Determination of the isotopic composition of silver by MC-ICP-MS using an improved full gravimetric isotope mixture...

We report independent primary measurements of silver isotope ratio by MC-ICP-MS using gravimetric mixtures prepared from near-pure silver-107 and silver-109 materials. To our knowledge, this study represents the first independent replication of the silver isotope measurements made in 1981 at the US National Bureau of Standards, now the National Institute of Standards and Technology (NIST). Although highly enriched silver isotopes of comparable chemical and isotopic purity to those employed in the original NIST study were employed, we adopted an improved calibration strategy based on the full gravimetric isotope mixture (FGIM) method for mass bias correction, rather than the traditional one-mixture-at-a-time calibration approach. This improved FGIM strategy eliminates the need to measure the two near-pure isotope materials directly, instead relying on additional gravimetric mixtures. As a result, it avoids potential systemic biases arising from any dependence of the isotope ratio correction factors on the isotopic composition of the mixtures. We obtain a silver isotope ratio of n(¹⁰⁷Ag)/n(¹⁰⁹Ag) = 1.07678 ± 0.00027 mol mol^-1 (95% confidence level) for the NIST reference material SRM 978a. This value differs slightly from the value certified by NIST in 1981 (1.07638 ± 0.00022 mol mol^-1 at 95% confidence level). Our intent is not to scrutinize the contributions of earlier work conducted four decades ago, rather to provide a rare independent determination of the isotopic composition of the same reference material, thereby offering an objective evaluation of the methods that underpin isotope ratio metrology.