Kinetics of solid-phase extraction and solid-phase microextraction in thinadsorbent layer with saturation sorption isotherm

The effects of sorbent saturation in thin adsorbent layers have been much o verlooked in earlier research and should be taken into account in both the theory and practice of solid-phase extraction (SPE) and solid-phase microex traction (SPME). The adsorption kinetics of a single analyte into a thin ad sorptive layer was modeled for several cases of agitation conditions in the analyzed volume. The extraction process in the adsorbent layer was modeled using a Langmuir isotherm approximated by the linear isotherm at low conce ntrations and by a saturation plateau at concentrations exceeding the criti cal saturation concentration. Laplace transformations were used to estimate the equilibration time and adsorbed analyte concentration profile for no a gitation, practical and perfect agitation in the analyzed volume. The equil ibration time may be significantly reduced at high degrees of oversaturatio n and/or agitation in the analyzed volume. The resulting models indicated t hat the adsorbent layer becomes saturated at some critical value of the ove rsaturation degree parameter. The critical value of the oversaturation para meter is affected by both the concentration of the analyte in the analyzed volume and the sorbent characteristics. It was also shown that the adsorpti on process is carried out via the propagation of the saturation adsorption boundary toward the inner boundary of the adsorbent layer. These new adsorp tion models should serve as "stepping stones" for the development of compet itive adsorption kinetic models far both SPE and SPME, particularly in case s where fast sampling is used. (C) 2000 Elsevier Science B.V. All rights re served.