Mass spectrometry 8/23/12. What are the principles behind MS? What do all MS instruments have in common? What are the different types of MS?. Lecture outline: Introduction to mass spectrometry sample introduction systems, mass analyzers popular combinations in geosciences.
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What are the principles behind MS?
What do all MS instruments have in common?
What are the different types of MS?
JJ Thomson’s cathode ray tube, 1897
Minimize collisions, interferences
Ion’s kinetic E function of accelerating voltage (V) and charge (z).
Applied magnetic field
balance as ion goes through flight tube
Combine equations to obtain:
Fundamental equation of mass spectrometry
Change ‘mass-to-charge’ (m/z) ratio by
changing V or changing B.
NOTE: if B, V, z constant, then:
B in gauss
r in centimeters
m in amu
V in volts
z in electronic charge
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 ions
into the collector?
You can scan in B or V to sweep masses
across a single detector.
You can put different masses into
multiple cups without changing B or V.
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
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
Gas stream passes through beam of e-,
positive ions generated.
Plasma: Gas stream passes through plasma
maintained by RF current and Ar.
Themal: Filament heated to ~1500C
Changes DC and RF
voltages to isolate
a given m/z ion.
PRO: cheap, fast, easy
Changes B and V to focus
a given m/z into detector.
PRO: turn in geometry means
less ‘dark noise’,
A) Faraday collector - long life, stable, for signals > 2-3e6 cps
B) Electron multiplier - limited life, linearity issues, high-precision, signals < 2e6 cps
1) 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, Chemistry)
- compound-specific ratio analyses
1) Thermal Ionization mass spec (multi-collector)
- heavy metals, REE
1) ICP quadrupole mass spec (at GT, Taillefert)
- trace metal analysis
2) Single collector magnetic sector ICPMS
- higher-precision trace metal
2) Multi-collector ICPMS (nearest at USC)
- U/Th dating, TIMS
Micromass IsoProbe - MC-ICPMS
of all ICPMS machines
and ion counter (electron multiplier)
Agilent 7500 ICPMS
The sample cone isolates the
torch from the interior.
2. High resolution ICPMS
aka double-focusing ICPMS
aka magnetic sector ICPMS
- same front end as Q-ICPMS
- combines magnet w
ions by charge
ion by mass
- same front end as other ICPMS
- same magnet and ES as
- multiple detectors spaced 1amu
apart enable simultaneous
measurement of many (~7) isotopes
-good for what kinds of systems?
very low concentrations
in environmental samples,
but high interest (why?)
Unfortunately, 56Fe has the
same atomic wt as ArO
Quadrupole measurement =
HR-ICPMS measurement =
can distinguish 56Fe from ArO
NOTE: most elements can be
distinguished with a low
ICPMS: Can determine concentration to ~1% R.E. using calibration curve (below)
Can monitor Sensitivity (signal response for given
solution concentration) over time
unknown sample =
conc ~ 10.5ppb
REMEMBER: all mass spectrometers are “black boxes” we really have no idea what goes on from sample container to detector signal
Isotope dilution is an analytical technique used in combination with mass spectrometry
to determine the concentration of element x in unknown samples.
A known amount of “spike” with
known elemental concentration
and isotopic abundances
(what’s the diff?)
is added to sample with unknown
elemental concentration but
known isotopic abundances.
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
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)
The AMS at University of Arizona (3MV)
The AMS at LLNL (10MV)
generates 2.5 million volts,
accelerates C- ions
b) INJECTOR MAGNET
separates ions by mass,
masses 12, 13, and 14 injected
C- ions interact with
‘stripper’ gas Ar,
become C+ ions,
molecular species CH
e) ELECTROSTATIC DEFLECTOR
specific charge of ions selected (3+)
a) ION SOURCE
by Cs sputtering
f) MAGNETIC SEPARATION
13C steered into cup, 14C
passes through to solid detector
g) Si BARRIER DETECTOR
pulse produced is proportional to the energy of ion, can differentiate b/t 14C and other ions count rate for modern sample = 100cps
1) Abundance sensitivity - ratio of signal at mass
m 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
- 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
- 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