Chromium speciation analysis in raw and cooked milk and meat samples by species-specific isotope dilution and...

This study aimed at the assessment of the impact of various culinary processes on the fate of chromium (Cr) species (Cr(III) and Cr(VI)) in infant formula milk, semi-skimmed milk and bovine meat samples. The cooking procedures were boiling at 70°C/100°C (milk samples) and frying without and with oil (95°C and 120°C) (bovine meat). The levels of Cr(III) and Cr(VI) in raw and cooked samples were determined by high-performance liquid chromatography (HPLC) coupled to inductively coupled plasma mass spectrometry (ICP-MS) using double spike species-specific-isotope dilution (SS-ID). The species were extracted by sequential complexation of Cr(III) with ethylenediaminetetraacetic acid and of Cr(VI) with 1,5-diphenylcarbazide in the same analytical run by heating at 70°C for 50 min. Anion exchange chromatography using a Dionex IonPac™ AG7 column and a mobile phase consisting of 10 mM HNO₃, 2.5% MeOH and 30 mM EDTA at pH 2 was employed for species separation. The quantification limits were 0.013 and 0.049 µg kg^-1 for Cr(III) and Cr(VI), respectively. ANOVA test used to compare the mean Cr species concentrations showed no significant differences between raw and cooked samples. The results obtained in the present study show that oxidation of Cr(III) to Cr(VI) does not occur during thermal cooking of milk and bovine meat samples. A selection of 10 samples of each type were analysed in terms of total Cr (Cr_total) as well as speciation (Cr(III) and Cr(VI)). Cr(VI) was not quantified in any of these samples, whereas Cr(III) levels ranged from 0.22 (infant formula milk) up to 80 µg kg^-1 (chorizo sausage). Additionally, Cr(III) and Cr_total levels were comparable hence demonstrating that in the samples analysed in this study, Cr is found exclusively as Cr(III) species.