Fate of free selenium atoms in externally heated quartz tube atomizers for hydride generation atomic absorption spectrometry and their reatomization at tube ends studied by means of the determination of longitudinal free atom distribution

Abstract

The longitudinal distribution of Se free atoms in externally heated quartz tube hydride atomizers was studied under analytical conditions, i.e. selenium concentrations of 10–100 μg l −1 and argon flow rates of 25–400 ml min −1. The selenium signal was measured through the walls of the optical tube with the tube perpendicular to the radiation beam at various positions along the tube length. The highest concentration of free atoms was found near the middle of the optical tube, where atomization proceeds. Under typical analytical conditions, all free atoms disappear before they reach the ends of the atomizer. The free atom decay rate is greatly enhanced at higher analyte concentrations. This is a major source of calibration graph curvature. The atomization efficiency was influenced much less. The presence of zones where once reacted free atoms are reatomized at the optical tube ends was proven by the spatially resolved observation. Using the measures originally meant to prevent flame ignition on the tube ends, such as quartz windows or prolonged unheated parts of the tube, no reatomization takes place. The effect of reatomization on the analytical performance of the atomizer was shown: the sensitivity is improved and the non-linearity of the calibration graph is substantially reduced, while the baseline noise remains at the same level. Recommendations concerning the quartz tube atomizer design and dimensions are given.