H ISOTOPE FRACTIONATION DUE TO HYDROGEN-ZINC REACTIONS AND ITS IMPLICATIONS ON DH ANALYSIS OF WATER SAMPLES

Abstract

After conversion of water to H2 gas with zinc for DH ratio determination, hydrogen gas can be absorbed by the zinc during cooling, resulting in H isotope fractionation. Thermal release and subsequent DH ratio determination of hydrogen bound to zinc revealed that the fractionation between the bound H and the H2 gas is extremely large with an empirical α-value of # 1.62. Differences in the fractionations observed using stopcock-sealed vessels and break-seal tubes indicate that the conversion method and H2Zn ratios should be the same for both samples and standards. The effect of hydrogen absorption can be eliminated (or lowered) by re-heating the zinc during transfer of hydrogen gas into the mass spectrometer. For measurement of hydrogen isotope composition of geological samples with varying and sometimes unpredictable water content (e.g., fluid inclusions of minerals) recording a calibration curve of varying waterzinc ratios and δD-values is proposed.