Determination of ultra-trace amounts of selenium (IV) by flow injection hydride generation atomic absorption spectrometry with on-line preconcentration by co-precipitation with lanthanum hydroxide .2. On-line addition of co-precipitating agent

A flow injection procedure for the determination of ultra-trace amounts of selenium(IV) is described, which combines hydride generation atomic absorption spectrometry (HGAAS) with on-line preconcentration of the analyte by co-precipitation-dissolution in a filterless knotted Microline reactor. Based on a previously published procedure that requires the off-line premixing of sample and co-precipitating agent, the present approach facilitates on-line addition of the co-precipitant to the time-based aspirated sample. The sample and the coprecipitating agent (lanthanum nitrate) are mixed on-line and merged with an ammonium buffer solution of pH 9.1, which promotes precipitation and quantitative collection on the inner walls of an incorporated knotted Microline reactor. The Se-IV preconcentrated by coprecipitation with the generated lanthanum hydroxide precipitate is subsequently eluted with hydrochloric acid, allowing an ensuing determination via hydride generation. At different sample flow rates, i.e., 4.8, 6.4 and 8.8 ml min(-1), enrichment factors of 30, 40 and 46, respectively, were obtained at a sampling frequency of 33 samples h(-1). The detection limit (3s) was 0.005 mu g l(-1) at a sample flow rate of 6.4 ml min(-1) and the precision (relative standard deviation) was 0.5% (n = 11) at the 0.1 mu g l(-1) level.