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

SYNTHESIS AND QUALITY CONTROL OF 13 C-ENRICHED UREA FOR Helicobacter pylori (HP) DIAGNOSIS. The aim of the study was to synthesize the urea (13 CO(NH 2) 2), with 99% 13 C atoms, and to perform a quality analysis for the diagnosis (breath test) of Helicobacter pylori. Furthermore, the process was submitted to economic analysis. The reaction was performed in a stainless steel reactor, lined with polytetrafluoroethylene, under low pressure and temperature. The synthesis method was shown to be appropriate (2.35 g; 81.9% yield), evidenced by physico-chemical and microbiological results, according to Brazilian legislation. The production and diagnosis costs were competitive compared with national and international market values, rendering this a valuable tool in clinical medicine.

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-Techniques that employ15 N have proved to be an important tool in many areas of the agronomic and biomedical sciences. Nevertheless, their use is limited by methodological difficulties and by the price of compounds in the international market. Nitric compounds ( 15 NO 3 -) have attracted the interest of researchers. However, these compounds are not currently produced in Brazil. Thus, in the present work H 15 NO 3 was obtained from the oxidation of anhydrous 15 NH 3 . The method we used differs from the industrial process in that the absorption tower is replaced with a polytetrafluoroethylene-lined, stainless-steel hydration reactor. The process output was evaluated based on the following parameters: reaction temperature; ratio of reagents; pressure and flow of 15 NH 3(g) through the catalyst (Pt/Rh). The results showed that, at the best conditions (500 ºC; 50 % excess O 2 ; 0.

Agricultural gypsum (CaSO 4 .2H 2 O) stands out as an effective source of calcium and sulfur, and to control aluminum saturation in the soil. Labeled as 34

CO(15NH2)2 enriched with the stable isotope 15N was synthesized based on a reaction involving CO, 15NH3, and S in the presence of CH3OH. The method differs from the industrial method; a stainless steel reactor internally lined with polytetrafluoroethylene (PTFE) was used in a discontinuous process under low pressure and temperature. The yield of the synthesis was evaluated as a function of the parameters: the amount of reagents, reaction time, addition of H2S, liquid solution and reaction temperature. The results showed that under optimum conditions (1.36, 4.01, and 4.48 g of 15NH3, CO, and S, respectively, 40 ml CH3OH, 40 mg H2S, 100 ºC and 120 min of reaction) 1.82 g (yield 76.5%) of the compound was obtained per batch. The synthesized CO(15NH2)2 contained 46.2% N, 0.55% biuret, melting point of 132.55 ºC and did not exhibit isotopic fractionation. The production cost of CO(15NH2)2 with 90.0 at. % 15N was US$ 238.60 per gram

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