Mass spectral and atomic absorption studies of selenium vaporization from a graphite surface

Selenium desorbs from a graphite surface primarily as SeO₂ which dissociates in the gas phase to form atomic Se with an appearance temperature of 1175 K in atmospheric GFAAS. Mass spectrometry studies in vacuum show that the SeO₂ desorption occurs at 425 K without the addition of any modifiers and that elemental Se does not desorb from the heated graphite flat.Vacuum MS studies show that the addition of Ni delays the appearance temperature of a significant amount of SeO₂ to 850 K. The SeO₂ lost at this temperature does not contribute to the Se absorbance signal which has a delayed appearance in atmospheric GFAA of 1575 K with the addition of Ni. Above 850 K, the remaining SeO₂ is attached to NiO which has formed on the surface. Then, NiO is reduced by the carbon forming CO and CO₂ which desorb at 1200 K while elemental Ni remains in the carbon. At the same time, SeO₂ is released from NiO and desorbs from the graphite in vacuum as SeO₂, SeO, Se and Se₂. Some reduction of SeO₂ on the surface has occurred at this high temperature. These Se species dissociate in the high temperature gas phase atmosphere of GFAA forming elemental Se which is then detected as a delayed Se AA signal. Much of the SeO₂ desorbed at low temperatures can be lost through diffusion from the furnace even when Ni is added, if the gas phase is not preheated.XPS and SEM studies confirm the existence of NiO on the surface at 1100 K with the remaining Se dispersed in the Ni so that it is not observable. Elemental Ni desorbs from the graphite at 1675 K. in vacuum MS, corresponding to the Ni signal observed in atmospheric GFAA. Following high temperature cleaning steps, elemental Ni is detected by XPS and MS to remain in the graphite. This amount of Ni is not significant enough to contribute to the Ni AA signal, nor does the elemental Ni affect the Se AA signal.An oxygenated surface, like that produced by the addition of Ni, also causes the appearance of the Se AA signal in atmosphere to be delayed to a higher temperature. The signal is enhanced, although not to the extent of that with the addition of Ni. SEM data show SeO_2(s,1) congregating at active sites on the oxygenated surface following a dry step. Under vacuum conditions, the desorption of SeO₂ from an oxygenated surface was not delayed. This indicates that atmospheric pressure is necessary to allow mobile SeO₂ species to move to oxygenated active sites which then delay the release. The mobile SeO₂ species are desorbed from the surface under vacuum conditions before reaching the active sites.