Adsorbent materials commonly used in air analysis for adsorptive enrichment and thermal desorption of volatile organic compounds

A review is given dealing with commonly used adsorbent materials in ambient air analysis of volatile organic compounds (VOCs). The adsorbents covered in the paper are selected in consideration of their compatibility with thermal desorption. Initially, we discuss the requirements that an adsorbent should fulfill, and useful parameters for the selection and evaluation of an appropriate material. Then, the most important materials are presented considering their properties, advantages, and drawbacks. A few applications are given, but a complete review of sampling techniques and applications dealing with adsorptive enrichment and thermal desorption is beyond the scope of this paper.     

Sample preparation for the analysis of volatile organic compounds in air and water matrices

This review summarizes literature data from the past 5 years on new developments and/or applications of sample preparation methods for analysis of volatile organic compounds (VOC), mainly in air and water matrices. Novel trends in the optimization and application of well-established airborne VOC enrichment techniques are discussed, like the implementation of advanced cooling systems in cryogenic trapping and miniaturization in adsorptive enrichment techniques. Next, focus is put on current tendencies in integrated sampling-extraction-sample introduction methods such as solid phase microextraction (SPME) and novel in-needle trapping devices. Particular attention is paid to emerging membrane extraction techniques such as membrane inlet mass spectrometry (MIMS) and membrane extraction with a sorbent interface (MESI). For VOC enrichment out of water, recent evolutions in direct aqueous injection (DAI) and liquid-liquid extraction (LLE) are highlighted, with main focus on miniaturized solvent extraction methods such as single drop microextraction (SDME) and liquid phase microextraction (LPME). Next, solvent-free sorptive enrichment receives major attention, with particular interest for innovative techniques such as stir bar sorptive extraction (SBSE) and solid phase dynamic extraction (SPDE). Finally, recent trends in membrane extraction are reviewed. Applications in both immersion and headspace mode are discussed.     

Accelerated solid-phase dynamic extraction for the analysis of biogenic volatile organic compounds in air

In this study, accelerated solid phase dynamic extraction (ASPDE) technique was used to identify biogenic volatile organic compounds (BVOCs) emitted from Norway spruce (Picea abies). Compounds that were determined in tree samples are: tricycylene, α-pinene, camphene, β-pinene, myrcene, 3-carene, p-cymene, limonene, cineole, α-phellandrene, α-terpinene, γ-terpinene and terpinolene. ASPDE showed a potential for the analysis of environmental samples as well as for field applications. This technique was further studied by using a gaseous mixture of BVOCs (sabinene, α-pinene, β-pinene, limonene, linalool, and (Z)-hexenyl acetate) and exhibited a good repeatability during all the experiments in the range of 2.5% (α-pinene) and 14.6% (linalool). However, during the analysis of samples it was observed that desorption at high temperature (230°C) can lead to the formation of artifacts, which were not observed at the desorption temperature of 100°C. Further experimental investigations revealed that monoterpenes appeared as unanticipated compounds during desorption of ASPDE samples; these compounds were degradation products of linalool.     

Development of second-generation sample enrichment probe for improved sorptive analysis of volatile organic compounds

The sample enrichment probe (SEP) has recently been introduced as a user-friendly and cost-effective method for the sorptive extraction of volatile organic analytes from gaseous and aqueous samples for GC and GC-MS analyses. In a further development of the SEP technique, thinner polydimethylsiloxane (PDMS) tubing on polyimide-coated fused silica, instead of stainless steel rods or stalks, were used to produce the second-generation SEPs. The new SEP does not require widening of the needle-guiding orifice of the septum cap and analytes are desorbed at a faster rate from the thinner sleeve, which reduces the risk of carry-over. The flowless period that was previously recommended for analyses of highly volatile analytes is made redundant by the faster desorption from the thinner sorptive medium. It was found that differences in the thermal histories of SEPs are not the cause of the high relative standard deviations (RSDs) reported in our first paper on the technique. Excellent reproducibility can be attained by careful handling and storing of loaded SEPs and by rigorously following a standardised analytical protocol.     

Potential of solid-phase microextraction fibers for the analysis of volatile organic compounds in air.

This work presents the usefulness of five different solid-phase microextraction fibers in the screening of volatile organic compound (VOC) traces in air samples. The performances of these fibers are compared by studying the sorption kinetics in an equimolar gaseous mixture of eleven VOCs. For each fiber, static and dynamic sampling are compared. It is shown that repeatability is better for the dynamic mode (less than 6% for dynamic sampling and 10% for static sampling). The equilibrium time and the sensitivity vary considerably from one fiber type to another. As an example, the classical polydimethylsiloxane (PDMS) coating presented the shortest equilibration time (5 min) but also the poorest sensitivity, whereas the PDMS-Carboxen showed the longest extraction time but the greatest sensitivity. The estimation of the quantity of VOCs fixed on the target fiber allows for the determination of the different affinities of the compounds with the involved sorbent and relates them with physicochemical properties of the molecules. Competitive sorption is observed for the fibers involved with the adsorption process (i.e., PDMS-divinylbenzene and PDMS-Carboxen fibers). These competitions can lead to SPME calibration problems and thus bad quantitative analysis.     

A versatile and uncomplicated method for the analysis of volatile organic compounds in ambient urban air

A method was developed for sampling and selective quantitative determination of typical volatile organic compounds (VOCs) in ambient urban air. A mobile and self-contained dual-channel air sampling tool based on solid phase adsorption was constructed. A simple calibration procedure circumventing the adsorption/desorption process was designed. The method was validated for seven “key-analytes”: n-hexane, 3-methyl-2-pentene, benzene, tetrachloroethene, styrene, 1,2,4-trimethylbenzene and acetophenone. The complete air sampling equipment is easily accommodated in a business suitcase. The lower limits of the practical working ranges are between 0.1 μg m^–3 (tetrachloroethene) and 1.2 μg m^–3 (benzene). Air samples were collected at a location in Salzburg with heavy motor vehicle traffic and measured in order to prove a satisfactory method performance under practical monitoring conditions.     

A versatile and uncomplicated method for the analysis of volatile organic compounds in ambient urban air

A method was developed for sampling and selective quantitative determination of typical volatile organic compounds (VOCs) in ambient urban air. A mobile and self-contained dual-channel air sampling tool based on solid phase adsorption was constructed. A simple calibration procedure circumventing the adsorption/desorption process was designed. The method was validated for seven “key-analytes”: n-hexane, 3-methyl-2-pentene, benzene, tetrachloroethene, styrene, 1,2,4-trimethylbenzene and acetophenone. The complete air sampling equipment is easily accommodated in a business suitcase. The lower limits of the practical working ranges are between 0.1 μg m^–3 (tetrachloroethene) and 1.2 μg m^–3 (benzene). Air samples were collected at a location in Salzburg with heavy motor vehicle traffic and measured in order to prove a satisfactory method performance under practical monitoring conditions. Received: 4 January 1998 / Revised: 14 September 1998 / Accepted: 21 October     

Sampling and analysis of volatile organic compounds in air using a dualsorbent trap

Summary A dualsorbent trap containing graphitized carbon blacks was used for the collection of volatile and semi-volatile organic pollutants from the atmosphere of different workplaces and from an above-ground parking lot. The method proved to be sensitive, simple and reliable. Thermal desorption and solvent extraction methods followed by GC-MS analysis were employed.     

Sorbent trapping of volatile organic compounds from air

The use of sorbents in trapping volatile organic compounds in air for subsequent analysis is reviewed. Sorbents are classified in accordance with the mechanism used to recover the trapped compounds, either solvent or thermal desorption. The use of sorbents is contrasted with other sampling procedures, such as collecting whole air samples using canisters. New developments such as solid-phase microextraction are described. In particular, emphasis is placed on a holistic approach to sampling and analysis, and communication is encouraged between those who take samples in the field, and those who perform the analysis.     

Solid-phase dynamic extraction for the enrichment of polar volatile organic compounds from water

Headspace solid-phase dynamic extraction coupled to gas chromatography-mass spectrometry (HS-SPDE-GC/MS) was evaluated for the trace determination of polar volatile organic compounds (PVOC) from aqueous matrices. The target compounds included 3 ethers and 12 alcohols. Four SPDE needle coatings with different phase polarities and sorption properties (WAX, 1701, PDMS, PDMS/AC) were tested. The effects of extraction temperature, number of extraction cycles, and ionic strength on partitioning of the target compounds have been investigated in detail, including the determination of salting-out constants for the investigated compounds. Lowest method detection limits (MDLs) were obtained with the WAX and the PDMS/AC phase. The WAX phase showed MDLs for ethers in the range of 0.06 microg/L (MTBE) to 0.8 microg/L (1,4-dioxane) and for alcohols between 0.02 microg/L (3-methyl-1-pentanol) and 3.5 microg/L (1-propanol). The evaluated MDLs for ethers with the PDMS/AC were in the range 0.06 microg/L (MTBE) to 1.2 microg/L (1,4-dioxane) and for alcohols between 0.004 microg/L (1-hexanol) and 4.9 microg/L (ethanol). Using either of these two phases, SPDE provides comparable or better sensitivities for the investigated compounds than other enrichment techniques, high sample throughput because of full automation, and short extraction times as well as a high robustness of the extraction phase because of its protection inside the steel needle. SPDE applicability has been demonstrated for the determination of fusel oils in different alcoholic beverages.     

Calibration of mass selective detector in non-target analysis of volatile organic compounds in the air

Volatile organic compounds (VOCs) play an important role in the chemistry of the atmosphere and in biogeochemistry. They contribute to the oxidative capacity of the atmosphere, particle and air pollutants, as well as to the production of greenhouse gases (for instance ozone). Among analytical techniques for their determination in the atmosphere gas chromatography coupled with mass spectrometry (GC-MS) offers several advantages. However, for an accurate quantification calibration with standard substances is necessary. A quantitative structure-property relationship (QSPR) model for the prediction of MS response factors was developed on basis of our experimental measurements for the quantification of ozone precursors present in the atmosphere. A linear correlation between chemical structures and response factors was established by using a 7-parameter MLR model. The average error in the prediction of response factors was calculated by cross-validation procedure and was below 20%, which is sufficient for the determination of VOCs in the air. The proposed procedure is time consuming so it is more suited for the quantification of tentatively identified organic compounds during the reprocessing of MS chromatograms in cases when the original sample is no longer available.     

Using mesoporous silica MCM-41 for in-line enrichment of atmospheric volatile organic compounds

A mesoporous silica MCM-41 with pore size of 29A was synthesized and assessed for its applicability as a sorbent for in-line trapping of volatile organic compounds (VOCs) from air samples. Several commercially available microporous carbon molecular sieves, i.e., Carbosieve SIII, Carboxen 1000, Carboxen 1003, and Carbotrap purchased from Supelco, were employed to form either single sorbent traps or multi-sorbent traps for comparing adsorption properties with those of the silica MCM-41. A standard gas mixture containing more than 50 target compounds with size varying from C(2) to C(12) was adsorbed by these sorbents and the per carbon response of flame ionization detection (FID) for the target compounds was calculated for obtaining the adsorption profiles. While the multi-carbon sorbents show very uniform adsorption ability across the entire carbon range from C(3) to C(12), the mesoporous silica MCM-41, however, shows little sorption for smaller molecules from C(3) to C(7), but exhibit comparable sorption ability for C(8)-C(12) compounds. Desorption at various temperatures indicates that C(8)-C(12) compounds once trapped can be easily released at moderate temperatures of about 150 degrees C, whereas for carbon sorbents the desorption temperatures for sufficient recovery need to go beyond 300 degrees C due to much tighter hold-up in the microporous structure. Sorption ability for MCM-41 is also reflected on linearity. Compounds with sufficient sorption as suggested by the adequate per carbon response also exhibit excellent precision and linearity with R(2) close to unity, an important requirement for quantitative analysis of ambient VOCs.     

Detection of volatile organic compounds indicative of human presence in the air

Volatile organic compounds were collected and analyzed from a variety of indoor and outdoor air samples to test whether human‐derived compounds can be readily detected in the air and if they can be associated with human occupancy or presence. Compounds were captured with thermal desorption tubes and then analyzed by gas chromatography with mass spectrometry. Isoprene, a major volatile organic compound in exhaled breath, was shown to be the best indicator of human presence. Acetone, another major breath‐borne compound, was higher in unoccupied or minimally occupied areas than in human‐occupied areas, indicating that its majority may be derived from exogenous sources. The association of endogenous skin‐derived compounds with human occupancy was not significant. In contrast, numerous compounds that are found in foods and consumer products were detected at elevated levels in the occupied areas. Our results revealed that isoprene and many exogenous volatile organic compounds consumed by humans are emitted at levels sufficient for detection in the air, which may be indicative of human presence.     

Dynamic versus static sampling for the quantitative analysis of volatile organic compounds in air with polydimethylsiloxane-carboxen solid-phase microextraction fibers

Polydimethylsiloxane-Carboxen solid-phase microextraction fibers are now well known to be very efficient trapping media for the analysis of volatile organic compound (VOC) traces in air. However, competitive adsorption, due to the nature of the coating, considerably limits analyte quantitation. In this contribution, different experimental conditions are investigated to achieve quantitative analysis. Static and dynamic sampling were compared for the analysis of 11 VOCs in a standard gaseous mixture at different extraction times (1, 5, 15 and 45 min). The same experiments were performed with four isolated compounds. Adsorption results from gas mixture and isolated compounds were compared and a common linear range (i.e., where quantitative analysis is conceivable) was determined. When sampling was in the dynamic mode, compounds with lower affinity for the coating showed a very narrow linear range, meaning that competition for adsorption was quickly discriminative. The same experiments in static mode allowed one to obtain wider linear ranges for all compounds, especially for lower-affinity compounds: for a 1 min sampling time, acetone showed a linear adsorption range from 3 to 60 microg m(-3) in the dynamic mode which extended from 5 to 300 microg m(-3) in the static mode.     

Novel multi-sorbent for sampling and determination of trace volatile organic sulfur compounds in ambient air

Novel adsorbent APSG-MW (average particle size 215?µm and specific surface 98 m²?g^?1) bonding multi-walled carbon nanotubes (MWCNTs) on silica gel are obtained. Then the sampling tubes filled with Tenax TA and APSG-MW are prepared and the adsorptive capacity of Tenax TA/APSG-MW for volatile organic sulfur compounds (VOSCs) is studied. The data show that the adsorption and desorption recoveries of multi-sorbent for VOSCs are satisfactory (>85%), and the breakthrough values are large (>16?L?g^?1) enough to absorb VOSCs in ambient air. The sampling precision of the sorbent tubes meets TO-17 criteria. The sampling tubes are successfully used to concentrate and analyze a sample of landfill air, and the major S compounds are identified.     

New methodologies in trace analysis of volatile organic compounds

Two methods for sampling and concentration of volatile organic compounds are reported. In the first method, traps coated with a very thick film (ca. 100 μm) of cross-linked silicone stationary phase are employed. Such thick films can be prepared with a modified dynamic coating procedure, which is briefly described. The low phase ratio traps can be utilized for enrichment of volatiles from gaseous as well as aqueous matrices. The second technique is based on chromatographic evaporation of a solvent in a capillary tube, where the process is sustained by a repeated sample injection and a cyclic flow reversal. In this way, large solvent volumes can be handled by a small volume system. Under optimal conditions, when using a solvent barrier, quantitative recovery is possible even for compounds of comparatively high volatility. Another important application of the technique is extraction of trace components from gases such as headspace samples, polluted air, etc.     

Determination of volatile organic compounds in ambient air. Comparison of methods

Two analytical methods for the determination of benzene vapour in ambient air are compared in this paper. The methods differ from each other in the sampling technique, type of sorbent, method of extraction and method of detection. The investigation of the methods using various techniques for sample analysing showed a significant influence of the way in which the analysis is carried out, on the final result of the analysis. Calculation of the standard deviations, relative standard deviations and confidence intervals allowed for assessment of the precision and repeatability of the methods. Of the two examined methods, that using an automated system of contaminant sampling and thermodesorption was more precise. This method has been applied to measurements of concentrations of benzene, toluene and xylenes in ambient air.     

Determination of volatile organic compounds in contaminated air using semipermeable membrane devices

Semipermeable membrane devices (SPMDs) were evaluated as passive samplers for the determination of 26 volatile organic compounds (VOCs) in contaminated air of occupational environments. A direct methodology based on the use of head-space-gas chromatography-mass spectrometry (HS-GC-MS) was developed for VOCs determinations in SPMDs, without any sample pre-treatment and avoiding the use of solvents. A desorption temperature of 150 °C for 10 min was sufficient for a sensitive VOCs determination providing limits of detection in the range of 15 ng SPMD⁻¹ for 21 of 26 studied compounds. Linear and equilibrium uptake models were established for each VOC from compound isotherms. Highly volatile compounds were slightly absorbed and moderately volatile compounds were strongly absorbed by SPMDs. This study is the first precedent of the use of SPMDs for the simultaneous sampling of a wide number of VOCs. The use of SPMDs is a simple and low cost alternative to ordinary sampling devices such as Radiello^® diffusive samplers or badge-type solid-phase supports.