Progress and challenges in dual- and triple-isotope (δ18O, δ2H, Δ17O) analyses of environmental waters: an international assessment of laboratory performance

RATIONALE Stable isotope analyses of environmental waters (δ2 H, δ18 O) are an important assay in in hydrology and environmental research with rising interest in δ17 O, which requires ultra-precise assays. We evaluated isotope analyses of six test water samples for 281 laboratory submissions measuring δ2 H and δ18 O along with a subset analysing δ17 O and Δ17 O by laser- and isotope-ratio mass spectrometry. METHODS Six test waters were distributed to laboratories spanning a wide δ-range of natural waters for δ2 H, δ 18 O and δ17 O and Δ17 O. One sample was a blind duplicate to test reproducibility and claims of analytical precision. RESULTS Results showed that ca. 83 % of the submissions produced acceptable δ18 O and δ2 H results within 0.2 ‰ (mUr) and 1.6 ‰ of the benchmark values, respectively. However, 17 % of the submissions gave questionable to unacceptable results. A blind duplicate revealed many laboratories reported overly optimistic precision, and many could not replicate within their claimed precision. For Δ17 O, dual-inlet IRMS using quantitative O2 conversion were accurate and highly precise, but the results for laser spectrometry ranged by ca. 200 per meg (μUr) for each sample, with ca. 70 % unable to replicate the duplicate to their claimed Δ17 O precision. One complicating factor is the lack of certified primary reference waters for δ17 O. CONCLUSIONS No single or combination of factors was identifiable for poor or good performance, and underperformance came from issues like data normalization including inadequate memory and drift corrections, compromised working reference materials, and underperforming instrumentation. We recommend isotope laboratories include high and low δ value controls of known isotope composition in each run. Progress in Δ17 O analyses by laser spectrometry requires extraordinary proof of performance claims and would benefit from the development of adoptable and systematic advanced data processing procedures to correct for memory and drift.