Analysis of the laboratory results and their metadata suggested inaccurate or imprecise performance stemmed mainly from skill- and knowledge-based errors including: calculation mistakes, inappropriate or compromised laboratory calibration standards, poorly performing instrumentation, lack of vigilance to contamination, or inattention to unreasonable isotopic outcomes. To counteract common errors, we recommend that laboratories include 1-2 'known' control standards in all autoruns; laser laboratories should screen each autorun for spectral contamination; and all laboratories should evaluate whether derived d-excess values are realistic when both isotope ratios are measured. Combined, these data evaluation strategies should immediately inform the laboratory about fundamental mistakes or compromised samples.

Rationale: Stable isotope analyses of environmental waters (δ 2 H, δ 18 O) are an important assay in hydrology and environmental research with rising interest in δ 17 O, which requires ultra-precise assays. We evaluated isotope analyses of six test water samples for 281 laboratory submissions measuring δ 2 H and δ 18 O along with a subset analyzing δ 17 O and Δ 17 O 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 δ 2 H, δ 18 O and δ 17 O and Δ 17 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 δ 18 O and δ 2 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 Δ 17 O, 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 (μUr) for each sample, with ca 70% unable to replicate the duplicate to their claimed Δ 17 O precision. One complicating factor is the lack of certified primary reference waters for δ 17 O.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 Δ 17 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.

The Cd-azide method yielded excellent results for routine determination of δ N, δ O and δ O values (and the O excess) of nitrate or nitrite by laser spectrometry. Disadvantages are the toxicity of Cd-azide chemicals and the lack of automated sampling devices for N O laser spectrometers. The N-enriched tracer test revealed potential for position-specific experimentation of aqueous nutrient dynamics at high N enrichments by laser spectrometry, but exposed the need for memory corrections and improved spectral deconvolution of O.

Rationale: The nitrogen and oxygen (δ 15 N, δ 18 O, and δ 17 O values) isotopic compositions of nitrate (NO 3 − ) are crucial tracers of nutrient nitrogen (N) sources and dynamics in aquatic systems. Current methods such as bacterial denitrification or Cd-azide reduction require laborious multi-step conversions or toxic chemicals to reduce NO 3 − to N 2 O for 15 N and 18 O isotopic analyses by isotope ratio mass spectrometry (IRMS). Furthermore, the 17 O composition of N 2 O cannot be directly disentangled using IRMS because 17 O contributes to mass 45 ( 15 N). Methods: We describe a new one-step chemical conversion method that employs Ti(III) chloride to reduce nitrate to N 2 O gas in septum sample vials. Sample preparation takes only a few minutes followed by a 24-h reaction producing N 2 O gas (65-75% recovery) which partitions into the headspace. The N 2 O headspace was measured for 15 N, 18 O and 17 O by IRMS or laser spectrometry. Results: IRMS and laser spectrometric analyses gave accurate and reproducible N and O isotopic results down to 50 ppb (3.5 μM) NO 3 -N, similar in precision to the denitrifier and Cd-azide methods. The uncertainties for dissolved nitrate reference materials (USGS32, USGS34, USGS35, IAEA-NO 3 ) were ±0.2‰ for δ 15 N values and ±0.3‰ for δ 18 O values using IRMS. For laser-based N 2 O isotope analyses the results were similar, with an δ 17 O uncertainty of ±0.9‰ without any need for 15 N correction. Conclusions: Advantages of the Ti(III) reduction method are simplicity, low cost, and no requirement for toxic chemicals or anaerobic bacterial cultures. Minor corrections may be required to account for sample nitrate concentration variance and potential chemical interferences. The Ti(III) method is easily implemented into laboratories currently using N 2 O headspace sampling apparatus. We expect that the Ti(III) method will promulgate the use of N and O isotopes of nitrate in important studies of nutrient dynamics and pollution in a wide range of aquatic ecosystems.

The ability to quickly and accurately measure extremely (2) H-enriched waters by laser spectrometry will facilitate the use of deuterium as a tracer in numerous environmental and other applications. For low-level tritium operations, this new analytical ability facilitated a 10-20 % increase in sample productivity through the elimination of spike standards and gravimetrics, and provides immediate feedback on electrolytic enrichment cell performance. Copyright © 2016 John Wiley & Sons, Ltd.