Determination of selenium (IV) at ultratrace levels in natural water samples by UV-assisted vapor generation - 'Collect and punch' - inductively coupled plasma mass spectrometry

A UV-assisted vapor generation combined with a sensitive 'collect and punch'-indtictively coupled plasma mass spectrometry (ICP-MS) method is described for the determination of Se(IV) at ultratrace levels in natural water samples. Volatile Se species were formed by UV irradiation in the presence of low molecular weight organic acids. The vapors were collected in a glass chamber before injection into the plasma in the form of a plug. This arrangement provided a 6-7 fold enhancement in sensitivity corny pared with continuous vapor injection. A detection limit (3 sigma) of close to 50 parts per trillion (0.05 mu g/L) was achieved. A precision of 1.2% (RSD, n=6) was obtained for a 0.5 mu g/L of Se(IV) solution. The method is specific for Se(IV) and was used for the determination of Se(IV) in tube well water samples of Punjab, India. The values obtained for Se(IV) by the UV assisted vapor generation method agreed well with the conventional HG-ICP-MS method. The concentration of Se(IV) ranged from 0.5-5.0 mu g/L in the water samples analyzed. However, the total selenium contents determined by HG-ICP-MS after reduction of Se(VI) to Se(IV) in many of these samples were greater than 50 mu g/L, demonstrating that the major species in these water samples are Se(VI).