Measurement of high precision isotope ratios for mercury from coals using transient signals

Abstract

We report on high precision measurements of mercury isotopes in natural samples. The natural isotopic Hg composition in cinnabar and coal was determined using different types of ICP-MS instrumentation. The performance of 4 different multicollector (MC) ICP-MS instruments was evaluated and compared to results obtained by collision cell ICP-MS and ICP-time-of-flight-MS. Hg in cinnabar (Almaden, Spain) was continuously introduced into the ICP plasma and Hg isotope ratios were corrected for mass fractionation by measuring the ²⁰³Tl/²⁰⁵Tl ratio, simultaneously introduced as a dry aerosol. The average corrected ratio of ²⁰¹Hg/²⁰²Hg in cinnabar using MC-ICP-MS was 0.44297 ± 0.00001 (2 SE, internal precision). This ratio differs significantly from the currently accepted IUPAC ratio for this isotope pair. Hg isotope ratios in different coal and fly ash samples were determined after the Hg in the samples was preconcentrated onto gold traps, from which the Hg was thermally desorbed into the plasma. Consequently, Hg ratios in coal were measured on transient signals. The ratios of Hg isotopes changed slightly during the evolution of the peak, suggesting a mass fractionation caused by the thermal desorption step. Hence, ratios were obtained from the integrated signal of the individual isotopes for the entire sample. The external precision between replicate samples was typically in the order of 300 to 4000 ppm (2 RSD). The external reproducibility of transient signals was similar to that from continuous signals, indicating that isotope ratio measurement on transient signals is a viable technique.