Solid phase microextraction in headspace analysis. Dynamics in non-steady state mass transfer

Previously proposed models (Ai, J. Anal. Chem. 1997, 69, 1230-6; 3260-6) dealing with the dynamics of solid-phase microextraction (SPME) were based on the assumption that a steady-state mass transfer is in effect throughout the SPME extraction process. In reality, a steady-state mass transfer may not be established at the initial stage of headspace SPME, which involves mass transfer across two interfaces. The rate of analyte transfer at the condensed phase/headspace interface may not be equal to the extraction rate at the headspace/SPME polymer film interface. An improved model is proposed in this report to handle the situation of the non-steady-state mass transfer for the headspace SPME. A mathematical solution is obtained for the dynamic process of the non-steady-state mass transfer by correlating the variation of the analyte concentration in the headspace with the analyte extraction rate. This solution provides an equation relating the extracted amount of analyte to the extraction time a little more complicated than the previous models. It also gives a better description of experimental observations.