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 (delta H-2, delta O-18) are an important assay in hydrology and environmental research with rising interest in delta O-17, which requires ultra-precise assays. We evaluated isotope analyses of six test water samples for 281 laboratory submissions measuring delta H-2 and delta O-18 along with a subset analyzing delta O-17 and Delta O-17 by laser spectrometry and isotope ratio mass spectrometry (IRMS). Methods Six test waters were distributed to laboratories spanning a wide delta range of natural waters for delta H-2, delta O-18 and delta O-17 and Delta O-17. 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 delta O-18 and delta H-2 results within 0.2 parts per thousand (mUr) and 1.6 parts per thousand 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 Delta O-17, dual-inlet results for IRMS using quantitative O-2 conversion were accurate and highly precise, but the results for laser spectrometry ranged by ca 200 per meg (mu Ur) for each sample, with ca 70% unable to replicate the duplicate to their claimed Delta O-17 precision. One complicating factor is the lack of certified primary reference waters for delta O-17. 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 delta value controls of known isotope composition in each run. Progress in Delta O-17 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.

Automated summary

Stable isotope analyses of environmental waters, including δH-2, δO-18, and δO-17, are crucial in understanding hydrology and environmental research. Most laboratories produced acceptable results for δO-18 and δH-2, but some struggled to replicate their claimed precision. Analysis of Delta O-17 showed varied results between IRMS and laser spectrometry, with many labs unable to achieve the claimed precision level.