11.2 11a. features of ams - yonsei universitychem.yonsei.ac.kr/~mhmoon/pdf/insanal/ch11.pdf ·...
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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
by Prof. Myeong Hee Moon
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
by Prof. Myeong Hee Moon
• 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 Prof. Myeong Hee Moon
By thermalor electrical
energy
TOFQuadrupoleMagnetic sector
Components of Mass Spectrometer
11.5
by Prof. Myeong Hee Moon
- similar to photomultiplier- Rugged & reliable- short response time (ns)
1. Electron multipliers (EM)
11B-1. Transducers for MS
11.6
by Prof. Myeong Hee Moon
11B-1. Transducers for MS
2. Faraday cup
11B-1. Transducers for MS
2. Faraday cup
11.7
by Prof. Myeong Hee Moon
11B-1. Transducers for MS
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
by Prof. Myeong Hee Moon
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
by Prof. Myeong Hee Moon
11B-2. Mass Analyzer
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
by Prof. Myeong Hee Moon
• 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
by Prof. Myeong Hee Moon
11B-2. Mass Analyzer
• 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
by Prof. Myeong Hee Moon
When AC voltage applied,lighter positive ions maintain in the space.
----- low pass mass filter
11B-2. Mass Analyzer
• 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
by Prof. Myeong Hee Moon
• 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
11B-2. Mass Analyzer
11.14
by Prof. Myeong Hee Moon
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
by Prof. Myeong Hee Moon
• 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
by Prof. Myeong Hee Moon
11B-4. Double focusing analyzer
focusing ion beams with electrostatic analyzer & magnetic sector
Focus ion beam havinga narrow band of K.E.
11.17
by Prof. Myeong Hee Moon
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
by Prof. Myeong Hee Moon
• 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
by Prof. Myeong Hee Moon
11C-2. AMS & Interferences
ICPMS vs. ICP optical methods for rare earth metals.
1000ppm Ce 10ppm Cemolecularbands
ICPAES ICPMS
Still holds interferences
11.20
by Prof. Myeong Hee Moon
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
11C-2. AMS & Interferences
11.21
by Prof. Myeong Hee Moon
• Polyatomic ion interferences
molecular ions (below m/z=82) generated in plasma
14N2+ - 28Si+, NOH+ - 31P+
16O2+ - 32S+, 40Ar2
+ - 80Se+
corrections: blank run
11C-2. AMS & Interferences
11.22
by Prof. Myeong Hee Moon
• 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
11C-2. AMS & Interferences
11.23
by Prof. Myeong Hee Moon
• 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
11C-2. AMS & Interferences
11.24
by Prof. Myeong Hee Moon
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
by Prof. Myeong Hee Moon
• 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