Thallium (Tl) and its compounds are toxic to all organisms. Relatively high levels of thallium of pedogeochemical origin have been found in some areas of the Czech Republic. The contents of .2 mg kg 21 of Tl have been found in aqua regia soil extracts of soils originated from melanocratic porphyric hornblende-biotite granite or from hornblende-biotite granodiorite. The contents of Tl in the range of 0.45-1 mg kg 21 have occurred mainly in soils originated from paragneiss. A detailed study of some of these sites has been made. Content of thallium in soils was determined after extraction
In our study the method for the determination of Tl by ICP-MS was optimized. Both isotopes, 203 Tl and 205 Tl, were used for the measurement. Lutetium at a concentration 0.4 mg L -1 was used as an internal standard. Soil extracts were measured after dilution (1+4 with the solution of lutetium 0.5 mg L -1 ). Several soils from the International Soil Exchange Programme (WEPAL) were analyzed for the comparison of the results. For all tested samples the achieved results were in a good agreement with the results reported by WEPAL. A detection limit 1.2 µg kg -1 was achieved for the samples. The method was used for the determination of thallium in soils collected in the national monitoring programme (200 sites) and in the monitoring programme of the contaminated areas (27 sites). Topsoil and subsoil were sampled on each site and analyzed separately (454 samples were analyzed). The median value for the tested soils was 0.25 mg/kg, lower and upper quartiles 0.19 and 0.32 mg kg -1 respectively, 10 th and 90 th percentiles 0.16 and 0.41 mg kg -1 respectively and the maximum value was 2.83 mg kg -1 .
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