Vacuum-assisted headspace solid phase microextraction of polycyclic aromatic hydrocarbons in solid samples

Vacuum-Assisted MonoTrapTM Extraction for Volatile Organic Compounds (VOCs) Profiling from Hot Mix Asphalt

MonoTrapTM was introduced in 2009 as a novel miniaturized configuration for sorptive sampling. The method for the characterization of volatile organic compound (VOC) emission profiles from hot mix asphalt (HMA) consisted of a two-step procedure: the analytes, initially adsorbed into the coating in no vacuum- or vacuum-assistance mode, were then analyzed following an automated thermal desorption (TD) step. We took advantage of the theoretical formulation to reach some conclusions on the relationship between the physical characteristics of the monolithic material and uptake rates. A total of 35 odor-active volatile compounds, determined by gas chromatography-mass spectrometry/olfactometry analysis, contributed as key odor compounds for HMA, consisting mainly of aldehydes, alcohols, and ketones. Chemometric analysis revealed that MonoTrapTM RGC18-TD was the better coating in terms of peak area and equilibrium time. A comparison of performance showed that Vac/no-Vac ratios increased, about an order of magnitude, as the boiling point of target analytes increased. The innovative hybrid adsorbent of silica and graphite carbon monolith technology, having a large surface area bonded with octadecylsilane, showed effective adsorption capability, especially to polar compounds.

Vacuum-assisted and multi-cumulative trapping in headspace solid-phase microextraction combined with comprehensive multidimensional chromatography-mass spectrometry for profiling virgin olive oil aroma

In the present work vacuum (Vac) and multiple cumulative trapping (MCT) headspace solid phase microextraction (HS-SPME) were evaluated as alternative or combined techniques for the volatile profiling. A higher extraction performance for semi-volatiles was shown by all three techniques. Synergic combination of Vac and MCT showed up to 5-times extraction power for less volatile compounds. The hyphenation of said techniques with comprehensive two-dimensional gas chromatography (GC × GC) enabled a comprehensive analysis of the volatilome. Firstly, 18 targeted quality markers, previously defined by means of classical HS-SPME, were explored for their ability to classify commercial categories. The applicability of such markers proved to be limited with the alternative sampling techniques. An untargeted approach enables the selection of specific features for each technique showing a better classification capacity of the commercial categories. No misclassifications were observed, except for one extra virgin olive oil classified as virgin olive oil in 3 × 10 min Vac-MCT-HS-SPME.

Investigation of operational fundamentals for vacuum-assisted headspace high-capacity solid-phase microextraction and gas chromatographic analysis of semivolatile compounds from a model solid sample

Vacuum-assisted headspace solid-phase microextraction (Vac-HS-SPME) is a technique used to enhance SPME sampling of semi-volatile organic compounds. Here, it was combined with a high-capacity SPME Arrow, which features a larger volume of extraction phase and a more rugged configuration than traditional extraction fibers. An in-depth assessment of the critical parameters was conducted to achieve optimal extraction of representative compounds from a model solid sample matrix (Ottawa sand). Operational fundamentals investigated included the types of seals needed to create a leak-free environment under vacuum conditions; the magnitude of the vacuum applied and time needed to activate the Vac kinetics; order of sample vial preparation methods (VPMs); and other standard variables associated with extract analysis by gas chromatography-mass spectrometry. When exploring the limits of sample VPMs, results indicated an ideal workflow requires the solid sample to be spiked before sealing the vial, allow the sample to rest overnight, then apply vacuum at a pressure of -677 mbar (out of -789 mbar maximum possible vacuum with pump and compressor used), exerted on the vial for 90 s. This work provides the necessary workflow for the optimization of Vac-HS-SPME sampling of analytes from solid matrices.

A comprehensive theory for vacuum-assisted headspace extraction of solid samples

Vacuum-assisted headspace extraction (VA-HSE) has proven to be an efficient solution for the rapid and effective extraction of volatile and semi-volatile species. The research has been mainly focused on liquid samples by considering differences in Henry's constants, while the accelerating effect of vacuum is more significant and practically more important in solid samples with complex matrices. Nevertheless, the lack of a comprehensive theory for VA-HSE in solid samples, based on the adsorption/desorption phenomena, is quite evident. This research was done with the aim of modeling VA-HSE of solid samples from a thermodynamic point of view. To understand the impact and mechanism of reduced pressure, the sampling space was divided into three separate areas including the solid matrix (the surface and cavities of the solid sample), the headspace of the sample, and the extraction phase (the surface and cavities of the adsorbent). The effects of vacuum on the movement of analyte molecules in all three areas were investigated and included in a comprehensive equation. According to the theoretical model, when a solid sample is subjected to vacuum conditions, the enthalpy of the analytes in their free state decreases. As a result, the analytes become more volatile. Additionally, vacuum reduces the thickness of the boundary layer in solid samples. This facilitates the diffusion of analyte molecules into the cavities within the solid material and eventually into the headspace of the sample. A similar effect is observed on the extractive phase side when vacuum is applied. The reduction in boundary layer thickness promotes the adsorption of analytes onto the extractive phase, thereby facilitating a quicker equilibrium of analyte concentration in the extraction phase. The proposed model was validated by correlating it with the experimental data found in the literature. The results of this analysis have shown a robust correlation between the theoretical model and the experimental data, bolstering the reliability of the model, and highlighting its practical relevance.

A simple and portable vacuum assisted headspace solid phase microextraction device coupled to gas chromatography based on covalent organic framework/metal organic framework hybrid for simultaneous analysis of volatile and semi-volatile compounds in soil

Various microextraction methods have demonstrated a positive effect when assisted by vacuum. However, working with such systems is often laborious, they often require expensive and non-portable vacuum pumps, and may even suck off some sample vapor or solid particles during the evacuation process. To address these issues, a simple, and affordable vacuum-assisted headspace solid-phase microextraction (HS-SPME) device was developed in this study. The device, named In Syringe Vacuum-assisted HS-SPME (ISV-HS-SPME), utilizes an adjustable 40 mL glass syringe as a vacuum provider and sampling vessel. A new fiber coating, made from a hybrid of covalent triazine-based frameworks and metal-organic frameworks (COF/MOF), was prepared and characterized by Fourier transform infrared spectrometry, field emission scanning electron microscopy, energy dispersive X-ray, X-ray diffraction, thermogravimetric analysis, and Brunauer-Emmett-Teller techniques for use in the ISV-HS-SPME. By optimizing parameters such as extraction temperature, extraction time, desorption temperature, desorption time, and, humidity using a simplex method, the ISV system was found to increase the extraction efficiency of polycyclic aromatic hydrocarbons (PAHs) and benzene, toluene, ethylbenzene, and xylenes (BTEX) in solid samples by up to 175%. The determinations were followed by GC-FID measurements. Compared to three commercially available fibers, the ISV-HS-SPME device with the COF/MOF (2DTP/MIL-101-Cr) fiber exhibited significantly higher peak areas for PAHs and BTEX. The linear dynamic ranges for BTEX and PAHs were 7.1-9000 ng g-1 and 0.23-9000 ng g-1, respectively, with limits of detection ranging from 2.1-5 ng g-1 for BTEX and 0.07-1.6 ng g-1 for PAHs. The relative standard deviation of the method was 2.6-7.8% for BTEX and 1.6-6.7% for PAHs. The ISV-HS-SPME was successfully used to simultaneously determine PAHs and BTEX in polluted soil samples with recoveries ranging from 80.4 to 108%.

Optimization of headspace solid-phase microextraction of volatile organic compounds from dry soil samples by porous coatings using COMSOL Multiphysics

Headspace solid-phase microextraction (HSSPME) is one of the simplest and cost-efficient sample preparation approaches for determination of volatile organic compounds (VOCs) in soil. This study was aimed at the development of the model for numerical optimization of HSSPME of volatile organic compounds from dry soil samples by porous coatings using COMSOL Multiphysics (CMP). ‘Transport of Diluted Species in Porous Medium’ physics was used for modeling. Effect of sample mass, pressure, fiber-headspace and soil-headspace distribution constants on extraction profiles and time of 95% equilibrium has been studied using the developed model. Equilibrium extraction under atmospheric pressure (1 atm) can take up to 97 min, while under vacuum (0.0313 atm) – 2.3 min. Equilibration time under vacuum was 42-43 times lower than under 1 atm at all studied distribution constants and sample masses. The developed model was modified for optimization of pre-incubation time using ‘Transport of Diluted Species’ physics. According to the obtained plots, 95% equilibration time can reach 13.3 min and depends on both sample mass and soil-headspace distribution constant of the analyte. The developed model can be recommended for optimization of pressure, preincubation and extraction time when fiber-headspace and soil-headspace distribution constants, soil porosity and density are known.

Quantitation of 1,4-Dichlorobenzene and Thymol in Beeswax Using Dynamic Headspace Vacuum Transfer in Trap Extraction Prior to Gas Chromatography-Mass Spectrometry

A method based on dynamic headspace vacuum transfer in trap extraction, followed by gas chromatography coupled with a mass spectrometer (DHS-VTT-GC-MS), was validated for the fast quantitation of 1,4-dichlorobenzene (p-dichlorobenzene; PDCB) and thymol residues in beeswax. The quantitation limits (LOQ) were 0.05 mg/kg (PDCB) and 0.25 mg/kg (thymol). Recoveries above 80% were obtained for PDCB concentrations between 0.05 and 10 mg/kg and for thymol concentrations between 0.25 and 200 mg/kg. Analysis of beeswax samples showed a good correlation between the results obtained by DHS-VTT-GC-MS analysis and those of a previous method based on SPE extraction followed by gas chromatography and triple- quadrupole mass spectrometry (GC-MS/MS) (R² = 0.9770 for PDCB and 0.9666 for thymol). However, the sample preparation and chromatography were much shorter using the DHS-VTT-GC-MS method. Forty comb foundations samples produced in Switzerland in 2019 and 2021 were analysed using DHS-VTT-GC-MS. Fourteen samples contained PDCB above the LOQ of 0.05 mg/kg, ranging up to a maximum of 1.53 mg/kg. The mean concentration of the positive samples was 0.22 mg/kg. All samples contained thymol residues ranging between 3.9 and 84.4 mg/kg with mean and median concentrations of 22.7 mg/kg and 17.4 mg/kg. Residue levels of PCDB and thymol in Swiss beeswax were substantially below those measured 20 (PDCB) and 10 (thymol) years ago, respectively.

Quantification of trace transformation products of rocket fuel unsymmetrical dimethylhydrazine in sand using vacuum-assisted headspace solid-phase microextraction

Quantification of unsymmetrical dimethylhydrazine transformation products in solid samples is an important stage in monitoring of environmental pollution caused by heavy rockets launches. The new method for simultaneous quantification of unsymmetrical dimethylhydrazine transformation products in sand samples using vacuum-assisted headspace solid-phase microextraction without addition of water followed by gas chromatography-mass spectrometry is proposed. Decreasing air evacuation time from 120 to 20 s at 23 °C resulted in increased responses of analytes by 25-46% and allowed obtaining similar responses as after evacuation at -30 °C. The best combination of responses of analytes and their relative standard deviations (RSDs) was achieved after air evacuation of a sample (m = 1.00 g) for 20 s at 23 °C, incubation for 30 min at 40 °C, and 30-min extraction at 40 °C by Carboxen/polydimethylsiloxane (Car/PDMS) fiber. The method was validated in terms of linearity (R²=0.9912-0.9938), limits of detection (0.035 to 3.6 ng g^-1), limits of quantification (0.12-12 ng g^-1), recovery (84-97% with RSDs 1-11%), repeatability (RSDs 3-9%), and reproducibility (RSDs 7-11%). It has a number of major advantages over existing methods based on headspace solid-phase microextraction-lower detection limits, better accuracy and precision at similar or lower cost of sample preparation. The developed method was successfully applied for studying losses of analytes from open vials with model sand spiked with unsymmetrical dimethylhydrazine transformation products. It can be recommended for analysis of trace concentrations of unsymmetrical dimethylhydrazine transformation products when studying their transformation, migration and distribution in contaminated sand.

Advances in pretreatment and analysis methods of aromatic hydrocarbons in soil

Benzene compounds that are prevalent in the soil as organic pollutants mainly include BTEX (benzene, toluene, ethylbenzene, and three xylene isomers) and PAHs (polycyclic aromatic hydrocarbons). These pose a severe threat to many aspects of human health. Therefore, the accurate measurement of BTEX and PAHs concentrations in the soil is of great importance. The samples for analysis of BTEX and PAHs need to be suitable for the various detection methods after pretreatment, which include Soxhlet extraction, ultrasonic extraction, solid-phase microextraction, supercritical extraction, and needle trap. The detection techniques mainly consist of gas chromatography (GC), mass spectrometry (MS), and online sensors, and provide comprehensive information on contaminants in the soil. Their performance is evaluated in terms of sensitivity, selectivity, and recovery. Recently, there has been rapid progress in the pretreatment and analysis methods for the quantitative and qualitative analyses of BTEX and PAHs. Therefore, it is necessary to produce a timely and in-depth review of the emerging pretreatment and analysis methods, which is unfortunately absent from the recent literature. In this work, state-of-art extraction techniques and analytical methods have been summarized for the determination of BTEX and PAHs in soil, with a particular focus on the potential and limitations of the respective methods for different aromatic hydrocarbons. Accordingly, the paper will describe the basic methodological knowledge, as well as the recent advancement of pretreatment and analysis methods for samples containing BTEX and PAHs.

Determination of BTEX Compounds in Contaminated Water Using the Novel Vacuum-Assisted-Total Vaporization SPME Method and GO-APTES Fiber

A novel and reliable microextraction technique was used for the fast determination of benzene, toluene, ethylbenzene and xylenes (BTEX) from contaminated water without any extra steps for the preparation or extraction of the aqueous sample. Vacuum-assisted-total vaporization-solid-phase microextraction (SPME) eliminated one of the partitioning steps in conventional headspace SPME and caused an increase in the sensitivity and speed of the method. A home-made graphene oxide/3-aminopropyl-triethoxysilane nanocomposite SPME fiber was synthesized and used as the extraction phase for efficient extraction. Several crucial parameters were optimized, such as the vaporization time and temperature, extraction time and desorption conditions. At the optimum experimental conditions, a linear wide range calibration curve over a wide range of 1-5,000 ng mL-1 and a relative standard deviation (n = 6) of 6.6-7.3% were obtained. The result of the determination of BTEX as a human health risk from real samples, using the proposed method, revealed an acceptable agreement with a valid method.

The effect of vacuum: an emerging experimental parameter to consider during headspace microextraction sampling

The effect of vacuum is an emerging experimental parameter to consider during optimization of a variety of headspace microextraction methodologies. The positive effect of vacuum was initially demonstrated for headspace solid-phase microextraction and was recently expanded to single-drop microextraction and higher capacity sorbents i.e. stir bar sorptive extraction. In all cases, sampling under vacuum greatly accelerated the extraction kinetics of analytes exhibiting long equilibration times under atmospheric pressure. At the same time, the extraction of analytes that reached equilibrium fast was not affected. In all optimized methods, extraction times were greatly reduced and/or sampling temperatures were lower to those reported with the standard methodology under atmospheric pressure. This work succinctly overviews the effect of vacuum on the different headspace microextraction technologies reported so far. The fundamental concepts describing the pressure dependence of each methodology are pulled together and presented in a simplified manner. The latest findings on the combined effects of vacuum and several selected experimental parameters typically examined during method optimization are then presented and the practical aspects of past outcomes are highlighted. The discussion also includes the air-evacuation step and the analysis of complex matrices. This article is intended for readers who are either new to the field of vacuum headspace microextraction sampling or its use and want to exploit this powerful approach. Graphical abstract.

Development and performance evaluation of a novel dynamic headspace vacuum transfer "In Trap" extraction method for volatile compounds and comparison with headspace solid-phase microextraction and headspace in-tube extraction

Headspace in-tube extraction (HS-ITEX) and solid phase microextraction (HS-SPME) sampling, followed by gas chromatography-mass spectrometry (GC-MS), are widely used to analyze volatile compounds in various food matrices. While the extraction efficiency of volatile compounds from foodstuffs is crucial for obtaining relevant results, these efficiency of these extraction methods limited by their long extraction times and requirements for large sample quantity. This study reports on the development and application of a new extraction technique based on HS-ITEX hardware, which improves the extraction rate and capacity by operating under reduced pressure, called Dynamic Headspace Vacuum Transfer In-Trap Extraction (DHS-VTT). The results of the study indicate that DHS-VTT improves the extraction of the target compounds. The area of the mass spectrometer signal for each compound can be up to 450 times more intense than the HS-SPME and HS-ITEX techniques performed in the same experimental conditions of extraction temperature and time. DHS-VTT runs in automated mode, making it possible to work with smaller sample quantity and also favors the HS extraction of all volatile compounds. In addition, the necessary modifications to the installation were cheap and the life of an ITEX trap is up to 10 times longer than an SPME fibre.

Opportunities for green microextractions in comprehensive two-dimensional gas chromatography / mass spectrometry-based metabolomics - A review

Microextractions have become an attractive class of techniques for metabolomics. The most popular technique is solid-phase microextraction that revolutionized the field of modern sample preparation in the early nineties. Ever since this milestone, microextractions have taken on many principles and formats comprising droplets, fibers, membranes, needles, and blades. Sampling devices may be customized to impart exhaustive or equilibrium-based characteristics to the extraction method. Equilibrium-based approaches may rely on additional methods for calibration, such as diffusion-based or on-fiber kinetic calibration to improve bioanalysis. In addition, microextraction-based methods may enable minimally invasive sampling protocols and measure the average free concentration of analytes in heterogeneous multiphasic biological systems. On-fiber derivatization has evidenced new opportunities for targeted and untargeted analysis in metabolomics. All these advantages have highlighted the potential of microextraction techniques for in vivo and on-site sampling and sample preparation, while many opportunities are still available for laboratory protocols. In this review, we outline and discuss some of the most recent applications using microextractions techniques for comprehensive two-dimensional gas chromatography-based metabolomics, including potential research opportunities.

Solvent-saturated solid matrix technique for increasing the efficiency of headspace extraction of volatiles

Due to the slow mass transfer rate of substance in solid media, very limited amount of volatiles can be released from the solid matrix to the headspace in the static headspace analysis. Thus, low sensitivity is often the main problem of static headspace analysis of the volatiles contained in a solid sample. Here, we reported on a solvent-saturated solid matrix (SSSM) technique which successfully enhanced the headspace extraction efficiency, and improved the sensitivity of the headspace analysis of the volatiles in solid sample. By adding a small amount of high-boiling-point solvent (e.g. glycerin) onto the solid sample to form a surface-covered solvent layer, the headspace extraction efficiency can be significantly increased by up to 2.5 times higher than that of the conventional one. Based on the experimental investigation of the performance of different amounts of solvent used for the headspace extraction of volatiles in air-dried lotus flower samples, the possible mechanism for the SSSM assisted headspace extraction has been proposed and validated, which showed that a saturation point of solvent existed for a given amount of solid sample, and the maximum extraction efficiency could be obtained at this saturation point. Moreover, positive results were also achieved when applying this new technique in the headspace extraction of the volatiles to the other two solid samples, which means this newly developed technique may open up a new avenue, and also serve as a general strategy for improving the sensitivity of headspace analysis of the volatiles entrapped in solid matrices.

Plastic pellets sorptive extraction: Low-cost, rapid and efficient extraction of polycyclic aromatic hydrocarbons from environmental waters

For the first time, plastic pellets, a low-cost and easy to reach industrial raw material, are reported as an efficient sorbent material for the laboratory extraction of polycyclic aromatic hydrocarbons (PAHs) from environmental waters. The proposed methodology, termed plastic pellets sorptive extraction (P2SE), consisted of a two-step procedure whereby target analytes were initially adsorbed onto the surface of three low-density polyethylene (LDPE) pellets and then desorbed using microliters of an organic solvent. Interphase mass transfer was greatly accelerated by means of vortex agitation. Organic extracts were analyzed by means of liquid chromatography-fluorescence detection. Different experimental parameters were controlled and the optimum conditions found were: three LDPE pellets (∼80 mg) added to 20 mL aqueous sample (20% w:v NaCl) followed by vortex agitation at 3000 rpm; for desorption, the three LDPE pellets were immersed in 100 μL of acetonitrile and the mixture was shaken at 3000 rpm for 5 min using the vortex agitator. The calculated calibration curves gave high levels of linearity yielding coefficients of determination (r(2)) greater than 0.9913. The precision of the proposed method was found to be good and the limits of the detection were calculated in the low ng L(-1) level. Matrix effects were determined by applying the proposed method to spiked river water, treated municipal wastewater and seawater samples. To compensate for the low recoveries of the more hydrophobic PAHs in spiked effluent wastewater and seawater samples the standard addition methodology was applied. The proposed method was applied to the determination of target pollutants in real seawater samples using the standard addition method. Overall, the performance of the proposed P2SE method suggests that the use of inexpensive and easy to reach sorbent materials for extracting analytes in the laboratory merits more intensive investigation.