The effect of hydride formation on instrumental mass discrimination in MC-ICP-MS: a case study of mercury (Hg) and thallium (Tl) isotopes

Abstract

Here we report evidence for enhanced hydride formation for Hg isotopes when using high sensitivity X skimmer cones and the Neptune MC-ICP-MS. The effect on the instrumental mass discrimination and the robustness of the Tl external mass-bias 20 correction was investigated. A series of experiments have been conducted to study the effect of variable Hg concentrations on the measured mass bias correction factor, ! Tl .The results of our experiments show a linear relation between the Hg concentration and the measured 205 Tl/ 203 Tl ratio of the internal standard. The resulting mass-bias correction factor ! Tl thus appears to be dependent on the analyte concentration, 25 questioning the applicability of Tl as an internal standard for high precision Hg isotope ratio measurements. These observations are consistent with the formation of Hg hydrides (HgH x where x = 1, 2). Numerical simulations show Hg hydride formation rates of 0.02 to 0.18% to be consistent with experimental observations, when using high-sensitivity X skimmer cones. Here we discuss the implications of Hg 30 hydride formation for Hg isotope ratio measurements and propose a set of precautionary steps that can help to identify and remediate analytical issues related to hydride formation. Whilst the effect of hydride formation on mass-dependent Hg isotope ratios can be controlled, mass independent Hg ratios are shown to be very sensitive to hydride formation. A comparison between different types of sediment 35 samples and standards also suggests a matrix dependency on hydride formation. We conclude that precise and accurate Hg isotope ratio determination using MC-ICP-MS is possible, however, probably not to low ppm-levels of precision.

Application of an Isotope Binary Mixing Model for Determination of Precise Mercury Isotopic Composition in Samples with Low Mercury Concentration

“…With the development and coupling of sample introduction systems to MC-ICPMS analysis, the volume and mercury concentration required for analysis by MC-ICPMS was reduced to 5–10 mL and 0.5–3.0 ng/mL, respectively. − However, environmental mercury concentrations are on the order of 1–100 pg/m 3 and 0.3 ng/L in reactive gaseous mercury (RGM) and marine seawater, respectively, and required large sample volumes for successful isotopic measurement. In order to reduce the large sample volume requirements, it was necessary to either directly improve the δ 202 Hg instrument signal or indirectly measure the mercury isotopic composition in these low mercury samples.…”

The River Runs Through It: the Athabasca River Delivers Mercury to Aquatic Birds Breeding Far Downstream

“…Details regarding mercury isotope analysis have been reported previously [33]. Mass-204 independent fractionation (MIF) of Hg isotopes is unrelated to isotope mass and is reported in 205 capital delta (Δ) notation.…”

Sensitivity and High-Precision Lead Isotopic Analysis by Multicollector Inductively Coupled Plasma Mass Spectrometry Based on Liquid Spray Dielectric Barrier Discharge Plasma-Induced Chemical Vapor Generation

“…Therefore, although it has been proved that Tl can be used to correct the MDF of Pb in LSDBD-CVG with sufficient precision, the small MIF effects may contribute to the relatively poor precision of the 207 Pb /204 Pb. Additionally, the Pb-hydride formation and analyte masses overlap in LSDBD-CVG, which may also lead to the poorer precision of the 207 Pb /204 Pb ratio …”

“…Additionally, the Pb-hydride formation and analyte masses overlap in LSDBD-CVG, which may also lead to the poorer precision of the 207 Pb /204 Pb ratio. 42 Isotopic Determination of Reference Materials and Geological Samples. To validate the accuracy of the proposed method, 208 Pb /204 Pb, 207 Pb /204 Pb, and 206 Pb /204 Pb isotope ratios of two basalt reference materials (BCR-2 and BHVO-2) were determined.…”