Application of microwave heating to the determination of free, combined and total sulphur in rubber

An atomic absorption spectrometry (AAS) method for the determination of total and combined sulphur in rubber and a catalytic method for the determination of free sulphur in rubber are described. The catalytic method is based on the iodine-azide reaction and is followed by an extraction of free sulphur with benzene, toluene or acetone. The catalytic determination, with the aid of gas chromatography, was performed in N,N-dimethylformamide after expelling the more volatile solvents used in the extraction. The calibration is linear over the range 2 × 10⁻⁶ – 3 × 10⁻⁴ M. Before the determination of total and combined sulphur, the rubber samples were decomposed by microwave assisted heating (5 min, 110 W) in a Teflon closed vessel. After microwave heating the solution was clear and homogenous and all sulphur compounds contained in the rubber were quantitatively oxidized to sulphate. The sulphates were then precipitated as BaSO₄ and an excess of barium was determined by AAS. From the difference in the results of the determinations of total and free sulphur the amount of combined sulphur can be calculated, and from the difference in the determinations of total and combined sulphur the amount of free sulphur can be calculated. In the latter case the catalytic analysis is not needed.     

Determination and fractionation of sulphur in a contaminated dredged sediment

Conversion of sulphur compounds in reduced, contaminated sediments that are brought in upland conditions may largely affect the potential mobility of heavy metal contaminants. We applied selected methods for the determination of total sulphur and sulphur fractions in a contaminated sediment in the reduced state and after gradual drying and oxidation during 1 month. Dry ashing, followed by ion chromatographic detection, was reliable for the determination of total sulphur. Good recoveries were obtained in both reference samples and spiked samples. Total sulphur was 14.5±0.6 g kg⁻¹ relative to dry sediment. Wet chemical methods that involve the determination of acid-volatile, Zn–HCl-reducible, chromium-reducible, acetone-soluble and HI-reducible sulphur were used to estimate specific sulphur forms. Results for acetone-soluble sulphur were to high and not consistent with results for other fractions. Major part of sulphur in the sediment (90%) was inorganic. Sulphides were converted partially to sulphate and to intermediary oxidised sulphur compounds upon oxidation of the sediment. The results suggested that the intermediary oxidised sulphur pool in the reduced sediment (2.5 g kg⁻¹) mainly consisted of pyrite, that was not converted during drying and oxidation. No significant changes in organic sulphur were detected.     

A sensor based on electrodes supported on ion-exchange membranes for the flow-injection monitoring of sulphur dioxide in wines and grape juices

A sensitive and fast responding electrochemical sensor is described for the determination of free and total sulphur dioxide in wines and grape juices which prevents interferences coming from ethanol and other natural components. It consists of a cell provided with a porous gold working electrode supported on one face of an ion-exchange membrane, acting as a solid polymer electrolyte (SPE), which allows gaseous electroactive analytes to be detected. This sensor was used as an amperometric detector for a flow injection system in which controlled volumes of headspace equilibrated with samples were injected. This approach was adopted to make also possible the determination of total SO₂, avoiding drawbacks caused by the high relative humidity generated by the sample heating resulting from the neutralization reaction of excess NaOH, whose addition was required to release sulphur dioxide from its combined forms. Factors affecting the detection process were examined and optimised. Under the identified optimal conditions, SO₂ detection resulted in sharp peaks which allowed to infer detection limits for a signal-to-noise ratio of 3, referred to liquid samples, of 0.04 and 0.02 mg L⁻¹ for free and total SO₂ which were determined at 20 and 35 °C, respectively. Moreover, the responses were found to be characterized by good repeatability (±2% and ±4%, respectively) and linear dependence on the SO₂ concentration over a wide range (0.2-500 mg L⁻¹ for both free and total SO₂). Finally, the long-term stability of the sensor turned out to be totally satisfactory in that responses changed of ±9% alone even after long periods of continuous use. The application to some commercial wines and grape juices is also presented.     

Investigation of sulphur compounds in coal tar using monodimensional and comprehensive two-dimensional gas chromatography

Coal is a non renewable fossil fuel, used mainly as a source of electrical energy and in the production of coke. It is subjected to thermal treatment, pyrolysis, which produces coke as a main product, in addition to a condensed liquid by-product, called tar. Tar is a complex mixture of organic compounds which contains different chemical classes, presenting aromatic and sulphur heterocyclic compounds. In general, identification of these compounds requires steps of isolation and fractionation, mainly due to co-elution of these compounds with polyaromatic hydrocarbons (PAH). The objective of this work is to characterize the sulphur compounds present in the coal tar obtained via pyrolysis, using comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry detector (GC×GC/TOFMS). Coal samples from the State of Paraná, Brazil were subjected to laboratorial scale pyrolysis. Several experimental conditions were tested, such as sample weight (5, 10 and 15g), heating ramp (10, 25 and 100°C/min) and final temperature (500, 700 and 900°C). Samples were analyzed by one dimensional gas chromatography (1D-GC) coupled to a quadrupole mass spectrometry detector (GC/qMS) and two-dimensional gas chromatography with time-of-flight mass spectrometry detector (GC×GC/TOFMS). The higher amount of sulphur compounds was obtained at a final temperature of 700°C and a heating ramp of 100°C/min. The main classes observed in the color plot were thiophenes, benzothiophenes and alkylated dibenzothiophenes. GC×GC/TOFMS allowed the identification of the greater number of compounds and the separation of several sulphur compounds from one another. Moreover, separation of sulphur compounds from polyaromatic hydrocarbons and phenols was achieved, which was not possible by 1D-GC. Comparing GC×GC/TOFMS and 1D-GC (SIM mode) also showed that 1D-GC, one of the most employed quantification tools for sulphur compounds, can be misleading for detection, identification and quantification, as the number of isomers of sulphur compounds found was greater than theoretically possible.     

Trace determination of sulphur mustard and related compounds in water by headspace-trap gas chromatography–mass spectrometry

A method for trace determination of sulphur mustard (HD) and some of its cyclic decomposition compounds in water samples has been developed using headspace-trap in combination with gas chromatography–mass spectrometry (GC–MS). Factorial design was used for optimisation of the method. The trap technology allows enrichment and focusing of the analytes on an adsorbent, hence the technique offers better sensitivity compared to conventional static headspace. A detection limit of 1 ng/ml was achieved for HD, while the cyclic sulphur compounds 1,4-thioxane, 1,3-dithiolane and 1,4-dithiane could be detected at a level of 0.1 ng/ml. The method was validated for the stable cyclic compounds in the concentration range from the limit of quantification (LOQ: 0.2–0.4 ng/ml) to hundred times LOQ. The within and between assay precisions at hundred times LOQ were 1–2% and 7–8% relative standard deviation, respectively. This technique requires almost no sample handling, and the total time for sampling and analysis was less than 1 h. The method was successfully employed for muddy river water and sea water samples.     

Determination and fractionation of sulphur in a contaminated dredged sediment

Conversion of sulphur compounds in reduced, contaminated sediments that are brought in upland conditions may largely affect the potential mobility of heavy metal contaminants. We applied selected methods for the determination of total sulphur and sulphur fractions in a contaminated sediment in the reduced state and after gradual drying and oxidation during 1 month. Dry ashing, followed by ion chromatographic detection, was reliable for the determination of total sulphur. Good recoveries were obtained in both reference samples and spiked samples. Total sulphur was 14.5±0.6 g kg⁻¹ relative to dry sediment. Wet chemical methods that involve the determination of acid-volatile, Zn–HCl-reducible, chromium-reducible, acetone-soluble and HI-reducible sulphur were used to estimate specific sulphur forms. Results for acetone-soluble sulphur were to high and not consistent with results for other fractions. Major part of sulphur in the sediment (90%) was inorganic. Sulphides were converted partially to sulphate and to intermediary oxidised sulphur compounds upon oxidation of the sediment. The results suggested that the intermediary oxidised sulphur pool in the reduced sediment (2.5 g kg⁻¹) mainly consisted of pyrite, that was not converted during drying and oxidation. No significant changes in organic sulphur were detected.     

Determination of sulphur in liquids obtained by thermal cracking of waste polymers and commercial fuels with different analytical methods

Low sulphur concentration in hydrocarbon products as fuels or lubricants is an important requirement for the high quality standards of refineries. A non-polarised energy dispersive X-ray fluorescence spectroscopy (EDXRFS) and sample combustion technique (ASTM D6428-99) was compared. A new application of energy dispersive X-ray spectrometry as analytical method for the determination of sulphur in fuels and fuel-like fractions was investigated. Low sulphur containing fuels and hydrocarbon mixtures obtained by thermal cracking of waste polymers were measured and the influence of C/H ratio on accuracy was studied. The concentration of sulphur in samples was measured with calibration graphs of different hydrocarbon matrices (commercial gasoline, diesel oil and white oil were used). Good correlation was observed between the different methods, but the correlation was depending on the characteristics of the matrices. Detection limits of 1.0 ppm, 1.1 ppm and 0.9 ppm were obtained for S in gasoline, diesel oil and white oil, respectively.     

Determination of sulphur in uranium matrix by total reflection X-ray fluorescence spectrometry

The possibility of sulphur determination in uranium matrix by total reflection x-ray fluorescence spectrometry (TXRF) has been studied. Calibration solutions and samples of sulphur in uranium matrix were prepared by mixing uranium in form of a standard uranyl nitrate solution and sulphur in the form of Na₂SO₄ standard solution, prepared by dissolving Na₂SO₄ in Milli-Q water. For major element analysis of sulphur, it was determined without separation of uranium whereas for the trace level determinations, uranium was first separated by solvent extraction using 30% tri-n-butyl phosphate (TBP) in dodecane as an extractant. In order to countercheck the TXRF results, a few samples of Rb₂U(SO₄)₃, a chemical assay standard for uranium, were diluted to different dilutions and sulphur content in these solutions were determined. The TXRF determined results for trace determinations of sulphur in these diluted solutions were counterchecked after addition of another uranium solution, so that sulphur is at trace level compared to uranium, separating uranium from these solution mixtures using TBP extraction and determining sulphur in aqueous phase by TXRF. For such TXRF determinations, Co was used as internal standard and W Lα was used as excitation source. The precision and accuracy of the method was assessed for trace and major element determinations and was found to be better than 8% (1σ RSD) and 15% at a concentration level of 1 μg/mL of sulphur measured in solutions whereas for Rb₂U(SO₄)₃, these values were found to be better than 4 and 13%, respectively.     

Simultaneous determination of nanomole amounts of sulphur dioxide and hydrogen sulphide by flow injection analysis with on-line preconcentration by means of capillary denuder tubes

A simple and rapid method for trace determination of SO2 and H2S in gaseous samples by using a flow injection system with on line preconcentration on capillary denuder is described. The gaseous samples are led through a 0.4 M sulphamic acid solution, retaining nitrogen dioxide, ammonia and hydrogen chloride. The sulphur dioxide is collected from the carrier gas stream (250 cm3 min-1) as sulphuric acid in a capillary denuder tube coated with a thin layer of 0.01-0.03 M hydrogen peroxide solution of 0.05 mM sulphuric acid; hydrogen sulphide passes into a second tube coated with 0.075 mM sodium sulphide solution of 0.1 M aqueous sodium hydroxide. The films containing the sulphuric acid and the sodium sulphide, respectively, are eluted with the corresponding circulating absorbent streams and pass through the detectors. Sulphuric acid is detected by conductimetry and sulphide is determined spectrophotometrically at 230 nm. If nanoequivalent amounts of H2S are present in the sample containing a large concentration of SO2 (SO2/H2S concentration ratio > 20), the sulphur dioxide is filtered out of the sample gas stream by solid sodium hydrogen carbonate. A limit of detection of 3.5 micrograms m-3 is obtained.     

Determination of total sulphur in fuel oils by ion chromatography

A method is described for the determination of total sulphur in fuel oils. The oils are burnt in an oxygen flask and the resulting sulphur dioxide is oxidised with hydrogen peroxide to sulphate which is subsequently determined by ion chromatography. With a 50-mg oil sample the limit of detection is 0.007% sulphur. The procedure is simple and has a r.s.d. of 2.7%. Blanks are negligible and the only interferences can be from metal ions that form insoluble sulphates. The method is compared with m.e.c.a, x.r.f. and titrimetric procedures.     

Volatile sulphur compounds in wines related to yeast metabolism and nitrogen composition of grape musts

The influence of nitrogen compounds in grape musts on the content of sulphur compounds of wines was studied. Different vinifications were performed with the addition of methionine (20 mg l⁻¹) and/or cysteine (40 mg l⁻¹) to grape musts before alcoholic fermentation. Six grape musts, with different nitrogen composition, from cultivars of the ‘Vinhos Verdes’ Region, in Portugal, were used. Addition of methionine to grape musts enhanced the content of wines in 3-(methylthio)-1-propanol, acetic acid-3-(methylthio)propyl ester, 3-(methylthio)propionic acid and some unidentified sulphur compounds. Increase of cysteine concentration in musts led to the production of wines with high levels of hydrogen sulphide and cis-2-methyltetrahydro-thiophene-3-OL and also unidentified sulphur compounds; however, the content of 3-(methylthio)propionic acid in the wines decreased considerately with the addition of cysteine to grape musts. This work showed that cultivars from the Vinho Verde Region show different sulphur compound contents. Avesso wines, elaborated from grape musts with low amino acids level, presented the highest total sulphur compounds content. Wines from Azal branco and Alvarinho were characterised by high contents of 4-(methylthio)-1-butanol and 3-(methylthio)propionic acid, respectively. A high content of N-(3-(methylthio) propyl)-acetamide and dimethylsulphone characterise the Loureiro wines. In contrast, Trajadura wines, produced from a must rich in amino acids, presented a low total sulphur compounds content; however, these wines were also characterised by high concentrations of 4-(methylthio)-1-butanol, acetic acid-3-(methylthio)propyl ester and hydrogen sulphide.     

Measurement of reduced sulphur compounds contained in aqueous matrices by direct injection into a gas chromatograph with a flame photometric detector

An analytical method was developed to measure the concentration of hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide contained in aqueous matrices (distilled water, tap water, kraft mill condensates and membrane bioreactor mixed liquor) by direct injection of aqueous samples into a gas chromatograph with a flame photometric detector. The analytical method requires a small sample volume (2 ml), sample preparation and analysis can be completed within 20 min and no complex sampling apparatus is needed. Consistent results and good recoveries were observed in all matrices investigated over the range of concentrations examined. The relationship between the normalized peak area obtained from GC–flame photometric detection and the concentration of the reduced sulphur compounds (RSCs) examined did not follow the theoretical power law exponent of two. The power law exponent appeared to decrease with the organic fraction associated with each RSC. The observed power law exponents for hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide were 1.92, 1.90, 1.66 and 1.72, respectively.     

低阶煤程序升温热解过程中钠、硅和硫间的相互作用

对一种澳大利亚低阶煤中,钠、硅和硫在热解过程中的相互作用进行了研究,以便深入了解钠的固硫作用和硫对流化床燃烧和气化中颗粒团聚的影响。制备了水洗、酸洗煤样和酸洗、乙酸钠离子交换煤样,样品在马弗炉中以17．7℃／min升至1000℃进行程序升温热解。对700℃-100℃热解半焦样品的依次水洗、酸洗、SEM—EDX和X射线元素扫描分析表明,部分有机结合的钠形成水溶性Na2O和NaOH化合物,水溶性的硅酸钠,以及与半焦中的硫形成酸不溶组分。最后一点解释了为何以往研究中发现,热解过程中有机结合的钠具有固硫作用,以及固硫历程中为何未发现Na2S的生成。在硅酸钠区域边缘存在的高硫区域,在流化床燃烧和气化过程中将形成含硫黏性相,导致床料的黏结。当用于酸洗、钠离子交换煤样的乙酸钠从0．25mol／L增加到1．0mol／L,固硫率将增加10％。相应的酸不溶钠组分的增加表明,酸不溶钠／硫组分随乙酸钠浓度的增加而增加,从而导致固硫率上升。     

Analysis of low-volatility organic sulphur compounds in wines by solid-phase microextraction and gas chromatography

A method for analysing low-volatility sulphur compounds using solid-phase microextraction has been developed. The analytes were extracted directly from the liquid sample using fibres coated with different stationary phases. The best extraction efficiency was obtained with Carboxen-polydimethylsiloxane coating. Ionic strength, sample volume, time and temperature of the extraction were optimised and the matrix effect studied. The method enables 15 sulphur compounds in wine to be determined at trace levels with recoveries close to 100% and limits of detection between 0.05 and 5 microg/L. The overall method was successfully applied to the determination of the sulphur compounds studied in several red, white and rosé wines.     

Emission quenching of binuclear pyrophosphito platinum(II) complexes in aqueous solution by sulphur dioxide. Spectroscopic measurements on sulphur dioxide addition and chromous ion reduction

The emission intensity at 517 nm from Pt₂(pop)₄⁴⁻ (pop = P₂O₅H₂²⁻) is quenched by the addition of sulphur dioxide. The sulphur dioxide coordinates at the axial platinum(II) sites by a η¹-SO₂ bond. This coordination is supported by ³¹p NMR and Raman spectroscopy of aqueous solutions. The electronic spectrum of a sulphur dioxide saturated solution of Pt₂(pop)₄⁴⁻ shows an absorption at 428.5nm ( = 4.1 × 10⁴). From the decrease in the chromophore for uncomplexed Pt₂(pop)₄⁴⁻ the equilibrium constant for SO₂ binding is estimated to be 1.74 M₂l⁻². The effect of adding different quenchers to aqueous solutions of Pt_2(pop)4⁴⁻ is discussed. The compound Pt₂(pop)₄⁴⁻ will undergo 2-electron reduction with chromous ion.     

Rapid and precise determination of total sulphur in soda-lime-silica glasses

A method is described for the determination of total sulphur in small amounts of soda-lime-silica glasses (100 mg or less). The crushed glass is mixed with vanadium pentoxide and decomposed at 1450 degrees under oxygen. The sulphur is quantitatively removed from the glass and determined by a conductometric technique. The method is standardized by accurately injecting sulphur dioxide into the furnace tube. The analysis time is about 10 min and the overall precision (2s) is of the order of 5%.     

A new approach for the determination of sulphur in airborne particles by HR-CS ETAAS

In this work, water and nitric acid extractable sulphur concentrations in PM2.5 fraction of urban aerosols were determined by high-resolution continuum source electrothermal molecular absorption spectrometry using most suitable CS molecular absorption band at 258.056 nm. For this purpose, the PM2.5 airborne particulates were collected on quartz filters using a high-volume sampler (500 L/min) in Istanbul (Turkey) during six months (January–June) of two consecutive years. The instrumental and experimental parameters (pyrolysis temperature, molecule formation temperature and leaching reagents) were optimised. The validity of the method for the sulphur was tested using standard reference material and the results were found to be in the uncertainty limits of the certified value.     

A new indirect spectrophotometric procedure for determination of sulphur dioxide

An operationally inexpensive and sensitive spectrophotometric procedure for sulphur dioxide is proposed. The reagent 5,5-dimethyl-1,2,3-cyclohexanetrione-1,2-dioxime-3-thiosemicarbaz one is used to determine trace amounts of sulphur dioxide indirectly by means of the reduction of Fe(III) to Fe(II). The method can determine down to 0.032 microg/ml of sulphur dioxide in the final solution and recoveries are better than 98%. The method can be applied to the determination of atmospheric SO(2) provided that interfering gases such as nitrogen dioxide and hydrogen sulphide are eliminated.     

Rapid microstill determination of free and total sulfite in wine with conductimetric detection

The development and optimization of on-line microdistillation for free and total sulfite (S(IV)) in grape juice and wine is reported. The microstill used both heat and an air stream to separate sulfur dioxide from the wine samples; the distillation product was captured in a peroxide solution, and converted to sulfuric acid, mirroring accepted industry practice. Measured from 1 to 300 mg L⁻¹ as SO₂ by conductance, sample throughputs of 60 h⁻¹ for free and 20 h⁻¹ for total sulfite were achieved. Data for bound S(IV) emphasises the slow kinetics of release reactions in some wines. The microstill method is more efficient for total sulfite than the accepted manual technique. Good correlation was found between the microstill and manual methods under specified control conditions.     

A practical method for the determination of sulphur in coal samples by high-resolution continuum source flame atomic absorption spectrometry

Sulphur in coal was determined using a high-resolution continuum source flame atomic absorption spectrophotometer (HR-CS-FAAS) with actylene/air flame. The C-S absorption band at 258.056 nm was found the most suitable analytical line with respect to sensitivity and spectral interferences. The instrumental parameters were optimized. The coal samples were dried and dissolved using microwave-assisted digestion technique. The validity of the method was tested using standard reference material and certified values were found in the limits of 95% confidence level. Since the concentrations of matrix elements of coal other than carbon are low enough not to cause any spectral interferences, the linear calibration method was applied in all quantifications without any problem. The calibration standards were prepared in sulphuric acid. The method was accurate, fast, simple and sensitive. The limit of detection (LOD, 3δ, N = 10) and the limit of quantification (LOQ, 10δ, N = 10) were found to be 0.01 and 0.03% (w/w), respectively. The sulphur concentrations of various kinds of the coal samples received around Turkey were determined. The sulphur contents of the coal samples were ranged from ≤LOQ to 1.2%.