verify that quantitative extraction is achieved.

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 I min-' cooling gas, 0.8 I min-' nebulizer gas, 0.6 I min-' intermediate gas and 0.5 ml min-' sample uptake rate. Arsenic, Se, Br and I showed linearity of more than 4 orders of magnitude. The detection limits (3s) were determined for 7 5 A ~, 78Se, 79Br and 1271, and were 0.05, 0.25, 0.30 and 0.07 pg I-', respectively. The influence of Na and K on the response of As was less than 20% up to concentrations of 1000 mg I-'.Analysis of 3% HCI causes an interference at a concentration of 1 pg I -' of As, analysis of 1000 mg I -' of Ca causes an interference at a concentration of 2 pg I-' of As. Both of these elements in the solution cause significant interference in the determination of 7 5 A ~, due to the formation of the 40Ca35CI 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 HNO,. 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 tetramethylammonium 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.