Simplifying silver isotope analysis of metallic samples: using silver nitrate precipitation to avoid perilous chloride formation

Abstract

Silver (Ag) isotopes have the potential to provide useful insights into a diverse range of geological, environmental, and archaeological processes. This manuscript presents a novel technique that provides a simple, time-efficient, and accurate method for obtaining Ag isotope compositions of metallic gold (Au) samples. Unlike previous methodologies that favoured multiple ion exchange columns to purify and isolate Ag in nitrate form. This technique instead uses a single anion-exchange column, followed by the chemical conversion of Ag from chloride to nitrate form using the widely available reagents, ammonium hydroxide and ascorbic acid. This chemical conversion not only speeds up and simplifies sample processing allowing increased sample throughput, but importantly also significantly reduces the risk of Ag loss (and therefore user-induced isotopic fractionation) while converting the samples into a medium suitable for mass spectrometry. In this study, both pure Ag and native Au samples have been investigated, with Ag isotope compositions given relative the NIST SRM978a Ag standard. The long-term reproducibility of the in-house Sigma MON Ag solution was ε¹⁰⁹Ag = 1.32 ± 0.31 (2 s.d.; n = 34), which is comparable to the precision achievable for unprocessed high-purity Ag samples with replicate analyses generating an average precision of ε¹⁰⁹Ag = ± 0.25 (2 s.d.; n = 5). Comparable levels of precision were also achieved for natural Au samples, indicating that this methodology has no resolvable effect on the precision of the Ag isotope measurements. The natural Au standard CEZAg was used to test the external reproducibility of the chemical separation and conversion technique, yielding an average value of ε¹⁰⁹Ag = 0.34 ± 0.13 (2 s.d.; n = 6), which is within analytical uncertainty of the previous determinations, demonstrating the accuracy of the new methodology. Furthermore, analysis of natural Au gold nuggets from the Fosterville Au Mine using the chemical conversion process described herein and a previously published multiple column method at a different institution produced consistent Ag isotope compositions, confirming the accuracy of the measurements generated using this method.