A digestion-purging-trapping method for precise stable mercury isotope measurements of natural carbonates†

Natural carbonates, such as the aragonite of corals and calcite of speleothems, are potential archives of mercury (Hg) isotope signatures of freshwater and seawater. However, the ultra-low Hg levels in carbonates pose a great challenge for their isotopic analysis. In this study, we developed a digestion-purging-trapping protocol to separate and trap Hg from carbonates for high-precision isotope analysis for the first time. High Hg recovery (100 ± 6%) was achieved by digestion of the carbonate sample and followed by Hg(II) reduction and Hg(0) purging and trapping into sulfidic KMnO₄. Our protocol was validated by testing synthetic carbonates spiked with varying amounts of certified Hg standards and interfering ions. The protocol could handle large sample volumes (up to 20 grams) in a short processing time (∼30 min), achieving high precision (0.08‰ (2SD) for δ²⁰²Hg), low blanks (<40 pg), and limited matrix interference in Hg isotopic analysis. For facilitating interlaboratory comparison, we measured the Hg isotope compositions of two geological standards (GBW 07129: limestone and GBW 07134: dolostone). Our measurements of coral and speleothem samples showed characteristic Hg isotope signatures indicative of their forming environments, highlighting their potential to record long-term Hg cycling.