Modeling solid-phase microextraction of volatile organic compounds by porous coatings using finite element analysis

Abstract

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.

The Effect of Sorbent Particles in a Binder on the Mass Transfer Kinetics in Separation Media: In Silico Study and Experimental Verification

“…5 Furthermore, findings have shown that in some cases, mass transfer is not improved by increasing agitation at the sample phase; this is especially true for gaseous samples where analytes are extracted using high-adsorption-capacity materials (e.g., CAR/PDMS). 11 In another study, Xu et al described a mathematical model for SPME kinetics in semisolid samples matrices, which yielded a number of key findings. Specifically, in semisolid samples, mass transfer resistance increases during extraction, preloaded analytes' extraction and desorption are isotropic, and analytes with higher distribution constants deviate more significantly from the first-order kinetics.…”

“…14 Other authors attempted to describe mass transport resistance in the heterogeneous coating but only considered tortuosity and porosity as parameters that contribute to effective diffusivity, which does not take into account analyte affinity for the sorbent, thus providing potentially skewing results as well. 11 The fundamental ideas of mass transfer into a liquid PDMS binder are well understood. In this paper, we utilize the finite element analysis software COMSOL Multiphysics (v.5.6) to develop numerical models that will provide a better understanding of how sorbents in a PDMS binder impact extraction kinetics, thereby resolving any misunderstandings regarding the mass transfer process.…”

“…Researchers have also attempted to describe the mass transfer resistance in the extraction phase, but these efforts have only yielded limited outcomes . Furthermore, findings have shown that in some cases, mass transfer is not improved by increasing agitation at the sample phase; this is especially true for gaseous samples where analytes are extracted using high-adsorption-capacity materials ( e.g ., CAR/PDMS) . In another study, Xu et al described a mathematical model for SPME kinetics in semisolid samples matrices, which yielded a number of key findings.…”

“…In addition, Chao et al also detailed how sample concentration and stirring speed affect the diffusion coefficient, but they failed to discuss whether the mass transfer resistance from outside of the extraction phase contributes to the overall mass transfer resistance, thus potentially skewing the results . Other authors attempted to describe mass transport resistance in the heterogeneous coating but only considered tortuosity and porosity as parameters that contribute to effective diffusivity, which does not take into account analyte affinity for the sorbent, thus providing potentially skewing results as well …”

Low-Cost Quantitation of Multiple Volatile Organic Compounds in Air Using Solid-Phase Microextraction

“…Extraction and desorption times are important parameters of VOC quantification by SPME, which have an impact on intensity of analyte responses. Speed of equilibration during the extraction stage depends on a vessel volume, the diffusion coefficient of an analyte and its distribution constant between the coating and the air [35]. The equilibrium between the fiber and air for almost all analytes was reached after 5-10 min of extraction ( Figure 1).…”