Eliminating the «memory effect» during mass spectrometric determination of mercury

Abstract

When estimating mercury content by inductively coupled plasma mass spectrometry (ICP-MS), this element adsorbs on the surface of the sample introduction system of the instrument, which creates problems in the determination of trace amounts of mercury. Various methods of cleaning the instrument have been proposed in the literature. Objective: to experimentally compare the effectiveness of trace mercury removal methods proposed in the literature for elemental analysis by ICP-MS and to select the optimal cleaning agent. Methods: Mercury content in solution was determined by the Agilent Technologies 7900 inductively coupled plasma mass spectrometer using 202Hg isotope. Diluted solutions of nitric and hydrochloric acids, solutions of gold chloride, potassium dichromate, thiourea, L-cysteine, potassium bromide, potassium bromate of different concentrations in water and 1% nitric and hydrochloric acids, aqueous solution of ammonium pyrrolidinedithiocarbomate were used as washing agents. Results: The background level of mercury content is achieved by using solutions of copper chloride (5%), thiourea (0.01, 0.1, and 0.5%), L-cysteine (2%), potassium bromide and bromate (0.0005 M) in 1.0% hydrochloric acid solution. The background is lowered by using 3 and 5% HCl, potassium dichromate (60 mg/liter), potassium bromide and bromate (0.01 and 0.05 M) solutions in 1% hydrochloric acid. Application of aqueous solution of ammonium pyrrolidinedithiocarbamate causes sharp degradation of plastic tubes of the sample introduction system. Conclusion: The efficiency of washing agents based on hydrochloric acid is higher than that based on nitric acid and water. The most promising washing agents are bromide-containing solutions and potassium bichromate solution in hydrochloric acid. The optimal way to clean the device is its washing with 0.5 mM potassium bromide solution in 1.0% hydrochloric acid. Its use allows to reduce the background content of mercury and does not complicate the analysis of the content of other elements.