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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
Introduction to Mass Spectrometry
Sample introduction
IonizationMinimize collisions, interferences
Separatemasses
Count ionsCollect results
Nier-type mass spec
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
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
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
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
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
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,
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
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
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
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
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
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?
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
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
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)
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.
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
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
Ion microprobe(orSecondaryIonMassSpectrometry SIMS)
-use an ion beam (usually Cs+1) to “sputter” a sample surface; secondary ions fed into mass spec
20μm
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.
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
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
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.)
Ex: NBS-19, O, C carbonate isotopic standard