Solid phase microextraction for quantitative analysis in nonequilibrium situations

Solid phase microextraction (SPME) is a convenient and efficient extraction method that involves using a thin polymer film coating on a fine silica fiber to adsorb analytes of interests from a sample matrix. A theoretical model is proposed to deal with the dynamic adsorption process of SPME. In this model, mass diffusion from the matrix to the SPME polymer film is considered as the rate-determining step in reaching an adsorption equilibrium, and a steady-state diffusion is assumed for SPME in an effectively agitated sampling medium. Mathematical treatment of the adsorption process generates an expression that can describe experimental adsorption time profiles of the SPME process. The expression also provides a directly proportional relationship between the amount of analyte adsorbed by the SPME fiber and its initial concentration in the sample matrix. This relationship indicates that SPME quantification is feasible before an adsorption equilibrium is reached, once the agitation condition and the sampling time are held constant. The theoretical model is fitted with experimental data, and there is a very good agreement between them. Data plots of the adsorbed amount vs the initial concentration showed excellent linearity, with a sampling time much shorter than that required for reaching an adsorption equilibrium.