ELUCIDATION OF MECHANISMS THAT CONTROL THE ELECTROTHERMAL ATOMIZATION OF TIN CHLORIDE

Mechanisms that control the electrothermal atomization of SnCl2 were investigated by monitoring real-time mass spectra of gaseous species generated in pyrolytic graphite coated graphite furnaces during the atomization cycle. Vacuum and atmospheric pressure vaporization were used to help differentiate between homogeneous gas-phase and condensed-phase interactions. During vacuum vaporization from tube atomizers, free Sn was not observed and the only molecular species detected were SnO(g) and SnCl2(g). During atmospheric pressure vaporization, the observed molecular species consisted primarily of SnCl2(g) and SnO(g). Free Sn, as determined by mass spectrometry, was absent from the centre of the atomizer. Free Sn was present, however, as determined by atomic absorption spectrometry. This suggests that atomization occurs at the regions of lowest temperature in the atomizer, i.e., the end regions. The thermal decomposition of homogeneous gas-phase species cannot account for the formation of free Sn because of the paucity of Sn(g) in the centre of the atomizer. However, SnO(g) interactions with the cooler surface regions of the graphite atomizer do explain these data. Desorption of the resulting Sn(ad) can then account for the free Sn that was observed as the atomizer temperature increased. This argument is supported by mass spectrometric results acquired when vaporizing at atmospheric pressure from capillary-like graphite cups that were heated to exhibit temperature gradients along their lengths.