Determination of highly volatile organic contaminants in water by the closed-loop gaseous stripping technique followed by thermal desorption of the activated carbon filters

Very volatile organic contaminants in water were determined by using closed-loop gaseous stripping combined with thermal desorption from the activated carbon filter into a high-resolution gas chromatograph. The operating parameters for quantitative applications were evaluated. The solvent-free thermal desorption procedure permits the determination of compounds that normally elute under the gas chromatographic solvent peak (e.g., dichloromethane and Freons). Sixteen volatile compounds with boiling points in the range -30 to 120 degrees C were determined with an overall recovery of 12-52%. Qualitative determinations of volatiles from a secondary sewage effluent were in good agreement with the results found by two more established methods.     

Analysis of airborne substances by means of microwave thermal desorption

A method has been developed to enable thermal desorption of airborne substances from solid adsorbents based on active charcoal by means of microwave energy, followed by on-line GC-FID analysis. Ceramic traps were filled with 50 mg of commercially available graphitized carbon black. Adsorption efficiency and desorption conditions for three substances covering a broad range of vapor pressure were investigated. The recovery rate, breakthrough, and linearity of the system were determined by direct fortification with standard solutions, followed by microwave thermal desorption and gas chromatographic separation. The recovery efficiencies were 80–84 %, with a standard deviation of 3–6 %. The correlation coefficient of the linear range between 1 (10 for one of the compounds) and 1000 ng was > 0.999. Initial experiments were performed to investigate the enrichment of the standards from the gaseous state. The recovery rates of the most volatile compound was increased by optimizing the density of the packing.     

Trace analysis of organics in aqueous samples by concentration in plastic tubing and multiplex gas chromatography

Summary The inside wall of an uncoated polyethylene capillary traps organic substances from a water sample pumped through it by a nitrogen gas stream. Heating the capillary in a chromatographic oven slowly releases the trapped organic substances from the wall. Nitrogen carrier gas transports sample substances released through a thermal desorption modulator and onto a chromatographic column. Pulsing the temperature of the modulator modulates the concentrations of sample components as they enter the column. Computing the cross correlation of the detector output signal against the applied modulation signal generates the chromatogram. Detection limits below 1 ppb are possible using a flame ionization detector. No sample pretreatment or cold trap is required.     

Construction of a cryogen‐free thermal desorption gas chromatographic system with off‐the‐shelf components for monitoring ambient volatile organic compounds

An automated gas chromatographic system aimed at performing unattended measurements of ambient volatile organic compounds was configured and tested. By exploiting various off‐the‐shelf components, the thermal desorption unit was easily assembled and can be connected with any existing commercial gas chromatograph in the laboratory to minimize cost. The performance of the complete thermal desorption gas chromatographic system was assessed by analyzing a standard mixture containing 56 target nonmethane hydrocarbons from C₂–C₁₂ at sub‐ppb levels. Particular attention was given to the enrichment efficiency of the C₂ compounds, such as ethane (b.p. = –88.6°C) and ethylene (b.p. = –104.2°C), due to their extremely high volatilities. Quality assurance was performed in terms of the linearity, precision and limits of detection of the target compounds. To further validate the system, field measurements of target compounds in ambient air were compared with those of a commercial total hydrocarbon analyzer and a carbon monoxide analyzer. Highly coherent results from the three instruments were observed during a two‐month period of synchronized measurements. Moreover, the phenomenon of opposite diurnal variations between the biogenic isoprene and anthropogenic species was exploited to help support the field applicability of the thermal desorption gas chromatographic method.     

Automated high-speed analysis of selected organic compounds in urban air by on-line isotopic dilution cryofocusing gas chromatography/mass spectrometry

An automated environmental air monitor has been developed to measure selected organic compounds in urban air. The instrument is based on a cryofocusing-thermal desorption gas chromatographic mass spectrometry technique where the mass spectrometer is a slightly modified residual gas analyzer (RGA). The RGA was chosen as a detector because the whole system must be robust for long periods, with 24-h continuous air monitoring. RCA are extremely simple and seemed the most reliable mass spectrometers for this purpose. Moreover, because they have no physically limited ion source, contamination is considerably reduced, so maintenance intervals are longer. The gas chromatograph is equipped with a computer-controlled six-way sampling valve, with a 100-mL sampling loop and thermal desorption cold trap injector. Environmental air is enriched with an isotopically labeled internal standard in the sampling line. This internal standard is added with a validated, custom-made, permeation tube device. The “on-line” internal standard provides for high quality quantitative data because all variations in instrument sensitivity in cryofocusing or in thermal desorption efficiency are taken into account. High repetition rates (down to 5 min for a full analytical cycle) are obtained with the use of an isothermal gas chromatography program, microbore capillary column, and environmental air sampling during the gas chromatography run.     

Thermal desorption cold trap-injection in high-resolution gas chromatography: multivariate optimization of experimental conditions.

In studies of low concentrations of volatile compounds in air, the method of adsorption on porous polymers and determination by thermal desorption cold trap-injection high-resolution gas chromatography is finding increasing application. Factors considered important for injection and chromatographic separation of volatile compounds by this method were investigated with the use of multivariate techniques. For the amount injected on to the chromatographic column, the factors of main importance were found to be the temperature of the injection block, the thickness of the internal coating of the cold trap and the flow-rate. Strong interaction effects were noted. For the sharpness of the chromatographic peaks, the flow-rate was the most important factor.     

Micropreparative system for enrichment of capillary GC effluents

An automated system for preparative gas chromatography with capillary columns is described. The effluet from the capillary column is switched to the FID detector or to the traps by means of a Live-T switching device. The pneumatics is controlled by a microprocessor so that repetitive sampling can be performed over a period of days in order to enrich sufficient amount of material for NMR or other spectroscopic methods. The effluent containing the compounds is collected in glass tubes filled with column packing material (e.g. Chromosorb coated with 3% OV - 101, crosslinked). The trap temperatue can be adjusted from + 20°C to ? 80°C, depending on the trapping material and volatility of trapped substances. The analysis of enriched substances or chromatographic fractions can be performed by thermal desorption of the same traps or by solven elution. The recovery of enriched substances is higher than 90%. High capacity and resolution for enrichment of trace components are obtained with the aid of a double column-double oven system. Examples of such applications are given.     

Evaluation of a hypercrosslinked polystyrene, MN-200, as a sorbent for the preconcentration of volatile organic compounds in air

Breakthrough volumes, average percentage recoveries, and storage stabilities were obtained for vapors of 8 volatile organic compounds (pentane, octane, undecane, isooctane, cyclohexane, toluene, methanol, and dichloromethane) on a new adsorbent material, Hypersol-Macronet, MN-200. Breakthrough volumes were estimated as half of the gas chromatographic specific retention volumes at 20 degrees C for the compounds. Recoveries of the adsorbates were determined by both solvent extraction and thermal desorption methods. The results obtained compare favorably with those for Tenax GR (values reported in the published literature and others obtained in our laboratory). Results of storage stability studies on MN-200 meet the criterion for acceptability (<10% loss). High adsorption capacity for very volatile and polar compounds, combined with ease of desorption of less volatile compounds, render MN-200 a highly promising adsorbent for sampling volatile organic compounds in indoor and outdoor air.     

Volatile organic compounds in urban atmospheres: Long-term measurements of ambient air concentrations in differently loaded regions of Leipzig

For the comprehensive characterization of ambient air concentrations of a broad spectrum of volatile organic compounds (VOCs) an analytical method is described, consisting of adsorptive enrichment, thermal desorption without cryofocusing, and capillary gas chromatographic separation. The method was applied during two-week measuring campaigns in winter and summer 1995, and in the winter of 1996. Long-term sampling was carried out at sampling points in residential areas in the suburbs and near the city center of Leipzig. About 70 VOCs – mainly hydrocarbons from propene to hexadecane – were identified both by GC-MS and chromatographic retention data and quantified after external calibration. Mean values of VOC concentrations obtained during the sampling periods are reported and discussed with regard to the topographical location of the sampling points in the Leipzig area, seasonal variations, and possible emission sources.     

用于昆虫外激素研究的微量顶空收集及热解吸气相色谱进样方法初探

描述了一个结合项空气流收集与无溶剂热解吸气相色谱进样的方法。用填充PorapskQ的微量注射器作为吸附管进行气流收集。将收集物不经溶剂洗脱直接进行热解吸进样。用人工合成的昆虫外激素化合物反-7-十二碳单烯乙酸酯（E-7-DA）及顺-5,反-7-十二碳二烯乙酸酯（Z-5,E-7-DDA）测定了方法的回收率,初步探索了运用于昆虫外激素分析的可行性,并讨论了提高回收率的途径。     

Implementation and optimisation of a high‐temperature loading strategy of liquid standards in the quantification of volatile organic compounds using solid sorbents

High‐temperature liquid standard loading strategy onto solid sorbent traps for calibration of thermal desorption–GC–MS techniques for the analysis of volatile organic compounds is evaluated and optimised. With this proposed set‐up, volatilised liquid‐loaded standards interact in gas phase with solid sorbent particles. Response factor for 15 volatile compounds with different vapour pressures have been evaluated and compared with common strategies based on liquid matrix interactions. Using gas matrix strategy improves signal output in the range 10–700%. Average increase for benzene, toluene, ethylbenzene and xylenes is 480%. Reported systematic bias between liquid standards and gas samples are reduced, enhancing thermal desorption methodologies on one of its more important issues. In addition, the proposed system improves the average repeatability to a 3.2%, over 13 times some reported data. The use of an ultra‐thin GC capillary column of 150 μm id performs better peak resolution in about 60% the time with usual 250 μm id capillary columns. The usefulness of this proposed optimised procedure has been proved in real air matrix samples, through a large study with the reliable characterisation of 93 different volatile compounds in the ambient air of a municipal solid waste treatment area     

Trace analysis of semi-volatile organic air pollutants using thick film silicone rubber traps with capillary gas chromatography

Established standard methods for analyzing semi-volatile pollutants by capillary gas chromatography are cumbersome, time-consuming, and expensive. With the longer term aim of replacing the sample preparation procedures with convenient concertrating and thermal desorption steps, thick film silicone rubber traps were constructed in a novel multi-channel configuration. Tests were performed to determine breakthrough volumes and thermal desorption characteristics for total transfer of the trap contents to a capillary column. Initial results are very promising.     

Experimental study on solvent-less sample preparation methods: Membrane extraction with a sorbent interface, thermal membrane desorption application and purge-and-trap

Different solvent-free sample preparation techniques for the enrichment of volatile and semivolatile organic compounds from aqueous samples for subsequent gas chromatographic separation and detection are compared. The methods under study are purge-and-trap, membrane extraction with a sorbent interface in two different configurations, and thermal membrane desorption application. The study has been performed with polar as well as with non-polar compounds in respect to sampling yield, enrichment, repeatability and analysis cycle rate. All experiments have been performed with a mobile GC–MS system.     

Sorbent-based sampling methods for volatile and semi-volatile organic compounds in air: Part 1: Sorbent-based air monitoring options

Sorbent tubes/traps are widely used in combination with gas chromatographic (GC) analytical methods to monitor the vapour-phase fraction of organic compounds in air. Target compounds range in volatility from acetylene and freons to phthalates and PCBs and include apolar, polar and reactive species. Airborne vapour concentrations will vary depending on the nature of the location, nearby pollution sources, weather conditions, etc. Levels can range from low percent concentrations in stack and vent emissions to low part per trillion (ppt) levels in ultra-clean outdoor locations. Hundreds, even thousands of different compounds may be present in any given atmosphere. GC is commonly used in combination with mass spectrometry (MS) detection especially for environmental monitoring or for screening uncharacterised workplace atmospheres. Given the complexity and variability of organic vapours in air, no one sampling approach suits every monitoring scenario. A variety of different sampling strategies and sorbent media have been developed to address specific applications. Key sorbent-based examples include: active (pumped) sampling onto tubes packed with one or more sorbents held at ambient temperature; diffusive (passive) sampling onto sorbent tubes/cartridges; on-line sampling of air/gas streams into cooled sorbent traps; and transfer of air samples from containers (canisters, Tedlar^® bags, etc.) into cooled sorbent focusing traps. Whichever sampling approach is selected, subsequent analysis almost always involves either solvent extraction or thermal desorption (TD) prior to GC(/MS) analysis. The overall performance of the air monitoring method will depend heavily on appropriate selection of key sampling and analytical parameters. This comprehensive review of air monitoring using sorbent tubes/traps is divided into 2 parts. (1) Sorbent-based air sampling option. (2) Sorbent selection and other aspects of optimizing sorbent-based air monitoring methods. The paper presents current state-of-the-art and recent developments in relevant areas such as sorbent research, sampler design, enhanced approaches to analytical quality assurance and on-tube derivatisation.     

Ultra trace detection of perfluorocarbon tracers in reservoir gases by adsorption/thermal desorption in combination with NICI-GC/MS

A new method for the analysis of perfluorocarbon tracers (PFTs) in reservoir samples based on adsorption/thermal desorption in combination with NICI-GC/ MS is presented. The tracer compounds were trapped in tubes filled with a carbon molecular sieve and in a two-step procedure thermally desorbed before they were analyzed with NICI-GC/MS. The chromatographic background noise, visible on GC/ECD-systems, due to compounds normally present in petroleum reservoir gases, could not be seen with NICI-GC/MS. Determination of the perfluorocarbons in reservoir gas samples confirmed the applicability of the method. Tracer concentrations as low as 42 femtoliter/liter were detected.     

Direct thermal desorption in the analysis of cheese volatiles by gas chromatography and gas chromatography-mass spectrometry: comparison with simultaneous distillation-extraction and dynamic headspace

Direct thermal desorption (DTD) has been used as a technique for extracting volatile components of cheese as a preliminary step to their gas chromatographic (GC) analysis. In this study, it is applied to different cheese varieties: Camembert, blue, Chaumes, and La Serena. Volatiles are also extracted using other techniques such as simultaneous distillation-extraction and dynamic headspace. Separation and identification of the cheese components are carried out by GC-mass spectrometry. Approximately 100 compounds are detected in the examined cheeses. The described results show that DTD is fast, simple, and easy to automate; requires only a small amount of sample (approximately 50 mg); and affords quantitative information about the main groups of compounds present in cheeses.     

Ultra trace detection of perfluorocarbon tracers in reservoir gases by adsorption/thermal desorption in combination with NICI-GC/MS

A new method for the analysis of perfluorocarbon tracers (PFTs) in reservoir samples based on adsorption/thermal desorption in combination with NICI-GC/MS is presented. The tracer compounds were trapped in tubes filled with a carbon molecular sieve and in a two-step procedure thermally desorbed before they were analyzed with NICI-GC/MS. The chromatographic background noise, visible on GC/ECD-systems, due to compounds normally present in petroleum reservoir gases, could not be seen with NICI-GC/MS. Determination of the perfluorocarbons in reservoir gas samples confirmed the applicability of the method. Tracer concentrations as low as 42 femtoliter/liter were detected.     

Determination of Teucrium chamaedrys volatiles by using direct thermal desorption-comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry

The direct qualification and quantification of the volatile components of Teucrium chamaedrys was studied using a direct thermal desorption (DTD) technique with comprehensive two-dimensional (2D) gas chromatography-time-of-flight mass spectrometry (GC x GC-TOF/MS). The GC x GC separation chromatographically resolved hundreds of components within this sample, and with the separation coupled with TOF/MS for detection, high probability identifications were made for 68 compounds. The quantitative results were determined through the use of internal standards and the desorption of differing amounts of raw material in the injector. The highest yield of volatile compounds (0.39%, w/w) was obtained at 150 degrees C thermal desorption temperature using 1.0mg of dried sample placed in a glass injector liner when studied over the range 1.0-7.0mg. Lowest yield of 0.33% (w/w) was found for the largest sample size of 7.0mg. Relative standard deviation (RSD) for 10 replicates at each size sample were in the range 3.9-21.6%. The major compounds identified were beta-pinene, germacrene D, alpha-pinene, alpha-farnesene, alpha-gurjunene, gamma-elemene and gamma-cadinene. All identified compounds were quantified using total ion chromatogram (TIC) peak areas. DTD is a promising method for quantitative analysis of complex mixtures, and in particular for quantitative analysis of plant samples, which can yield data without the traditional obligation for costly and time-consuming extraction techniques.     

The determination of chlorinated aliphatic hydrocarbons in air, natural waters, marine organisms, and sediments

A gas Chromatographic procedure is described for the determination of chlorinated aliphatic hydrocarbons in the atmosphere, natural waters, aquatic organisms and sediments. Air samples are passed through activated carbon traps and the chloro compounds are later desorbed by heating in a current of nitrogen. Chloro compounds are stripped from water samples by bubbling with nitrogen and from bio-materials and sediments by heating in a current of nitrogen. In each instance, the chlorinated compounds are trapped in copper columns packed with Chromosorb coated with silicone oil, and cooled to -78°. The chloro compounds are subsequently swept off these columns into a gas chromatographic column with a current of argon. Detection of the Chromatographic peaks is performed with an electron-capture detector. The procedure gives near quantitative recoveries of a range of chlorinated hydrocarbons from natural samples.     

Isolation,Concentration and Subsequent Analysis by Capillary Gas Chromatography of Trace Amounts of Organophosphorus Compounds from Aqueous Samples

Abstract

Trace amounts (ppb or less) of phosphorus containing compounds present in aqueous samples are adsorbed on XAD-4 and subsequently eluted by means of ethyl acetate. The solvent and the eluted compounds are evaporated and swept over a Tenax-GC tube. This gas stripping method traps the phosphorus containing compounds together with only a small amount of the solvent whereas the water entrapped in the XAD step is removed simultaneously. The compounds are desorbed from the Tenax-GC tube and injected into the gas chromatograph using the combination of thermal desorption, cold trapping and flash heating. The subsequent analysis is carried out on a capillary column and the compounds are detected by means of a flame photometric detector. The various steps of the analytical procedure are discussed, including the recoveries of the different compounds studied and some instrumental aspects.