Solvent extraction method for determination of thorium in soft tissues

A simple, precise and accurate analytical technique has been developed for the determination of thorium isotopes in soft tissues. The method consists of preliminary nitric acid digestion of tissues after adding 229Th tracer, followed by a mixture of nitric and sulfuric acid with occasional addition of hydrogen peroxide; thorium is then coprecipitated with iron carrier by ammonium hydroxide. The precipitate is washed until free of sulfate ions, dissolved in 1:1 HNO3 and finally adjusted to 4 M HNO3. Thorium is extracted twice into 25% trilaurylamine (TLA) in xylene (pre-equilibrated with 4 M HNO3) and backwashed twice with 10 M HCl. The aqueous phase is evaporated to almost dryness, treated with H2SO4 with frequent addition of a few drops of HNO3, and electrodeposited onto a platinum planchet prior to α spectrometry with a surface-barrier silicon detector. The final total recovery ranged from 24 to 93% with a mean of 65% in 28 samples. Yield appeared to be independent of total iron when 10 to 100 mg Fe were added, and independent of total iron when 10 to 100 mg Fe were added, and independent of the amount of added tracer. The natural 228Th content of three different beef liver samples was 1.3, 1.4, and 3.0 pCi/kg wet weight. 4 tables