A critical overview of Cr speciation analysis based on high performance liquid chromatography and spectrometric techniques
The role and impact of chromium (Cr) on the environment and living organisms depends primarily on its chemical form. High toxicity of hexavalent Cr is well documented, while trivalent Cr is an essential micronutrient. In the last decades numerous analytical procedures have been developed for the determination of Cr(VI) in different sample matrices. To obtain reliable speciation data it is important to preserve species integrity during the sample storage, pre-treatment, extraction and the determination of Cr species. Among different speciation methods combination of high performance liquid chromatography (HPLC) with atomic spectrometry techniques provides comprehensive information on the presence of Cr species in a variety of sample matrices, while hyphenation of HPLC to inductively coupled plasma mass spectrometry (ICP-MS) represents the most powerful and the most sensitive analytical tool for Cr speciation. Precise isotope ratio measurement enables the application of isotope dilution techniques for the quantification of trace amounts of individual Cr species in various environmental and biological samples. Furthermore, enriched stable isotopes can be introduced as tracers to investigations on the fate and role of Cr in the environment and living organisms or to monitor the species transformation during the analytical procedure. Despite general understanding of Cr chemistry, which is closely related to its trivalent and hexavalent oxidation states and knowledge on conditions that may influence species transformation, there are still open questions that should be addressed to obtain reliable speciation analysis data. So, this article is focused mostly to overview recent developments in methodological approaches for Cr speciation in different sample matrices by applying HPLC and spectrometric techniques. Different procedures for preparation of isotopically enriched Cr spike solutions are critically evaluated. The advantages of their use as tracers to follow and account for species transformation during sample preparation and for the quantification of Cr species by HPLC-ID-ICP-MS are discussed. The importance of the use of adequate analytical methodologies and speciation analysis in the determination of Cr(VI) is highlighted in order to avoid inadequate conclusions to be made based on wrongly applied analytical methodologies. An increasing need to develop speciation procedures for selective determination of Cr(III) species is also emphasized.