Ultra-trace determination of mercury in water by cold-vapor generation isotope dilution mass spectrometry

Abstract

Determination of total Hg at the ppt level in water using cold vapor generation and inductively coupled plasma mass spectrometry for calibration by isotope dilution (ID) with both sector field (SF-ICP-MS) and time-of-flight MS (ICP-TOF-MS) detection is described. Samples were reduced on-line in a flow injection manifold with SnCl₂ to generate Hg vapor for both steady-state and transient sample introduction following gold trapping. No significant difference in final Hg concentrations was detected using either approach. Certified reference material ORMS-3 river water (NRCC, Ottawa, Canada) was used for method validation. Mercury concentrations of 12.75 ± 0.24 and 12.34 ± 0.11 pg g⁻¹ (mean and one standard deviation, n = 4) were obtained using steady-state generation with SF-ICP-MS and ICP-TOF-MS, respectively, in agreement with the certified value of 12.6 ± 1.1 pg g⁻¹ (combined uncertainty, k = 2). Concentrations of 12.55 ± 0.41 and 12.65 ± 0.30 pg g⁻¹ (one standard deviation, n = 4) were obtained following gold trapping and SF-ICP-MS and ICP-TOF-MS detection, respectively. Compared with SF-ICP-MS, a 2-fold enhancement in the precision of the measured ²⁰⁰Hg/²⁰¹Hg and ²⁰²Hg/²⁰¹Hg ratios in a 100 ppt Hg standard solution was obtained using ICP-TOF-MS. Consequently, improved precisions of 0.87–2.33% RSD in Hg concentrations obtained by steady-state and transient signal acquisition, respectively, in ORMS-3 were obtained using ICP-TOF-MS, compared with 1.88–3.25% RSD obtained with SF-ICP-MS. Method detection limits (LODs, three times standard deviation) of 0.024 and 0.15 pg g⁻¹ for Hg using direct CV and following gold trapping, respectively, were obtained using SF-ICP-MS. These were superior to corresponding LODs of 0.30 and 0.87 pg g⁻¹ obtained using ICP-TOF-MS as a result of its 15-fold lower sensitivity.