SUMMARYSulphur plays an essential role in plants and is one of the main nutrients in several metabolic processes. It has four stable isotopes ( 32 S, 33 S, 34 S, and 36 S) with a natural abundance of 95.00, 0.76, 4.22, and 0.014 in atom %, respectively. A method for isotopic determination of S by isotope-ratio mass spectrometry (IRMS) in soil samples is proposed. The procedure involves the oxidation of organic S to sulphate (S-SO 4 2-), which was determined by dry combustion with alkaline oxidizing agents. The total S-SO 4 2-concentration was determined by turbidimetry and the results showed that the conversion process was adequate. To produce gaseous SO 2 gas, BaSO 4 was thermally decomposed in a vacuum system at 900 ºC in the presence of NaPO 3 . The isotope determination of S (atom % 34 S atoms) was carried out by isotope ratio mass spectrometry (IRMS). In this work, the labeled material (K 2 34 SO 4 ) was used to validate the method of isotopic determination of S; the results were precise and accurate, showing the viability of the proposed method.Index terms: sample preparation, soil samples, isotopic dilution, labeled material, stable isotope, 34 S.
RESUMO: DETERMINAÇÃO ISOTÓPICA DE ENXOFRE POR ESPECTROMETRIA DE MASSAS (IRMS) EM AMOSTRAS DE SOLO