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 δ2H, δ18O and δ17O 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 δ2H, δ18O, δ17O, d-excess and Δ17O, 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 δ2H, δ18O, δ17O, d-excess and Δ17O). 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 δ2H analysis.
Keywords: Cavity ring-down spectroscopy; QA/QC; hydrogen-2; isotope measurements; methods and equipment; oxygen-17; oxygen-17 excess; oxygen-18; uncertainty.