CORRECTION OF ION-SOURCE NONLINEARITIES OVER A WIDE SIGNAL RANGE IN CONTINUOUS-FLOW ISOTOPE RATIO MASS-SPECTROMETRY OF WATER-DERIVED HYDROGEN

Ion source nonlinearities are characterized over a wide range of signa l intensities characteristic of complex mixtures, and correction schem es are proposed and evaluated for high-precision determinations of D/H ratios from water via an on-line reduction system facilitating contin uous-now isotope ratio mass spectrometry, Hydrogen isotope ratios are shown to be sensitive to analyte pressure in the IRMS ion source with or without carrier gas admitted with analyte, indicating that analyte level must be taken into account for isotope ratio calculation, Two ex perimentally simple ''peakwise'' correction schemes, in which hydrogen isotope ratios are corrected after peak identification and ratio calc ulation, are compared to the method routinely applied to static dual-i nlet IRMS measurements, It is demonstrated that traditional linear cor rection applied to continuous-now peaks is adequate over small signal ranges, about mit 2 +/- 0.5 V; however, a second order correction is r equired for acceptable accuracy and precision over larger ranges, In a ddition, tests of the peakwise algorithms were made using a set of liq uid water samples with delta(DTp Water) over the range of 39-407 parts per thousand with uncorrected data with precisions of SD-(delta D-Tap Water) water) < 34 parts per thousand and accuracy within 11 parts pe r thousand. Peakwise correction using a linear calibration model resul ted in substantial improvements in precision (SD < 10 parts per thousa nd) and accuracy (<4 parts per thousand), Peakwise-corrected data, cal ibrated using a second-order regression to account for unmatched detec tor response, are still further improved to accuracy within 2 parts pe r thousand from the calibration curve, The peakwise correction schemes are advantageous because of experimental simplicity when applied to p eaks of the same or similar shapes, This study shows that ion source n onlinearities in hydrogen analysis require correction for optimal anal ytical performance and can successfully be handled using straightforwa rd procedures over the wide signal range required for chromatographic analysis.