Trace analysis in high matrix aqueous solutions using helium microwave induced plasma mass spectrometry

Abstract

The practical aspects of a He microwave induced plasma (MIP) mass spectrometry system for the determination of As, Se, Br and I in biological samples were investigated. A prototype instrument with a toroidal helium MIP coupled to a quadrupole mass spectrometer was used. Liquid samples were analysed by pneumatic nebulization with a Meinhard nebulizer without desolvation. The plasma was operated at 380 W with 3.6 l min^–1 cooling gas, 0.8 l min^–1 nebulizer gas, 0.6 l min^–1 intermediate gas and 0.5 ml min^–1 sample uptake rate. Arsenic, Se, Br and I showed linearity of more than 4 orders of magnitude. The detection limits (3s) were determined for ⁷⁵As, ⁷⁸Se, ⁷⁹Br and ¹²⁷I, and were 0.05, 0.25, 0.30 and 0.07 µg l^–1, respectively. The influence of Na and K on the response of As was less than 20% up to concentrations of 1000 mg l^–1. Analysis of 3% HCl causes an interference at a concentration of 1 µg l^–1 of As, analysis of 1000 mg l^–1 of Ca causes an interference at a concentration of 2 µg l^–1 of As. Both of these elements in the solution cause significant interference in the determination of ⁷⁵As, due to the formation of the ⁴⁰Ca³⁵Cl polyatomic ion. Biological reference materials (liver, muscle, milk, flour, diet and fish) and different fish samples were analysed for As and Se after high-pressure ashing with HNO₃. Bromine was found to be a good internal standard for these elements. The results obtained with the He-MIP showed good agreement with the certified values for most of the materials investigated. Bromine and I were determined after alkaline digestion with tetramethylam-monium hydroxide. The determination of Br was complicated by severe memory effects. The results obtained for the few available reference materials showed acceptable agreement with the certified values.