m/z 227 [M]-)
-8.00
-6.00
-4.00
-2.00
0.00
2.00
4.00
6.00
8.00
-4.5 -4 -3.5 -3 -2.5 0 2.5 3 3.5 4 4.5
DV(kV)
CV
(V
olts
)
0%
10%
20%
30%
40%
50%
Peak Width vs DV at varied carrier gas composition TNT(m/z 227 [M]-)
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
2.00
-4.5 -4 -3.5 -3 -2.5 0 2.5 3 3.5 4 4.5
DV(kV)
Pea
k W
idth
(V
olts
) 0%
10%
20%
30%
40%
50%
C
A
B
Figure 3-19. Graph of (A) CV, (B) peak width, and (C) signal intensity versus DV for the [M]- ions of TNT in N2/He mixture. Red circle shows that TNT presents an even stronger type C ion behavior in high helium content.
107
Intensity vs DV at varied carrier gas composition Tetryl(m/z 241 [M-NO2]-)
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
-4.5 -4 -3.5 -3 -2.5 0 2.5 3 3.5 4 4.5
DV(kV)
Sign
al in
tens
ity (×
10 4 c
ount
s)
0%10%20%30%40%
CV vs DV at varied carrier gas composition Tetryl(m/z 241 [M-NO2]-)
-3.00
-2.00
-1.00
0.00
1.00
2.00
3.00
-4.5 -4 -3.5 -3 -2.5 0 2.5 3 3.5 4 4.5
DV(kV)
CV
(V
olts
)
0%
10%
20%
30%
40%
Peak Width vs DV at varied carrier gas composition Tetryl(m/z 241 [M-NO2]-)
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
-4.5 -4 -3.5 -3 -2.5 0 2.5 3 3.5 4 4.5
DV(kV)
Peak
Wid
th (V
olts
)
0%
10%
20%
30%
40%
C
A
B
Figure 3-20. Graph of (A) CV, (B) peak width, and (C) signal intensity versus DV for the [M-NO2]- ions of tetryl in N2/He mixture. Red circle shows that Tetryl presents an even stronger type C ion behavior in high helium content.
108
50
60
70
80
90
100
110
6.5 7 7.5 8 8.5 9
Ele
ctri
c fie
ld (E
/N,T
d)
Electric field vs Radial position in cell
30-4500V
60-4500V
70-4500V
90-4500V
30-5000V
Inner cylinder
Outer cylinder
Radial distance(mm)
Figure 3-21. Calculated electric field as a function of radial distance between cylindrical FAIMS inner/outer cylinders at different temperature and DV.
109
Signal intensity vs electrode temperature
0.0
2.0
4.0
6.0
8.0
10.0
40 50 60 70 80
Outer electrode temperature()
Sign
al in
tens
ity(×
10 4 c
ount
s)
TNT2,6-DNT
Peak width vs electrode temperature
0.00
0.50
1.00
1.50
2.00
40 50 60 70 80
Electrode temperature()
Peak
wid
th(V
olts
)
TNT
2,6-DNT
CV vs electrode temperature
0.00
2.00
4.00
6.00
8.00
10.00
12.00
40 50 60 70 80
Electrode temperature()
CV
(Volts
)
TNT
2,6-DNT
A
C
B
Figure 3-22. Graph of (A) CV, (B) peak width, and (C) signal intensity versus cell temperature for the [M]- ions of TNT and 2,6-DNT.
110
50
60
70
80
90
100
110
6.5 7 7.5 8 8.5 9
Ele
ctri
c fie
ld(E
/N, T
d)
Radial distance(mm)
Electric field vs Radial position in cell
I40/O40
I40/O90
I90/O40
I90/O90
I40-Planar
Inner cylinder
Outer cylinder
Figure 3-23. Calculated electric field as a function of radial distance between cylindrical FAIMS
inner/outer cylinders at DV of −4500 V. (I: inner electrode temperature, O: outer electrode temperature, Planar: planar FAIMS cell)
111
Signal intensity vs electrode temperature
0.0
2.0
4.0
6.0
8.0
10.0
40 50 60 70 80 90
Outer electrode temperature()
Sign
al in
tens
ity(×
10 4 c
ount
s)
4050607080
Peak width vs electrode temperature
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
40 50 60 70 80 90
Outer electrode temperature()
Peak
wid
th(V
olts
)
40
50
60
70
80
CV vs electrode temperature
0.00
1.00
2.00
3.00
4.00
5.00
6.00
40 50 60 70 80 90
outer electrode temperature()
CV
(Vol
ts)
40
50
60
70
80
A
C
B
Inner temperature
Inner temperature
Inner temperature
Figure 3-24. Graph of (A) CV, (B) peak width, and (C) signal intensity versus inner and outer electrode temperatures () for the [M]- ions of TNT.
112
Signal Intensity vs electrode temperature
0.0
2.0
4.0
6.0
8.0
10.0
40 50 60 70 80 90
Outer electrode temperature()
Sign
al in
tens
ity(×
10 4 c
ount
s)
4050607080
Peak width vs electrode temperature
0.00
0.50
1.00
1.50
2.00
2.50
40 50 60 70 80 90
Outer electrode temperature()
Peak
wid
th(V
olts
) 40
50
60
70
80
CV vs electrode temperature
0.00
2.00
4.00
6.00
8.00
10.00
12.00
40 50 60 70 80 90
Outer electrode temperature()
CV
(Vol
ts)
40
50
60
70
80
A
C
B
Inner temperature
Inner temperature
Inner temperature
Figure 3-25. Graph of (A) CV, (B) peak width, and (C) signal intensity versus inner and outer electrode temperatures () for the [M]- ions of 2,6-DNT.
113
CHAPTER 4 PERFORMANCE OF APCI-FAIMS-MS FOR ANALYSIS OF EXPLOSIVES
Introduction
FAIMS is able to separate analyte ions from chemical background noise and enhance
analyte sensitivity.112 The previous chapter described the characterization of APCI-FAIMS-MS
for nitrate ester, nitramine and nitroaromatic compounds. This chapter describes the validation
of this method, was carried out in this research, evaluating the repeatability of CV values,
resolving power (RP), resolution (RS), linear dynamic range (LDR), and limit of detection
(LOD).
Experimental
In this research, experiments were performed employing a FAIMS-MS system, comprising
a cylindrical FAIMS device (Thermo Scientific, San Jose, CA) and a commercial ion trap mass
spectrometer (LCQ, Thermo Scientific). Gas-phase explosive ions were generated by
atmospheric pressure chemical ionization (APCI) using a corona discharge needle that is
positioned at an angle of 45° and ~1 cm from the opening in the curtain plate of FAIMS device.
The cylindrical FAIMS cell consists of two electrodes, inner and outer electrodes. The
combination of inner electrode having an outer radius of 6.5 mm and outer electrode having an
inner radius of 9.0 mm makes a gap of 2.5mm for ion transmission. The asymmetric waveform
(750 kHz) and the DC compensation voltage (CV) were both applied to the inner electrode of the
FAIMS. The dispersion voltage (DV) was set in the range between −2500 and −4500 V for type
A and B ions and at the range between +2500 and +4500 V for type C ions. The CV was
scanned between -20.0 to 20.0 V at scan rate of 10.0 V/min. A constant DC bias voltage of
−25 V was applied to the outer cylinder of the FAIMS device and to the inlet of the mass
spectrometer. In order to connect the Thermo FAIMS cell, designed for Thermo TSQ mass
114
spectrometer, onto the LCQ, a brass capillary extender (i.d. = 0.76 mm, o.d. = 22 mm) was
designed to serve as an interface. The curtain plate was held at −1000V to assist negative ions to
transit across the desolvation region. The nitrogen carrier gas was introduced into the region
between the curtain plate and the orifice into the FAIMS analyzer at flow rate of 2.0 L/min. The
inner and outer electrode temperatures were not heated (left at room temperature).
For the APCI source, the vaporizer temperature was set to 150°C. The heated capillary
temperature and voltage were set to 130°C and −25.0 V, respectively. The discharge current was
set at 5 μA and the tube lens offset was set to 30.0 V. The sheath gas was set to 20.0 (arbitrary
units) and the injection flow rate of the analyte was maintained at 20.0 μL/min.
Eleven explosive compounds (TNT, TNB, Tetryl, 1,3-DNB, 2,4-DNT, 2,6-DNT, 4-DNT,
RDX, HMX, NG and PETN) were studied. These explosives were provided by Dr. Jehuda
Yinon of the Weizmann Institute of Science, and were obtained from the Analytical Laboratory
of the Israeli Police Headquarters. To build up the calibration curve, standard solutions of each
of the explosive compounds were prepared by serial dilution of the stock solutions (in
acetonitrile) with 65:35 methanol/water. The concentrations ranged between 0.001 and 10
μg/mL. Five replicate CV scans were collected for each sample at each concentration. The
average peak area and relative standard deviation values were acquired and calculated.
Results and Discussion
Repeatability of CV Values
Repeatability is one of the crucial elements for an analytical approach, which describes the
consistency of the measurement. The nature of the compound and the composition of the carrier
gas dictate the combination of DV and CV that will permit successful transmission of a
particular ion through the FAIMS cell. Similar to retention time (RT) in chromatography
approach, a repeatable CV value can be treated as a judging index from the FAIMS data to
115
identify the explosive compounds under investigation. Additionally, the CV value could be set
for a known explosive compound for rapid identification in screening.
The repeatability of CV value was evaluated at different DV values with five repetitive
injections of a 10 μg/mL standard mixture while employing the same conditions over a period of
a few hours. Standard deviations (SDs) and relative standard deviations (RSDs) were calculated
by analysis of variance. As shown in Table 4-1, type A or type B ions gave the better
repeatability at DV values with negative (N1 mode) polarity, and, on the contrary, type C ions
are considerably more repeatable at DV values with positive polarity (N2 mode). The better
repeatability was obviously acquired as higher DV applied. With higher DV, the effect of ion
focusing increases the ion transmission of target ions, resulting in a more intense and
symmetrical CV peak which generates more precise and reproducible CV values. For TNT,
TNB, DNT, Tetryl, and NG at a DV of −4500 V, the SDs of CV values are distributed from 0.06
V to 0.2 V and the RSDs of CV values range from 1.0% to 5.3%, and the average of SD and
RSD are 0.12 V and 2.2%. For RDX, HMX, and PETN at a DV of 4500 V, the SDs of CV
values are from 0.02 V to 0.16 V and the RSD of CV values are from 4.4% to 8.1%, and the
average of SD and RSD are 0.09 V and 6.9%. In general, the results indicate a high degree of CV
values repeatability while utilizing this method.
In addition to the DV, the scan rate is the other crucial factor affecting the repeatability of
CV value. Generally speaking, higher scan rates can be expected to decrease the repeatability
because the chance of missing the most abundant point of the CV peak increases. However,
slower scan rates also suffer from the problem of asymmetrical or zigzag peak shapes which
worsen the accuracy of CV values. Therefore, moderate scan rates need to be applied to obtain
better repeatability, which was 10 V/min in this research. The concentration of analytes may
116
also influence the accuracy and repeatability of CV values. Too high an analyte concentration
may lead to saturation of the detector, resulting in an inaccurate determination of the optimum
CV value, while too low an analyte concentration will lead to poor ion statistics and a noisy peak,
leading to an erroneous CV assignment.52 In addition, for some compounds, a high analyte
concentration can lead to multimer formation, resulting in multiple CVs observed per
compound.49
Separation
The ions of different types are separated in FAIMS by ion mobility increments that depend
on electric field strength.100 Resolution (RS) is an important characteristic of any analytical
method, and accounts for its capacity to separate specific components of a mixture; resolving
power (RP) indicates the ability of the analytical method to produce narrow and well-resolved
peaks. The RP for FAIMS was calculated from equation 4-1, where the CV is divided by the
peak width at full width half maximum.
RP = CV/ W1/2max (4-1)
The RS between peaks of explosives was calculated by Equation 4-2, used to quantify the
degree of a two-component separation.
Rs =2ΔCV
(Wb2+Wb1) (4-2)
where ΔCV is the difference in compensation values of maximum intensity of the two peaks and
Wb1 and Wb2 are the peak width at 10% height for the two species, respectively. In
chromatography, a condition of an adequate separation of two peaks is the equality: RS = 1. For
a complete separation RS 1.5, where as an RS 0.5 denotes that separation is unavailable.
117
Resolving power
To assess the impact of changing the electric field on resolving power of the FAIMS,
varying DVs were applied to acquire the RP for individual explosive compounds. The RP is
higher for narrower peaks at CV values away from DV. A high value of RP corresponding to a
good separation of peaks is similar to the convention used with chromatography separations. The
RP values for the eleven explosives obtained with the FAIMS system are shown in Table 4-2 and
indicate that six of the eight type A and type B explosive ions (TNT, TNB, DNT, DNB, tetryl,
and NG) yielded the best RP values at DV of -4500 V. The other five explosives (some of type B
and the type C explosive ions) gave a higher value of RP at DV of 4500 V. As described in
Chapter 3, broader peaks normally can be observed at higher DV, but this is also accompanied
the appearance at higher CV values which benefit to increase the RP. Of significant interest in
this table is that higher RP of TNB and 2,4-DNT can be seen at DV of 4500 V than at DV of
−4500 V due to a narrower peak produced in CV spectra. However, the peaks for type A or type
B ions acquired at DV with positive polarity are not recommended for neither qualitative nor
quantitative analysis because of reduced transmission and lower reproducibility of CV value and
peak area.
From previous studies, some strategies can be applied to improve RP, which include
adding helium into the carrier gas to decrease peak width for type A ions or to increase the CV
shift to greater positive values for type C ions, or using less curvature, like a planar FAIMS cell,
to decrease the focusing effect to generate a narrower peak.
Separation and resolution between isomeric explosives
The relative percentage composition within the total nitroaromatic component has been
shown to be most useful for the characterization of explosive samples in addition to the usual
preliminary tests.118 Identification of patterns within the nitroaromatic isomeric explosives can
118
be use to differentiate explosives from similar batch types.37 Three nitroaromatic isomeric
explosives (Figure 4-1), 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6-DNT) , and 3,4-
dinitrotoluene (3,4-DNT) were investigated by FAIMS to evaluate the capability to separate
isomeric and related explosives.
The capability to separate the isomeric and related nitroaromatic compound by FAIMS is
shown in Table 4-3 and Figure 4-2 to 4-4. Among the DNT isomers, only 2,6-DNT and 3,4-
DNT achieved baseline separation at DV of -5000 V in pure nitrogen carrier gas; however, the
pair 2,4-DNT and 3,4-DNT reached adequate separation as the content of helium in carrier gas
was increased to 20%, as shown in Figure 4-2 and 4-3(B). Meanwhile, the resolution between
2,4-DNT and 2,6-DNT raised to 0.75, which indicated that the resolution between these two
peaks still remained incomplete. The lower peak intensity of 2,4-DNT relate to the two other
isomers may have been caused by lower ionization efficiency as shown in Figure 4-2 and 4-4.
This further worsened the identification of the less intense isomer peak. Fortunately, separation
of DNT isomers in FAIMS is significantly orthogonal to MS dimensions. 2,4-DNT and 2,6-
DNT can be identified by extracting the mass chromatogram by selecting the disparate major
ions (m/z 181 and m/z 182) as mentioned in Chapter 3. It can be seen in Figure 4-2 that all three
isomers were separated from one another and from the background signals when selective ions
(m/z 181 and m/z 182) were monitored.
TNT and TNB are two major components which are usually accompanied with DNT
isomers. As presented in Figure 4-3 and 4-4 and Table 4-3, TNT and TNB were both resolved
completely from these DNT isomers with the content of helium in carrier gas up to 10% at DV of
−5000 V.
119
Separation and resolution of explosive mixtures
Military explosives, in which several explosives are mixed to create an anticipated
lethality, are often found among terrorist attacks and crime scenes. The explosives most
frequently blended in these explosive mixtures are NG, TNT, RDX, and HMX.119 For example,
HMX or RDX powders may be mixed (typically by melt-casting) with TNT to form Octol or
Cyclotol. The above compositions may describe the majority of the explosive material, but a
practical explosive will often include small percentages of other materials. To examine the
FAIMS method for a mixture sample analysis, several solution mixtures of explosives were
evaluated in this research.
Table 4-4 summarizes the resolution observed experimentally between explosives, which
indicated that the mixture of TNT, RDX, and HMX achieved better resolution at DV of 4500 V
with carrier gas of 30:70 helium/nitrogen (Figure 4-5), and the mixture of TNT, PETN, and NG
was well separated at DV of 4500 V with nitrogen carrier gas (Figure 4-6). From previous
experimental experience, optimum resolution for explosive compounds can be always expected
at higher DV or with helium content in the carrier gas. However, for the mixture of TNT, RDX,
and HMX, superior resolution was acquired at DV of 4500 V instead of 5000 V. This is results
from the CV shifting more rapidly than the peak width decreasing. For the mixture of TNT,
PETN, and NG, improved resolution was observed when pure nitrogen carrier gas was applied
instead of a gas mixture including helium. That is because the apparent increase of peak width
for type C ions, such as PETN, and the lower CV values for type B ions, such as TNT and NG,
both decrease the resolution as the He composition in the carrier gas is increased. In addition,
the dramatically reduced transmission of type B ions with increased helium in the carrier gas also
obstructs the identification of NG from PETN. In conclusion, the resolution for explosive
mixtures needs to be optimized experimentally for most occasions.
120
Figure 4-7 illustrates an ion-selected CV (IS-CV) spectrum of seven nitroaromatic
explosives. The spectrum was collected by setting the DV to −4500 V and scanning the CV
from 0 to 20 V while monitoring the m/z values of the most abundant ion (most of them are [M]-
or [M-H]- ion ) for the analytes. The results shown in Figure 4-7 demonstrate that all
nitroaromatic explosives explored in this research can be identified and separated by APCI-
FAIMS-MS. Although 2,4-DNT, 2,6-DNT and DNB cannot be separated by FAIMS alone,
selected ions collected by mass spectrometer can be used to resolve these target compounds from
mixture. This method is operated as a two dimension separation, FAIMS and mass spectrometry,
which provides two kinds of orthogonal information to strengthen the power of separation.
Quantitation
Reproducibility
Reproducibility of intensity (peak area) is essential for achieving reliable quantification. In
this study, reproducibility of selected ion peak area for different compounds under varied DV
values was assessed by analyzing FAIMS-MS data sets. The RSD for each compound was
calculated from five analyses of a 10 μg/mL sample. Since replicate runs used the same amount
of explosives from the same sample, lower RSD is favorable and indicates better reproducibility
of peak area between replicate runs.
Table 4-5 shows that the RSD of peak area between replicate runs at different DV values
for different explosive compounds ranged from 0.9% for 3,4-DNT at DV of −4000 V to 33% for
2,6-DNT at DV of 4500 V. Typically, type A or type B ions gave better reproducibility with
negative DV (N1 mode) polarity, whereas type C ions are more reproducible at DV with positive
polarity (N2 mode). The RSD range for type A or type B ions at DV of −4500 V was from 1.0%
to 4.7% and for type C ions at DV of 4500 V was from 2.1% to 4.2%, indicating that the peak
area for each explosive ion in replicate runs was similar when higher DV with adequate polarity
121
was applied. Table 4-5 also denotes that the heavier explosives, such as RDX, HMX, PETN and
NG, usually defined as type B or type C ions, achieve better reproducibility at negative polarities.
However, similar conclusions cannot be said for type A ions at N2 mode. The reason is that
these type B ions are converting from type A behavior to type C behavior in N1 mode as DV
increases; however, type A ions will have an evident decrease in transmission in N2 mode. The
optimum RSD for these ions in N1 mode was 3.5% at −3500 V for RDX, 3.0% at −3500 V for
HMX, 1.7% at −3500 V for PETN, and 2.2% at −4000 V for NG.
Limit of detection and linear dynamic range
The ability to quantify a trace element or molecule using specific analytical methods is
often viewed in terms of the limit of detection (LOD). The LOD is a value, expressed in units of
concentration (or amount), that describes the lowest concentration level (or amount) of the
element that an analyst can determine statistically to be different from an analytical blank.120 In
this research, the LOD was taken as three times the standard deviation of the blank signal,
expressed in concentration. Seven explosive compounds (TNT, TNB, Tetryl, 1,3-DNB, 2,4-
DNT, 2,6-DNT, and 4-DNT) were studied. Solutions at concentrations of 0, 1, 10, 50, 100, 250,
500, 1000, and 10000 ng/mL for each of the compounds were made, and the mass spectrometer
was scanned in the full scan and selected ion monitoring (SIM) mode, using characteristic ion or
ions for each compound. The CV was scanned from 0 to 20.0 V at scan rate of 10.0 V/min. The
LOD was also collected at varied concentrations by setting to the optimum CV for transmission
of the nitro aromatic explosives for the collection time of 1 minute, 30 and 10 seconds.
Linear dynamic range (LDR) was evaluated by five repetitive injections of standard
compound at eight different levels of concentration ranging from 1 ppb to 10ppm. The lower
point of the LDR is equal to the limit of quantitation, namely, the concentration yielding a signal
122
10 times the standard deviation of the blank. The higher point refers to the highest concentration
that shows a linear dependence of the intensity on concentration.121
Results for LOD and LDR for explosives evaluated by this method are given in Table 4-6
and 4-7. Based on the data shown in Table 4-6, the LOD of explosive compounds collected in
full scan MS ranged from 1 to 28 ng/mL in concentration and from 51 to 1113 pg in amount. The
correlation coefficient values for the regression ranged from 0.9624 to 0.9943, demonstrating a
moderate LDR for the FAIMS peak area generation of these explosive compounds. Notably, for
most explosives, the curves begin to flatten at the high concentrations presumably because of
saturation of the ion source region, a complication in all uses of gas-phase ion chemistry with
sources at ambient pressure, resulting in the decrease on the linearity at higher concentration.32
As expected, the LOD decreased significantly when scanning in SIM mode, which ranged from 2
to 7 ng/mL in concentration and from 72 to 276 pg in amount. The linearity for the calibration
curves also improved, which was supported by the correlation coefficient from 0.9847 to 0.9990.
Although the full scan gave higher LOD and decreased linearity, it is still indispensable for
screening tests because more information about molecular and fragment ions will be provided by
this detection mode.
The result in Table 4-7 show the LOD and LDR collected by setting FAIMS at the
optimum CV for transmission for the nitro aromatic explosives with varied detection time. The
average LODs in concentration and the correlation coefficients are similar (0.996 and 11 ng/mL,
respectively) for the detection time of 1minute and 30 seconds. However, the amounts of
explosives required to be identified within 30 seconds vary from 32 to 343 pg. The amount
required for identification in 10 seconds is 95 pg on average; however the LOD in concentration
and the linearity of calibration curve have both deteriorated with only a 10-second acquisition.
123
Therefore, the preferable detection time to monitor specific explosive compounds by setting the
CV at a fixed value at which maximum transmission of target ions are achieved is no less than 30
seconds.
In this research, the LOD was also restrained by the sensitivity of LCQ mass spectrometer,
which was reported down to 5 ppb for each explosive with a LDR that reaches up to 1000 ppb.63
However, the LOD and LDR for explosive compounds obtained in this research are maintained
at the same level to the results reported previously63 despite the loss of 95 % ions in the FAIMS
cell and extender capillary, indicating that FAIMS can really improve the sensitivity by filtering
out background noise.
Although FAIMS generate chromatography-style data, its separation is based upon a
different principle from chromatographic separation. In contrast to gas chromatography or liquid
chromatography, in which almost all the analyte pass the column and can be detected, in FAIMS,
only some of the analyte ions will pass through the cell and the rest of them will be discharged
on the electrodes. Therefore, the quantitative results acquired by FAIMS without applying
internal standards are only relative concentrations (or amount) and can serve for semi-
quantitative analysis method.
Figure 4-8 and Figure 4-9 compare the mass spectra collected by APCI-MS and APCI-
FAIMS-MS. Figure 4-8 presents the mass spectra for analytes containing 50 ng/mL explosives
collected by full-scan APCI-MS and APCI-FAIMS-MS from m/z 50 to 500. As seen in Figure
4-8, the filtering capability is evident in the low levels of background noise in this system as
compared with those collected without FAIMS. Recall that only the ions which are compensated
correctly by a specific CV are transmitted through FAIMS cell and detected. Nevertheless, some
less abundant ions were still observed in the spectra collected with FAIMS. Some of these are
124
fragment ions produced from the molecular ions which have passed the FAIMS cell in the
extender or vacuum area of mass spectrometer.
In Figure 4-9, the concentration of explosive compounds was further reduced to 10 ng/mL
and the mass scan range was restricted between m/z 150 and m/z 300 to avoid the fragment or
background noise produced in the low mass region. The selected CV value mass spectra
collected by APCI-FAIMS-MS in Figure 4-9 all demonstrate a significant reduction in the
background ion signal. The target ion of each explosive compound, which is pointed out by a
red square, is still the most abundant peak in each spectra and much easier to identify than the
target peak in spectra collected without FAIMS.
Conclusion
In this chapter, the performance of APCI-FAIMS-MS in separation and detection was
evaluated. The best repeatability of CV value was obtained at DV of −4500 V for type A or type
B ions and at DV of 4500 V for type C ions. The SDs and RSDs of CV values were distributed
from 0.02 V to 0.2 V and 1.0% to 8.1%, respectively. The ability of FAIMS to separate
explosives from mixtures has been demonstrated. Although higher DV was shown to provide
increased CV value and resolving power, it also yielded broader peaks. Addition of helium to
the carrier gas improved the separation between isomeric and similar explosives; however, it
decreased the transmission of explosive ions for type A and type B ions. The ratio of helium
required to resolve explosive peaks needs to be optimized experimentally for different occasions.
Two scan modes, full scan and SIM, and three lengths of detection time, 1minute, 30 seconds,
and 10 seconds, were tested for LOD and LDR. The method proved to be sensitive for
nitroaromatic explosives down to the average concentration of 14 ppb for full scan and 4 ppb for
SIM and to extend a linear dynamic range up to 1000 ppb for most nitroaromatic explosive
compounds. Although full-scan gave higher LOD and decreased LDR, it is still essential for
125
explosive detection because it provides more information about molecular and fragment ions.
The preferred detection time to monitor specific explosive compound by setting to the optimum
CV is 30 seconds and the amount of explosive compound required for this accumulation time
was shown to be as low as 32 pg.
126
Table 4-1. Repeatability of CV values from five replicate analyzes of explosive compounds. DV(kV) -4.5 -4 -3.5 -3 -2.5 2.5 3 3.5 4 4.5
CV(Volts) 5.7 4.3 2.8 1.0 0.9 -1.0 -1.6 -2.6 -3.6 -5.5SD 0.06 0.06 0.10 0.00 0.06 0.26 0.17 0.26 0.21 0.15RSD % 1.0 1.3 3.6 0.0 6.2 26.5 10.8 10.2 5.7 2.8CV(Volts) 7.3 5.5 3.9 2.5 1.3 -1.3 -2.1 -3.4 -5.2 -6.8SD 0.10 0.12 0.06 0.06 0.06 0.17 0.20 0.21 0.42 0.50RSD % 1.4 2.1 1.5 2.3 4.6 13.3 9.5 6.2 8.0 7.4CV(Volts) 9.6 7.3 5.3 3.2 1.7 -1.6 -2.5 -4.3 -6.6 -9.2SD 0.20 0.10 0.12 0.05 0.10 0.29 0.09 0.10 0.54 0.45RSD % 2.1 1.3 2.2 1.7 5.9 18.5 3.8 2.4 8.2 4.9CV(Volts) 10.3 7.7 5.3 3.3 1.7 -1.8 -2.9 -4.6 -7.2 -9.5SD 0.15 0.21 0.15 0.06 0.10 0.25 0.15 0.21 0.36 0.44RSD % 1.5 2.7 2.9 1.7 5.9 14.2 5.2 4.5 5.0 4.6CV(Volts) 8.0 5.8 3.8 2.3 1.3 -1.4 -1.9 -3.6 -5.4 -7.4SD 0.12 0.12 0.10 0.12 0.20 0.10 0.06 0.06 0.31 0.67RSD % 1.4 2.0 2.6 4.9 15.4 7.1 3.0 1.6 5.7 9.0CV(Volts) 10.6 7.9 5.6 3.4 2.0 -1.9 -2.8 -4.8 -7.4 -10.0SD 0.15 0.20 0.12 0.00 0.12 0.15 0.06 0.36 0.45 0.74RSD % 1.4 2.5 2.1 0.0 5.9 7.9 2.0 7.5 6.1 7.4CV(Volts) 2.7 2.2 1.5 0.9 0.5 -0.5 -0.8 -1.2 -1.8 -2.5SD 0.15 0.13 0.06 0.11 0.04 0.05 0.20 0.05 0.16 0.00RSD % 5.3 5.9 4.0 11.9 8.7 10.8 25.0 4.3 8.8 0.0CV(Volts) 2.2 1.9 1.5 1.4 0.8 0.5 0.1 0.3 0.3 0.3SD 0.07 0.06 0.10 0.18 0.14 0.11 0.05 0.07 0.10 0.05RSD % 3.2 3.3 6.4 13.2 17.4 22.0 50.3 22.8 33.5 16.7
0.12 0.12 0.10 0.07 0.10 0.17 0.12 0.17 0.32 0.372.2 2.7 3.2 4.5 8.7 15.1 13.7 7.4 10.1 6.6
CV(Volts) 2.5 2.2 2.0 1.4 1.0 0.4 0.4 0.4 0.3 0.2SD 0.21 0.25 0.20 0.20 0.24 0.06 0.05 0.05 0.03 0.02RSD % 8.4 11.7 10.4 14.5 24.8 13.2 12.2 13.4 10.2 8.3CV(Volts) -0.4 -0.4 0.1 0.7 0.7 0.5 0.7 1.1 1.1 2.3SD 0.11 0.10 0.04 0.16 0.14 0.04 0.11 0.07 0.03 0.10RSD % 26.7 24.2 27.4 23.3 21.4 6.9 16.0 7.0 3.2 4.3CV(Volts) 0.0 0.3 0.5 0.5 0.4 0.8 1.1 1.1 1.5 2.0SD 0.43 0.12 0.14 0.15 0.08 0.13 0.14 0.18 0.08 0.16RSD % 18.5 43.3 26.8 28.2 20.1 16.2 13.3 16.6 5.3 8.1
0.25 0.16 0.13 0.17 0.16 0.07 0.10 0.10 0.05 0.0917.9 26.4 21.5 22.0 22.1 12.1 13.8 12.4 6.2 6.9
Type A or Type B ions
Average RSD %Average SD
1,3-DNB
Tetryl(-NO2)
RDX(+Cl)
HMX(+Cl)
PETN(+Cl)
NG(+Cl)
Average SD
Type C ions
TNT
TNB
2,4-DNT
2,6-DNT
3,4-DNT
Average RSD %
127
Table 4-2. Resolving power for explosive compounds. DV(kV) -4.5 -4 -3.5 -3 -2.5 2.5 3 3.5 4 4.5TNT 4.46 3.33 2.55 1.03 0.82 0.81 1.14 1.63 2.10 3.09TNB 5.93 4.45 3.62 2.40 1.23 1.15 1.79 2.41 4.12 7.852,4-DNT 4.52 4.07 3.76 2.44 1.28 0.89 1.19 2.18 2.87 5.432,6-DNT 5.44 4.33 3.55 2.78 1.38 1.33 2.14 2.17 3.55 4.923,4-DNT 4.89 3.77 2.92 1.66 0.95 0.98 1.26 1.70 2.93 3.061,3-DNB 6.01 4.73 3.90 2.68 1.68 1.93 2.83 4.80 5.85 5.76Tetryl(-NO2) 2.05 1.59 1.22 0.73 0.45 0.34 0.58 0.80 1.20 1.73RDX(+Cl) 0.80 0.63 0.64 0.41 0.28 0.22 0.17 0.17 0.13 0.08HMX(+Cl) 0.12 0.10 0.04 0.20 0.22 0.17 0.26 0.44 0.50 1.03PETN(+Cl) 0.00 0.09 0.18 0.20 0.13 0.26 0.35 0.38 0.53 0.80NG(+Cl) 0.71 0.70 0.56 0.48 0.28 0.29 0.06 0.16 0.18 0.19
128
2,4-DNTMW=182.13
3,4-DNTMW=182.13
2,6-DNTMW=182.13
CH3
N+
O-
O
N+
O-
O
CH3
N+
O-
ON
+
O-
O
CH3
N+O
-
O N+
O-
O
Figure 4-1. Structures of the isomeric explosives studied in this research.
Table 4-3. Resolution between TNT, TNB and DNT isomers. DV(kV) -2.5 -3 -3.5 -4 -4.5 -5 -5 -5
Carrier gas He/N2 0/100 0/100 0/100 0/100 0/100 0/100 10/90 20/802,4-DNT TNT 0.36 1.13 1.17 1.14 1.37 1.76 2.59 3.03
TNB 0.20 0.35 0.67 0.72 0.82 0.98 1.67 1.942,6-DNT 0.01 0.08 0.02 0.11 0.20 0.44 0.38 0.753,4-DNT 0.16 0.37 0.64 0.55 0.52 0.59 0.98 0.91
2,6-DNT TNT 0.39 1.27 1.15 1.28 1.73 2.50 3.05 3.64TNB 0.22 0.46 0.67 0.87 1.14 1.67 2.14 2.67
3,4-DNT 0.18 0.45 0.64 0.68 0.79 1.19 1.44 1.683,4-DNT TNT 0.17 0.66 0.49 0.60 0.94 1.48 2.06 2.35
TNB 0.01 0.07 0.03 0.13 0.28 0.47 0.85 1.10
129
RT: 4.00 - 5.98 SM: 5B
4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8Time (min)
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m/z 181[DNT-H]-
m/z 182[DNT]-
3,4-DNT 2,6-DNT 2,4-DNT
0 2 4 6 8 10 12 14 16 18 20CV (Volts)
Figure 4-2. CV spectra of a solution mixture of 2,4-DNT, 2,6-DNT, and 3,4-DNT at DV of -5000 V and in the carrier gas of 20:80 helium/nitrogen.
130
RT: 2.00 - 4.00 SM: 5B
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m/z 227[TNT]-
m/z 182[DNT]-
3,4-DNT 2,6-DNT
TNT
0 2 4 6 8 10 12 14 16 18 20CV (Volts)
RT: 2.00 - 4.00 SM: 5B
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m/z 182[DNT]-
3,4-DNT 2,6-DNT
TNT
0 2 4 6 8 10 12 14 16 18 20CV (Volts)
A
B
Figure 4-3. CV spectra of a solution mixture of TNT, 2,6-DNT, and 3,4-DNT at DV of -5000 V and in (A) the nitrogen carrier gas, (B) the carrier gas of 20:80 helium/nitrogen.
131
RT: 2.00 - 4.00 SM: 5B
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4Time (min)
0
50
1000
50
1000
50
1000
50
100
Full Scan
m/z 181[DNT-H]-
m/z 213[TNB]-
m/z 182[DNT]-
2,4-DNT
2,6-DNT
TNB
0 2 4 6 8 10 12 14 16 18 20CV (Volts)
Figure 4-4. CV spectra of a solution mixture of TNB, 2,4-DNT, and 2,6-DNT at DV of −5000 V and in the carrier gas of 10:90 helium/nitrogen.
132
Table 4-4. Resolution of explosive mixtures. DV(kV) 2.5 3 3.5 4 4.5 4.5 4.5 4.5 5
Carrier gas He/N2 0/100 0/100 0/100 0/100 0/100 10/90 20/80 30/70 30/70RDX TNT 0.52 0.64 0.89 1.12 1.57 1.09 0.96
HMX 0.02 0.07 0.16 0.20 0.52 0.96 0.71HMX TNT 0.42 0.68 1.08 1.43 2.30 2.15 1.57PETN TNT 0.49 0.70 0.98 1.33 2.06 1.59 1.69
NG 0.07 0.25 0.19 0.31 0.49 0.54 0.77NG TNT 0.60 0.67 0.98 1.35 2.02 0.92 0.89
133
Qualifying Examination April 18, 2008
0 .0 0 .5 1 .0 1 .5 2 .0 2 .5Tim e (m in)
0
5 0
1 000
5 0
1 000
5 0
1 00
Rel
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5 0
1 00Full Scan
m/z 257[RDX+Cl]-
m/z 226[TNT-H]-
m/z 331[HMX+Cl]-
RDX HMX
TNT
-20 -10 0 10 20CV (Volts)
Figure 4-5. CV spectra of a solution mixture of TNT, RDX, and HMX at DV of 4500 V with the
carrier gas of 30:70 helium/nitrogen.
134
RT: 0.00 - 3.00 SM: 3B
0.0 0.5 1.0 1.5 2.0 2.5 3Time (min)
0
50
1000
50
1000
50
1000
50
100
Qualifying Examination April 18, 2008
Full Scan
m/z 351[PETN+Cl]-
m/z 226[TNT-H]-
m/z 262[NG+Cl]-
TNTNG
PETN
-20 -10 0 10 20CV (Volts)
Figure 4-6. CV spectra of a solution mixture of TNT, NG, and PETN at DV of 4500 V with the nitrogen carrier gas.
135
RT: 10.00 - 12.00 SM: 3B
10.0 10.2 10.4 10.6 10.8 11.0 11.2 11.4 11.6 11.8 12Time (min)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Rel
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unda
nce
Tetryl
TNT TNB 2,6-DNT
3,4-DNT
2,4-DNT
DNB
0 2 4 6 8 10 12 14 16 18 20CV (Volts)
Figure 4-7. IS-CV spectrum of nitroaromatic explosives at DV of −4500 V and in the nitrogen
carrier gas.
136
Table 4-5. Reproducibility of peak areas from five replicate analyzes of explosive compounds. DV(kV) -4.5 -4 -3.5 -3 -2.5 2.5 3 3.5 4 4.5Peak area 468814 430507 359906 305778 225219 226627 208609 238505 241822 165238RSD % 4.2 9.8 5.6 6.8 12.7 8.7 6.5 17.9 11.9 21.7Peak area 820227 703567 486363 235923 104048 72029 67152 39299 27722 19791RSD % 2.3 5.5 8.9 8.5 2.0 8.6 8.6 22.2 12.9 27.6Peak area 1392630 1122380 678174 326222 191489 100343 79480 49228 34117 14313RSD % 2.4 2.2 5.0 11.9 11.4 4.5 9.8 7.3 9.0 19.6Peak area 1725286 1248024 698343 336120 163783 128165 90506 50921 19882 18977RSD % 1.0 4.5 4.0 6.5 18.6 7.5 12.1 14.3 15.9 32.5Peak area 1026866 805471 459985 256334 142445 119176 81529 60984 39872 21790RSD % 4.7 0.9 10.3 5.4 16.1 14.3 4.3 14.6 15.1 29.8Peak area 1344159 990463 626242 296082 157249 81181 62840 36329 19181 13408RSD % 1.4 5.2 2.5 6.3 14.4 1.1 25.0 20.8 20.5 29.2Peak area 635514 633901 495748 300800 192854 256080 315220 306151 307642 403898RSD % 2.4 1.0 1.7 7.0 13.4 21.9 3.8 12.1 7.6 5.5Peak area 3698229 3747959 3972949 3374851 2704275 1649147 1871024 2062641 2336755 2670661RSD % 7.4 4.7 3.5 5.9 5.3 10.1 10.9 5.7 4.9 2.1Peak area 3453817 4088608 4369903 4464514 4143868 2555905 3458119 4439756 5676881 6578504RSD % 8.3 4.7 3.0 5.2 4.1 8.7 10.1 7.2 2.4 4.2Peak area 4637077 5045016 5736454 4425278 3895087 2250896 5694710 6294470 6462891 6666631RSD % 5.4 4.4 1.7 6.7 12.8 16.3 5.5 2.4 1.8 2.7Peak area 3509916 4320895 4787442 4212408 3543002 1954903 2317536 1819609 1529655 1262850RSD % 4.5 2.2 3.2 3.4 5.4 9.3 9.4 11.9 12.1 8.2
4.0 4.1 4.5 6.7 10.6 10.1 9.6 12.4 10.4 16.6
TNT
TNB
2,4-DNT
2,6-DNT
3,4-DNT
1,3-DNB
Tetryl(-NO2)
RDX(+Cl)
HMX(+Cl)
PETN(+Cl)
NG(+Cl)
Average RSD %
137
Table 4-6. Linear dynamic range and limits of detection for the nitroaromatic explosives collected by full scan and SIM mode.
linear dynamicrange (ng/mL)
corr coef(R2)
concn(ng/mL)
amt injected(pg)
linear dynamicrange (ng/mL)
corr coef(R2)
concn(ng/mL)
amt injected(pg)
TNT 145-10000 0.9932 16 637 41-10000 0.9972 6 223TNB 31-10000 0.9943 28 1113 22-10000 0.9978 2 97
2,4-DNT 49-1000 0.9869 11 427 35-1000 0.9945 5 2062,6-DNT 13-1000 0.9914 1 51 25-1000 0.9972 2 923,4-DNT 47-1000 0.9711 9 380 14-10000 0.9990 7 2761,3-DNB 22-1000 0.9828 10 385 9-1000 0.9930 2 87
Tetryl(-NO2) 45-1000 0.9624 22 880 6-10000 0.9847 2 72Average 0.9832 14 553 0.9948 4 150
limit of detection limit of detectionFull scan SIM
Table 4-7. Linear dynamic range and limits of detection at the optimum CV for transmission of the nitro aromatic explosives for
varied collection time.
linear dynamicrange (ng/mL)
corr coef(R2)
concn(ng/mL)
amt injected(pg)
linear dynamicrange (ng/mL)
corr coef(R2)
concn(ng/mL)
amt injected(pg)
linear dynamicrange (ng/mL)
corr coef(R2)
concn(ng/mL)
amt injected(pg)
TNT 15-10000 0.9977 4 81 30-10000 0.9986 7 72 49-10000 0.9971 27 91TNB 19-10000 0.9963 17 345 46-10000 0.9946 34 343 56-10000 0.9952 38 125
2,4-DNT 18-10000 0.9955 6 121 20-1000 0.9980 3 32 70-1000 0.9526 20 652,6-DNT 22-1000 0.9964 15 294 17-1000 0.9994 9 90 17-1000 0.9962 5 173,4-DNT 19-1000 0.9990 6 119 35-1000 0.9918 6 63 33-1000 0.9954 11 381,3-DNB 12-1000 0.9928 5 102 20-1000 0.9970 7 70 80-1000 0.9651 39 130
Tetryl(-NO2) 29-10000 0.9949 21 419 20-10000 0.9953 15 151 91-10000 0.9750 59 197Average 0.9961 11 211 0.9964 12 117 0.9824 28 95
limit of detection limit of detection limit of detection1 min 30 s 10 s
138
100 150 200 250 300 350 400 450 500m/z
56
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87
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227.07
89.2075.13
120.9361.13 93.20 228.07178.87135.07 164.93 258.0089.13
75.13
213.13120.87
63.00 106.87 178.87135.07 164.87 214.13 244.0789.20
75.13
182.07120.87
61.13 106.80 178.80133.00 183.07
182.07
89.2075.13
59.13120.93 183.07106.87 178.87135.00 212.93
89.2059.13
182.0775.13
120.93134.93 178.87106.80 183.00
89.20
75.13
120.87199.0061.13 106.80 168.07 178.93133.00
89.20
75.13
120.93242.9361.13 106.73 178.87 226.00135.07 164.67 243.93
100 150 200 250 300 350 400 450 500m/z
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197.20228.13209.93192.07
213.07
183.27
115.20 164.93
181.20
152.07
182.00
152.27184.00
182.00
152.20168.00
168.13
138.27
241.13
226.13
185.33
213.27166.00 242.33 432.00268.67136.07 482.67292.87
APCI-MS APCI-FAIMS-MS
A
B
C
D
E
F
G
Figure 4-8. Mass spectra for analytes containing 50 ng/mL explosives collected by APCI-MS
and APCI-FAIMS-MS ranging from m/z 50 to 500: (A) TNT, (B) TNB, (C) 2,4-DNT, (D) 2,6-DNT, (E) 3,4-DNT ,(F) 1,3-DNB, (G) Tetryl.
139
APCI-MS APCI-FAIMS-MS
150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300m/z
32
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20
239.00227.00
165.07178.87
182.07177.93 235.07 240.00212.27150.93 163.87 167.07 220.27 249.40 253.13227.80194.73182.93 275.13265.07209.07 291.13199.13 285.40 297.87
178.87
177.80 213.13164.87
150.87 177.00 235.13163.13 239.00227.07221.07182.07 210.80194.80 275.27249.07 267.33 285.93 293.27203.27 263.27 296.93182.07
178.87
164.93 239.07177.93235.27150.87 163.53 177.13 227.13183.00 221.33 249.40212.20 240.07194.67 265.13209.07 286.27259.13 291.07276.00 297.13
182.07
178.93177.80
164.67150.73 183.07 212.87 235.07176.87 220.40 227.07 249.80239.00199.07 264.93193.13 210.27182.07
178.93
177.93164.93
150.87 177.07163.00 235.13221.13183.07 239.13227.13196.87 249.27166.87 209.13 275.27253.93 265.13 290.93297.33285.27178.80
177.80164.80
168.13150.87 198.93163.87 235.20 239.07220.40176.87 227.07180.93 249.27212.33162.33 196.80 291.13240.07208.00 265.13 275.00 286.20 297.07253.13178.93
242.13177.80
150.87 164.80227.00158.80 177.13 235.07 243.00220.47 249.80196.87181.87 212.27 275.07207.80191.00 287.47265.13257.20 294.93
150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300m/z
58
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226.93
197.20177.13 228.07210.00173.33167.73 180.27 191.93151.27 226.40 279.93197.87 263.00 287.00239.73 293.67271.07245.87
213.07
183.27212.73155.07 257.67166.20 215.20177.93 272.73200.53 247.13 298.33193.40 260.33241.93 284.67 291.53217.80206.07 223.20 237.27
181.27
182.13
151.20 166.27159.47 231.60183.07179.87 207.93 256.27 289.27279.13
182.00
152.13182.80 276.20153.60 179.87174.47 233.80 282.13238.67245.93163.07
182.07
168.00228.47 273.60183.00 223.87 234.67198.00152.00 243.40212.13 297.80176.93165.07 279.53250.27 291.00270.13200.93
168.07
169.07150.40 167.40 237.73180.53 203.13 260.93241.07
185.33
218.00217.67 242.00184.67 218.93177.07 186.07 201.20 225.87164.93 293.47209.67150.40 289.67248.07 266.80235.00
A
B
C
D
E
F
G
Figure 4-9. Mass spectra for analytes containing 10 ng/mL explosives collected by APCI-MS
and APCI-FAIMS-MS ranging from m/z 150 to 300: (A) TNT, (B) TNB, (C) 2,4-DNT, (D) 2,6-DNT, (E) 3,4-DNT ,(F) 1,3-DNB, (G) Tetryl.
140
CHAPTER 5 CONCLUSIONS AND FUTURE WORK
Conclusions
As demonstrated in this dissertation, FAIMS is a promising technique for separating gas-
phase explosive ions at atmospheric pressure, based on changes in their mobility at high electric
fields relative to low electric field. FAIMS can behave as an ion filter, capable of transmitting
selected compounds in a mixture on to a mass spectrometer. Mass spectrometry is by far the
most widely used technique for explosive identification and provides information orthogonal to
that provided by FAIMS. In addition, both FAIMS and mass spectrometry instrumentation can
be manufactured in a small scale, offering the potential for API-FAIMS-MS instruments to be
portable. An instrument of this kind may be able to replace conventional ion mobility
spectrometers (IMS) in the field for explosive detection. In this study, the combination of a mass
spectrometer with the FAIMS cell, in which only selected ions are transmitted through the cell,
has been proved to greatly simplify mass spectra over those acquired using a conventional mass
spectrometer, significantly improving the sensitivity and selectivity of this method.
Two API sources, APCI and DPIS, were investigated and were observed to produce
different characteristic ions, and relative intensities for analysis of explosives. Typically, the
DPIS gave more structural information over APCI through increased fragmentation, presumably
due to more abundant O2-, NO2
- and NO3-. In addition, spectra which present either more
information about structure or more abundant molecular ion can be obtained from DPIS by
adjusting the components in the surrounding air. Overall, the mass spectra of explosive
compounds produced by DPIS are comparable to those formed by APCI, although the formation
of nitrate and nitrite adduct ions with the explosives is more pronounced with the DPIS source.
This phenomenon will benefit explosive investigation especially in the field, where additives
141
may not be available for use. Although the signal intensity generated by DPIS is somewhat lower
than APCI due to the spatial obstruction of the neon bulb, the merits presented by DPIS, such as
rich ion patterns and decreased complexity of spectra for nitramines and nitrate esters, makes
DPIS an attractive alternative to APCI for explosive investigation.
The use of FAIMS as a potential method for characterizing explosive compounds has been
evaluated. A thorough understanding of ion behavior influenced by experimental parameters was
obtained in order to gain the optimum separation or transmission of ions. This knowledge will also
be beneficial for the further development of devices for explosives detection based on the
FAIMS technique. In this work, the CV scan rate, curtain gas flow rate, dispersion voltage (DV),
carrier gas composition and electrode temperature were optimized. The effect of these parameters
on the signal intensity (sensitivity), peak width (resolution) and compensation voltage (peak capacity)
was studied for explosive compounds of interest.
Experiments showed that a CV scan rate of 10 V/s and curtain gas flow rate from 2.0-2.5
L/min were optimal for both resolution and transmission. An increase in CV value and
sensitivity was observed at higher DV values for both type A and type B ions in N1 and type C
ions in N2 modes. Although the ion focusing mechanism in the cylindrical cell improves the
sensitivity, it also decreases the resolution due to peak broadening. Furthermore, the separation
and sensitivity can also be controlled in FAIMS by changing the carrier gas composition. A
mixture of helium and nitrogen was demonstrated to improve resolution and sensitivity. For type
A ions, the peak width was reduced and the CV was shifted to more positive value in N1 mode
with increasing helium content in carrier gas, improving the resolution between those ions. In
addition, the sensitivity and peak width were increased dramatically for type C ions in N2 mode
with increased content of helium in carrier gas. A broader peak width and increased signal
intensity was also seen for type B ions in the same mode at higher DV, indicating that a type B
142
ion was transformed to a type C ion at higher electric field as the helium content in carrier gas
increased. Finally, variations were also observed in CV value, peak width, and signal intensity
with varied electrode temperatures. Two type B ions, the [M]- ions of TNT and 2,6-DNT, have
decreasing mobilities at lower electric field with increased temperature, which implies that the
raised temperature on electrodes not only reduced the number density of the curtain gas in the
FAIMS cell but also affected the interaction of these ions with the gas.
The performance of APCI-FAIMS-MS in separation and detection was evaluated in this
research. The best repeatability of CV value was obtained at DV of −4500 V for type A or type
B ions and at DV of 4500 V for type C ions. The SDs and RSDs of CV values were distributed
from 0.02 V to 0.2 V and 1.0% to 8.1%, respectively. The ability of FAIMS to separate
explosives from mixture has been demonstrated. Although higher DV was proved to increase
CV values and resolving power, it also leads to peak broadening. Addition of helium to the
carrier gas improved the separation between similar explosives and isomers; however, it also
decreased the transmission of explosive ions for type A and type B ions. In this research, most
explosives can be resolved by the combination of CV spectrum from FAIMS separation and
mass select-ion from the mass spectrometry. In brief, the optimum condition of each parameter
required to resolve explosive peaks needs to be discovered experimentally for different analytical
situations.
Although quantitation of these compounds was not the purpose of this research, calibration
curves were constructed in order to test linearity and sensitivity of the APCI-FAIMS-MS method.
Two scan modes, full scan and SIM, and three lengths of detection time, 1minute, 30 seconds,
and 10 seconds, were tested for LOD and LDR. The quantitative study showed that FAIMS was
sensitive for nitroaromatic explosives down to the average concentration of 14 ppb for full scan
and 4 ppb for SIM, and provided linear calibration for at least 3 orders of magnitude. Although
143
the full scan gave higher LOD and decreased LDR, it is still essential for explosive detection
because of more information about molecular and fragment ions provided by this detection mode.
The preferred detection time to monitor specific explosive compound by setting to the optimum
CV is 30 seconds and the amount of explosive compound required for this accumulation time
can be down to 32 pg.
The ultimate goal of this research was to evaluate the feasibility of API-FAIMS-MS to
detect explosive compounds, and to ascertain methodologies to eventually do so in a field
environment. According to the experimental data shown in this research, the integration of
FAIMS with mass spectrometry for the analysis of explosive compounds was very fruitful,
permitting a sensitive, selective, and rapid analysis of explosives. The method is able to perform
a fast separation of explosive ions and a selective and sensitive detection of different classes of
explosives, pointing to the development of a powerful portable device for monitoring explosives
in field. The development of fieldable explosive device based on this concept could make a
contribution toward the protection of first responders and emergency personnel, diagnosis of the
nature of the attack, and gathering of forensic data, or even for the prevention of terrorist
activity.
Future Work
Preliminary results have shown that API-FAIMS-MS is a viable method for the analysis of
explosive compounds, but further studies are required to improve the sensitivity, resolution,
portability, and reliability of the method to be employed in the field. This work would likely
include further research in several modifications on each component (ion source, FAIMS, and
mass spectrometer) to allow the instrument to serve its purposes.
144
Ionization Source
The DPIS has presented the superiority over APCI on the explosives detection by
generating diversified and selectable ion pattern on mass spectra. It may be potentially used for
the fast detection of explosives in the field with the advantages that include design configuration
flexibility, dimensional stability, simplicity and ruggedness of design, and extended source
lifetime. However, in this research, the ion intensity produced by DPIS is only 10% to 50% of
ion intensity by APCI due to the inefficiency on the transmission of the ions from ionization
source to detector, which is primary caused by the spatial obstacle of the DPIS neon bulb itself.
A possible resolution is to reshape the DPIS neon bulb into a cylinder of tube, which may
increase the efficiency of both ionization and ion transport by directing the analytes through the
ionization area inside the cylinder.
The other important issue for the desirable ionization source employing in the field is the
capability to ionize explosives directly from various matrices. A design based on similar
mechanism termed dielectric barrier discharge ionization (DBDI)17, 74 has been demonstrated to
permit desorption and ionization of the explosives from solid surfaces. The uses of discharge
gases to assist desorption and ionization may provide an alternative to integrate with DPIS in the
future design.
FAIMS
How to downsize the FAIMS cell without compromising resolution and sensitivity will be
a concern when developing a portable explosive detector. Ongoing efforts in our group are
aimed at developing FAIMS cells of varied geometries (planar, cylindrical, hemispherical and
spherical cells), and varied dimensions in order to fabricate a field-portable explosive detector
for providing early and timely detection of different classes of explosives. Current research
145
demonstrates that the planar geometry provides better resolution and the spherical geometry
gives higher transmission. The resolution and sensitivity are also influenced by the dimensions
(gap width, height and length) of the cell. Therefore, further studies are necessary to determine
the ideal FAIMS cell design which may combine several geometry and optimum dimensions,
which will provide the best combination of resolution and sensitivity.
Additional experiments using FAIMS could include the evaluation of different waveform types
and frequencies, curtain gas combinations, and designs for introduction of analyte ions into the
FAIMS cell. It would be advantageous to improve our understanding of how ions behave under the
influence of different parameters of FAIMS in order to apply the most optimum condition when more
sensitivity is required for the application of interest.
Mass Spectrometer
As suggested in the previous chapter, the limit of detection of the API-FAIMS-MS
method was limited primarily by the mass spectrometer. For this reason, the use of more
sensitive mass spectrometer, such as triple quadrupole, can be expected to increase the sensitivity
of this method. In addition, modification of the commercial FAIMS cell or the mass
spectrometer so the FAIMS cell could be attached directly onto the inlet of mass spectrometer
could eliminate the capillary extender used in this research that caused a 75% decrease on signal
intensity.
146
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BIOGRAPHICAL SKETCH
Alex Ching-Hong Wu was born in 1972, in Kaohsiung County, Taiwan. He attended
Central Police University where he received a bachelors degree in forensic science in 1994 and
started undergraduate research on glass evidence analysis by ICP-AES under the supervision of
Dr. Chien-Min Hsu. After three years of working experience in the forensic science field, he
came back to Central Police University to purse his master’s degree in 1997 and focused his
research on amphetamines analysis by SPE and GC/MS under the direction of Dr. Sheng-Meng
Wang.
After graduating from Central Police University, he worked for the Forensic Science
Center of the Criminal Investigation Bureau in Taiwan, which is responsible for the investigation
of major crimes nationwide. With the Bureau, he served as a forensic expert and police officer
while performing crime scene investigations and forensic evidence analyses. In addition to his
laboratory experience, he also spent a few years working with practical crime case investigation.
In 2004, he married Rosalind Yi-Chun Lin in Taipei. During this time, he was selected for
the students studying abroad by the Taiwanese government, sponsored by the Ministry of
Education, to study for his Ph.D. in the United States. In fall 2006, he chose to pursue his Ph.D.
degree in analytical chemistry at the University of Florida and joined the Yost Lab. He received
his PhD degree in August 2009 under the supervision of Dr. Richard A. Yost.
