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

The use of stable isotopes (<i>δ</i>²H and <i>δ</i>¹⁸O) 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 ‰ <i>δ</i>¹⁸O and 0 to –115 ‰ <i>δ</i>²H. 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 <i>δ</i>²H but only 54% achieved similar scores for <i>δ</i>¹⁸O. Only a minor fraction of results (12% for <i>δ</i>²H and 15% for <i>δ</i>¹⁸O) were unsatisfactory. The larger number of questionable results for <i>δ</i>¹⁸O 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.