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.

Comparative evaluation of 2H- versus 3H-based enrichment factor determination on the uncertainty and accuracy of low-level tritium analyses of environmental waters.

Analysis of low-level tritium (3H) in environmental waters requires pre-concentration using electrolytic enrichment prior to decay counting. Accurate and precise electrolytic enrichment factors (EF) are required to determine the sample's environmental 3H concentration. Two methods are used to determine EFs: i) the Spike Proxy Method (SPM) and ii) the Deuterium Method (DM) with each having several modalities. We conducted a comparative assessment of four EF strategies using 250 mL and 500 mL electrolytic enrichment of three low-level 3H proficiency water standards (0.5-7 TU) to see which strategy gave the most accurate 3H results based on z- and Zeta-scores. Our comparative evaluation revealed the DM offers consistently superior 3H results, with more precise EF determinations compared to the three SPM strategies. The DM gave the best z-scores with an EF relative combined uncertainty of about 0.5‰ and a negligible contribution to the overall uncertainty budget due to the EF determination. Moreover, the DM can improve productivity by eliminating the spike and gravimetric procedures from routine analyses and can give rapid cell enrichment performance feedback prior to decay counting. We recommend low-level tritium laboratories consider adopting the DM into their 3H sample enrichment and analysis operations.

Correcting Laser-Based Water Stable Isotope Readings Biased by Carrier Gas Changes.

Recently, laser-based water stable isotope spectrometers have become popular as they enable previously impossible approaches of environmental observations. Consequently, they have been subjected to increasingly heterogeneous atmospheric conditions. However, there is still a severe lack of data on the impact of nonstandardized gas matrices on analyzer performances. Against this background, we investigated the influence of changing proportions of N2, O2, and CO2 in the carrier gas on the isotope measurements of a typical laser-based water stable isotope analyzer (Picarro L2120-i). We combined environmentally relevant mixtures of N2, O2, and CO2 with referenced, flash-evaporated water and found that isotope readings of the same water were altered by up to +14.57‰ for δ(18)O and -35.9‰ for δ(2)H. All tested relationships between carrier gas changes and respective isotope readings were strongly linearly correlated (R(2) > 0.99). Furthermore, an analyzer-measured variable allowed for reliable postcorrection of the biased isotope readings, which we additionally tested on field data. Our findings are of importance for environmental data obtained by analyzers based on the same technology. They are relevant for assays where inconsistent gas matrices or a mismatch in this regard between unknown and reference analyses cannot be excluded, which is in particular common when investigating the soil-vegetation-atmosphere continuum.