Preparation of Cr(VI) and Cr(III) isotopic spike solutions from 50Cr and 53Cr enriched oxides without the use of oxidizing and/or reducing agents

Abstract

The use of enriched stable isotopes as tracers in speciation procedures by ion-exchange chromatography coupled to ICP-MS enables to follow the oxidation-reduction processes of Cr. The most commonly available Cr stable isotopes are (50)Cr and (53)Cr enriched oxides or metallic Cr. For application of Cr enriched stable isotopes, adequate preparation of isotopic spike solutions is necessary. To ensure that Cr species present in the sample investigated are not compromised, no excess of the reducing neither oxidizing agents should remain in the isotopic spike solutions. Cr(VI) isotopic solutions are mostly prepared by dissolving of Cr oxide in HClO(4), followed by the addition of ammonia and H(2)O(2) to quantitatively oxidize Cr, while the excess of H(2)O(2) is removed by boiling or UV irradiation. If traces of H(2)O(2) still remains, such isotopic spike solution may cause artefacts in Cr speciation in the sample investigated. In the present work, new procedure based on alkaline melting of (50)Cr enriched oxide for preparation of pure (50)Cr(VI) spike solution was developed. Cr(III) was quantitatively oxidized to Cr(VI) with air oxygen without use of other oxidizing agents. Moreover, the microwave assisted digestion procedure of (53)Cr enriched oxide was applied for preparation of (53)Cr(III) spike solution without use of reducing agents. The purity of (50)Cr(VI) and (53)Cr(III) isotopic spike solutions was verified by the speciation analysis applying hyphenation of anion-exchange FPLC to ICP-MS. Speciation analysis demonstrated suitability of the proposed procedures for preparation of Cr isotopic spike solutions. In addition, the artefacts in Cr speciation, which may be initiated by traces of oxidizing and/or reducing agents present in Cr spike solutions, were demonstrated. The outcomes of our investigation highlighted the importance of the adequate preparation of spike solutions of Cr isotopes that may be used as reliable tracers in the investigations of the oxidation-reduction processes of Cr in wide range of environmentally relevant pH values.