Correction of ion source nonlinearities over a wide signal range in continuous-flow isotope ratio mass spectrometry of water-derived hydrogen.

Abstract

Ion source nonlinearities are characterized over a wide range of signal intensities characteristic of complex mixtures, and correction schemes are proposed and evaluated for high-precision determinations of D/H ratios from water via an on-line reduction system facilitating continuous-flow 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 experimentally simple "peakwise" correction schemes, in which hydrogen isotope ratios are corrected after peak identification and ratio calculation, are compared to the method routinely applied to static dual-inlet IRMS measurements. It is demonstrated that traditional linear correction applied to continuous-flow peaks is adequate over small signal ranges, about m/z 2 +/- 0.5 V; however, a second order correction is required for acceptable accuracy and precision over larger ranges. In addition, tests of the peakwise algorithms were made using a set of liquid water samples with delta D Tap Water over the range of 39-407/1000 with uncorrected data with precisions of SD-(delta D Tap Water < 34/1000 and accuracy within 11/1000. Peakwise correction using a linear calibration model resulted in substantial improvements in precision (SD < 10/1000) and accuracy (< 4/1000). Peakwise-corrected data, calibrated using a second-order regression to account for unmatched detector response, are still further improved to accuracy within 2/1000 from the calibration curve. The peakwise correction schemes are advantageous because of experimental simplicity when applied to peaks of the same or similar shapes. This study shows that ion source non-linearities in hydrogen analysis require correction for optimal analytical performance and can successfully be handled using straightforward procedures over the wide signal range required for chromatographic analysis.