High-precision measurements of δ 2 H, δ 18 O and δ 17 O in water with the aid of cavity ring-down laser spectroscopy.

A thorough evaluation of measurement uncertainty together with control of short-term and long-term precision of measurements should be a basis of any successful quality assurance/quality control (QA/QC) strategy aimed at maintaining a high quality of the analytical process. Here we present the results of a comprehensive assessment of the analytical performance of a Picarro L2140-i CRDS laser spectrometer analysing δ ² H, δ ¹⁸ O and δ ¹⁷ O in water. The assessment is based on results obtained during 15 months of continuous operation of this instrument (February 2017 to May 2018). The short-term precision of measured and derived quantities was 0.11, 0.036, 0.028, 0.23 ‰ and 11 per meg, for δ ² H, δ ¹⁸ O, δ ¹⁷ O, d-excess and Δ ¹⁷ O, respectively, and is comparable to the precision reported by the manufacturer. The long-term precision of the L2140-i, defined as standard uncertainty of the time series of 153 analyses of a laboratory standard conducted throughout 15 months, was roughly two times lower (0.24, 0.053, 0.038, 0.37 ‰ and 21 per meg, for δ ² H, δ ¹⁸ O, δ ¹⁷ O, d-excess and Δ ¹⁷ O). In-depth assessment of the measurement uncertainty of a single analysis revealed that assigned uncertainty of the calibration standards is an important component of the uncertainty budget, especially in case of δ ² H analysis.