Abstract Cobalt in sludge of domestic and industrial ori- gin, with high iron contents (> 17 g/kg), was determined by slurry sampling graphite furnace atomic absorption spectrometry (GF-AAS). Slurries prepared by ultrasonic stirring were adequately diluted to cover the variation in cobalt content in the sludge samples. The diluent was 5% HNO 3 . Standard atomisation conditions for cobalt deter- mination were used and no matrix modifier was applied. Slurry sampling GF-AAS results in the sludge were veri- fied by analysing totally digested samples by inductively coupled plasma atomic emission spectrometry (ICP-AES) and by GF-AAS. The procedure was validated by analysing the certified reference material BCR 146 R, a sewage sludge of industrial origin. Recoveries for cobalt in the spiked slurried sludge samples ranged from 92 to 96%, with a relative standard deviation of 10%. Recoveries in the certified sludge using slurry sampling GF-AAS tech- nique were about 103% for a cobalt content of 7.39 mg/kg. 1 Introduction Sludge from the wastewater treatment plants of São Paulo City is generated in a rate of 100 t/day. Final dis- posal of sludge generally involves some type of applica- tion to the land: at sanitary landfill, on agricultural land or for land reclamation [1]. Heavy metals, e.g. As, Cd, Co, Cr, Cu, Hg, Pb, Zn are primary pollutants due to their potential hazardous effects on plants and humans. Among them Co has to be monitored because it is an essential trace element (as Cr and Ni) but can be toxic above cer- tain limits. The concentration of Co in sludge can vary from 10 to 2,500 mg/kg (median value = 30 mg/kg) [2]. The analyti- cal technique to be chosen for routine analysis should cover this range with a minimum sample manipulation and minimum changes in the operational parameters. Be- sides, the methodology should be extendable to the other pollutant elements whose concentrations can be as low as 1 mg/kg (As, Cd, Se) with variations of one to two orders of magnitude. Samples of sludge can be dry ashed or acid digested with concentrated nitric acid and hydrofluoric acid. Typi- cal procedures for complete decomposition of these matri- ces in acid mixtures use conventional or microwave-as- sisted heating [3–6]. Direct analysis of the sludge is possible by X-ray fluo- rescence spectrometry (XRF) with minimum sample ma- nipulation and detection limits in the μg/g range or lower. However, the typical variation in the matrix composition and impurities contents for sludge, and some interelement interferences make standardisation and routine analysis a laborious task. Specifically for cobalt determination, there is an overlap of the Fe K β 0.17903 nm line and the Co K α 0.17566 nm line. The use of suspensions (slurries) as a method of intro- ducing solid samples in GF-AAS is widely recognised for numerous matrices [7–20] but only a few authors present data for Co determination in sewage sludge. Sterrit and Lester [21] compared flame AAS analysis of dry-ashed sludge samples that were digested with nitric acid and sul- furic acid and GF-AAS analysis of homogenized sludge samples. Martinez-Avila et al. [22] applied flow-injection flame AAS for the analysis of sewage slurries. Significant improvement of the precision of the slurry technique has been achieved by Miller-Ihli [23] using a small ultrasonic probe to mix the slurry in the autosampler cup just before the pipette withdraws the aliquot. The wide spread in composition of sludge sample matrix and trace elements make the slurry sampling ideal for preparation of diluted Rosa Ana Conte · Margaretha T. C. de Loos-Vollebregt Determination of cobalt in sewage sludge using ultrasonic slurry sampling graphite furnace atomic absorption spectrometry Fresenius J Anal Chem (2000) 367 : 722–726 © Springer-Verlag 2000 Received: 7 March 2000 / Revised: 25 April 2000 / Accepted: 26 April 2000 ORIGINAL PAPER R. A. Conte Departamento de Engenharia de Materials, Faculdade de Engenharia Quimica de Lorena, P.O. Box 116, 12600-000 Lorena, SP, Brazil e-mail: rosaconte@demar.faenquil.br M. T. C. de Loos-Vollebregt () Delft University of Technology, Faculty of Applied Sciences, DelftChemTech, Julianalaan 136, 2628 BL, Delft, The Netherlands e-mail: g.deloos@tnw.tudelft.nl