USING DIRECT COMBUSTION AND SYRINGE INJECTION FOR THE ISOTOPIC ANALYSIS OF MERCURY

Mercury (Hg) isotopic composition is traditionally determined by acid digestion sample preparation and Hg cold vapor generation by stannous chloride (SnCl₂) reduction coupled to a multi-collector inductively-coupled-plasma mass-spectrometer (MC-ICP-MS). This method produces hazardous waste, requires lengthy sample preparation, and can result in inconsistent isotope data for samples containing high organic content. Direct combustion has been utilized as an alternative preparation technique to release Hg vapor from sample materials with a wide range of matrices. Released Hg vapor can be captured in an acidic KMnO₄ solution for analysis by SnCl₂ cold vapor generation or onto a gold trap for subsequent release, syringe collection, and introduction into the gas stream of the MC-ICP-MS. Combustion preparation reduces variability in samples containing high organic content, but solution capture suffers from the same drawbacks of SnCl₂ cold vapor generation. Here we present work using Hg combustion, gold trap capture and release, and syringe introduction by using commercially available combustion instrumentation (DMA-80) to combust, capture, and release Hg into a temporary storage vessel for subsequent sampling by syringe. Three Hg reference materials (NIST-SRM 2711, SARM-20, and TORT-1) were prepared by three different techniques: (1) bench top acid digestion and (2) microwave acid digestion with SnCl₂ cold vapor generation, and (3) combustion preparation with gas introduction via syringe. Initial results verify reduced sample preparation time and hazardous waste, but isotope composition indicates decreased precision. Sample combustion and syringe gas introduction yielded larger standard deviations than bench top acid digestion with cold vapor generation with respect to δ²⁰²Hg: -0.143 ± 0.173‰ compared to -0.199 ± 0.095‰ for NIST-SRM 2711, -0.705 ± 0.485‰ compared to -1.025 ± 0.066‰ for SARM-20, and -0.639 ± 0.315‰ compared to -0.815 ± 0.174‰ for TORT-1. Future work will focus on improving combustion efficiency of organics and reduce simultaneous collection of matrix components with Hg vapor. If the precision can be increased, this method can be a low-waste alternative particularly useful for initial sample screening to reduce sample preparation workloads.