Isotope Ratio Mass Spectoscopy同位素比质谱.pptVIP

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 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 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 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: Wavelength-Scanned Cavity Ring Down Spectroscopy (WS-CRDS) Picarro Inc Off-axis integrated cavity output spectroscopy (OA-ICOS) Los Gatos Research HDO H2O ?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 Do one thing really well – eg H2O N