Investigation of Artefact Formation During Analysis of Volatile Sulphur Compounds Using Solid Phase Microextraction (SPME)

The occurrence and origin of artefact formation during the analysis of volatile sulphur compounds in air by PDMS/Carboxen fibre were investigated. Among the studied compounds (hydrogen sulphide, carbon disulphide, diethyl sulphide, methyl ethyl sulphide, isopropanethiol, methanethiol, dimethyl sulphide, dimethyl disulphide, carbonyl sulphide), essentially mercaptans were shown to react to form the corresponding dimers. However, in the presence of several oxygenated and amines compounds which are common components of industrial effluents, no further artefact formation or reaction was noticed. Artefact formation was therefore considered to be independent of the sample matrix. Thermal oxidation occurring during the desorption step in the GC injection port was assessed. As metallic elements were previously suspected to catalyse the reaction, a specially deactivated SPME needle was tested, but no significant difference was noticed compared to the original needle. It was therefore assumed that metallic elements naturally present on Carboxen may act as catalysts. Similar results were obtained by using Carboxen in adsorbent tubes, as artefacts were increased by comparison with Tenax TA and molecular sieve 5A.     

Determination of flavone C-glucosides in antioxidant of bamboo leaves (AOB) fortified foods by reversed-phase high-performance liquid chromatography with ultraviolet diode array detection

A sensitive solid-phase microextraction and gas chromatography-pulsed flame photometric detection technique was developed to quantify volatile sulfur compounds in wine. Eleven sulfur compounds, including hydrogen sulfide, methanethiol, ethanethiol, dimethyl sulfide, diethyl sulfide, methyl thioacetate, dimethyl disulfide, ethyl thioacetate, diethyl disulfide, dimethyl trisulfide and methionol, can be quantified simultaneously by employing three internal standards. Calibration curves were established in a synthetic wine, and linear correlation coefficients (R2) were greater than 0.99 for all target compounds. The quantification limits for most volatile sulfur compounds were 0.5 ppb or lower, except for methionol which had a detection limit of 60 ppb. The recovery was studied in synthetic wine as well as Pinot noir, Cabernet Sauvignon, Pinot Grigio, and Chardonnay wines. Although the sulfur compounds behaved differently depending on the wine matrix, recoveries of greater than 80% were achieved for all sulfur compounds. This technique was applied to analyze volatile sulfur compounds in several commercial wine samples; methionol concentrations were found at the ppm level, while the concentrations for hydrogen sulfide, methanethiol, and methyl thioacetate were at ppb levels. Only trace amounts of disulfides and trisulfides were detected, and ethanethiol was not detected.     

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.     

Comparison of various detection limit estimates for volatile sulphur compounds by gas chromatography with pulsed flame photometric detection

This paper addresses the variations that presently exist regarding the definition, determination, and reporting of detection limits for volatile sulphur compounds by gas chromatography with pulsed flame photometric detection (GC-PFPD). Gas standards containing hydrogen sulphide (H(2)S), carbonyl sulphide (COS), sulphur dioxide (SO(2)), methyl mercaptan (CH(3)SH), dimethyl sulphide (DMS), carbon disulphide (CS(2)), and dimethyl disulphide (DMDS) in concentrations varying from 0.36ppb (v/v) up to 1.5ppm (v/v) in nitrogen were prepared with permeation tubes and introduced in the gas chromatograph using a 0.25-ml gas sampling loop. After measuring the PFPD response versus concentration, the method detection limit (MDL), the Hubaux-Vos detection limit (x(D)), the absolute instrument sensitivity (AIS), and the sulphur detectivity (D(s)) were determined for each sulphur compound. The results show that the MDL determined by the US Environmental Protection Agency procedure consistently underestimates the minimum concentrations of volatile sulphur compounds that can be practically distinguished from the background noise with the PFPD. The Hubaux-Vos detection limits and the AIS values are several times higher than the MDL, and provide more conservative estimates of the lowest concentrations that can be reliably detected. Sulphur detectivities are well correlated with AIS values but only poorly correlated with MDL values. The AIS is recommended as a reliable and cost-effective measure of detection limit for volatile sulphur compounds by GC-PFPD, since the AIS is easier and faster to determine than the MDL and the Hubaux-Vos detection limit. In addition, this study confirmed that the PFPD response is nearly quadratic with respect to concentration for all volatile sulphur compounds.     

The properties of calibration errors in the analysis of reduced sulfur compounds by the combination of a loop injection system and gas chromatography with pulsed flame photometric detection

In order to evaluate the extent of analytical biases involved in the GC calibration, we conducted a series of experiments to examine the calibration methods of trace gas components. For the purpose of this comparative study, gaseous standards of reduced sulfur compounds (RSC) including hydrogen sulfide (H₂S), methanethiol (CH₃SH), dimethyl sulfide (DMS), carbon disulfide (CS₂), and dimethyl disulfide (DMDS) were calibrated by the combination of a GC/PFPD technique and a loop-injection method. In the course of this study, two different types of calibration methods were tested and compared: incremental-injection of a given standard with the fixed standard concentration (FSC) versus injection of multiple standards (with different concentrations) at the fixed standard volume (FSV). In the case of the FSV calibration, a notable increase in the GC sensitivity is apparent with decreasing loop size (or injection volume). For instance, the calibration slope for RSC obtained using a 10 μl loop system was approximately three times higher than that for a 250 μl one. However, the results obtained by the FSC method exhibit much poorer sensitivity than its counterpart with slight differences in their sensitivities across different standard concentrations (due to such factors as the matrix effect from varying injection volumes). Thus, the overall results of this study confirm that the detailed characterization of the selected calibration method (e.g., the use of FSV approach relative to FSC) is of primary significance to perform an accurate quantification of trace gases.     

Investigation of membrane dryers and evaluation of a new ozone scrubbing material for the sampling of organosulphur compounds in air.

The applicability of two different types of Nafion membrane dryers (based on counter-current flow and desiccant drying) and of a new ozone scrubbing material, polyphenylene sulphide wool (noXon-S), to adsorptive sampling of selected volatile sulphur compounds (methanethiol, dimethyl sulphide, isopropanethiol and isobutanethiol) is investigated at the low ppb (v/v) level (1-5 ppb). No analyte losses occur with either type of dryer at relative humidities (RH) of < or = 50%, while at higher RH values particularly the thiols tend to be lost (between 6 and 32%) even after conditioning. The actual losses depend more on the state of the individual permeation membrane rather than on the type of dryer. NoXon-S is a highly suitable ozone scrubber material for sulphur compounds since it efficiently removes ozone without retainment of the analytes and without the formation of blanks or artefacts from the scrubber material. The combined use of a Nafion membrane dryer and a noXon-S ozone scrubber is thus recommended for artifact-free sampling of sulphur compounds.     

The vibrational spectra of the boron halides and their molecular complexes Part 10. The complexes of boron trifluoride with ammonia and its methyl derivatives. An ab initio study

Ab initio calculations, at the level of second order M?ller-Plesset perturbation theory, and using a triple-zeta Gaussian basis set with polarization and diffuse functions on all atoms, have been carried out on the donor-acceptor complexes of boron trifluoride with ammonia and its mono-, di- and trimethyl derivatives. The structures, interaction energies and vibrational spectra of the complexes have been determined. An eclipsed and a staggered conformer have been examined for each complex, and the preferred conformer was found to be the staggered species in each case. The computed data have been compared with those for some similar complexes containing boron trifluoride and a series of oxygen and sulphur electron donors (water, hydrogen sulphide, methanol, methanethiol, dimethyl ether and dimethyl sulphide) and the effect of successive methyl substitution in all three series has been investigated.     

Capillary gas chromatography-atomic emission spectroscopy-mass spectrometry analysis of sulphur mustard and transformation products in a block recovered from the Baltic Sea.

A block of yperite fished up from the Baltic Sea was analysed by gas chromatography coupled with atomic emission spectrometry and mass spectrometry. In the samples of the block about 50 compounds were detected, out of which 30 were identified. The identification of the compounds was performed by using the element chromatograms of the investigated compounds, and the data obtained by mass spectrometric detection. Thiodiglycol was not found among the compounds present in the investigated block. The calculations of the contents of sulphur mustard and some products in the block were performed by an external calibration method using bis(2-chloroethyl) sulphide as the standard. A satisfactory precision of elements determinations was obtained (RSD from 4.4 to 14.3%).     

Automated determination of sulphite and sulphur dioxide by cool flame molecular emission spectrometry after reduction to hydrogen sulphide with sodium tetrahydroborate III

An automated method for the determination of sulphite and sulphur dioxide by cool flame molecular emission spectrometry is described. The method is based on the reduction of both compounds to hydrogen sulphide with sodium tetrahydroborate III. The sample which is mixed with NaBH(4) is acidified with 6M hydrochloric acid and carried by a continuous-flow stream into a gas-liquid separator where the evolved hydrogen sulphide is swept by nitrogen into a cool, hydrogen-nitrogen-entrained air flame. The intensity of the blue diatomic S(2) emission generated is measured at 384 nm. The proposed method has a detection limit for sulphite of 0.029 mug/ml and relative standard deviations of 1.2 and 1.5% for 1 and 5 mug/ml respectively. The calibration graph is linear up to 24 mug/ml sulphite and samples can be analysed at a rate of about 40/hr. The method has been applied to the determination of SO(2) in air and sulphite in wines.     

Odour‐causing compounds in air samples: Gas–liquid partition coefficients and determination using solid‐phase microextraction and GC with mass spectrometric detection

A quantification method based on solid‐phase microextraction followed by GC coupled to MS was developed for the determination of gas–liquid partition coefficients and for the air monitoring of a group of odour‐causing compounds that had previously been found in wastewater samples including dimethyl disulphide, phenol, indole, skatole, octanal, nonanal, benzothiazole and some terpenes. Using a divinylbenzene/carboxen/polydimethylsiloxane fibre, adsorption kinetics have been studied to define an extraction time that would avoid coating saturation. It was found that for an extraction time of 10 min, external calibration could be performed in the range of 0.4–100 μg/m³, with detection limits between 0.1 and 20 μg/m³. Inter‐day precision of the developed method was evaluated (n = 5) and RSD values between 12 and 24% were obtained for all compounds. The proposed method has been applied to the analysis of air samples surrounding a wastewater treatment plant in Catalonia (Spain). In all air samples evaluated, dimethyl disulphide, limonene and phenol were detected, and the first two were the compounds that showed the highest partition coefficients.     

Zur colorimetrischen Bestimmung flüchtiger Schwefelverbindungen in Lebensmitteln

The quantitative determination of volatile mercaptans and hydrogen sulphide with bis-(p-nitrophenyl)-disulphide is described. The influence of various other volatile sulphur compounds, which may be found in food, and of ethanol has been examined. It is possible to determine down to 0.1 mMol/kg of mercaptans and hydrogen sulphide in food with a standard deviation of 8.4%. A few examples are given.     

Calibration transfer between electronic nose systems for rapid In situ measurement of pulp and paper industry emissions

Electronic nose systems when deployed in network mesh can effectively provide a low budget and onsite solution for the industrial obnoxious gaseous measurement. For accurate and identical prediction capability by all the electronic nose systems, a reliable calibration transfer model needs to be implemented in order to overcome the inherent sensor array variability. In this work, robust regression (RR) is used for calibration transfer between two electronic nose systems using a Box–Behnken (BB) design. Out of the two electronic nose systems, one was trained using industrial gas samples by four artificial neural network models, for the measurement of obnoxious odours emitted from pulp and paper industries. The emissions constitute mainly of hydrogen sulphide (H₂S), methyl mercaptan (MM), dimethyl sulphide (DMS) and dimethyl disulphide (DMDS) in different proportions. A Box–Behnken design consisting of 27 experiment sets based on synthetic gas combinations of H₂S, MM, DMS and DMDS, were conducted for calibration transfer between two identical electronic nose systems. Identical sensors on both the systems were mapped and the prediction models developed using ANN were then transferred to the second system using BB–RR methodology. The results showed successful transmission of prediction models developed for one system to other system, with the mean absolute error between the actual and predicted concentration of analytes in mg L⁻¹ after calibration transfer (on second system) being 0.076, 0.1801, 0.0329, 0.427 for DMS, DMDS, MM, H₂S respectively.     

A sensor for hydrogen sulphide

A sensor for hydrogen sulphide is described. It is based on the catalytic effect of the gas on the iodine-azide reaction. An iodine-azide solution is exuded from a sintered-glass ball (10mm diameter) at a flow-rate of 3.30 ml min whilst its potential is monitored by two platinum electrodes. one inside and the other outside the ball. Sample is carried in a gas at a flow-rate of 350 ml min . Carbon disulphide, methyl mercaptan or sulphur dioxide in amounts up to a hundred times that of the hydrogen sulphide do not interfere. The detection limit is 5 ng and the determination limit is 8 ng, with a relative standard deviation of 10%.     

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.     

Comparison of different calibration approaches in the application of thermal desorption technique: A test on gaseous reduced sulfur compounds

In this study, the reliability of thermal desorbing technique was investigated using the gaseous standards of five reduced sulfur compounds (RSCs: hydrogen sulfide, methane thiol, dimethyl sulfide, carbon disulfide, and dimethyl disulfide). A series of calibration experiments for RSCs were performed using gas chromatography (GC) with pulsed flame photometric detector (PFPD) that is interfaced with a thermal desorber (TD) unit. These calibration data were evaluated by means of two contrasting concepts: fixed standard concentration method (FSC: variable volumetric injection of standard gases prepared at a given concentration) and fixed standard volume method (FSV: injection of multiple standards with varying concentrations at a given volume). When the results of both methods were compared, RSCs generally showed enhanced sensitivity with increasing concentration (FSC) and sample loading volume (FSV). This study highlights that TD-based calibration properties are practically undistinguishable between different sample transfer approaches (e.g., FSV and FSC). As a result, the calibration properties of RSCs derived by thermal desorption technique are greatly distinguished from those of direct injection into GC.     

A study of the effect of some pollutant gases on cathodes used in membrane-covered amperometric oxygen detectors

The effects of various gaseous pollutants on the electrochemical activity of materials used as cathodes for membrane-covered amperometric oxygen detectors are described. It is shown that gold and platinum cathodes are not unduly affected by concentrations of sulphur dioxide and chlorine many times greater than those likely to be encountered in test solutions, and that a gold cathode is also unaffected by hydrogen sulphide. A platinum cathode is rapidly and significantly poisoned on contact with hydrogen sulphide; an analysis of the fall in the rate of oxygen reduction as a function of time indicates that the poisoning occurs by the blocking of surface sites by sulphide. The effect of hydrogen sulphide on silver, nickel and nickel sulphide electrodes is also reported. Of these materials, only nickel sulphide is an effective electrocatalyst for oxygen reduction in the presence of sulphide.     

Quantitative determination of wine highly volatile sulfur compounds by using automated headspace solid-phase microextraction and gas chromatography-pulsed flame photometric detection. Critical study and optimization of a new procedure

The quantitative determination of wine volatile sulfur compounds by automated headspace solid-phase microextraction (HS-SPME) with a carboxen-polydimethylsiloxane (CAR-PDMS) fiber and subsequent gas chromatography-pulsed flame photometric detection (GC-PFPD) has been evaluated. The direct extraction of the sulfur compounds in 5 ml of wine has been found to suffer from matrix effects and short linear ranges, problems which could not be solved by the use of different internal standards or by multiple headspace SPME. These problems were attributed to saturation of the fiber and to competitive effects between analytes, internal standards and other wine volatiles. Another problem was the oxidation of analytes during the procedure. The reduction in sample volume by a factor 50 (0.1 ml diluted with water or brine) brought about a reduction in the amount of sulfur compounds taken in the fiber by a factor just 3.3. Consequently, a new procedure has been proposed. In a sealed vial containing 4.9 ml of saturated NaCl brine, the air is thoroughly displaced with nitrogen, and the wine (0.1 ml) and the internal standards (0.02 ml) are further introduced with a syringe through the vial septum. This sample is extracted at 35 degrees C for 20 min. This procedure makes a satisfactory determination possible of hydrogen sulfide, methanethiol, ethanethiol, dimethyl sulfide, diethyl sulfide and dimethyl disulfide. The linear dynamic ranges cover the normal ranges of occurrence of these analytes in wine with typical r2 between 0.9823 and 0.9980. Reproducibility in real samples ranges from 10 to 20% and repeatability is better than 10% in most cases. The method accuracy is satisfactory, with errors below 20% for hydrogen sulfide and mostly below 10% for the other compounds. The proposed method has been applied to the analysis of 34 Spanish wines.     

Determination of sulphide, polysulphide, thiocarbonates and sulphur by extraction with tributyltin hydroxide, and of thiosulphate by hydrogenation

The methods presented involve the separation of sulphur compounds by means of a hexane solution of tributyltin hydroxide (TBT), followed by titration with o-hydroxymercuribenzoic acid in the presence of dithizone as indicator. Free sulphide is selectively extracted from strongly alkaline solution, whereas the polysulphides and thiocarbonates are extracted at pH 9-10. The polysulphide and thiocarbonate extracts decompose to form TBT-sulphide, sulphur and carbon disulphide. Treatment with sulphite, stannate(II), copper, vinyl cyanide and ethylenediamine, and hydrogenation with zinc and hydrobromic acid have been applied in the course of the analysis. The sulphur is determined by its attack on copper to form copper(I) sulphide which is subsequently dissolved in aqueous potassium cyanide solution and the sulphide separated by extraction.     

Coulometric argentometric determination of microamounts of sulphur in iron and steel after reduction to hydrogen sulphide with iron(II) in strong phosphoric acid medium

A coulometric method was developed for the determination of microamounts of sulphur in iron and steel. Hydrogen sulphide is quantitatively evolved by reduction with iron(II) in strong phosphoric acid medium and is titrated with electrolytically generated silver ion from a silver anode. Microamounts of sulphide (2.96–224.3 μg) in sodium sulphide standard solutions could be determined with an error of only a few percent. Sulphur in a potassium sulphate standard solution is quantitatively reduced to hydrogen sulphide and could be separated from the solution by heating and determined accurately. Trace amounts of sulphur (7–100 μg g^?1) in iron and steels could be determined with a standard deviation of 0.7–2.1 μg g^?1.     

Quality control in quantification of volatile organic compounds analysed by thermal desorption-gas chromatography-mass spectrometry

This paper presents a detailed study on the calibration of a thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS)-based methodology for quantification of volatile organic compounds (VOCs) in gaseous and liquid samples. For the first time, it is documented to what extent three widely encountered problems affect precise and accurate quantification, and solutions to improve calibration are proposed. The first issue deals with the limited precision in MS quantification, as exemplified by high relative standard deviations (up to 40%, n=5) on response factors of a set of 69 selected VOCs in a volatility range from 16 Pa to 85 kPa at 298 K. The addition of [(2)H(8)]toluene as an internal standard, in gaseous or liquid phase, improves this imprecision by a factor of 5. Second, the matrix in which the standard is dissolved is shown to be highly important towards calibration. Quantification of gaseous VOCs loaded on a sorbent tube using response factors obtained with liquid standards results in systematic deviations of 40-80%. Relative response factors determined by the analysis of sorbent tubes loaded with both analytes and [(2)H(8)]toluene from liquid phase are shown to offer a reliable alternative for quantification of airborne VOCs, without need for expensive and often hardly available gaseous standards. Third, a strategy is proposed involving the determination of a relative response factor being representative for a group of analytes with similar functionalities and electron impact fragmentation patterns. This group method approach indicates to be useful (RSD approximately 10%) for quantifying analytes belonging to that class but having no standards available.