Cavity Ring-Down Spectroscopy Performance and Procedures for High-Throughput δ18O and δ2H Measurement in Water Using "Express" Mode.

Cavity ring-down spectroscopy (CRDS) is rapidly becoming an invaluable tool to measure hydrogen (δ²H) and oxygen (δ¹⁸O) isotopic compositions in water, yet the long-term accuracy and precision of this technique remain relatively underreported. Here, we critically evaluate one-year performance of CRDS δ²H and δ¹⁸O measurements at ETH Zurich, focusing on high throughput (~200 samples per week) while maintaining required precision and accuracy for diverse scientific investigations. We detail a comprehensive methodological and calibration strategy to optimize CRDS reliability for continuous, high-throughput analysis using Picarro's "Express" mode, an area not extensively explored previously. Using this strategy, we demonstrate that CRDS achieves long-term precision better than ±0.5‰ for δ¹⁸O and ±1.0‰ for δ²H (±1σ) on three United States Geological Survey (USGS) reference materials treated as unknowns.¹⁸ Specifically, reported results for each reference material over this one-year period are: (i) USGS W-67444: ${\delta }^2$H = $-399.32\pm 0.96\text{‰}$, ${\delta }^{18}$O = $-51.07\pm 0.45\text{‰}$ ($n=30$), (ii) USGS W-67400: ${\delta }^2$H = $2.55\pm 0.49\text{‰}$, ${\delta }^{18}$O = $-1.85\pm 0.13\text{‰}$ ($n=140$), and (iii) USGS-50: ${\delta }^2$H = $33.68\pm 0.91\text{‰}$, ${\delta }^{18}$O = $5.03\pm 0.04\text{‰}$ ($n=21$). We also address challenges such as aligning our analytical uncertainties with the narrower uncertainties of International Atomic Energy Agency reference materials, and mitigating inherent CRDS issues like memory and matrix effects when analyzing environmental samples. Our review provides a practical framework for CRDS applications in hydrology, paleoclimatology, and biogeochemistry, underscoring the importance of continuous evaluation and methodological refinement to ensure accuracy and precision in δ²H and δ¹⁸O analyses.¹⁸.