Mechanised flow system for on-line microwave digestion of food samples with off-line catalytic spectrophotometric determination of cobalt at ng l–1 levels

Abstract

A mechanised system for on-line slurry food sample digestion was developed and an off-line cobalt determination was performed. The stabilised slurry sample was introduced into an air carrier stream until reaching the digestion coils located inside a household microwave oven. Software written in Visual Basic 3.0 was developed to permit the transport of the slurry samples and the programming of the microwave oven and also the control of the mineralization valve. The proposed system was optimized for determination of cobalt in certified samples such as mussels, bovine liver and fish and also uncertified fish samples. The digestion parameters were established as 3 mol l^–1 HNO₃ for mussels, 3 mol l^–1 HNO₃ plus 0.16% v/v H₂O₂ for bovine liver and 12 mol l^–1 HNO₃ for fish employing maximum power for 5 min of microwave actuation. In the subsequent spectrophotometric method for the catalytic determination of cobalt, the Tiron and hydrogen peroxide concentrations were 1.8 × 10^–3 and 3.0 × 10^–4 mol l^–1, respectively, and the sample residence time was 300 s as determined by an optimisation process. The proposed method features a linear range from 10 to 200 ng l^–1 Co (r > 0.996) with detection and quantification limits of 1.7 and 5.5 ng l^–1 Co, respectively. The precision, expressed as RSD, was 2.4% (n = 12) for repeatability and 5.2% (n = 10) for reproducibility and the accuracy of the proposed method was assessed by using certified samples and an alternative technique (ETAAS).