Remarks on the Interference of Sulfate in the Determination of Selenium by Electrothermal Atomic Absorption Spectrometry

Abstract

The determination of selenium by electrothermal atomic absorption spectrometry (ETAAS) still remains a very difficult task.1 This reason, together with the beneficial and toxicological roles of this element in living systems, has forced many researchs to use selenium ETAAS. The main problems with selenium ETAAS are related to its high volatility and subsequent losses during the ashing step2, as well as to spectral interference from different chemical species.3 In this respect, studies have been performed in order to overcome these troublesome problems. Thus, developments in both instrumentation and analytical methodology4 have shown important improvements in the determination of selenium by ETAAS in complex matrices. In spite of these efforts, several interfering problems remain unclear5'6, the interference of sulfate6_g being one of the most important. Although some papers have dealt with this question, different interpretations were derived. Thus, the expulsion of selenium atoms together with a violently volatilized matrix early in the atomization stage causes the loss of selenium due to formation and volatilization of Se02 in the presence of an oxidizing SO3 matrix in the ashing stage; also spectral interference as a result of the absorption of light at the selenium analytical wavelength by some metal sulfides formed by a gas-phase reaction under reducing conditions (argon atmosphere)9 is another usual explanation. In this paper, we try to explain the vaporization/ decomposition processes of some metal sulfates in the graphite tube during the atomization of selenium. A spectral interfering effect due to the formation of gasphase sulfur species during the atomization step of selenium is assumed.