Evaluation, mechanism and application of solid-phase extraction using a dithiocarbamate resin for the sampling and determination of mercury species in humic-rich natural waters

Abstract

A method for the simultaneous determination of mercury species at sub-ng l^–1 levels in humic-rich natural waters was evaluated. Methyl- and inorganic mercury were preconcentrated from a freshly collected humic-rich water sample following batchwise addition of pre-cleaned dithiocarbamate resin. The sample was shaken for 8–22 h, allowing mercury complexed by the humic substances to be transferred to and enriched on the resin. A mathematical model describing the recovery as a function of shaking time was developed and the model derived indicated that the rate-determining step for collection of mercury species is the release of these species from two major groups of sites present on the humic substances. The dithiocarbamate resin was collected by filtration and transferred to a column which was then installed in a closed flow injection system where the mercury species were eluted with an acidic thiourea solution. Next, the mercury species were extracted into 500 µl of toluene as diethyldithiocarbamate complexes and butylated with a Grignard reagent. The butylated forms were injected (⩽13 µl) on to a gas chromatograph equipped with a non-polar capillary analytical column, separated and detected at 253.7 nm by atomic emission spectrometry after excitation in a microwave-induced helium plasma. The recovery of methylmercury was 82.6 ± 5.6% at a dissolved organic carbon (DOC) concentration of 44 mg l^–1, and increased to over 92.5% at lower DOC levels. For inorganic mercury, the recovery was poorer, 45.8 ± 3.3% at a DOC concentration of 12 mg l^–1. The detection limits for the methyl- and inorganic mercury were 0.04 and 0.28 ng l^–1(3s criterion), respectively, for a 1 l sample, the latter being limited by the poorer recovery and contamination problems. The accuracy of this mercury speciation method was assessed, in part, by means of a laboratory intercomparison and use of an independent analytical technique.