Far UV Ionization Photo Ionization) and Absorbance Detectors for CapillaryGC

8/4/2019 Far UV Ionization Photo Ionization) and Absorbance Detectors for CapillaryGC

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52 FAR-UV IONIZATION AND ABSORBANCE DETECTORSHPLC, and SFC, where the lower flow rates ean increase the sensiti,:.such detectors.

Background information, analytical applications and sources f ( ' ~ far-UV radiation are described in detail in the following sections.

1. PHOTOIONIZATION DETECTION1.1. Background

The use of far-UV ionization (photoionization) detection for GC wasdescribed in the late 19505 along with flame ionizat ion (FID), electron c:.;(ECD), mass spectrometer, and cross-section detection (l). During the _>, .there was considerable research on the PID ( 2 ~ 4 ) but the FID gained pority so quickly that all commereial gas chromatograph manufacturers 0::: __the FID along with the thermal conductivity detector (TCD). In the mea!".:. - _the PID continued to be a sensitive, novel detector but required a ,:1":( 1 ~ l O T o r r ) for operation, was easily fouled by column bleed, was uns- - .and required a skilled operator. By the later 1960s, a decade after its discc :the PID had been virtually abandoned. By the mid-1970s, however, the -: was reborn when several researchers (5,6) found that separating the ~ . : - discharge from the ion chamber improved the stability of the PID, an":detector was simplified. Driscoll and Spaziani (7) described additional im F .ments to the PID that resulted in an improved range {> 10 7 instead 0:- :improved stability, and lower background. It also minimized the probiccolumn bleed that plagued earlier PIDs. After a slow start in the 1960,.-. PID was finally on its way in the later 19708 to becoming an importan: in the arsenal of the analytical chemist Davenport and Adlard have rey;;:

19891976

1960'811

150 200 3 0 0 - ~ - - - -Figure 4.1. Comparison of PID geometry before and after 1970.

Tmax


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PHOTOIONIZATION DETECTION 53the PID for its application with respect to analysis of alkenes and aromaticcompounds; they found that it has a high sensitivity and selectivity to thesematerials (8). Some suggestion for an improved detector for analysis viacapillary columns was also included.

1.2. Theory of OperationIn 1976, the first commercial PID was described by HNU Systems, Inc.,Newton, Massachusetts (9). Schematic drawings of that detector and a recent(third-generation) PI D are shown in Figure 4.1. The process of photonioniza tion starts with the absorption ofa photon (hv) by a molecule R. I f the energyof the photon is more than or equal to the ionization potential of species Rand the carrie r gas is given by C, then ionization and the other processes showshere occur:

DirectR + h v ~ R + +e

IndirectC + h v ~ C *

C* + R---> R+ + e- + CR + hV'---7 R*

QuenchingR++e----+Re'+C---+C

(k d

(k2)(k3)(k 4 )(k s )

(k6)(k7 )(k s )

The number of ions formed is equal to the summation of the direct andindirect process minus the quenching process:

Note that in ks , ions may also result from a photochemical rearrangementprocess leading to the formation of R! . This process may account for thephotoionization signal observed for the 10.2-e V lamp with methylene chloridelionization potential = 11.3 eV), Here. the rearrangement and resultant loss ofel 2 may form an ethylenic species with a lower ionization potential. The


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54 FAR-UV IONIZATION AND ABSORBANCE DETECTORSP " .nergy of a 10.2-eV lamp, which corresponds to more than 230 kcal m: .greater than the 80 kcal/mol required to break a typical carbon-carbon c:

One can calculate the energy (E) in eV or wavelength (nm) from the follo\;.equation:

;;iW(nm) = 1234.5/E(eV) ' ~ r Thus, a 10.2-eV lamp has an energy of 121 nm.

The photoionization yield (Plyield) is the number of ions producedphoton absorbed:

= 100(Rs:m)/(number of photons absorbed)lyicldFrom this equation, the number of ions produ ced (Rs:m) is proportion::.. .the product of the absorption coefficient and the intensity of the d i s c h . : . ~ ; . lamp.

Electrodes

High l~ n " ~S h i e l d ~ d UVW;odowTemperatureSeal

Collector LElectrode pertureccelerating A ;Ii

Figure 4.2. Exploded view of ion chamber for PID.


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PHOTOIONI ZA TION DETECTION 55j, - -.ypicai PID has two functional parts: an excitation source and anbond .:.:ion chamber. An expanded view of the lamp and ionization chamberfollowing - . xn in Figure 4.2. A potential of 100-200 V is applied to the accelerating_ -:de to push the ions formed by UV ionization (above) to the collection_ :de where the current (proportional to concentration) is measured.

:hat the collection electrode is shielded to reduce the background (9);; configuration of the ion chamber is axial. It can be shown that the most.


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56 FAR-CV IONIZATION AND ABSORBANCE DETECTORS

106,7

ii i:zUJ;;104,8

........ _ . ~ - .. , - ~ > - ..'-". '-"-"'-

WAVELENGTH (nm)Figure 4.3. Emission spectrum of an 1 L7-eV lamp.

periods. This lamp has a lifetime of > 5000 h. The lifetimes of the 9.5- 2::..8.3-eV lamps are similar. The 11.7-eV lamp, however, has a lithium fluor::::window that has a problem with solarization (color center formation) tr..limits its useful lifetime to a few hundred hours. A recently improved laICshown in Figure 4.4, has a nickel cathode to bleach the color centers ( f o r r r : ~ ' by absorption of the short-wavelength photons and trapping of electronsbelow the conduction band of lithium fluoride) and extends the lifetime5 0 0 ~ 1000 h. The trapped electrons are released by irradiation with UV (abc'.7 eV) from the Ni lines in the lamp. Another problem with this lamp is :':-.:shift in short-wavelength cutoff to a longer wa velength as the temperaturethe window increases. Since the argon emission lines (104.4 and 106.6 nm)close to the cutoff at ambient temperatures, heating the window above 125reduces the transmission of lithium fluoride and therefore the photon flux.

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PHOTorONIZATION DETECTION 57

of the 9.5- an..::!hium fluoride:'ormation) tha

::1proved lam!"-::enters (forme..::electrons j u s ~

the lifetime L,ith UV (aboL:his lamp is thetemperature

::-.d 106.6nm) aI:.\\ above 125 C:Jhoton flux. A

.c igure 4.4. Construction of an improved 1L7-10V lamp: #18, metal cathode; #28. lithium fluoride::dow; #14 and #26, discharge electrodes.

2

FID 11.7 10.2 9.5 8.3Increasing Selectivity ...

. ~ c r e 4.5. Comparison oflhe relative response of the FID with a PID containing various lamps, ;ies).

. - same time, the solarization of the window increases as a result of the. . :eased absorption of photons by lithium fluoride.The most popular PID lamp is the 1O.2-eV model, which has the highest:on flux and therefore the greatest sensitivity. This lamp uses a magnesium

. -ide window. Figure 4.5 illustrates the variation in sensitivity relative to.;:' energy. In comparing the two chromatograms in Figure 4.6, the differ, -c in sensitivity (attenuat ion) of benzene should be noted (10). The 10.2-eV


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58 FAR-UV IONIZATION AND ABSORBANCE DETECTORS

PIDID11.7 eV0.2 eV1 x 80 x 16

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PHOTOIO?>;IZATION DETECTION 59

excited state

Figure 4.7. Morse curve of potent ial energyground state vs interatomic distance for the ground state(RoJ and the first electronically excited state

Atomic Distance (RdI'I, I

1( : sensitivity of the PID is proportional to the intensity of the light source,.::.sers will have an inherently higher sensitivity. The development of high- ~ a k - p o w e r and high-repetition-rate tunable lJV lasers has made multiJoton ionization (MPI) feasible (II). In this process, the first photon excitesmolecule from the ground state to the first excited electronic state and thecJsorption of the second photon produces ionization. Typical Morse curves. r photoionization and two-photon ionization are shown in Figure 4.7. In~ . e two-photon ionization process, the sum of the energy of the two photons.. :lst be greater than the ionization potential of the molecule and the two~ J o t o n s can have either identical or different frequencies. Since many large":".0Iecules have ionization potentials from 7 to 11 eV (176 to 112 nm), twoJoton ionization can be initiated with near-UV laser sources. Typical laser; :,urccs can be excimer pumped frequency doubled dye lasers.Two-photon ionization can produce ionization yields of several percent. )mpared with 10 - for conventional photoionization processes (above).:>,ovided the repetition rate (duty cycle) is sufficiently high, the "potential"; :nsitivity is higher than that obtained with a typical discharge lamp.~ . i t h o u g h Klimetand Wessel (12) showed that laser ph6toionization is':lotentially" more sensitive than ionization with a discharge lamp, their':3 ta is not conclusive since the low-picogram detection limits reported can be.::hieved using a conventional UV discharge lamp in the PID. Ogawa et al.:3) also report detection limits of 6 pg for pyrene using laser two-photon~ n i z a t i o n . Two major problems with a laser replacing a discharge lamp in a PID are: ~ e cost ($50,000 vs. $300) and the size (several cubic meters vs several cubic~ ; : n t i m e t e r s ) . Should these problems be overcome, the possibility of selectivity.::ctermining isomers or classes of eompounds by two-photon ionization will-;:come a reality


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62 FAR-tiV IONIZATION AND ABSORBANCE DETECTORS1.4. Gas Chromatography Applications

The 11.7-eV lamp will respond to many low-molecular-weight comp('_.that have more tightly bound electrons and hence higher ionization poter .Typical applications include the detection of low- or sub-ppm ( n a n o ~ ~ _ levels of formaldehyde (14), which cannot be measured directly by any.:detector at these levels. The 11.7-eV lamp is also useful for the d e t e c t ; ~ low-molecular-weight compounds (< 150 amu) such as CC1 4 , CHCI3 ,and CZH2 that have IPs> 10.5 eV. A comparison of the response fortypical hazardous waste components using a 10.2- and a 11.7-eV l a r . ~ shown in Figure 4.6. Chloroalkanes are not detected with the 1O.2-eV L:-The 11.7-eV lamp PID is similar in response to the FID for manycarbons (except for methane, which does not respond), and it also resp: .to inorganic compounds such as C1 2 , PH 3 , and 12 , The FID only resp.:-_to carbon-containing compounds. A list of ionization potentials (IPs) is l_ !in Table 4.1. IThe major difference between the 10.2- and 9.5-eV lamps is the a b s ~ intensity of the lines. There are certain applications where the 9.5-e V lar:preferable to the I0.2-e V lamp. These applications include aromatics I:. _aliphatic matrix such as pentane, mercaptans in the presence of H 2S. c. _amines in the presence of ammonia 05). With the 8.3-eV lamp thereconsiderable increase in selectivity compared to the 10.2-eV lamp. A ber.::or toluene solvent prod uces no response on a PID (8.3 eV). The 8.3-e V l.: was selected for the determination of polyaromatic hydrocarbons (16) bee::_of the increased selectivity. Typical detection limits were low- to s u b - n 2 ~ gram levels. Chromatograms of complex mixtures using these three lamp:'.;. .the FID are shown in Figures 4.8 and 4.9. The pattern for the 10.2-eV PID i,,:is similar to that of the FID, as noted previously (17), whereas the chrorr :.grams are simpler for the 8.3-eV lamp. In a previous publication (101.reported a lower "apparent" sensitivity for biphenyl (IP 8.27 eV) use: _a normalization compound. In later work, we found that if a n t h r a ~ : (lP = 7.5 eV) is used for normalization, a 10-fold higher response is obta::for the 8.3-e V lamp, which is attributable to the increased efficienc:.anthracene. II.Langhorst (18) determined the sensitivities for nearly two hundred 'f'""pounds for a PID with a 10.2 eV lamp. She found that the PID was a car:counter (on a molar basis); that the sensitivity for alkanes < alkenes < . : . ~ matics; tha t sensitivities for cyclic> monocyclic and branched> r:: branched; and that for substituted benzenes, ring activators increased:sensitivity whereas ring deactivators decreased the sensitivity. Figure..:.depicts a typical curve for a homologous series of hydrocarbons compareebenzene. A comparison of the sensitivity (normalized to benzene) for a var::

-

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(10),cV) usedanthrace:is obtaiL

cc:'a cart .

n(Figure ,.l.

.::omparedfor a varic

U.--_w ,''''._lllll111i11'- - - - - - - - P H O T O I O ~ I Z A T I O ~ D E T E C T I O ~ 63

: ;:ure 4.8. Chromatogram of leaded gasoline by FID compared with Chr(lmatograms by PID:. 9.5. and 8.3 eV).


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64 FAR-UV IOKIZATION AND ABSORBANCE DETECTORS

2 ,4 ,4 TRIMETHYLPENTANEOCTENEtrans 1 ,4 DIMETHYLCYCLOHEXANE

4 DIMETHYLCYCLOHEXANEPID 9,5 eV

:r~ I ~

"

1 HEXENE2 3 1 4 5 6 7 TOLUENE 8 m - XYLENE 9 p . XYLENE 10 C U ~ E N E 11 0 - XYLENE12 MESITYLENE

Figure 4.9. Chromatogram of components of gasoline by FlD compared with chromatogri.:: by PID (10.2, 9,5, and 8.3eV).


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PHOTOIONIZAnON DETECTION 65;,a-SltiVlty (MR) ReI. to Benzene

: h r o m a t o g r a ~

Carbon NumberAliphatic HC Olefinic HC Aromatic HC

" 10. Comparison of relative molar response (MR)VB. carbon numberfor the PID (10.2 eV).

-':ituted benzenes is shown in Figure 4.11. The GC-PID response for:.: ted benzenes changes slightly with electronegative substituents, yet-_


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66 fAR-UV rONrZATION AND ABSORBANCE DETECTORS

P OOH . I I IPhGOOH

Ph"NO, "0"'" Ph"Br"E

" I""el

u"

! Iehil " II I I_I j I IP h " H ~ I : I' i ~ __ 4-'Figure 4.11. Compari son of the molar response for 'j-! 10 2')L",j" 5011GC-PlD with LC-PrD. ~ l I 1 o i a ~ Respor.se C o m p a ~ i s o r , :E0>'iii.


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- ~ . - - - - - - - - - - - - - - - - - - - - - - - - . PHOTOIO NIZA TION DETECTION 67

3.7 nglcmpd.NO MAKEUPAttn: 10X32

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FAR-lJV IONIZATION AND ABSORBANCE DETECTORS68C10

L"'9caPIO cscs C10.2 eV_+ JL\ 7. _ L _ - , L _ - - ~ ' - ' - " - -FlO

c9C10

c8C7

Cs c6Figure 4.14. Comparison of capillary chromatograms for the FI D and PI D for alkanes (Nordibond SE54, 25 M x 0.32mm ; temp. prog. 50-180'C at 10C/min). Courtesy ofHNU-Nordion.

used a similar cryogenic trap approach but with smaller diameter (O.OS-mm)capillary columns and longer retention times (minutes vs. seconds) as a resultof the molecular weight of the components selected for analysis. Results for apeppermint oil sample obtained using a O.OS-mm capillary column are shownin Figure 4.17. Van Es and Rijks (25) used makeup gas for the O.OS-mmcolumns with no adverse effect on the performance of the detector. They founda minimum detectable quantity of 0.2 pg for anthracene. This is an order ofmagnitude lower than packed column detection limits and about two to threetimes better than detection with the tow dead volume cell and no makeup gas(10). Although the reduction of cell volume and detector pressure are equivalentalternatives for other concentration detectors such as the TCD, van Es andRijks (2S) found that, for the PID, the reduction of dead volume was morecritical than reducing the pressure. They also found that the effect of additionof makeup gas and reduced pressure on detection limits was equivalent.


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alkanes (N c' ,ofHNU-Norc,

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effect of adZ:equivalent

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..mparison of capillary chromatograms for the FID and PID for aromatic hydro: :ibond SE54, 25 M x 0.32 mm; temp. 40 C/3 min, 40-120 DC at 5CjminJ,, :';:--;CNordion.

PHOTOIO]\,lZATION DETECTION 69

'"": "'"ll :"" ""

o 2 4R E T E ~ T I O N TIME (secends)

c::romatogram with high-speed PID. Courtesy of S. Levine. University of


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70 FAR-UV IONIZATION AND ABSORBANCE DETECTORS

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Fro

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o---I---r------a---4----s-=:::niln--7--Figure 4.17. Fast chroma togram of peppermint oil using PID and FID in series (50-JlM column).Courtesy of J. Rijks, University of Eindhoven.

1.5. Liquid ChromatographyMany papers have been published over the past 15 years on interfacing a PIDto a liquid chromatograph (26), yet there are no commercial PIDs availablefor liquid chromatography (Le). This detector remains one of the moreinteresting detectors for HPLC and awaits development, as noted severalyears ago by the author (to). One interesting feature of the PID is that with


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PHOTOIONIZATION DETECTION 71TabJe 4.2. IOllizatioll Potelltials of Some CommonSo}yents fOF HPLC

Ionization PotentialSolvent (eV)\Vater 12.35\fethanol 10.85Chloroform 11.42Carbon tetrachloride 11.47Acetonitrile 12.22Pentane 10.35

..::TiP, most of the common solvents used for reversed phase LC. , 'nized (see Table 4.2).-.:..:' for LC have involved flowing the liquid directly into the cell or- ; ::-:'c liquid before passing it through the cell (27). Both these ap

'. i: drawbacks, and neither solves the problem of quenching, which. ::.; D to the detection of quantities above the low-microgram (by the. ~ . : .:ch) or nanogram level (by the vaporization approach), in LC.. 'land, low-picogram quantities are detected in Gc. If these GC. c. ,,]s were approached more closely, this detector would become_ .. LC., ..::-.d Jorgensen (28) have described a PID that uses open tubular., :C'Jd of conventional LC columns. The sample is vaporized before. ::le PID. Berthold et aL (29) have used laser two-photon photo- . : detect 28 polyaromatic hydrocarbons (PAHs) in an LC effluent.: : = ~ : i o n limits were 50-70pg for the PAHs compared with 6pg. ' : - :rted by Ogawa et al. (13) for GC-PID. Note that the quenching_:


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172 FAR-liV IONIZATION AND ABSORBANCE DETECTORS

r-----------------INJECTOR IIIVAPORCOIL

COLUMN III!!IIIL__

Figure 4.18. Schematic or LC-PfD,

........PID --------1,II

-"OVEN

Table 4.3. Selected Detection Limits Obtained by LC-PIDDetection Limits

Compound (ng)Bromobenzene 4Chlorobenzene 4Nitrobenzene 2Phenol 3Benzoic acid 2Dioctyl phthalate 16Benzene 15Anthracene 75

mechanism will undoubtedly predominate:5 + hv - - .5* (kg)5* R ----> R+ + e - 5 (k 10 )

The quenching will be considerably greater in the LC mode than the GCmode because of the higher concentrations of electronegative species in thecondensed solvents compared to the inert gases in the GC mode. This leadsto the following quenching reactions:

S+e----+S* (kill5* R- - .R - '-e-+S (k12l

..........


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I 74 FAR-CV IONIZATION AND ABSORBANCE DETECTORS- - ~ - ~ - ~ -

Dichlorobiphenylsulfone in Dichloromethane, - - - - - - - ~ . - - - - ~ - - - - - - - - ,

-1.7 pg Injection Triazopan in AcetronitrileFigure 4.19. SFCPID of Jow-picogram levels of triazopan. Reprinted with permission froml Hayhurst and B. Magill, Int. Labmate 14. (Aug., 1988). Copyright [988, International Labmate.

results attainable by Gc. The results of Hayhurst and Magill (34) show subnanogram levels of anthracene detected by PIO using similar conditions. Thissuggests some quenching or loss due to the CO 2 carrier gas. Hayhurst andMagill (34) used a 6 m x 50 11m capillary column system and detected low- orsub-picogram quantities of dichlorobiphenyl sulfone and triazopan. A typicalchromatogram of a low-picogram sample oftriazopan is shown in Figure 4.19.Advantages of SFCPIO include reduced analysis time by eliminatingsolvent tailing, use of a wider range of solvents that produce "no response"(such as methanol, methylene chloride, acetonitrile, and Freons), detection ofa wide range of compounds that do not absorb in the UV, and increasedsensitivity over the FlO.


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Far UV Ionization Photo Ionization) and Absorbance Detectors for CapillaryGC

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