2 - Theory of Solid-Phase Microextraction

Abstract

This chapter describes the thermodynamics and the kinetics of the extraction process. An understanding of solid-phase microextraction (SPME) theory provides insight and direction when developing methods and identifies parameters for rigorous control and optimization. Effective use of the theory minimizes the number of experiments that need to be performed. The theory has been developed to understand the principal processes involved in SPME by applying basic fundamentals of thermodynamics and mass transfer. The amount of analyte extracted at equilibrium conditions can be calculated using thermodynamic principles, while the extraction time can be estimated by solving differential equations describing mass transfer conditions in the system. Three basic extraction modes can be performed using SPME: a direct extraction, a headspace extraction, and an extraction involving membrane protection. The theory for ideal extraction conditions can be very accurate for trace concentrations in simple matrices like air or drinking water at ambient conditions, when secondary factors, such as thermal expansion of polymers, changes in diffusion coefficients due to the presence of solutes in polymers and heterogeneity of the matrix, can be neglected. In the direct extraction mode, the coated fiber is inserted into the sample and the analytes are transported directly from the sample matrix to the extracting phase. In headspace sampling, the fiber is inserted into the headspace above the aqueous matrix. For samples containing both non-volatile target analytes and high-molecular-weight interfering compounds the application of direct or headspace SPME may be challenging. In such cases, use of restricted-access materials (RAMs) or membrane-protected SPME results in better reproducibility and accuracy.