A new high pressure flow digestion system has been developed for sample digestion at a pressure of up to 40 bar and a temperature of about 230 C. The reaction with acids takes place in a PFA tube and is heated by microwave radiation in a multimode cavity. As the PFA tube cannot withstand the harsh digestion conditions without support, it is placed inside a coiled glass tube pressurized by 40 bar nitrogen thus forming an autoclave. Corrosion of system components by acid fumes and related sample contamination is circumvented by establishing a slow but steady flow of the high pressure nitrogen countercurrent to the sample flow. The presented system does not constrain the selection of the digestion reagent. Acid cocktails of nitric acid with hydrochloric and/or hydrofluoric acid as well as hydrogen peroxide were successfully used for the digestion of various samples. The method accuracy was validated with five certified reference materials (BCR 62, DORM-2, NIST SRM 1515, NIST SRM 1567, NIST SRM 1568) and good agreement between the determined and the certified values was obtained for Al, Ca, Cr, Cu, Fe, Mg, Mn, Ni, Pb, and Zn using inductively coupled plasma optical emission spectrometry (ICP-OES) for analyte quantification. The flow digestion of the CRMs resulted in clear solutions with residual carbon concentrations (RCC) between 11 and 40%. Spike recoveries of Al,

In inductively coupled plasma based techniques the signal enhancing effect of carbon on high ionization potential elements is not only caused by changes in the nebulization efficiency and charge exchange reactions, but also by increased plasma power density and the state of matter of the introduced carbon species.

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 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 HO with HNO increased the digestion efficiency of phenylalanine, and the residual carbon content (RCC) was around 50% when 6.0% V/V HCl or HO was used in combination with 32% V/V HNO. Juice samples were digested with 3.7 mol L HNO and 0.3 mol L HCl, and the RCC was 16 and 29% for apple and mango juices, respectively. Concentrated HNO (10.5 mol L) 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. Graphical abstract Application of a high-pressure microwave-assisted flow digestion system for fruit juice and milk sample preparation.