Multipoint normalization of δ¹⁸O of water against the VSMOW2-SLAP2 scale with an uncertainty assessment

In this study, we measured the oxygen stable isotope ratio of drinking water using gas chromatography isotope ratio mass spectrometry. The δ¹⁸O value of drinking water was normalized based on the Vienna Standard Mean Ocean Water 2 (VSMOW2), Standard Light Antarctic Precipitation 2 (SLAP2), and Greenland Ice Sheet Precipitation (GISP) scale by CO₂ equilibrium for 24 h. The isotope ratio responses of a dummy sample drifted as much as 0.145‰ due to a significant decrease in the amount of injected sample. The autodilution technique improved measurement precision of the δ¹⁸O of dummy sample two-fold compared to that without autodilution to 0.025‰. The autodilution of an injected concentration of equilibrated CO₂ also helped improve the measurement precision of the isotope ratio response. The drift of the ratio responses was tested using linear model regression to validate linearity within the sample concentration and isotope ratio ranges. Measurement reliability was assessed using various statistical approaches. One-way analysis of variance verified non-reproducible results of individual measurements. Normalization uncertainties were then assessed by various normalization schemes including two-point and three-point values consisting of the VSMOW2, SLAP2, and GISP standards, showing equivalent results associated with similar extent of normalization uncertainties among various normalization methods. In particular, the uncertainty of the GISP (0.09‰) contributed to one-third of the total normalization uncertainty, implying that the three-point normalization can be improved by a potential standard of which uncertainty is equivalent to the bracketing standards, VSMOW2 (0.02‰) and SLAP2 (0.02‰).