How are hydrogen isotope ratios measured on water samples?
Since May 1, 1990, hydrogen-isotope-ratio analyses have been performed using a hydrogen equilibration technique (at 30°C) (Coplen et al, 1991), rather than the zinc technique used prior to that date (Kendall and Coplen, 1985). The hydrogen equilibration technique measures deuterium activity, whereas the zinc technique measures deuterium concentration. For the majority of WRD samples, the difference in reported isotopic compositions between the two techniques is not significant. However, in brines, the difference may be significant (Sofer and Gat, 1972, 1975). Reported delta hydrogen-2 values of activity are more positive than delta hydrogen-2 values of concentration, and this difference is proportional to molalities of the major dissolved solids. Some examples of the differences between activity ratios and concentration ratios for delta hydrogen-2 and delta oxygen-18 in 1 molal salt solutions are as follows (Horita et al, 1993). The data for individual salts may be multiplied by molali
