Direct Chromium Speciation Using Thermospray: Preliminary Studies with Inductively Coupled Plasma Mass Spectrometry

Abstract

Thermospray (TSP) sample introduction methods have been found to selectively deposit Cr(III) in the presence of Cr(VI) and thus provide a direct, nonchromatographic speciation capability for these chromium oxidation states. Previous results have shown that the sensitivity for Cr(III) could be reduced to a negligible level compared with the sensitivity for Cr(VI) by adjusting sample and thermospray operating parameters, with detection by inductively coupled plasma atomic emission spectrometry (ICP-AES). Hence a rapid, sensitive chromium speciation method with limits of detection (LODs) at the 0.5 ng/ml level was developed. In this paper, application of this approach to inductively coupled plasma mass spectrometry (ICP-MS) was intended to further reduce the LODs for Cr detection and further elucidate the kinetics and optimum conditions for efficient, quantitative Cr(III) deposition. These preliminary results show that for Cr(III) concentrations below 10 ng/ml, LODs and accurate, selective measurements of Cr(VI) down to the 50 pg/ml are possible. However, the deposition process of Cr(III) is incomplete for Cr(III) concentrations above 10 ng/ml under the currently established conditions, restricting the sub-nanogram-per-milliliter accuracy of the selective detection of Cr(VI) with these higher concentrations of Cr(III). The results also suggest that the process is not kinetically limited or at equilibrium in the present configuration.