Transcript Making Isotopic measurements – mass spectrometry

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:
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
Separate
masses
Ionization
Nier-type
mass spec
Count ions
Minimize collisions, interferences
Collect results
Basic equations of mass spectrometry
1 2
mv  zV
2
Ion’s kinetic E function of accelerating voltage (V) and charge (z).
F  mv 2 / R
F  Bzv
mv 2 / R  Bzv
Centrifugal force
Applied magnetic field
balance as ion goes through flight tube
Combine equations to obtain:
m / z  B 2 R 2 / 2V
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:
r m
If:
B in gauss
r in centimeters
m in amu
V in volts
z in electronic charge
then….
2 2
m
B
r
 4.825x105
z
V
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?
Examples of mass spec data output
Ex: B
You can scan in B or V to sweep masses
across a single detector.
OR
You can put different masses into
multiple cups without changing B or V.
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 Ionization
Plasma: Gas stream passes through plasma
maintained by RF current and Ar.
Themal: Filament heated to ~1500C
Mass Analyzers - the quadrupole vs. magnetic sector
Quadrupole:
Changes DC and RF
voltages to isolate
a given m/z ion.
PRO: cheap, fast, easy
Magnetic Sector:
Changes B and V to focus
a 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 combinations
Gas source
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
Solid source
1) Thermal Ionization mass spec (multi-collector)
- heavy metals, REE
ICP
1) 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 (nearest at USC)
- U/Th dating, TIMS
replacement
Micromass IsoProbe - MC-ICPMS
Inductively Coupled Plasma Mass Spectrometry
detector
high vacuum
10-7 bar
mass/charge
discriminator
Shared components
of all ICPMS machines
sample cone
skimmer cone
“fore” vacuum
10-4 bar
instrument housing
atmospheric
pressure
1.
Quadrupole ICPMS
- measure concentrations
as low as several ppt
- no fuss sample preparation
(dissolve in 5% HNO3)
- get beam intensity
vs. mass/charge ratio
or magnetic
sector
Faraday cup
and ion counter (electron multiplier)
spray chamber
Ar feed
torch
RF coil
The sample cone isolates the
torch from the interior.
The torch box of an
Agilent 7500 ICPMS
ICPMS plasma torch schematic
plasma components
High-resolution ICPMS
2.
electrostatic
analyzer
separates
ions by charge
High resolution ICPMS
aka double-focusing ICPMS
aka magnetic sector ICPMS
- same front end as Q-ICPMS
- combines magnet w
electrostatic analyzer
magnet
separates
ion by mass
Faraday cup
and EM
Multi-collector ICPMS
3.
MC-ICPMS
- same front end as other ICPMS
- same magnet and ES as
HR-ICPMS
- multiple detectors spaced 1amu
apart enable simultaneous
measurement of many (~7) isotopes
-good for what kinds of systems?
Low vs. High – resolution ICPMS and Interferences
56Fe
very low concentrations
in environmental samples,
but high interest (why?)
Unfortunately, 56Fe has the
same atomic wt as ArO
(40Ar+16O)
Quadrupole measurement =
INTERFERENCE!
HR-ICPMS measurement =
can distinguish 56Fe from ArO
NOTE: most elements can be
distinguished with a low
resolution 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 given
solution 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 measure
samples 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 spectrometry
to 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 unknown
elemental concentration but
known 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 detection limits for a variety of elements
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)
Perkin Elmer Quadrupole ICPMS Instrument Detection limits, 3σ
Ion microprobe
(or
Secondary
Ion
Mass
Spectrometry
 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 demonstrating
the utility of attaching an accelerator to a conventional mass spectrometer
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.
The AMS at LLNL (10MV)
c) ACCELERATOR
generates 2.5 million volts,
accelerates C- ions
b) INJECTOR MAGNET
separates ions by mass,
masses 12, 13, and 14 injected
a) ION SOURCE
generates negative
carbon ions
by Cs sputtering
d) TERMINAL
C- ions interact with
‘stripper’ gas Ar,
become C+ ions,
molecular species CH
destroyed
e) ELECTROSTATIC DEFLECTOR
specific charge of ions selected (3+)
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
http://www.physics.arizona.edu/ams/education/ams_principle.htm
Hurdles in mass spectrometry
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
isotope values
- can correct with known (natural)
isotope ratios within run, or with
known standards between runs
Hurdles in mass spectrometry (cont.)
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
Ex: NBS-19, O, C carbonate isotopic standard