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A versatile flow injection system for spectrophotometric determination of silicon (Si) in agronomic samples is proposed. For plant and slag analysis (1.0-10.0 mg L 21 Si), the method involves monitoring the yellowish molybdosilicic acid at 410 nm. Soil, fertilizer, water, and sugarcane juice analysis (0.5-5.0 mg L 21 Si) were accomplished by adding a reducing agent, and the molybdenum blue compound that formed was monitored at 735 nm. Flexibility of the method allows determination in a variety of matrices involving a wide range of concentrations. Beer's law is followed up to 20.0 mg L 21 Si (r , 0.9997; n ¼ 6) for analysis at 410 nm and up to 10.0 mg L 21 Si (r , 0.9998; n ¼ 6) at 735 nm. For the yellow-color and blue-color methods, the detection limits were estimated as 0.5 and 0.1 mg L 21 Si. Measurement frequency for both methods is approximately 75 h 21 using 48 mg of ammonium heptamolybdate, 80 mg of oxalic acid, and 24 mg of ascorbic acid per determination. Results are precise (r.s.d. . 0.1%, n ¼ 10) and in agreement with inductively coupled plasma-optical emission spectroscopy (ICP-OES). Statistical differences between data sets were not confirmed after applying the Student's ttest at the 95% confidence level (t exp ¼ 0.195 . t tab ¼ 2.57) related to n ¼ 6.

A method for isotopic determination of silicon by mass spectrometry in plants and soils labeled with 30 Si is reported. The development of this method is for use with studies involving the physiological process of absorption, transport, and redistribution of Si in the soil-plant system by use of the stable isotope 30 Si as a tracer. The procedure leads to SiF 4 formation, and the isotopic determination of Si was based on the measurements of the 28 SiF 3 þ , 29 SiF 3 þ , and 30 SiF 3 þ signals. Relative standard deviation of 30 Si abundance measurements (n ¼ 6) were lower than 0.1%, and the detection limit was 0.5 mg Si (dry mass).

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

Single superphosphate is currently one of the mostly used fertilizers as an alternative source for phosphorus and sulphur. SO 2 gasoso foi realizada em linha de vácuo a 900ºC na presença de NaPO 3 . A determinação isotópica do S (% em átomos de 34 S) foi realizada no espectrômetro de massas. O rendimento da produção do Ca(H 2 PO 4 ) 2 e do superfosfato simples marcado foi em média 97 e 99%, respectivamente, e a pureza do superfosfato marcado foi estimada como 96%. Não foi observado fracionamento isotópico no processo de produção do superfosfato simples marcado com 34 S. Palavras-chave: determinação isotópica, enxofre, isótopos estáveis, espectrometria de massas, compostos marcados