Speciation analysis of Cr(III) and Cr(VI) in bread and breakfast cereals using species-specific isotope dilution and HPLC-ICP-MS

This study reports the development and validation of a analytical approach for simultaneous Cr(III) and Cr(VI) speciation analysis in bread and breakfast cereals using species specific isotope dilution (SS-ID) and high-performance liquid chromatography (HPLC) coupled to inductively coupled plasma-mass spectrometry (ICP-MS). The species were extracted by sequential complexation of genuine Cr(III) with ethylenediaminetetraacetic acid (EDTA) and subsequently of Cr(III) originating from reduction of Cr(VI) with 1,5-diphenylcarbazone (DPCO) in the same analytical run. Efficient HPLC separation of Cr(III)-EDTA and Cr(III)-DPCO complexes was carried out by anion exchange HPLC in less than 3 min. The method was validated by means of the accuracy profile approach by carrying out 3 measurement series in duplicate on 5 different days over a timespan of one month. The quantification limits were 0.014 μg kg−1 for Cr(III) and 0.047 μg kg−1 for Cr(VI), respectively. The measurement bias ranged from 0.31 to 0.49 %, whereas the coefficient of variation in terms of repeatability (CVr) varied from 1.3–4.4 % for Cr(III) and from 0.6–7.9% for Cr(VI). Similarly, the coefficient of variation in terms of intermediate reproducibility (CVR) ranged from 1.3–4.4% for Cr(III) and from 2.0–8.9% for Cr(VI), respectively. The method was successfully applied to the analysis of a selection of bread and breakfast cereals samples. Cr(VI) was not detected in any of these samples while Cr(III) levels ranged between 5.2 and 176 μg kg−1 for bread samples and between 23.8 and 350 μg kg-1 for breakfast cereals. These results were comparable with the levels of total Cr analysed in the same samples by ICP-MS. The method developed in this study on the basis of SS-ID and sequential species complexation is a powerful analytical tool for accurate and precise quantification of Cr(III) and Cr(VI) at trace levels and allows for correction of any species interconversion during sample preparation.