Staying Focussed..
An introduction to stable
isotope mass spectroscopy
Stable Isotope analysis
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Sample preparation
Chemically convert sample material (ie rocks, water, biological materials)
into gas
Quantitative
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Measurement of isotope ratios
Mass spectroscopy
Laser cavity molecular spectroscopy
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Normalization of results
Laboratory references
International standards
Stable Isotope mass specs are
gas-source
D/H
18O/16O
13C/12C
15N/14N
34S/32S
37Cl/35Cl
H2
CO2, CO, O2
CO2, CO
N2
SO2, SO, SF6
CH3Cl
Combust:
(C6H10O5)n + O2
CO2 + H20
(C6H10O5)n + C
CO + H2
Reduce:
H20 + Zn
H2 + ZnO
React:
SiO2 + BrF5
O2 + SiF2
O2 + C
CO2
Equilibrate:
C16O2 + 2H218O
C16O18O + 2H216O18O
Purification
Vacuum lines
Cryogenic (LN2) traps for
separation of gasses
Reaction vessels for chemical
reactions in vacuum
Usually used in conjunction
with “off-line” isotope analysis
Necessary for some analyses
ie silicate analyses
Gas Chromatography
Uses a GC column to separate gasses
He
TCD detectors
usually in a singe instrument as a
preparatory inlet to a mass
spectrometer
Combustion/Reduction
Automation
“On-Line”
Mass
Spec
Mass spectrometers
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JJ Thomspon 1910 – parabola
spectrograph
• Discovered first stable isotopes (Ne mass 20-22)
• Discovered the electron
• Awarded nobel prize 1906
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F.W.Aston – mass spectrograph
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Discovered 21Ne
212 out of 287 naturally occurring isotopes
Mass defect – binding energies
Awarded nobel prize 1922
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A Nier
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First stable isotope abundance instrument
Electron-impact source
Dual detectors
Magnetic sector
Electronic rather than photographic ion counting
A lot has changed, a lot has
stayed the same……
Inlet System:
DI or CF
Source of ions
M+
Analyser =
Magnetic sector
Detectors
-Faraday cups
-electronic ion
counting
Pumping system
Diffusion or turbo
pumps
Ion Source
Electron Impact source: M + e-
M+ + 2e-
Electron energies ca. 100 eV
Electron emission 1mA or 6x1015 e-/s
Efficiency = 1 in 2400 molecules ionized
Problems:
-Linearity
current
≠ const
measured ratios
-memory
-stability
-chemical inertness of hot filiament
Ionization efficiency - sensitivity
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Electrons about 70 eV – de
Broglie l is about equal to
molecule bond lengths
About 1 in 1000 impacts give
ionization
Emission is about 1 mA
Cross section is low – 10-7 mm2
About 1nA ion current from
1mA emission
Increased source pressureclosed ion box
Potential across ion box – too
high – variable ion energies –
too low ion-molecule
interactions.
fragmentation
Ca 70V
Ion optics
• About 50% efficient
• Burn marks
• Extraction
• Half plate focussing
• Fine-tuning –
• Generally
empirically tuned
Problem areas of source design
1.
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Deviations from linear behavior – “discrimination”
ion-molecule interactions forming isobaric interferences
– ie H3+
collimating magnetic field can lead to non-linear
response
changes in number of ions – affects space charge of
ion-source – therefore extraction conditions
careful source design
more important in CF instruments.
Problem areas of source design
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2. Gas exchange
• Minimize gas exchange in ion source
• Pumping efficiency of source region
• Avoidance of dead volumes
• Chemical inertness of source materials
• Filament – chemical inertness and
conditioning
Problem areas in ion sources
3. High stability over time
 electrostatic potentials need to be
stable to 200 ppm
 insulating surface layers lead to
charging – source cleaning
Analyser
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Magnetic Sector
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U=HT, B=Mag Field, z = # charge, e= charge, m=mass
m/z 44 (CO2) at 5KV = 13.5 cm, B=0.5T
Permanent/electromagnets
Magnetic field more uniform with electromagnets
Need two magnets for low mass
Image broadening by inhomogeneous ion energy
Large-radius – high energy (10kV) less affected
DE/U less in large radius instruments
Early instruments x-only focussing
All modern instruments X-Y (cross)focussing
Cross focussing
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Stigmatic focussing
Ions enter and exit magnet at an
offset angle rather than 90°
Fringing fields at the magnet
pole gap result in y-direction
focussing
Mat 250 – 1977
Permanent vs Electromagnets
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From a theoretical veiwpoint both are identical
Limited mass selection with permanent magnets –
5KV for N2 (typically designed at high end of HT range)
3.2 kV for CO2
2.2 kV for SO2 (v. low HT- lower resolution)
cannot scan lower than mass 28 without magnet change
HD separate magnet
Detectors
Faraday cups
Mechanically simple
Named after Micheal Farrady who first theorized ions about 1830
Error sources
-Secondary electrons
-backscattering
Circuit where charged ions are
the charge carriers in vacuum
Cup gains charge that can be
measured as current when
discharged
N/t = I/e
N/t= #of ions/sec, I= current,
e= elementary charge(1.6x1019 C)
1nA = 6x109 ions/sec
Electrometers
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Measure charge or currrent (charge/sec)
Solid state – transistors
Ohms law E=IR
1V = 10-9A x 109 ohms
Different resistors – different currents - similar
voltages
Measured by ADC
High amplifications – shielded
r m
Masses not evenly spaced – so
cant get a collector array for
more than element
-compromise – triple array or
moving collectors
Flat-topped peaks – defining slit width
Inlets
McKinney (1950) – introduced changeover valve, thereby eliminating most
instrumental effects allowed measurement
of O2 and CO2 to 0.1 per mil
-Measured d13C to precisions of about 0.1
per mil
-Has essentially remained unchanged in 50
years
-smallest sample limited by
requirement to maintain viscousflow conditions
-Practical limit about 15-20 mbar
-Cold fingers for small volumes.
-Smallest sample size about 0.2
mmol
- With a few exceptions, most
sample preparations are “off-line”
Continuous Flow or IR monitoring
inlet
•GC techniques coupled to MS
•No change over or dual inlet
•Viscous flow in GC stream
•Smaller sample sizes
•Completely taken over most modern
analyses
•Well suited for automated analyses
Things to be aware of:
o Linearity effects
o small measurement times
o absolute sensitivity
o isotope chromatography
o statistical limits on precision
o large He background (HD)
o Background corrections
Isotope Chromatography
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Transport of gas through GC
not only separates chemical
species but also isotopic species
Cannot measure instantaneous
isotope ratios, but must
integrate entire peaks to “count
ions”
Makes correct background
subtraction, and peak
integration algorithms essential
HD measurement in He
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Large mass 4 (He+) tails into m/z
3
Generally reduced by modern
instrument design
Differential pumping
Increased abundance sensitivity by
increased dispersion
Energy filters to homogenize
minimize DU
Statistical Limits to precision
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IRMS is basically measuring ion
currents
Ion currents have a standard deviation
as a result of “shot noise”
For low ion currents - poisson
distribuion
s- = 1/√N
Implications for CF-IRMS
-Typically small total ion numbers
are measured
For example typical 10nA CO2
peak
– about 3x1011 ions for mass 44
-about 3x109 ions for mass 45
s= 1/√3x109 = 2x10-5 or about
0.02 per mil
Reference peak has similar
precisions so that minimum
statistical limit of s is about 0.05
permil or so. Minimum.
Mass Resolution
Abundance Sensitivity
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Basically, how much does one mass
peak overlap the other
About 10-5 on modern instruments for m/z=45
=0.001 per mil
Instrument Corrections
Corrections to measured d values based
on instrument properties
Not as important on newer instruments as
manufacturing and materials have
improved
Should be monitored to evaluate
instrument performance
1. Leak correction (zero
enrichment)
- corrects for differences in the
two viscous leaks
- crimp is adjusted so that
enrichment is zero
2. Abundance sensitivity
- effect of one mass on the
adjacent mass
- principally controlled by
instrument design
- is different for each mass
- dependent on inlet pressure
3. Valve mixing
-mixing of reference and sample
gasses in the changeover valve
by cross seat leakage
- new changeovers minimize
this
Computation of d values
Mass spectrometers measure abundance ratios or mass enrichments
Need to correct for isobaric interferences to get isotope ratios
Optical Methods
Two Competing technologies:
1. Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS)
Picarro Inc
2. Off-axis integrated cavity output spectroscopy (OA-ICOS)
Los Gatos Research
HDO
H2 O
•Absorption spectrometry is a
direct measure of concentration
•Very selective - C2H2 absorbs
light between 1510 - 1545 nm
•Fast – laser can be reproducibly
swept at > 100 Hz
•For a 1 meter sample containing
100 torr of 1 ppm acetylene,
ΔI/I0 ~ 10-5
•Increase pathlength by (1-R)-1
~ 10,000 times, giving several
kilometers of effective path
•Single-pass ΔI/I0 ~ 10-5 􀄺
Multipass ΔI/I0 ~ 10-1 (a
considerable absorption)
Pros - cons
Pros
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Do one thing really well – eg
H2O
No compressed gasses
No moving parts
Cheap
Simple mechanically
Cons
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Do one thing
Difficult to calibrate
Like IRMS – instrumental
effects
Wikipedia – A modern stable isotope ratio mass spectrometer