The first IAEA inter-laboratory comparison exercise in Latin America and the Caribbean for stable isotope analyses of water samples

Assessing the performance of international laboratories analysing the stable isotope composition (δ15 N, δ18 O, δ17 O) of nitrate in environmental waters

RATIONALE

Stable-isotope analyses of nitrate (NO3 - ) in various water sources are crucial for understanding nitrogen pollution and its impact on aquatic ecosystems. We evaluated the accuracy and precision of stable-isotope analyses of nitrate conducted by international laboratories.

METHODS

Six samples with nitrate (2 mg L-1 NO3 - -N) were sent to 47 laboratories. The NO3 - had a 30-50 ‰ range of δ values for δ15 N, δ18 O and δ17 O. One blind duplicate evaluated reproducibility and the effect of water δ18 O. Laboratories used diverse methods to convert nitrate to N2 O, N2 , CO or O2 for stable-isotopic measurements (microbial, cadmium, titanium and elemental analysis) and isotope-ratio mass spectrometry or laser-based technologies.

RESULTS

Thirty-six international laboratories (83 %) reported results; however, 23 % did not analyze the test samples due to technical difficulties. Of the reporting laboratories, 79 % and 84 % produced accurate δ15 N and δ18 O results falling within ±0.8 ‰ and ±1.1 ‰ of the benchmark values, respectively. Three laboratories produced only outliers. The duplicate revealed most laboratories gave internally reproducible results at appropriate analytical precision. For δ17 O, six laboratories reported results, but 67 % could not reproduce results within their claimed analytical measurement precision. One complication is a lack of nitrate reference materials for δ17 O.

CONCLUSIONS

Analyst experience contributed to better performance, and underperformance was from compromised standards or inappropriate δ range of working reference materials. The stable isotope community must develop new nitrate reference materials for δ15 N spanning -20 ‰ to +80 ‰ and new materials for δ17 O.

Balancing precision and throughput of ..17O and .÷...17O analysis of natural waters by Cavity Ringdown Spectroscopy

δ ¹⁷O and Δ'¹⁷O are emerging tracers increasingly used in isotope hydrology, climatology, and biochemistry. Differentiating small relative abundance changes in the rare ¹⁷O isotope from the strong covariance with ¹⁸O imposes ultra-high precision requirements for this isotope analysis. Measurements of δ ¹⁷O by Cavity Ringdown Spectroscopy (CRDS) are attractive due to the ease of sample preparation, automated throughput, and avoidance of chemical conversions needed for isotope-ratio mass spectrometry. However, the CRDS approach requires trade-offs in measurement precision and uncertainty. In this protocol document, we present the following:•New analytical procedures and a software tool for conducting δ ¹⁷O and Δ'¹⁷O measurements by CRDS.•Outline a robust uncertainty framework for Δ'¹⁷O determinations.•Description of a CRDS performance framework for optimizing throughput, instrumental stability, and Δ'¹⁷O measurement precision and accuracy.

Progress and challenges in dual- and triple-isotope (δ18 O, δ2 H, Δ17 O) 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 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 analyzing δ¹⁷ O and Δ¹⁷ 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 δ² H, δ¹⁸ 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 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 Δ¹⁷ O precision. One complicating factor is the lack of certified primary reference waters for δ¹⁷ 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 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.

Performance of low-cost stainless-steel beverage kegs for long-term storage integrity and easy dispensing of water isotope (δ18 O, δ2 H) reference materials

RATIONALE

A widespread problem observed in global water isotope (δ¹⁸ O, δ² H) proficiency tests is compromised working reference materials due to storage-dispensing evaporation effects. Proper storage requires no evaporation or leakage, which causes isotopic drift and bias. Surveys by the International Atomic Energy Agency (IAEA) show most isotope laboratories use glass or plastic bottles to store working reference materials, with frequent opening and closings that pose evaporation risks. Practical small (ca. 2-5 L) storage-dispensing solutions free of air exposure, evaporation, and leakage are needed. We also tested several smaller-scale bottles for day-to-day aliquots.

METHODS

We tested low-cost, conveniently sized (4 L) adaptations of a common stainless-steel beverage keg with a liquid dispenser, with minor modifications to facilitate low-flow dispensing and pressurization (1-2 bar) with Ar or N₂ . We tested three kegs (100%, 75%, 50% initial fills) for a 2-year period along with monthly dispensing to assess long-term storage viability for maintaining δ¹⁸ O and δ² H integrity and dispensing, and day-to-day aliquot bottles for 6 months.

RESULTS

Test results showed these small keg storage systems fully maintained the isotopic integrity of water over the 2-year testing period with no trend in the isotopic data that would reveal evaporative loss or leakage (e.g., pressure or mass loss) regardless of starting fill level. However, evaporated water in the outlet tube assembly must be eliminated by discarding 15-20 mL before dispensing into appropriate daily-use laboratory standard bottles (30-100 mL). Glass bottles for daily aliquots showed good integrity properties, but only if their fill level was >50%.

CONCLUSIONS

The use of a low-cost pressurized metal beverage keg dispensing system provides a robust solution to enable laboratories to maintain the integrity of their water isotope working reference materials over several years.