SUITABILITY OF OPEN SYSTEM DIGESTIONS FOR THE DETERMINATION OF THALLIUM IN ENVIRONMENTAL-SAMPLES

The determination of thallium (Tl) content in environmental samples is important because of the high toxicity of this metal. Fast and accura te analytical methods are required. Therefore, various open system dig estion methods were investigated for the rapid determination of Tl in plant and soil samples. For soils, all procedures were tested by analy sis of two National Institute of Standards and Technology (NIST) stand ard reference materials. For plant samples, the accuracy of the method s was evaluated using three digested materials, with and without a sta ndard addition of 1 mu g Tl on each one before digestion. Thallium was determined directly on digested solutions, without separation or prec oncentration procedures and with two electrothermal atomic absorption spectrophotometers (ETAAS). Results showed that only one procedure was suitable. The method is based on the solubilization of sample by heat ing a solution made from 30% hydrogen peroxide (H2O2) and 65% nitric a cid (HNO3). The procedure gave good agreement with the NIST certified concentration for soils and adequate recoveries for plant samples. No further hydrofluoric acid (HF) digestion was required on soils samples . Thallium concentrations numerically were higher but the increase was not significant. Volatility of Tl compounds was a drawback with the o ther digestion methods (aqua regia digestion, HNO3-H2SO4 digestion and sample dry-ashing at 480 degrees C) where Tl losses occurred. After v olumation in flasks, changes in Tl concentration in digested solutions were not detected during 10 days. The detection limit for the wet dig estion method in 10% HNO3 is 60 ng TI/L which is adequate with other i nvestigations.