A method to test the detectability of GC/PFPD for an extended concentration range with respect to reduced sulfur compounds

In this study, a possible means to extensively expand the quantifiable range of reduced sulfur compounds (RSC) in air has been investigated by a combined application of GC with pulsed flame photometric detector (PFPD) and a multifunction thermal desorber (TD) system. To comply with the purpose of this study, gaseous RSC standards containing the equimolar concentrations of H(2)S, CH(3)SH, DMS, DMDS, and CS(2 )were prepared at 11 concentration levels (i. e., 10 ppb-10 ppm (over 10(3 )range)). These standards were then used to derive three-point calibrations based on the modified injection through a TD (MITD). If the mean calibration slopes of each concentration level are normalized to that of CH(3)SH, the relative ordering is found as 0.65 (H(2)S): 1 (CH(3)SH): 1.34 (DMS): 2.24 (DMDS). The reproducibility of MITD method, when assessed in terms of relative standard error (RSE) for all calibration slopes, had the most stable pattern for DMDS (5.77%) and the least stable one for H(2)S (12.8%). The sensitivity of the MITD-based calibration generally improved with an increase in concentration levels of standard gas. Based on our study, the MITD technique is useful to extend quantification of GC/PFPD by allowing RSC detection over a 10(3) range.     

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.     

Identification of control parameters for the sulfur gas storability with bag sampling methods

Air samples containing sulfur compounds are often collected and stored in sample bags prior to analysis. The storage stability of six gaseous sulfur compounds (H₂S, CH₃SH, DMS, CS₂, DMDS and SO₂) was compared between two different bag materials (polyvinyl fluoride (PVF) and polyester aluminum (PEA)) at five initial concentrations (1, 10, 100, 1000, and 10,000 ppb). The response factors (RF) of these samples were determined after storage periods of 0, 1, and 3 days by gas chromatography–pulsed flame photometric detector (GC–PFPD) combined with an air server (AS)/thermal desorber (TD) system. Although concentration reduction occurred more rapidly from samples of the high concentration standards (1000 and 10,000 ppb), such trends were not evident in their low concentration counterparts (1, 10, and 100 ppb). As such, temporal changes in RF values and the associated loss rates of most sulfur gases were greatly affected by their initial concentration levels. Moreover, the storability of oxidized sulfur compound (SO₂) was greatly distinguished from that of reduced sulfur compounds (RSCs), as the former almost disappeared in the PVF bag even after one day. The results of our study confirm that storability of gaseous sulfur species is affected interactively by such variables as initial gas concentration level, bag material type, and oxidation status with the associated reactivity.     

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.     

Analysis of sulfur-containing compounds in ambient air using solid-phase microextraction and gas chromatography with pulsed flame photometric detection

A new analysis method for sulfur-containing compounds in air using solid-phase microextraction (SPME), gas chromatography and pulsed flame photometric detection (PFPD), SPME-GC-PFPD method, has been developed. The analysis method is simple, fast and easily performed. To demonstrate the usefulness and versatility of the method air samples collected in geothermal areas in Rotorua, at a muddy beach in Auckland (cities in New Zealand), and in a wastewater treatment plant were analysed. COS, H₂S, CS₂, SO₂, CH₃SH, (CH₃)₂S and CH₃(CH₂)₂CH₂SH were identified in the samples from Rotorua. It was noted that air quality in residential areas with respect to sulfur compounds was better than that around geothermal sources. Samples from the wastewater treatment plant contained COS, H₂S, CS₂, SO₂, CH₃SH, (CH₃)₂S and (CH₃)₂S₂. It was found that the emission of sulfur compounds was reduced in the course of the wastewater treatment process. The potential impact of the detected sulfur compounds on human health is briefly discussed.     

Organotin analysis by gas chromatography–pulsed flame-photometric detection (GC–PFPD)

Monobutyltin (MBuT), dibutyltin (DBuT), and tributyltin (TBuT) mixtures have been separated and quantified by gas chromatography with pulsed flame-photometric detection (GC–PFPD). The compounds were first derivatized with NaBEt₄, then extracted with hexane and injected into the GC in splitless mode. Optimum GC and detector conditions were established. For GC, various injector temperatures and oven temperature programs were tested. For the PFPD detector, gate settings (gate delay and gate width) and detector temperature were optimized. A very good linearity was obtained up to 100–150 ppb for all organotin compounds. The detection limits obtained were: MBuT (0.7 ppb), DBuT (0.8 ppb), and TBuT (0.6 ppb). RSD for repeatability and reproducibility were well below 20% when the instrument was in routine operation. A biological sample (CRM 477) was also analyzed for organotins. Extraction from the biological matrix was performed with TMAH. Besides the increased risk of contamination, the derivatization step seemed to be critical. pH and amount of derivatizing agent were tested. When using an internal standard (TPrT) between 90% and 110% of the certified amounts of organotin were recovered.     

The loss patterns of reduced sulfur compounds in contact with different tubing materials

To collect or transfer samples of airborne pollutants, tubings made of various materials are used. To analyze the reactive loss patterns of reduced sulfur compounds (RSC) in the use of tubing fittings, a series of laboratory experiments were conducted so that the concentration changes were induced by the physical contact between gas samples and tubing walls. For the purpose of this study, a total of five tubing materials were investigated in reference to silcosteel (S1) tubing: stainless steel (S2), silicone (S3), PTFE Teflon (T1), tygon (T2), and copper (C). This comparative experiment was made using gaseous standards containing equimolar concentrations of four RSCs (H(2)S, CH(3)SH, DMS, and DMDS). The loss patterns of RSC were then evaluated and compared in terms of their calibration slope values across different tubing materials. The results of this comparative analysis indicated that except for a few cases, a fairly good compatibility was seen consistently among different tubing types and different RSCs. The results generally showed that the magnitude of calibration slope values obtained from different tubing materials tend to increase with an increase in RSC molecular weights. If the results are compared between different tubing materials, a highly contrasting pattern was evident. For instance, C tubing shows significant losses of light RSCs (H(2)S and CH(3)SH), while S3 tubing experiences the large losses of DMS and DMDS. A line of evidence found in this study thus suggests that most tubing types have their unique loss mechanism for RSCs, when they contact RSCs in sample gas stream passing through the tubing walls.     

Difference in Conversions Between Dimethyl Sulfide and Methanethiol in a Cold Plasma Environment

This study compared the conversion of two malodorous substances, dimethyl sulfide (CH₃SCH₃, DMS) and methanethiol (CH₃SH) in a cold plasma reactor. The DMS and CH₃SH were successfully destroyed at room temperature. DMS decomposed less than CH₃SH at the same conditions. In oxygen-free condition, CS₂ and hydrocarbons were the major products, while SO₂ and CO_x were main compounds in oxygen-rich environments. The DMS/Ar plasma yielded more hydrocarbons and less CS₂ than that of CH₃SH/Ar plasma. In the CH₃SH/O₂/Ar plasma, rapid formation of SO and CO resulted in the yields much more amounts of SO₂ and CO₂ than those in the DMS/O₂/Ar plasma; and remained only a trace of total hydrocarbons, CH₂O, CH₃OH, CS₂, and OCS. The major differences between the reaction mechanisms of DMS and CH₃SH were also proposed and discussed.     

Determination of organotins in aquatic food by gas chromatography with pulsed flame photometric detection

A method based on gas chromatography (GC)-pulsed flame photometric detection (PFPD) was developed to determine the levels of organotins in aquatic food. After being purified by gel-permeation chromatography in ethyl actate-tetrahydrofuran, the organotin compounds were derivatized by pentylmagnesium bromide. The derivative products were injected into the GC system and detected by PFPD (sulfur mode). The method was validated by analysis of the certified reference material and spiked samples. Recoveries of organotins ranged from 84.1 to 116.6% with relative standard deviation between 1.3 and 16.0% when spiked at levels of 2, 10, and 40 microg/kg. The limits of detection varied from 0.1 to 1.2 microg/kg for shellfish and 0.1 to 0.5 microg/kg for fish. The proposed method was suitable for determining organotins in aquatic foods.     

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.     

Solubility of dimethyldisulfide (DMDS) in aqueous solutions of Fe(III) complexes of trans-1,2-cyclohexanediaminetetraacetic acid (CDTA) using the static headspace method

Total reduced sulfurs quartet (H₂S, CH₃SH, CH₃SCH₃ and CH₃S₂CH₃) is part of a well-known environmental problem afflicting pulp mills exploiting the Kraft mill sulfate-pulp process. Utilization of ferric chelate complex of trans-1,2-cyclohexanediaminetetraacetic acid (CDTA) for the oxidative scrubbing of H₂S and CH₃SH in Kraft mill streams is beneficial from the standpoints of iron protection against precipitation and oxygen-mediated regenerative oxidation of the ferrous chelate CDTA. The remaining two sulfur-bearing compounds, considered not oxidizable by CDTA–Fe(III), undergo only physical absorption in such solutions, so their solubility in aqueous CDTA–Fe(III) alkaline solutions is a crucial parameter for designing the complete scrubbing-absorption process. This investigation was carried out to determine the Henry's law constants of dimethyldisulfide (DMDS) in pure water, in aqueous iron-free CDTA solutions and CDTA–Fe(III) complex solutions using the static headspace method with an estimated accuracy of 2%. Experiments with aqueous solutions of chelate concentrations varying between 38 and 300 mol m⁻³ were carried out at temperatures between 298 and 333 K and atmospheric pressure. It was shown that DMDS solubility decreases with increasing temperature for all systems and is not much influenced by the CDTA concentration and solution pH.     

Rapid and accurate determination of trace volatile sulfur compounds in human halitosis by an adaptable active sampling system coupling with gas chromatography

Halitosis with the main components of trace volatile sulfur compounds widely affects the quality of life. In this study, an adaptable active sampling system with two sample‐collection modes of direct injection and solid‐phase microextraction was developed for the rapid and precise determination of trace volatile sulfur compounds in human halitosis coupled with gas chromatography–flame photometric detection. The active sampling system was well designed and produced for efficiently sampling and precisely determining trace volatile targets in halitosis under the optimized sampling and detection conditions. The analytical method established was successfully applied for the determination of trace targets in halitosis. The limits of detection of H₂S, CH₃SH, and CH₃SCH₃ by direct injection were 0.0140–23.0 μg/L with good recoveries ranging from 82.2 to 118% and satisfactory relative standard deviations of 0.4–9.5% (n = 3), respectively. The limit of detections of CH₃SH and CH₃SCH₃ by solid‐phase microextraction were 2.03 and 0.186 × 10^?3 μg/L with good recoveries ranging from 98.3 to 108% and relative standard deviations of 5.9–9.0% (n = 3). Trace volatile targets in positive real samples could be actually found and quantified by combination of direct injection and solid‐phase microextraction. This method was reliable and efficient for the determination of trace volatile sulfur compounds in halitosis.     

Gas analyzer for continuous monitoring of trace level methanethiol by microchannel collection and fluorescence detection

The highly odorous compound methanethiol, CH₃SH, is commonly produced in biodegradation of biomass and industrial processes, and is classed as 2000 times more odorous than NH₃. However, there is no simple analytical method for detecting low parts-per-billion in volume ratio (ppbv) levels of CH₃SH. In this study, a micro gas analysis system (μGAS) was developed for continuous or near real time measurement of CH₃SH at ppbv levels. In addition to a commercial fluorescence detector, a miniature high sensitivity fluorescence detector was developed using a novel micro-photomultiplier tube device. CH₃SH was collected by absorption into an alkaline solution in a honeycomb-patterned microchannel scrubber and then mixed with the fluorescent reagent, 4-(N,N-dimethylaminosulfonyl)-7-fluoro-2,1,3-benzoxadiazole (DBD-F). Gaseous CH₃SH was measured without serious interference from other sulfur compounds or amines. The limits of detection were 0.2 ppbv with the commercial detector and 0.3 ppbv with the miniature detector. CH₃SH produced from a pulping process was monitored with the μGAS system and the data agreed well with those obtained by collection with a silica gel tube followed by thermal desorption–gas chromatography–mass spectrometry. The portable system with the miniature fluorescence detector was used to monitor CH₃SH levels in near-real time in a stockyard and it was shown that the major odor component, CH₃SH, presented and its concentration varied dynamically with time.     

The evaluation of recovery rate associated with the use of thermal desorption systems for the analysis of atmospheric reduced sulfur compounds (RSC) using the GC/PFPD method

In this work, the recovery rate (RR) of preconcentration technique was examined using a combination of the Peltier cooling (PC) and thermal desorption (TD) system for the gas chromatographic (GC) analysis of reduced sulfur compounds (RSC) in air. The possible loss or gain of analytes resulting from the use of the PC/TD system was estimated by analyzing equimolar standards (10 ppm) of four S compounds including H₂S, CH₃SH, DMS, and DMDS in two different manners: (1) by injecting directly the four S compounds into the GC via injector and (2) by introducing them through the PC/TD system. When a series of tests were conducted on different types of gas media (ultrapure air versus N₂) and across varying relative humidity (RH), it was found that the RR values for the four S compounds vary from 80 to 110% range. The overall results of our study thus indicate that the RR for the PC/TD system is fairly good and that subtle differences in their RR values may reflect the combined effects of different factors investigated in this study such as types of gas media, RH change, and properties of target analytes (e.g., recovery rate of the least (H₂S) versus the highest compound (DMDS)).     

Studies of the naturally occurring biomethylation of selenium and the determination of the products

A systematic study of the biomethylation of selenium and the determination of the methylated species indicates preliminarily that selenium is susceptible to natural biomethylation under certain environmental conditions. Detectable levels of methylated selenium species, including dimethyl selenide [(CH₃)₂Se], dimethyl diselenide [(CH₃)₂Se₂] and dimethylselenone [(CH₃)₂SeO₂] have been detected by gas chromatography – graphite furnace atomic absorption spectrophotometry (GC – GF AA) from a variety of environmental samples. Findings of naturally methylated selenium species from both soil samples and related air samples suggest that there may exist a localized cycle of selenium between ground soil and the ambient air. Factors that influence the sensitivity and accuracy for the determination of alkyl selenide compounds by GC – GF AA have also been investigated. Flashlike injection mode and addition of about 10% of hydrogen gas to the argon carrier gas provide for highly sensitive detection. Reproducible determination can be obtained with a precision of about 6% and the detection limits are 0.3 ng Se m^?3.     

Organotin speciation in environmental samples by capillary gas chromatography and pulsed flame photometric detection (PFPD)

Pulsed flame photometric detection (PFPD) for gas chromatography was applied to organotin compounds as standards and in environmental samples. Ethylated organotin species (n-propyl-, n-butyl- and phenyl-) were extracted from spiked artificial seawater and from an environmental sample. Selectivity towards tin is shown in the analysis of highly polluted seawater samples from a commercial port where no significant interferences are found. The self-cleaning capability and long-term stability of PFPD is shown in this work during 140 days of continuous operation. The absolute limit of detection for this capillary GC–PFPD technique ranged from 0.2 to 0.4 pg (Sn) for tetraethyl- to tetraphenyl-tin, allowing determination of sub-nanogram/litre concentrations of organotin compounds. © 1997 John Wiley & Sons, Ltd.     

Surface structure of silica gel reacted with 3-mercaptopropyltriethoxysilane and 3-aminopropyltriethoxysilane: formation of the S–S bridge structure and its characterization by Raman scattering and diffuse reflectance Fourier transform spectroscopic studies

 Silica gel samples, modified with 3-mercaptopropyltriethoxysilane, and their S–S-bridged samples have been prepared. In order to characterize the microstructure of the surface of these silica gel samples, Raman scattering and diffuse reflectance Fourier transform IR spectra of these samples have been examined by comparison with Raman spectra of various n-alkyl disulfides and their related silane polymers. The S–S and C–S stretch modes characteristic of the CH₂SSCH₂ segment, in addition to the SH and C–S stretch modes of the CH₂SH segment, have been assigned for these silica gel samples. It has been found that, even on the surface of the silica gel, a specific conformer is stabilized about the CH₂SSCH₂ segment. Received: 1 May 2001 Accepted: 16 June 2001     

Routine analysis of mercury species using commercially available instrumentation: chemometric optimisation of the instrumental variables

Mercury speciation analysis (inorganic mercury, Hg²⁺, methylmercury, CH₃Hg⁺ and dimethylmercury, (CH₃)₂Hg) by gas chromatography (GC) coupled to atomic emission spectroscopy with microwave induced plasma as excitation source (MIP-AES), after ethylation of the sample and extraction of the derivatised species into an organic phase, has been optimised using factorial design, analysis of variance and MultiSimplex techniques. Standard conditions were used in the derivatisation step with sodium tetraethylborate (NaB(C₂H₅)₄) and in the extraction step into hexane. Good separation of the species investigated and maximum sensitivity was achieved using an OV-1701 capillary column. The sensitivity was found to be maximum with an helium flow rate (make-up flow) of 100 ml min⁻¹. Procedures for a correct cleaning of glass and plastic ware, as well as for the purification of reagents used throughout the analytical process, are also suggested in order to avoid unacceptably high blank signals. The effect that ageing of stock solutions used in calibrations has on the artefact formation of CH₃Hg⁺ has been also investigated. Using the optimum conditions found, good quality calibration curves (R²>0.995) for the three mercury species were obtained. Absolute detection limits of 0.5, 3 and 15 pg of (CH₃)₂Hg, CH₃Hg⁺ and Hg²⁺, respectively, were estimated. The repeatability of the analysis was found to be better than 5% (n=5) in relative standard deviation (R.S.D.) units. The optimised procedure for the speciation of mercury in standard samples is the first step in the development of a method for routine analysis of mercury species in aquatic environmental samples.     

Decomposition of Gaseous Sulfide Compounds in Air by Pulsed Corona Discharge

The effectiveness of applying a pulsed corona discharge to the destruction of olfactory pollution in air was investigated. This paper presents a comparative study of the decomposition of three representative sulfide compounds in diluted concentrations: hydrogen sulfide (H₂S), dimethyl sulfide (DMS), and ethanethiol (C₂H₅SH), which could be completely removed when a sufficient but reasonable energy density was deposited in the gas. DMS showed the lowest energy cost (around 30 eV/molecules); C₂H₅SH and H₂S had an EC of respectively 45 eV and 115 eV. The efficiency of the non-thermal plasma process increased with decreasing the initial concentration of sulfide compounds, while the energy yield remained almost unchanged. SO₂ was the only identified byproduct of H₂S decomposition, but the sulfur balance suggests the formation of undetected SO₃. The byproducts analyzed during the degradation of DMS and C₂H₅SH enabled to propose a reaction mechanism, starting with radical attack and breaking of C–S bonds.