Eliminating the Memory Effect for Correct Determination of Mercury by Inductively Coupled Plasma Mass Spectrometry

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: the memory effect distorts the results of determination of trace amounts of mercury. Experimental comparison of the effectiveness of the proposed in the published sources methods for removal of trace amounts of mercury for its correct determination by ICP-MS is conducted, and the optimum washing agent is chosen. 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, 1% nitric and hydrochloric acids, aqueous solution of ammonium pyrrolidine dithiocarbamate, are used as washing agents to clean the sample introduction system of an Agilent Technologies 7900 mass spectrometer. It is found that the background level of mercury content is achieved by using solutions of gold chloride (5%), thiourea (0.01, 0.1, 0.5%), L-cysteine (2%), and potassium bromide and bromate (0.5 mM) in 1.0% hydrochloric acid. The background is lowered by using 3 and 5% HCl, potassium dichromate (60 mg/L), potassium bromide and bromate (0.01 and 0.05 M) solutions in 1% hydrochloric acid. Application of an aqueous solution of ammonium pyrrolidinedithiocarbamate causes severe degradation of the plastic tubes of the sample introduction system. The efficiency of washing agents based on hydrochloric acid is higher when compared to nitric acid and water. The most promising washing agents are bromine-containing solutions and a solution of potassium dichromate in hydrochloric acid. The optimum way to clean the instrument appears to be its washing with a 0.5 mM solution of potassium bromide in 1.0% hydrochloric acid which makes it possible to reduce the intensity of the background signal of ²⁰²Hg isotope and does not challenge the determination of other elements.