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

After conversion of water to H-2 gas with zinc for D/H ratio determination, hydrogen gas can be absorbed by the zinc during cooling, resulting in H isotope fractionation. Thermal release and subsequent D/H ratio determination of hydrogen bound to zinc revealed that the fractionation between the bound H and the H-2 gas is extremely large with an empirical alpha-value of similar to 1.62. Differences in the fractionations observed using stopcock-sealed vessels and break-seal tubes indicate that the conversion method and H-2/Zn 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 water/zinc ratios and delta D-values is proposed.