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Rationale: Stable isotope analyses of environmental waters (δ²H, δ18O) are an important assay in hydrology and environmental research with rising interest in Δ17O, which requires ultra-precise assays. We evaluated isotope analyses of six test water samples for 281 laboratory submissions measuring δ²H and δ18O along with a subset analyzing Δ17O and Δ17O by laser spectrometry and isotope ratio mass spectrometry (IRMS). Methods: Six test waters were distributed to laboratories spanning a wide δ range of natural waters for δ²H, δ18O and Δ17O and Δ17O. 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 δ18O and δ²H 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 Δ17O, dual-inlet results for 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 Δ17O precision. One complicating factor is the lack of certified primary reference waters for Δ17O. Conclusions: No single factor 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 Δ17O 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. [ABSTRACT FROM AUTHOR]

The use of stable isotopes (δ2H and δ18O) is widespread in water resources studies. In the Latin America and the Caribbean (LAC) region, the application of isotope techniques has increased in the past decade, but there remains room to gain self-reliance in environmental isotope studies, necessitating easy and fast access to good-quality isotope data. To that end, in 2018 the IAEA carried out the first regional interlaboratory comparison exercise, testing the analytical performance of 25 laboratories using isotope-ratio mass spectrometry and laser absorption spectroscopy. The three test samples covered a commonly observed range of 0 to –16 ‰ δ18O and 0 to –115 ‰ δ2H. z- and ζ-scores were used to benchmark laboratories' performance against a strict criterion. We found that 81% of the laboratories had satisfactory performance ( | z | ¯ ≤ 2) for δ2H but only 54% achieved similar scores for δ18O. Only a minor fraction of results (12% for δ2H and 15% for δ18O) were unsatisfactory. The larger number of questionable results for δ18O confirmed the challenges in laser absorption spectroscopy for this isotope. Besides instrumental performance, the sample throughput, laboratory reference materials, and data post-processing were contributing factors to inaccurate or imprecise performance. [ABSTRACT FROM AUTHOR]

The stable isotope composition of water (d2H, d18O) is a crucial tracer of hydrological processes, hence accurate assays are required to yield sound scientific information. We tested the performance of 235 worldwide laboratories conducting isotope analyses of water by IRMS and laser spectrometers. Eight samples comprised the d-range of natural waters including a highly depleted, enriched, saline, and methanol contaminated sample to assess contamination effects. Results showed 73 % of laboratories gave acceptable results, but 27 % produced unacceptable results. Methanol contamination resulted in extreme outliers for laser spectrometers, but also affected reactor-based IRMS systems. Analysis of results and metadata suggested poor performance stemmed mainly from skill and knowledge-based errors including: calculation mistakes, compromised calibration standards, faulty instrumentation, lack of vigilance to contamination, and inattention to unreasonable isotopic outcomes. To counteract these, we recommend laboratories include 1-2 isotopically known controls in all runs; laser spectrometry laboratories should screen for spectral contamination, and all should evaluate if derived d-excess values are realistic if both isotopes are measured. Together, these strategies should immediately inform the laboratory about fundamental mistakes or compromised samples. [ABSTRACT FROM AUTHOR]