11.2 11A. Features of AMS - Yonsei Universitychem.yonsei.ac.kr/~mhmoon/pdf/InsAnal/Ch11.pdf ·...

11.1

by Prof. Myeong Hee Moon

• Useful for really all elements.– Detection limit of AMS : better than Atomic optical method

3 orders of magnitude better– unique, simple spectra– atomic isotope ratio

• weakness– More expensive– Instrument drift 5~10%/hr– Interference effects

Ch.11 Atomic Mass Spectrometry

11.2


11A. Features of AMS

C126

steps:1. atomization2. conversion of atom to ions3. separating ions by m/z4. counting the # of ions or measuring ion I

11A-1. Atomic weights in MS

: MS distinguishes isotopes: Atomic mass units (amu) or Daltons

1/12 of the mass of one neutral

1 amu = 1 Da

Catoms 6.0221x10

C/mol g12

12

1126

23

126

Cg/atoms 1.66054x10 126

-24

11.3


• m/z ratio : mass to charge ratio12C1H4

+ m/z=16.035/1=16.035

This chapter covers QuadrupoleTime-of-FlightDouble Focusing

11A-1. Atomic weights in MS

11.4


By thermalor electrical

energy

TOFQuadrupoleMagnetic sector

Components of Mass Spectrometer

11.5


- similar to photomultiplier- Rugged & reliable- short response time (ns)

1. Electron multipliers (EM)

11B-1. Transducers for MS

11.6



2. Faraday cup


2. Faraday cup

11.7



3. Array transducer- Analogy between optical spectrometers and MS enhances detection systems

- Microchannel plates : an electrooptical ion detector

x 1000 e

MCP~ 6 m

flash of light

11.8


11B-2. Mass Analyzer

1. Quadrupole : Common in AMS, scan rates <100ms

ionacceleration

By 5~10V

Mass filter- selectively pass specific m/z

• rate of ion movement along z-axis

• m/z• frequency & magnitude

of AC signal

x zy

11.9



In the absence of dc voltage, ions converge in z axis during positive half of ac cycle

diverge during negative half.

During negative cycle, positive ion strikes the rod and neutralized disappearing

: depends on rate of movementm/z, frequency & magnitude of

ac signal

11.10


• If an ion in the channel is heavy orthe frequency of ac voltage is large,: Ions will not respond significantly to the alternating

potential and will be influenced largely by the dc potential.Under these circumstances,

ions maintain between the rods. Ion deflection depends largely on dc potential.

: lighter ions deflect easier than heavier ions

Under positive dc potential superimposed on theac signal (rf:radio frequency) (at xz plane)

11.11



• In case of lower ac freq. or lighter ions (at xz plane): light ions collide with the rods and disappear.

- high pass mass filter

11.12


When AC voltage applied,lighter positive ions maintain in the space.

----- low pass mass filter


• At rods of negative dc potential (two rods at yz plane): In the absence of ac field, all positive ions will be

drawn to the rods.

11.13


• Quadrupole transmits a band of ions of limited m/z range

resolution of Quad: ratio of ac V and dc U potential

Maximum --- at 6

by varying V and U (fixed V/U), scan m/z ratio

dc: 0~ +250VAc: 0~1500V


11.14


11B-3. Time of Flight (TOF) mass analyzer

• ionization: positive ion by electrons, laser pulse, photons10~50 kHz, 0.25ms

ion acceleration1~10kV

drift time1~30ms

electronmultiplier

11.15


• Resolution & Reproducibility

not as good as Q or magnetic sector

simplicityruggednessease of accessibility to ion sourceunlimited mass rangerapid data acquisition rate

Improvement in resolution - using reflector

11B-3. Time of Flight (TOF) mass analyzer

11.16


11B-4. Double focusing analyzer

focusing ion beams with electrostatic analyzer & magnetic sector

Focus ion beam havinga narrow band of K.E.

11.17


11C. Inductively Coupled Plasma MS

1980s, for elemental analysis low detection limitshigh degree of sensitivitygood precision & accuracy

atomizer &ionizer

10-4 torr

sampling cone (water cooled, 1torr)Rapid gas expansion-cooling

(<1.0mm)

Inside chamber+ ions separated fromelectrons & molecules

focused to ion lensto Q

11.18


• Features - ICP mass range: 3~300- 90% of Periodic Table- 10s/element meas. 0.1~10ppb- RSD 2-4%

(m/z)=1

11C. Inductively Coupled Plasma MS

11.19


11C-2. AMS & Interferences

ICPMS vs. ICP optical methods for rare earth metals.

1000ppm Ce 10ppm Cemolecularbands

ICPAES ICPMS

Still holds interferences

11.20


1. Spectroscopic interferencesin case of same m/z for ionic species

• Isobaric interferences

isobaric species: two elements that have isotopesof the same mass

for Q, mass difference within 1 unit.

113In+ = 113Cd+

115In+ = 115Sn+

40Ar+ overlap with 40Ca+(97%) : use 44Ca+(2.1%)

58Ni+ isotope - 58Fe+ 56Fe+

corrections by software


11.21


• Polyatomic ion interferences

molecular ions (below m/z=82) generated in plasma

14N2+ - 28Si+, NOH+ - 31P+

16O2+ - 32S+, 40Ar2

+ - 80Se+

corrections: blank run


11.22


• Oxide & hydroxide species interferences

: most serious in ICPMS from matrix, solvent,

plasma gas

MO+, MOH+

Efforts in reduction of

injector flow rateRF powerskimmer spacingorifice gas comp.oxygen elimination


11.23


• Matrix effects

in case of conc. larger than 500~1000 g/mL

Solved by using more dilute solutionsaltering the sample introduction procedureseparating out the offending species

- Internal standards (major)

same mass & ionization potential with analyte


11.24


11C-3. Application of ICPMS

Qualitative and Semi-quantitative analysis

• D.L.: lower thanICPAES, similar to ET, AAS

0.02~0.7 ppb with majority 0.02~0.1 ppb

11.25


• Quantitative analysis

internal standards: In, Rh.

Isotope dilution method: accuracy improvement.: measure the change in

isotope intensity ratio for known quantity added

or for stds. enriched in one ofthe isotope

- science- medicine- archeologist- geologist

11C-3. Application of ICPMS

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11.2 11A. Features of AMS - Yonsei Universitychem.yonsei.ac.kr/~mhmoon/pdf/InsAnal/Ch11.pdf ·...

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