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     

Large Volume Headspace Analysis Using Solid-Phase Microcolumn Extraction

A rapid and simple large volume headspace (HS) sampling technique termed headspace solid-phase microcolumn extraction (HS-SPMCE) is described. HS gas above a liquid or solid sample is aspirated by attaching a gas-tight syringe onto a glass thermal desorption tube filled with Tenax sorbent. The trapped analytes are recovered by thermal desorption for gas chromatography–mass spectrometry (GC–MS) analysis. Benzene, toluene, ethylbenzene and the xylene isomers (BTEX) are used as model compounds to demonstrate the application of the extraction procedure for water samples. The results of the tests of the effect of agitation time and aspiration rate on recovery of the analytes show a good robustness of the method. BTEX are determined in the linear range from 0.5 to 50.0 μg L^?1 with limits of detection (3 σ) ranging within 0.09–0.14 μg L^?1 (MS was in scan mode). The method provides a good repeatability (RSD < 9%) and only a negligible carryover effect was observed ( ≤0.05%) when analysing BTEX at concentration 50.0 μg L^?1.     

Development of a method for the monitoring of odor‐causing compounds in atmospheres surrounding wastewater treatment plants

This study describes the development of an analytical method based on active collection in a multisorbent Tenax TA/Carbograph 1TD tube, followed by thermal desorption and GC‐MS for the determination of 16 volatile organic compounds in air samples. The analyzed compounds include ozone precursors and odor‐causing compounds belonging to different chemical families (sulfur‐ and nitrogen‐containing compounds, aldehydes, and terpenes). Two types of sorbents were tested and desorption conditions (temperature, time, and sampling, and desorption flow) were evaluated. External calibration was carried out using the multisorbent bed. Method detection limits in the range 0.2–2.0 μg m^?3 for 1 L samples were obtained. The method was applied for determining the target compounds in air samples from two different wastewater treatment plants. Most compounds were detected and toluene, limonene, and nonanal were found in particularly high concentrations with maximum values of 438, 233, and 382 μg m^?3, respectively.     

Simple preparative gas chromatographic method for isolation of menthol and menthone from peppermint oil, with quantitative GC-MS and (1) H NMR assay

The quantitative performance of a simple home‐built preparative gas chromatography (prep‐GC) arrangement was tested, incorporating a micro‐fluidic Deans switch, with collection of the target compound in a deactivated uncoated capillary tube. Repeat injections of a standard solution and peppermint sample were made into the prep‐GC instrument. Individual compounds were eluted from the trapping capillary, and made up to constant volume. Chloronaphthalene internal standard was added in some cases. Recovered samples were quantitatively assayed by using GC‐MS. Calibration linearity of GC‐MS for menthol standard area response against number of injections (2–20 repeat injections) was excellent, giving R² of 0.996. For peppermint, menthol correlation over 2–20 repeated injections was 0.998 for menthol area ratio (versus IS) data. Menthone calibration for peppermint gave an R² of 0.972. ¹H NMR spectroscopy was conducted on both menthol and menthone. Good correspondence with reference spectra was obtained. About 80 μg of isolated menthol and menthone solute was collected over a sequence of 80 repeat injections from the peppermint sample, as assayed by 600 MHz ¹H NMR analysis (～100% recovery for menthol from peppermint). A procedure is proposed for prediction of number of injections required to acquire sufficient material for NMR detection.     

Experimental validation of an effective carbon number-based approach for the gas chromatography–mass spectrometry quantification of ‘compounds lacking authentic standards or surrogates’

For the quantitative analysis of ‘compounds lacking authentic standards or surrogates’ (CLASS) in environmental media, we previously introduced an effective carbon number (ECN) approach to develop an empirical equation for the prediction of their response factor (RF). In this research, a series of laboratory experiments were carried out to benchmark the reliability of an ECN approach for sorbent tube/thermal desorption/gas chromatography (GC)/mass spectrometry (MS) applications. First, the ECN values were determined using external calibration data from 25 reference volatile organic compounds (VOCs) using two MS dectectors (quadrupole (Q) and time-of-flight (TOF)). Then, a certified standard mixture of 54 VOCs was analyzed by each system as a simulated unknown sample. The analytical bias, assessed in terms of percentage difference (PD) between the certified and ECN-predicted mass values, averaged 19.2 ± 16.1% (TOF-MS) and 28.2 ± 27.6% (Q-MS). The bias using a more simplified carbon number (CN)-based prediction increased considerably, yielding 53.4 ± 53.3% (TOF-MS) and 61.7 ± 81.3% (Q-MS). However, the bias obtained using the ECN-based prediction decreased significantly to yield average PD values of 9.84 ± 7.28% (TOF-MS) and 16.8 ± 8.35% (Q-MS), if the comparison was limited to 26 (out of 54) VOCs with CN ≥ 4 (i.e., 25 aromatics and hexachlorobutadiene).     

Membrane protected conductive polymer as micro‐SPE device for the determination of triazine herbicides in aquatic media

A micro‐SPE technique was developed by fabricating a rather small package including a polypropylene membrane shield containing the appropriate sorbent. The package was used for the extraction of some triazine herbicides from aqueous samples. Solvent desorption was subsequently performed in a microvial and an aliquot of extractant was injected into GC‐MS. Various sorbents including aniline‐ortho‐phenylene diamine copolymer, newly synthesized, polypyrrole, multiwall carbon nanotube, C18 and charcoal were examined as extracting media. Among them, conductive polymers exhibited better performance. Influential parameters including extraction and desorption time, desorption solvent and the ionic strength were optimized. The developed method proved to be rather convenient and offers sufficient sensitivity and good reproducibility. The detection limits of the method under optimized conditions were in the range of 0.01–0.04 ng/mL. The RSDs at a concentration level of 0.1 ng/mL were obtained between 4.5 and 9.3% (n=5). The calibration curves of analytes showed linearity in the range of 0.05–10 ng/mL. The developed method was successfully applied to the extraction of selected triazines from real water samples. The whole procedure showed to be conveniently applicable and quite easy to manipulate.     

Critical role of a pre‐purge setup in the thermal desorption analysis of volatile organic compounds by gas chromatography with mass spectrometry

In general, volatile organic compounds in ambient air are quantified by following a well‐defined standard calibration procedure using a gas‐/liquid‐phase standard. If the liquid standard is analyzed by a thermal desorption, the solvent effect is unavoidable through the alteration of breakthrough properties or retention times. To learn more about the variables of the thermal desorption‐based analysis, the effect of pre‐purge conditions was evaluated for 18 volatile organic compounds with different types of sorbent tube materials by fixing standard volume (1 μL) and flow rate (100 mL/min). The gas phase calibration was also carried out as reference for the non‐solvent effect. A single tube filled with Tenax TA exhibited the least solvent effect with the short pre‐purge (1 min), while being subject to the breakthrough at or above 10 min pre‐purge. For a three‐bed sorbent tube with Carboxen 1000, at least 10 min of pre‐purge was needed for the compounds with a retention time close to methanol (e.g., propanal). Another three‐bed tube with Carbopack X reduced the solvent effect efficiently for a short pre‐purge (2 min) without the breakthrough. As such, the solvent effect can be adjusted by the proper control of the sorbent tube application.     

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.     

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.     

Selective determination of alkylmercury compounds in solid matrices after subcritical water extraction,followed by solid‐phase microextraction and GC–MS

A method for the extraction and determination of methylmercury (MeHg) in solid matrices is presented. Combining the advantages of two extraction techniques—subcritical water extraction (subWE) and solid‐phase microextraction (SPME)—selective separation of MeHg from soils is possible. The procedure is based on extraction with subcritical water without using organic solvents, followed by in situ aqueous‐phase derivatization with sodium tetraethylborate and headspace SPME with a silica fiber coated with poly(dimethylsiloxane). The optimization of the extraction parameters is described. The identification and quantification of the extracted alkylmercury compounds from spiked soil samples is performed by GC–MS after thermal desorption. Copyright © 2000 John Wiley & Sons, Ltd.     

Needle‐trap device for the sampling and determination of chlorinated volatile compounds

A needle‐trap device, with immobilized sorbent inside the syringe, coupled with GC–MS was applied for air sampling and determination of chlorinated volatile organic compounds such as dichloromethane, trichloromethane, and tetrachloromethane. The application of a needle trap packed with combination of three sorbents including Tenax TA, Carbopack X, and Carboxen 1000 resulted in detection limits of few pg for chlorinated volatile compounds and recoveries of 99.2–102.8%. The extraction and desorption parameters were optimized within the study. As a result, the precision determined as RSD was equal to 5.05 and 3.03 and 6.52% for dichloromethane, trichloromethane, and tetrachloromethane, respectively. The storage time for chlorinated compounds up to 48 h and reusability of the needle‐trap device were verified. The obtained results have proved the ability of needle traps to compete with other solventless sampling and sample preparation extraction techniques.     

Needle microextraction trap for on-site analysis of airborne volatile compounds at ultra-trace levels in gaseous samples

Different capillary needle trap (NT) configurations are studied and compared to evaluate the suitability of this methodology for screening in the analysis of volatile organic compounds (VOCs) in air samples at ultra-trace levels. Totally, 22 gauge needles with side holes give the best performance and results, resulting in good sampling flow reproducibility as well as fast and complete NT conditioning and cleaning. Two different types of sorbent are evaluated: a graphitized carbon (Carbopack X) and a polymeric sorbent (Tenax TA). Optimized experimental conditions were desorption in the GC injector at 300°C, no make-up gas to help the transport of the desorbed compounds to the GC column, 1 min splitless time for injection/desorption, and leaving the NT in the hot injector for about 20 min. Cross-contamination is avoided when samples containing high VOC levels (above likely breakthrough values) are evaluated. Neither carryover nor contamination is detected for storage times up to 48 h at 4°C. The method developed is applied for the analysis of indoor air, outdoor air and breath samples. The results obtained are equivalent to those obtained with other thermal desorption devices but have the advantage of using small sample volumes, being simpler, more economical and more robust than conventional methodologies used for VOC analysis in air samples.     

Development of solid-phase extraction method for the determination of aromatic sensitisers in paper mill effluent

In this paper, a procedure for the determination of 11 aromatic hydrocarbon-type sensitisers and their related compounds from water samples, used in the manufacture of thermal paper, is presented. The compounds were extracted using a solid-phase extraction (SPE) cartridge with an octadecyl (C18) or a phenyl-bonded silica (PH) sorbent and then determined by gas chromatography–mass spectrometry (GC–MS). Factors affecting the performance of the extraction steps were thoroughly evaluated, and their effects on the yield of the sample preparation were discussed. Under optimised experimental conditions, SPE cartridges were conditioned with 10?mL hexane followed subsequently by 10?mL methanol, loaded with water sample at 2?mL?min^?1, and eluted with 10?mL hexane at 1.5?mL?min^?1. The limits of detection and quantification, calculated for signal-to-noise ratios of 3 and 10, were in the range of 1–5?µg?L^–1 and 2.5–10?µg?L^–1, respectively. Recovery yields of the present method using river water were in the range of 88%–112% with a C18 sorbent and 86%–116% with a PH sorbent. The repeatability, expressed as a relative standard deviation, was in the range of 2.8%–11% with a C18 sorbent and 0.7%–9.7% with a PH sorbent (n?=?4). Analysis of paper mill effluents revealed the presence of aromatic hydrocarbon-type sensitisers with maximum concentrations of up to 5.2?µg?L^?1.     

Determination of the volatiles from tobacco by capillary gas chromatography with atomic emission detection and mass spectrometry

A new gas chromatograph‐atomic emission detector (GC‐AED) coupled with Deans switching technique for analyzing volatiles from tobaccos were developed. The detector operating parameters (reagent gas pressure and make‐up gas flow rate) were optimized. The detection limits for the elements carbon (193 nm), hydrogen (486 nm) and oxygen (171 nm) ranged 0.05–0.2, 0.05–0.3 and 1–11 ng, respectively, depending on the compound. The sensitivity and linearity for the elements carbon (193 nm), hydrogen (486 nm) and oxygen (171 nm) decreased in the order O>H>C. Calibration curves were obtained by plotting peak area versus concentration, and the correlation coefficients relating to linearity were at least 0.9359. Elemental response factors measured on these channels, relative to the carbon 193‐nm channel, were hydrogen, 0.38–0.48 (mean %RSD=5.64), and oxygen, 0.085–0.128 (mean %RSD=14.9). The evaluation was also done for the new technique and for an established GC‐MS technique for the same real samples. The results of GC‐AED and GC‐MS showed that there was a relatively good agreement between the two sets of data.     

Sorbent-based sampling methods for volatile and semi-volatile organic compounds in air. Part 2. Sorbent selection and other aspects of optimizing air monitoring methods

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. Applications range from atmospheric research and ambient air monitoring (indoor and outdoor) to occupational hygiene (personal exposure assessment) and measuring chemical emission levels. Part 1 of this paper reviewed the main sorbent-based air sampling strategies including active (pumped) tube monitoring, diffusive (passive) sampling onto sorbent tubes/cartridges plus sorbent trapping/focusing of whole air samples that are either collected in containers (such as canisters or bags) or monitored online. Options for subsequent extraction and transfer to GC(MS) analysis were also summarised and the trend to thermal desorption (TD)-based methods and away from solvent extraction was explained. As a result of this trend, demand for TD-compatible sorbents (alternatives to traditional charcoal) is growing. Part 2 of this paper therefore continues with a summary of TD-compatible sorbents, their respective advantages and limitations and considerations for sorbent selection. Other analytical considerations for optimizing sorbent-based air monitoring methods are also discussed together with recent technical developments and sampling accessories which have extended the application range of sorbent trapping technology generally.     

The perspectives of ethanol usage as an internal standard for the quantification of volatile compounds in alcoholic products by GC‐MS

The potential use of ethanol as an internal standard (IS) for GC‐MS analysis was studied. The paper describes the analysis of spirit drinks and other alcoholic products which consist of a mixture of water, ethanol, and volatile compounds. In the suggested method, ethanol was employed as an IS for the common procedure of volatile compounds quantification. A number of standard solutions of nine compounds with different concentrations was prepared in a water‐ethanol matrix and measured with GC‐MS in the SIM mode. Two possible approaches were suggested to avoid detector saturation during ethanol detection. The first one consisted in using less abundant m/z 47 as quantifiers. These ions mainly correspond to unfragmented heavy ethanol molecules containing one ¹³C isotope. The second method consisted in reduction of the voltage of MS electron multiplier. The experiment also included the preparation and subsequent dilution of the standard solution and ethanol with water, which determined the linearity of the modified MS response relative to the ethanol content. Analysis of the obtained results revealed that volatile compounds can be successfully accurately determined with GC‐MS by employing ethanol as an IS. Application of the suggested method is not limited to the reported volatile compounds and alcoholic products.     

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.     

Solid‐phase extraction using bis(indolyl)methane‐modified silica reinforced with multiwalled carbon nanotubes for the simultaneous determination of flavonoids and aromatic organic acid preservatives

A novel bis(indolyl)methane‐modified silica reinforced with multiwalled carbon nanotubes sorbent for solid‐phase extraction was designed and synthesized by chemical immobilization of nitro‐substituted 3,3′‐bis(indolyl)methane on silica modified with multiwalled carbon nanotubes. Coupled with high‐performance liquid chromatography analysis, the extraction properties of the sorbent were evaluated for flavonoids and aromatic organic acid compounds. Under optimum conditions, the sorbent can simultaneously extract five flavonoids and two aromatic organic acid preservatives in aqueous solutions in a single‐step solid‐phase extraction procedure. Wide linear ranges were obtained with correlation coefficients (R²) ranging from 0.9843 to 0.9976, and the limits of detection were in the range of 0.5–5 μg/L for the compounds tested. Compared with the silica modified with multiwalled carbon nanotubes sorbent and the nitro‐substituted 3,3′‐bis(indolyl)methane‐modified silica sorbent, the developed sorbent exhibited higher extraction efficiency toward the selected analytes. The synergistic effect of nitro‐substituted 3,3′‐bis(indolyl)methane and multiwalled carbon nanotubes not only improved the surface‐to‐volume ratio but also enhanced multiple intermolecular interactions, such as hydrogen bonds, π–π, and hydrophobic interactions, between the new sorbent and the selected analytes. The as‐established solid‐phase extraction with high‐performance liquid chromatography and diode array detection method was successfully applied to the simultaneous determination of flavonoids and aromatic organic acid preservatives in grape juices with recoveries ranging from 83.9 to 112% for all the selected analytes.     

Exploration of the problems and solutions related to reference introduction prior to calibration of thermal desorber–gas chromatography

Reference introduction in thermal desorption with gas chromatography is a critical aspect. It is mostly performed by offline liquid calibration using a micro syringe to inject a liquid standard solution on the sorbent in the thermal desorption tube. This is based on the assumptions that the adsorption‐desorption process is quantitative and that no sample is lost in manipulating the tube. However, for analytical procedures involving thermal extraction of solid matrices, the adsorption‐desorption processes for sample and reference differ and the assumptions are not always fulfilled. This is explored in this work. First, issues related to the online liquid calibration were investigated. With tubes containing only quartz filters, a relative loss of over 80% was noticed for some solvents due to tube manipulation processes. Enclosing a bed of mesoporous silica as sorbent limited the losses to about 25% when samples were immediately analysed, and even better results were obtained when tubes were stored for several hours so that proper adsorption could take place. An additional sweep gas during loading boosted the transfer of analytes with recoveries above 95%. Next, an inline injection system was installed on the thermal desorber instrument. This sorbent free, independent calibration tool avoids the drawbacks of other approaches.     

Novel sample preparation technique with needle-type micro-extraction device for volatile organic compounds in indoor air samples

A novel needle-type sample preparation device was developed for the effective preconcentration of volatile organic compounds (VOCs) in indoor air before gas chromatography–mass spectrometry (GC–MS) analysis. To develop a device for extracting a wide range of VOCs typically found in indoor air, several types of particulate sorbents were tested as the extraction medium in the needle-type extraction device. To determine the content of these VOCs, air samples were collected for 30 min with the packed sorbent(s) in the extraction needle, and the extracted VOCs were thermally desorbed in a GC injection port by the direct insertion of the needle. A double-bed sorbent consisting of a needle packed with divinylbenzene and activated carbon particles exhibited excellent extraction and desorption performance and adequate extraction capacity for all the investigated VOCs. The results also clearly demonstrated that the proposed sample preparation method is a more rapid, simpler extraction/desorption technique than traditional sample preparation methods.