Oxidative extraction versus total decomposition of soil in the determination of thallium

An aqua regia extraction and a total decomposition of soil were compared in terms of thallium determination. A sequential extraction of soil, according to the BCR protocol, was also performed for additional information on thallium distribution in soil fractions. Certified reference material—soil GBW 07401 of Chinese origin, containing 1 ± 0.2 ppm of thallium was used in these experiments. Thallium was determined by flow injection-differential pulse-anodic stripping voltammetry (FI-DP-ASV). Only 35% of total thallium was extracted in the aqua regia extraction, while the total decomposition led to satisfactory recovery. The sequential extraction showed that only 5% of thallium in GBW 07401 is dissolvable in the four BCR procedure fractions, and that 95% of the element is entrapped in the residual parent matter. These results show that the aqua regia extraction does not ensure complete thallium extraction from soil. Surprisingly, the total decomposition is significantly less time consuming than the aqua regia extraction.