Application of microwave digestion to trace organoelement determination in water samples

Abstract

Microwave digestion is a powerful tool for converting various types of samples into a solution ready for instrumental measurement. In trace analysis for heavy metals in water and waste water it may show some limitations due to the comparatively high dilution of the digestion reagent by the water sample itself. This is proved in this study with organically bound elements to be analyzed, after digestion, by cold vapor or hydride generation atomic absorption spectrometry, because these techniques require the elements to be completely mineralized. The experiments were carried out with 2-[(ethylmercury)thio]benzoic acid, d,l-selenomethionine and tetraphenylarsonium chloride using microwave digestion devices allowing high- and low-pressure digestion. The selenium compound yielded satisfactory results with both digestion modes; in contrast the arsenic compound yielded very low recoveries under all test conditions applied. The mercury compound gave good recoveries only with the high-pressure digestion device. Low-pressure digestion revealed significantly lower results. High concentrations of organic matter may lead to incomplete digestion and low results because of the development of high carbon dioxide pressure which causes the feed-back system of the microwave device to reduce microwave power and, hence, temperature in the batch. Also the digestion mixture may play an important role as shown by the organoarsenic compound. The quoted limitations should be taken into account in water analysis particularly if it is intended to incorporate microwave digestion in standardized procedures.