The on-fiber standardization technique for solid-coated solid-phase microextraction (SPME) was studied, and a theoretical model is proposed for the isotropic behavior of adsorption and desorption, based on Fick's law of diffusion and the Langmuir model. The isotropy of the adsorption and desorption of analytes onto and from the surface of porous solid SPME fiber was validated with the use of a commercially available fiber, a 50-µm Carbowax/ templated resin for carbamate pesticide analysis in various sample matrixes, and a self-made polypyrrole fiber for diazepam analysis in blood samples. Time constants were comparable for the adsorption and desorption processes. Equilibrium constants and fiber capacities were calculated with the Langmuir isotherm model. A kinetic method was developed to calibrate adsorption using desorption. This calibration corrected for the sample matrix effects and minimized displacement effects as a pre-equilibrium extraction. The technique was successfully applied to the analysis of pesticides in river water and white wine as well as drug analysis in clinical plasma and whole blood samples.
Solid-phase microextraction (SPME) was first documented byPawliszyn's group in 1990. 1 The valuable sampling technology is fast, simple, and solvent-free and combines sampling, sample preparation, and preconcentration to the extraction phase into one single step. 2 It has been widely applied in environmental monitoring, industrial hygiene, process monitoring, and clinic, forensic, food, flavor, fragrance, and drug analyses, in laboratory, and onsite analyses. 3 There are two types of SPME coatings, including liquid and solid phases. Poly(dimethylsiloxane) (PDMS) and polyacrylate (PA) are liquid coatings. PDMS is the most widely used coating for the analysis of volatile organic compounds (VOCs) such as benzene, toluene, ethylbenzene, and xylene (BTEX), polycyclic aromatic hydrocarbons, odor, and flavor. Both PDMS and PA