Technical Note: Evaluation of between-sample memory effects in the analysis of δ 2H and δ 18O of water samples measured by laser spectroscopes
This study evaluated between-sample memory in isotopic measurements of δ 2H and δ 18O in water samples by laser spectroscopy. Ten isotopically depleted water samples spanning a broad range of oxygen and hydrogen isotopic compositions were measured by three generations of off-axis integrated cavity output spectroscopy and cavity ring-down spectroscopy instruments. The analysis procedure encompassed small (less than 2‰ for δ 2H and 1‰ for δ 18O) and large (up to 201‰ for δ 2H and 25‰ for δ 18O) differences in isotopic compositions between adjacent sample vials. Samples were injected 18 times each, and the between-sample memory effect was quantified for each analysis run. Results showed that samples adversely affected by between-sample isotopic differences stabilised after seven–eight injections. The between-sample memory effect ranged from 14% and 9% for δ 2H and δ 18O measurements, respectively, but declined to negligible carryover (between 0.1% and 0.3% for both isotopes) when the first ten injections of each sample were discarded. The measurement variability (range and standard deviation) was strongly dependent on the isotopic difference between adjacent vials. Standard deviations were up to 7.5‰ for δ 2H and 0.54‰ for δ 18O when all injections were retained in the computation of the reportable δ-value, but a significant increase in measurement precision (standard deviation in the range 0.1‰–1.0‰ for δ 2H and 0.05‰–0.17‰ for δ 18O) was obtained when the first eight injections were discarded. In conclusion, this study provided a practical solution to mitigate between-sample memory effects in the isotopic analysis of water samples by laser spectroscopy.