Determination of ultratrace dissolved arsenite in water - selective coprecipitation in the field combined with HGAFS and ICP-MS measurement in the laboratory

Because stabilization of arsenite in water samples during transit and stora ge is troublesome, this work deals with a method to prevent this by on-site selective coprecipitation of arsenite with dibenzyldithiocarbamate and rec overy of the coprecipitate by filtration through a 0.45-mum membrane filter . In the laboratory arsenic on the filter is quantitatively released by oxi dation of arsenite to arsenate with H2O2 (6%) in alkaline medium (8 mmol L- 1 NaO) at elevated temperature (85 degreesC) for 30 min followed by ultratr ace determination by routine HGAFS and ICP-MS. It is shown that arsenate co ntamination of the coprecipitate is so low that arsenate concentrations thr ee orders of magnitude higher than the arsenite concentration do not interf ere; this is essential, because arsenate is usually the dominant arsenic sp ecies in water. Because significant preconcentration can be achieved in the solution obtained from the leached filter (normally a factor 20 but easily increased to 100) very low detection limits can be obtained (only limited by the purity of the materials and the cleanliness of working); a realistic limit of determination is 0.01 mug L-1 arsenite. The procedure was used fo r the determination of arsenite in two ground waters from an ash depository site in the Salek valley (Slovenia). The matrix contained some elements at very high levels but this did not impair the efficiency of arsenite coprec ipitation. The results obtained by use of HGAFS and ICP-MS were not signifi cantly different at the 5% level for sub-mug L-1 arsenite concentrations.