Determination of chromium in feeds by automated microwave digestion and atomic absorption spectrophotometry

Chromium, as trivalent organic form, has been recently proposed as feed additive to improve animal welfare and the quality-quantity of meat productions. To control its use in animal production we developed an analytical method that consisted of sample digestion by a commercially available focused microwave system and, subsequently, metal determination by graphite furnace atomic absorption spectrophotometry. The mineralization step was performed by treating the samples with a nitric acid-sulphuric acid-hydrogen peroxide mixture. Complete sample digestion was achieved in 26 min, as compared to 8-12 h or more required by dry ashing. To determine method accuracy and precision, we analyzed both feeds spiked at different Cr concentrations and a reference plant material. We found that mean metal recoveries ranged from 98.7 to 102.7% and for the reference material we obtained an excellent agreement between calculated and certified value. Quantitative Cr yields and relative coefficients of variation between 0.77 and 9.7% were very good and accounted for method accuracy and reliability. Finally, we applied the developed method to Cr analysis of some commercial feeds and the results were compared with those obtained by dry-ashing procedure.