Determination of Thallium Isotope Ratios in Ten Geological Reference Materials, and of Thallium Isotopic Fractionation Effects Related to Sample Digestion by Dry Ashing

Abstract

We report for the first time Tl isotope ratios for ten geological reference materials (RMs), namely SGR-1b (from the USGS), and GBW07302, GBW07303a, GBW07401a, GBW03104, GBW07103, GBW07109, GBW07110, GBW07111 and GBW07113 (from China). The principal matrix characteristics of these materials range from sediments to igneous rocks, including stream sediment, soil, shale, granite, nepheline syenite, trachyte, granodiorite and rhyolite. Analytically, the procedure we developed includes a sample digestion step, a simplified single-stage anion exchange matrix separation step and measurements by MC-ICP-MS. For sample digestion we compared a dry ashing-based process with an acid digestion method under high-pressure conditions. We observed that the RMs with high organic content (e.g., SGR-1b = ~ 25%) showed very high values of loss on ignition (LOI) (~ 40%) during dry ashing. Moreover, Tl isotopic measurement results were heavier after dry ashing than after application of the high-pressure bomb method. We infer that the combustion conditions in case of large amounts of organic matter are probably the cause of Tl isotopic fractionation effects (preferential evaporation of ²⁰³Tl). We thus recommend the high-pressure bomb method for the sample digestion step. We validated our entire measurement procedure by applying it to well-documented RMs, including GSP-2 (granodiorite), Nod-A-1 (Marine sediment), Nod-P-1 (Marine sediment) and SCo-1 (shale). Our measurement results were in agreement with literature values.