Accurate and Precise Determination of Silver Isotope Fractionation in Environmental Samples by Multicollector-ICPMS

Abstract

High precision silver isotope ratios in environmental samples were determined by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Purification of Ag from sample matrixes was performed by a two stage tandem column setup with use of anion and cation exchange resin, sequentially. It was found that 1% HNO(3) and 3% HCl was efficient to stabilize Ag in the final purified sample digests prior to MC-ICPMS determination. Pd at 2 mug g(-1) was added to both sample and Ag standard solution as a common doping matrix as well as an internal standard for mass bias correction. Mass discrimination and instrument drift were corrected by a combination of internal normalization with Pd and standard-sample-standard bracketing, without assuming identical mass bias for Pd and Ag. NIST SRM 978a (silver isotopic standard reference material) was used for method validation and subjected to column separation and sample preparation processes. A value of -0.003 +/- 0.010 per thousand for delta(107/109)Ag (mean and 2SD, n = 4) was obtained, confirming accurate results can be obtained using the proposed method. To the best of our knowledge, this is the first report on delta(107/109)Ag variations in environmental samples. Significant differences in Ag isotope ratios were found among NIST SRM 978a standard, sediment CRM PACS-2, domestic sludge SRM 2781, industrial sludge 2782, and the fish liver CRM DOLT-4. The sediment CRM PACS-2 has a very small negative delta(107/109)Ag value of -0.025 +/- 0.012 per thousand (2SD, n = 4). The domestic sludge SRM 2781 has a negative delta(107/109)Ag value of -0.061 +/- 0.010 per thousand (2SD, n = 4), whereas industrial sludge SRM 2782 has a positive delta(107/109)Ag value of +0.044 +/- 0.014 per thousand (2SD, n = 4), which may indicate the contribution of Ag from different anthropogenic inputs. DOLT-4 has a much larger negative value of -0.284 +/- 0.014 per thousand (2SD, n = 4), possibly caused by biological processes. These observations confirm that Ag isotope fractionation may provide a useful tool for fingerprinting sources of Ag in the environment and for studying a wide variety of chemical and biological processes in nature. High precision of better than +/-0.015 per thousand (2SD, n = 4) obtained in real sample matrixes makes the present method well suited for monitoring small Ag isotope fractionation in nature.