Performance evaluation of a high-pressure microwave-assisted flow digestion system for juice and milk sample preparation

Acid digestion is usually required for metal determination in food samples. However, this step is usually performed in batch mode which is time consuming, labor intensive, and may lead to sample contamination. Flow digestion can overcome these limitations. In this work, the performance of a high-pressure microwave-assisted flow digestion system with a large volume reactor was evaluated for liquid samples high in sugar and fat (fruit juice and milk). The digestions were carried out in a coiled perfluoroalkoxy (PFA) tube reactor (13.5 mL) installed inside an autoclave pressurized with 40 bar nitrogen. The system was operated at 500 W microwave power and 5.0 mL min⁻¹ carrier flow rate. Digestion conditions were optimized with phenylalanine, as this substance is known to be difficult to digest completely. The combinations of HCl or H₂O₂ with HNO₃ increased the digestion efficiency of phenylalanine, and the residual carbon content (RCC) was around 50% when 6.0% V/V HCl or H₂O₂ was used in combination with 32% V/V HNO₃. Juice samples were digested with 3.7 mol L⁻¹ HNO₃ and 0.3 mol L⁻¹ HCl, and the RCC was 16 and 29% for apple and mango juices, respectively. Concentrated HNO₃ (10.5 mol L⁻¹) was successfully applied for digesting milk samples, and the RCCs were 23 and 25% for partially skimmed and whole milk, respectively. Accuracy and precision of the flow digestion procedure were compared with reference digestions using batch mode closed vessel microwave-assisted digestion and no statistically significant differences were encountered at the 95% confidence level. [Figure not available: see fulltext.].