An improved digestion coil arrangement for high-pressure microwave-assisted flow digestion

Abstract

A high pressure flow digestion system for microwave assisted sample digestion at a pressure of 40 bar and a digestion temperature of about 230 °C has been developed. The sample mineralization took place in a 2 mm inner diameter PFA digestion tube heated by microwave radiation in a pressurized microwave applicator. By employing computer simulation of the microwave field, the position and geometry of the coiled perfluoralkoxy (PFA) digestion tube was optimized. Thereby, a uniform absorption of the microwave radiation over the length of the digestion tube was attained, whereby the formation of hot- and cold spots was avoided. The high-pressure flow digestion system had a heated volume of 22 mL and was operated at 500 W microwave power and a carrier flow rate of 5 mL min⁻¹. Digestion coil and connections were made of PFA allowing any kind of digestion acid mixture. Acid mixtures of nitric acid with hydrochloric and/or hydrofluoric acid were successfully used for sample digestion. The accuracy was evaluated with three certified reference materials (NIST SRM 1547 – peach leaves, IAEA-A-13 – animal blood and BCR-185R – bovine liver) digested with a mixture of 5 mL 6 mol L⁻¹ HNO₃ and 3 mol L⁻¹ HCl. After 5 minutes sample mineralization in the high-pressure flow digestion system, the residual carbon concentration in these digests was <50 mg L⁻¹. The agreement between the determined and the certified values ranged from 90–110% for Al, As, B, Ba, Ca, Cu, Fe, K, Mg, Mn, Na, Rb, Sr, and Zn using inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) for analyte quantification. For Fe in SRM 1547 and Cd in BCR-185R inferior agreement of 86% and 121% with the certified values was encountered. The sample throughput of the fully automated system was 12 samples per hour.