Determination of time-weighted average (TWA) concentrations of volatile organic compounds (VOCs) in air using solid-phase microextraction (SPME) is advantageous over other sampling techniques, but is often characterized by insufficient accuracies, particularly at longer sampling times. Experimental investigation of this issue and disclosing the origin of the problem is problematic and often not practically feasible due to high uncertainties. This research is aimed at developing the model of the TWA extraction process and optimization of TWA air sampling by SPME using finite element analysis software (COMSOL Multiphysics, Burlington, MA, USA). It was established that sampling by porous SPME coatings with high affinity to analytes is affected by slow diffusion of analytes inside the coating, an increase of their concentrations in the air near the fiber tip due to equilibration, and eventual lower sampling rate. The increase of a fiber retraction depth (Z) resulted in better recoveries. Sampling of studied VOCs using 23 ga Carboxen/polydimethylsiloxane (Car/PDMS) assembly at maximum possible Z (40 mm) was proven to provide more accurate results. Alternative sampling configuration based on 78.5 × 0.75 mm internal diameter SPME liner was proven to provide similar accuracy at improved detection limits. Its modification with the decreased internal diameter from the sampling side should provide even better recoveries. The results obtained can be used to develop a more accurate analytical method for determination of TWA concentrations of VOCs in air using SPME. The developed model can be used to simulate sampling of other environments (process gases, water) by retracted SPME fibers.

Experimental optimization of analytical methods based on solid-phase microextraction (SPME) is a complex and labor-intensive process associated with uncertainties. Using theoretical basics of SPME and finite element analysis software for the optimization proved to be an efficient alternative. In this study, an improved finite element analysis-based model for SPME of volatile organic compounds (VOCs) by porous coatings was developed mainly focussing on the mass transport in coatings. Benzene and the Carboxen/ polydimethylsiloxane (Car/PDMS) coating were used as the model VOC and a porous SPME coating, respectively. It has been established that in the coating, volumetric fractions of Carboxen, PDMS, and air are 33, 42 and 25%, respectively. It has been proven that Knudsen diffusion in micropores can slow down a mass transport of analytes in the coating. For Car/PDMS coating, mass transport of benzene is mostly characterized by a molecular diffusion, which can be explained by a large fraction of macro-and mesopores. It has been shown that the developed model can be used to model the extraction of VOCs from air and water samples encountered in a typical SPME development method procedure. It was possible to determine system equilibration times and use them to optimize sample volume and Henry's law constant. The developed model is relatively simple, fast, and can be recommended for optimization of extraction parameters for other analytes and SPME coatings. The diffusivity of analytes in a coating is an important property needed for improved characterization of existing and new SPME polymers and analytical method optimization.

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This study explored the possibility of applying different carbon-based adsorbents for removal of benzene, toluene, ethylbenzene and o-xylene (BTEX) from indoor air in static and dynamic modes. To determine BTEX removal effectiveness, the approach based on solid-phase microextraction (SPME) in combination with gas chromatography -mass spectrometry (GC-MS) was used. In static mode, removal effectiveness of BTEX from indoor air using different carbon-based adsorbents (shungite, walnut shell, saxaul, apricot pits, activated charcoal, Tenax, carbon black) varied from 80% to 100%. Optimal preparation conditions for shungite-based adsorbent are no activation and addition of NaOH at 1:0.8 ratio. Shungite-based adsorbent was not able to remove BTEX from polluted indoor air at the flow rate 300 mL/min corresponding to the linear flow rate 25 cm/s, a minimum value for most commercial air purification systems. At the flow rate 75 mL/min (6.25 cm/s), a saturation time of shungite-based adsorbent made up 368 min for benzene and 437 min for toluene. At this flow rate, BTEX adsorption capacities of the shungitebased adsorbent were 0.3, 2.1, 0.2 and 0.3 µg/g, respectively. Compared to shungite, activated charcoal allowed the complete removal of BTEX at both flow rates in the whole studied time frame. Thus, shungite-based adsorbents are not recommended for BTEX removal from air because of much greater efficiencies of classic activated charcoal adsorbents. Applied methodology based on SPME-GC-MS allowed simple, fast and inexpensive collection of data and can be recommended as the analytical tool for developing new adsorbents and systems for air purification.