Chromium speciation analysis in bread and breakfast cereals
Researchers from France and Denmark now aimed at clarifying conflicting results for the presence of hexavalent chromium in bread and breakfast cereals. Using highly selective separation by HPLC coupled with ICP-MS detection and species-specific isotope dilution for quantification they found no Cr(VI) in any of their samples.
Chromium is highly abundant in the environment both because of industrial use but also natural processes. In the environment it exists mainly in two oxidation states Cr(III) and Cr(VI) which differ with respect to chemical properties and biological activities. While Cr(VI) is recognized as carcinogenic, Cr(III) has been considered to be beneficial for human health for a long time. In more recent time the beneficial effect of Cr(III) has been questioned for healthy people. Anyhow, Cr(VI) should not be present in common food for the health risks accompanied. While the European Food Safety Authority (EFSA) reported that Cr exists in food exclusively as Cr(III), some researchers reported the presence of Cr(VI) in some foodstuffs including bread (see the EVISA news below). We commented that such results call for a validation by highly selective and sensitive analysis. Four years later, a Slovenian group of researchers proved that the reported hexavalent chromium is just an artifact of a technique not sufficiently selective to differentiate between trivalent and hexavalent chromium.
The new study
Even when high performance liquid chromatography is used for the separation of species, Cr speciation analysis is challenged by many error sources. The most critical step for Cr speciation in food by LC-ICP-MS is the extraction of the species from the sample. The problem is related to the incomplete extraction efficiency and the transformation of species during the analysis. In order to address such problems, researchers from France and Denmark optimized the extraction step for bread and breakfast cereals. In order to avoid errors due to species transformation they used species-specific isotope dilution, an approach that can correct for such species transformation during analysis.
Photo: rye bread from Denmark
The method used by them is an adaptation and further development of an approach recently reported for simultaneous speciation analysis of Cr(III) and Cr(VI) in milk and bovine meat samples (Saraiva et al., 2021). Both species are stabilized during the analytical procedure by complexation. Cr(III) is complexed with EDTA while Cr(VI) is reduced to Cr(III) by 1,5-diphenylcarbazide (DPC) and subsequently complexed by the oxidized form of DPC, namely 1,5-diphenylcarbazone (DPCO). Both complexes are separated by an isocratic ion chromatography system using a short AG7 column. Limits of quantification obtained by the developed method were 14 ng/kg and 47 ng/kg for Cr(III) and Cr(VI) respectively. Cr(VI) was not detected in concentrations surpassing the quantification limit in any of the samples analyzed hence supporting the EFSA statement related to the absence of Cr(VI) in foods, due to the reducing power of such matrices.
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