Download - lecture2_massspec
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Mass spectrometry 8/31/10
What are the principles behind MS?
What do all MS instruments have in common?
What are the different types of MS? Lecture outline:1) Introduction to mass spectrometry
2) sample introduction systems, mass analyzers
3) popular combinations in geosciences
JJ Thomson’s cathode ray tube, 1897
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Introduction to Mass Spectrometry
Sample introduction
IonizationMinimize collisions, interferences
Separatemasses
Count ionsCollect results
Nier-type mass spec
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Basic equations of mass spectrometry
21
2mv zV
2 /F mv R
F Bzv2 /mv R Bzv
2 2/ / 2m z B R V
Ion’s kinetic E function of accelerating voltage (V) and charge (z).
Centrifugal force
Applied magnetic field
balance as ion goes through flight tube
Fundamental equation of mass spectrometry
Combine equations to obtain:
Change ‘mass-to-charge’ (m/z) ratio bychanging V or changing B.
NOTE: if B, V, z constant, then:
r m
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If: B in gaussr in centimetersm in amuV in voltsz in electronic chargethen….
What magnetic field strength would be required to focus a beam of CO2+ ions on
a collector of a mass spectrometer whose analyzer tube as a radius of 31.45cm,assuming a voltage of 1000V?
Change your magnetic field strength by -10%, what voltage puts the CO2 ionsinto the collector?
V
rBx
z
m 22510825.4
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Examples of mass spec data output
You can scan in B or V to sweep massesacross a single detector.
OR
You can put different masses intomultiple cups without changing B or V.
Ex: B
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Sample Introduction Systems (aka “front ends”)
1) Gas source (lighter elements)dual inlet - sample purified and measured with standard gas at identical conditions
precisions ~ ±0.005%continous flow - sample volatized and purified (by EA or GC) and injected into
mass spec in He carrier gas, standards measured before and after,precisions ~ 0.005-0.01%
2) Solid source (heavier elements)TIMS - sample loaded onto Re filament, heated to ~1500°C, precisions ~0.001%laser ablation - sample surface sealed under vacuum, then sputtered with laser
precisions ~0.01%?
3) Inductively coupled plasma (all elements, Li to U)ICPMS - sample converted to liquid form,
converted to fine aerosol in nebulizer,injected into ~5000K plasma torch
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Ionization occurs in the ‘source’
Electron Ionization
Gas stream passes through beam of e-,positive ions generated.
Thermal IonizationPlasma: Gas stream passes through plasmamaintained by RF current and Ar.
Themal: Filament heated to ~1500C
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Mass Analyzers - the quadrupole vs. magnetic sector
Quadrupole:Changes DC and RFvoltages to isolatea given m/z ion.PRO: cheap, fast, easy
Magnetic Sector:Changes B and V to focusa given m/z into detector.PRO: turn in geometry means
less ‘dark noise’,higher precision,
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Two types of ion detectors
A) Faraday collector - long life, stable, for signals > 2-3e6 cps
B) Electron multiplier - limited life, linearity issues, high-precision, signals < 2e6 cps
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Popular combinationsGas source1) Dual inlet isotope ratio mass spec (at GT, Lynch-Steiglitz and Cobb)
- O, C, H ratio analyses
2) Elemental analyzer IRMS (at GT, Montoya)- N, C, S ratio analyses
3) Gas chromatograph IRMS (at GT, ????)- compound-specific ratio analyses
Solid source1) Thermal Ionization mass spec (multi-collector)
- heavy metals, REE
ICP1) ICP quadrupole mass spec (at GT, Taillefert)
- trace metal analysis
2) Single collector magnetic sector ICPMS- higher-precision trace metal analysis
2) Multi-collector ICPMS- U/Th dating, TIMS replacement
Micromass IsoProbe - MC-ICPMS
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Inductively Coupled Plasma Mass Spectrometry
sample cone
skimmer cone
instrument housing
mass/chargediscriminator
detector
atmosphericpressure
“fore” vacuum10-4 bar
high vacuum10-7 bar
Shared componentsof all ICPMS machines
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or magneticsector
Faraday cupand ion counter (electron multiplier)
1. Quadrupole ICPMS- measure concentrations
as low as several ppt
- no fuss sample preparation(dissolve in 5% HNO3)
- get beam intensityvs. mass/charge ratio
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2. High resolution ICPMSaka double-focusing ICPMSaka magnetic sector ICPMS
- same front end as Q-ICPMS
- combines magnet welectrostatic analyzer
electrostaticanalyzer
separatesions by charge
magnetseparates
ion by mass
Faraday cupand EM
High-resolution ICPMS
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Multi-collector ICPMS
3. MC-ICPMS
- same front end as other ICPMS
- same magnet and ES as HR-ICPMS
- multiple detectors spaced 1amuapart enable simultaneous
measurement of many (~7) isotopes
-good for what kinds of systems?
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56Fevery low concentrations
in environmental samples,but high interest (why?)
Unfortunately, 56Fe has thesame atomic wt as ArO
(40Ar+16O)
Quadrupole measurement =INTERFERENCE!
Low vs. High – resolution ICPMS and Interferences
HR-ICPMS measurement =can distinguish 56Fe from ArO
NOTE: most elements can bedistinguished with a lowresolution quadrupole
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The importance of standards in mass spectrometry
ICPMS: Can determine concentration to ~1% R.E. using calibration curve (below)
Can monitor Sensitivity (signal response for givensolution concentration) over time
unknown sample = 8.2e7 cps,
conc ~ 10.5ppb
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REMEMBER: all mass spectrometers are “black boxes” we really have no idea what goes on from sample container to detector signal
Ex: you measure a count-rate of 10,000 cps for a given element, but you need to know how many atoms of that element, or its concentration, were in your sample
- measuring isotope ratios is a powerful approach because we can measuresamples against standards with known isotopic ratios (it’s much more difficult to change a material’s isotopic ratios than it is to change its elemental concentration!)
- isotope dilution takes advantage of ability to precisely measure ratios
- ALL measurements need to include blanks and standards (either concentration or ratio standards)
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Isotope dilution principle
Isotope dilution is an analytical technique used in combination with mass spectrometryto determine the concentration of element x in unknown samples.
ex: Rb
A known amount of “spike” with known elemental concentration
and isotopic abundances(what’s the diff?)
is added to sample with unknownelemental concentration butknown isotopic abundances.
Requirements: 1) The sample has natural (or known) isotopic abundance (usually true).2) The spike and sample isotopic ratios are different.
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More Commonly used ICPMS terms
Nebulization efficiency – the amount of solution that reaches the plasma (~1%)- varies with sample matrix
- surface tension, viscosity, and density of solution will affect neb. eff.- usually all standards, spikes, and samples are introduced as 2-5% HNO3
- an acid solution reduces complexation, surface adsorption
Matrix effects – the changes in ICP characteristics with variable matrices- largely black box (we see these effects, cannot wholly explain/predict them)
- you must carefully match the matrices of your standards/samples to obtain quantitative results
Ionization efficiency – the amount of ions produced per atoms introduced- depends on matrix, focusing of beam through cones, lenses
- usually no better than 1/1000
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ICP-OES ICP-MS
Detection limit – defined as 3 x the S.D. of the signal as the concentration of the analyte
approaches 0 (measure stability at a variety of conc’s, extrapolate to 0; or measure
5% HNO3 blank solution)
ICP detection limits for a variety of elements
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Ion microprobe(orSecondaryIonMassSpectrometry SIMS)
-use an ion beam (usually Cs+1) to “sputter” a sample surface; secondary ions fed into mass spec
20μm
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Accelerator Mass Spectrometry
The AMS at University of Arizona (3MV)-prior to AMS samples were 14C-dated by counting the number of decays
- required large samples and long analysis times
-1977: Nelson et al. and Bennett et al. publish papers in Science demonstratingthe utility of attaching an accelerator to a conventional mass spectrometer
The AMS at LLNL (10MV)
Principle:You cannot quantitatively remove interferring ions to look for one 14C atom among several
quadrillion C atoms.Instead, you
a) destroy molecular ions (foil or gas)b) filter by the energy of the ions (detector)
to separate the needle in the haystack.
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a) ION SOURCEgenerates negative
carbon ionsby Cs sputtering
b) INJECTOR MAGNETseparates ions by mass,
masses 12, 13, and 14 injected
http://www.physics.arizona.edu/ams/education/ams_principle.htm
c) ACCELERATORgenerates 2.5 million volts,
accelerates C- ions
d) TERMINALC- ions interact with
‘stripper’ gas Ar,become C+ ions,
molecular species CHdestroyed
e) ELECTROSTATIC DEFLECTORspecific charge of ions selected (3+)
f) MAGNETIC SEPARATION13C steered into cup, 14C
passes through to solid detector
g) Si BARRIER DETECTORpulse produced is proportional to the energy of ion, can
differentiate b/t 14C and other ions count rate for modern sample = 100cps
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Hurdles in mass spectrometry
1) Abundance sensitivity - ratio of signal at massm to signal at m+1
- better with better vacuum- acceptable values: 1-3ppm at 1amu
2) Mass discrimination
- heavier atoms not ionized as efficiently as light atoms
- can contribute 1% errors to isotope values
- can correct with known (natural) isotope ratios within run, or with known standards between runs
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3) Dark Noise - detector will register signal even without an ion beam- no vacuum is perfect
and- no detector is perfect
- must measure prior to run to get “instrument blank” if needed
4) Detector “gain” - what is the relationship between the electronic signal recorded by the detector and the number of ions that it has counted?- usually close to 1 after factory calibration- changes as detector “ages”- must quantify with standards
Cardinal rule of mass spectrometry:Your measurements are only as good as your STANDARDS!
Standards (both concentration and isotopic) can be purchased from NIST
Hurdles in mass spectrometry (cont.)
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Ex: NBS-19, O, C carbonate isotopic standard