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Staying Focussed. An introduction to stable isotope mass spectroscopy. Stable Isotope analysis. Sample preparation Chemically convert sample material (ie rocks, water, biological materials) into gas Quantitative Measurement of isotope ratios Mass spectroscopy

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staying focussed

Staying Focussed..

An introduction to stable isotope mass spectroscopy

slide2

Stable Isotope analysis

  • Sample preparation

Chemically convert sample material (ie rocks, water, biological materials) into gas

Quantitative

  • Measurement of isotope ratios

Mass spectroscopy

Laser cavity molecular spectroscopy

  • Normalization of results

Laboratory references

International standards

stable isotope mass specs are gas source
Stable Isotope mass specs are gas-source

D/H H2

18O/16O CO2, CO, O2

13C/12C CO2, CO

15N/14N N2

34S/32S SO2, SO, SF6

37Cl/35Cl CH3Cl

slide4
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

slide5
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

slide6
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
Mass spectrometers
  • JJ Thomspon 1910 – parabola spectrograph
    • Discovered first stable isotopes (Ne mass 20-22)
    • Discovered the electron
    • Awarded nobel prize 1906
  • F.W.Aston – mass spectrograph
    • Discovered 21Ne
    • 212 out of 287 naturally occurring isotopes
    • Mass defect – binding energies
    • Awarded nobel prize 1922
slide8
A Nier
    • First stable isotope abundance instrument
    • Electron-impact source
    • Dual detectors
    • Magnetic sector
    • Electronic rather than photographic ion counting
slide10

Inlet System:

DI or CF

Source of ions

M+

Pumping system

Diffusion or turbo pumps

Analyser =

Magnetic sector

Detectors

-Faraday cups

-electronic ion counting

ion source
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
Ionization efficiency - sensitivity

Ca 70V

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 pressure- closed ion box

Potential across ion box – too high – variable ion energies – too low ion-molecule interactions.

fragmentation

slide13

Ion optics

  • About 50% efficient
  • Burn marks
  • Extraction
  • Half plate focussing
  • Fine-tuning –
  • Generally empirically tuned
slide14

Problem areas of source design

  • 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
Problem areas of source design
  • 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
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
Analyser
  • Magnetic Sector
    • 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
Cross focussing

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
Permanent vs Electromagnets

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
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(1.6x10-19 C)

1nA = 6x109 ions/sec

electrometers
Electrometers

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

slide22

Masses not evenly spaced – so cant get a collector array for more than element

-compromise – triple array or moving collectors

slide25

McKinney (1950) – introduced change-over 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 viscous-flow 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”
slide26

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:
  • Linearity effects
  • small measurement times
  • absolute sensitivity
  • isotope chromatography
  • statistical limits on precision
  • large He background (HD)
  • Background corrections
isotope chromatography
Isotope Chromatography

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
HD measurement in He

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
Statistical Limits to precision
  • 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
slide31

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.
abundance sensitivity
Abundance Sensitivity

About 10-5 on modern instruments for m/z=45

=0.001 per mil

Basically, how much does one mass peak overlap the other

slide34

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

  • Leak correction (zero enrichment)
  • - corrects for differences in the two viscous leaks
  • - crimp is adjusted so that enrichment is zero
  • Abundance sensitivity
  • - effect of one mass on the adjacent mass
  • - principally controlled by instrument design
  • - is different for each mass
  • - dependent on inlet pressure
  • Valve mixing
  • -mixing of reference and sample gasses in the changeover valve by cross seat leakage
  • - new changeovers minimize this
slide35

Computation of d values

Mass spectrometers measure abundance ratios or mass enrichments

Need to correct for isobaric interferences to get isotope ratios

optical methods
Optical Methods
  • Two Competing technologies:
  • Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS) Picarro Inc
  • Off-axis integrated cavity output spectroscopy (OA-ICOS)
  • Los Gatos Research

HDO

H2O

slide45

•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 - cons

Pros

Cons

  • Do one thing
  • Difficult to calibrate
  • Like IRMS – instrumental effects

Do one thing really well – eg H2O

No compressed gasses

No moving parts

Cheap

Simple mechanically