High-precision measurement of mercury isotope ratios in sediments using cold-vapor generation multi-collector inductively coupled plasma mass spectrometry

Abstract

An on-line Hg reduction technique using stannous chloride as the reductant was applied for accurate and precise mercury isotope ratio determinations by multi-collector (MC)-ICP/MS. Special attention has been paid to ensure optimal conditions (such as acquisition time and mercury concentration) allowing precision measurements good enough to be able to significantly detect the anticipated small differences in Hg isotope ratios in nature. Typically, internal precision was better than 0.002% (1 RSE) on all Hg ratios investigated as long as approximately 20 ng of Hg was measured with a 10-min acquisition time. Introducing higher amounts of mercury (50 ng Hg) improved the internal precision to <0.001%. Instrumental mass bias was corrected using (205)Tl/(203)Tl correction coupled to a standard-sample bracketing approach. The large number of data acquired allowed us to validate the consistency of our measurements over a one-year period. On average, the short-term uncertainty determined by repeated runs of NIST SRM 1641d Hg standard during a single day was <0.006% (1 RSD) for all isotope pairs investigated ((202)Hg/(198)Hg, (202)Hg/(199)Hg, (202)Hg/(200)Hg, and (202)Hg/(201)Hg). The precision fell to <0.01% if the long-term reproducibility, taken over 11 months (over 100 measurements), was considered. The extent of fractionation has been investigated in a series of sediments subject to various Hg sources from different locations worldwide. The ratio (202)Hg/(198)Hg expressed as delta values (per mil deviations relative to NIST SRM 1641d Hg standard solution) displayed differences from +0.74 to -4.00 per thousand. The magnitude of the Hg fractionation per amu was constant within one type of sample and did not exceed 1.00 per thousand. Considering all results (the reproducibility of Hg standard solutions, reference sediment samples, and the examination of natural samples), the analytical error of our delta values for the overall method was within +/-0.28 per thousand (1 SD), which was an order of magnitude lower than the extent of fractionation (4.74 per thousand) observed in sediments. This study confirmed that analytical techniques have reached a level of long-term precision and accuracy that is sufficiently sensitive to detect even small differences in Hg isotope ratios that occur within one type of samples (e.g., between different sediments) and so far have unequivocally shown that Hg isotope ratios in sediments vary within approximately 5 per thousand.

Tracing historical trends of Hg in the Mississippi River using Hg concentrations and Hg isotopic compositions in a lake sediment core, Lake Whittington, Mississippi, USA

“…Based on the concentration of Hg in the sediment samples, 0.200 to 1.0 g of sample was weighed and digested in concentrated HNO 3 and HCl (9:1) using a previously published method (Foucher and Hintelmann, 2006). In addition, 1 mL of concentrated H 2 O 2 was added to aid in the digestion of organic matter.…”

(Pre-) historic changes in natural and anthropogenic heavy metals deposition inferred from two contrasting Swiss Alpine lakes

“…It is meaningful to note that the geochemical composition of the heavy metals entrapped in sediment deposits, as produced by either anthropogenic or natural activities, differs according to their geological sources. Despite the recent analytic developments made in the field of Hg isotopes in natural samples (Foucher and Hintelmann, 2006;Gehrke et al, 2009), the current technical development does not allow to discriminate among different Hg sources (Bergquist and Blum, 2009;Kritee et al, 2009). Conversely, the isotopic composition of lead (Pb), which is unaffected by natural fluxes variability and by anthropogenic processes (e.g., smelting), consistently traces the changing sources of pollution (Sturges and Barrie, 1987;Veysseyre et al, 2001).…”

Sedimentary records of mercury stable isotopes in Lake Michigan

“…Hg) (Bergquist and Blum, 2009;Blum et al, 2014;Yin et al, 2014a). Technological advances in the past two decades [particularly multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS)] have enabled high precision analysis of Hg isotope ratios (Foucher and Hintelmann, 2006;Blum and Bergquist, 2007;Yin et al, 2010a). Laboratory experiments have observed both mass dependent fractionation (MDF) and mass independent fractionation (MIF) of Hg isotopes.…”