Staying Focussed An introduction to stable isotope

Staying Focussed. . An introduction to stable isotope mass spectroscopy

Stable Isotope analysis n Sample preparation Chemically convert sample material (ie rocks, water, biological materials) into gas Quantitative n Measurement of isotope ratios Mass spectroscopy Laser cavity molecular spectroscopy n Normalization of results Laboratory references International standards

Stable Isotope mass specs are gassource D/H 18 O/16 O 13 C/12 C 15 N/14 N 34 S/32 S 37 Cl/35 Cl H 2 CO 2, CO, O 2 CO 2, CO N 2 SO 2, SO, SF 6 CH 3 Cl

Combust: (C 6 H 10 O 5)n + O 2 CO 2 + H 20 (C 6 H 10 O 5)n + C CO + H 2 Reduce: H 20 + Zn H 2 + Zn. O React: Si. O 2 + Br. F 5 O 2 + Si. F 2 O 2 + C CO 2 Equilibrate: C 16 O 2 + 2 H 218 O C 16 O 18 O + 2 H 216 O 18 O

Purification Vacuum lines Cryogenic (LN 2) 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 n JJ Thomspon 1910 – parabola spectrograph • Discovered first stable isotopes (Ne mass 20 -22) • Discovered the electron • Awarded nobel prize 1906 n F. W. Aston – mass spectrograph • • Discovered 21 Ne 212 out of 287 naturally occurring isotopes Mass defect – binding energies Awarded nobel prize 1922

n A Nier • • • 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+ + 2 e- Electron energies ca. 100 e. V Electron emission 1 m. A or 6 x 1015 e-/s Efficiency = 1 in 2400 molecules ionized Problems: -Linearity current ≠ const measured ratios -memory -stability -chemical inertness of hot filiament

Ionization efficiency - sensitivity n n n n Electrons about 70 e. V – de Broglie l is about equal to molecule bond lengths About 1 in 1000 impacts give ionization Emission is about 1 m. A Cross section is low – 10 -7 mm 2 About 1 n. A ion current from 1 m. A emission Increased source pressureclosed ion box Potential across ion box – too high – variable ion energies – too low ion-molecule interactions. fragmentation Ca 70 V

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

Problem areas of source design 1. u u u Deviations from linear behavior – “discrimination” ion-molecule interactions forming isobaric interferences – ie H 3+ 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 n 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 n Magnetic Sector • n n n n n U=HT, B=Mag Field, z = # charge, e= charge, m=mass m/z 44 (CO 2) at 5 KV = 13. 5 cm, B=0. 5 T Permanent/electromagnets Magnetic field more uniform with electromagnets Need two magnets for low mass Image broadening by inhomogeneous ion energy Large-radius – high energy (10 k. V) less affected DE/U less in large radius instruments Early instruments x-only focussing All modern instruments X-Y (cross)focussing

Cross focussing n n 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 n n From a theoretical veiwpoint both are identical Limited mass selection with permanent magnets – 5 KV for N 2 (typically designed at high end of HT range) 3. 2 k. V for CO 2 2. 2 k. V for SO 2 (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. 6 x 1019 C) 1 n. A = 6 x 109 ions/sec

Electrometers n n n n Measure charge or currrent (charge/sec) Solid state – transistors Ohms law E=IR 1 V = 10 -9 A x 109 ohms Different resistors – different currents - similar voltages Measured by ADC High amplifications – shielded

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

Mc. Kinney (1950) – introduced changeover valve, thereby eliminating most instrumental effects allowed measurement of O 2 and CO 2 to 0. 1 per mil -Measured d 13 C 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 n n n 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 n n n 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 § § 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 10 n. A CO 2 peak – about 3 x 1011 ions for mass 44 -about 3 x 109 ions for mass 45 s= 1/√ 3 x 109 = 2 x 10 -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 n 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 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 H 2 O

• Absorption spectrometry is a direct measure of concentration • Very selective - C 2 H 2 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/I 0 ~ 10 -5 • Increase pathlength by (1 -R)-1 ~ 10, 000 times, giving several kilometers of effective path • Single-pass ΔI/I 0 ~ 10 -5 Multipass ΔI/I 0 ~ 10 -1 (a considerable absorption)

Pros - cons Pros n n n Do one thing really well – eg H 2 O No compressed gasses No moving parts Cheap Simple mechanically Cons n n n Do one thing Difficult to calibrate Like IRMS – instrumental effects

Wikipedia – A modern stable isotope ratio mass spectrometer