Mikrochim.Acta 119, 233-241 (1995) Mikrochimica Acta 9 Springer-Verlag 1995 Printed in Austria Comparison of Three Digestion Methods for the Determination of the Aqua Regia Soluble Content of Lead, Cadmium and Chromium in Sewage Sludges by ETAAS Nikolaos S. Thomaidis, Efrosini A. Piperaki, and Panayotis A. Siskos* Laboratory of AnalyticalChemistry,Department of Chemistry,Universityof Athens, Panepistimiopolis, 15771 Athens, Greece Abstract. A procedure for the determination of the aqua regia soluble content of lead, cadmium and chromium in a sewage sludge reference material (CRM 145R) by electrothermal atomic absorption spectrometry (ETAAS) is described. A com- parison of the dissolution procedure proposed in the certification report, to an oven-assisted digestion and a proposed microwave digestion procedure is per- formed. In the ETAAS method developed, 1 gg of Pt proved to be an appropriate modifier for each of the above heavy metals. Possible sources of error at each analytical step are addressed. The metal contents obtained with the proposed method are in a good agreement at 95% significance level with the certified values given for CRM 145R. Key words: sewage sludge, trace metals, microwave digestion, electrothermal atomic absorption spectrometry, chemical modifiers. The determination of heavy metals in sewage sludges is of great importance because these materials can be used as fertilizers in agriculture, provided that the content of hazardous substances is low [1]. Hence, monitoring of the concentration of metals such as lead, cadmium and chromium in such samples is an everyday practice for many laboratories. The most commonly used techniques for these studies are atomic absorption spectrometry with flame (FASS) or electrothermal (ETAAS) atomization and induc- tively coupled plasma-atomic emission spectrometry (ICP-AES) or -mass spec- trometry (ICP-MS). These techniques have recently been compared for the determination of metals in acid digests of solids [2]. Most analytical techniques require the sample in a liquid form. The conventional wet digestion procedures involve heating of the sample for some hours with several mixtures of acids and oxidizing agents such as HNO3/H20 2 (EPA method 3050) [3], * To whom correspondence should be addressed