Nonequilibrium solid phase microextraction for determination of the freelydissolved concentration of hydrophohic organic compounds: Matrix effects and limitations

Solid-phase microextraction (SPME) has recently been applied to measure the freely dissolved concentration; as opposed to the total concentration, of hydrophobic substances in aqueous solutions. This requires that only the fr eely dissolved analytes contribute to the concentration in the SPME fiber c oating. However, for nonequilibrium SPME the sorbed analytes that diffuse i nto the unstirred water layer (UWL) adjacent to the SPME fiber can desorb f rom the matrix and contribute to the flux into the fiber. These processes w ere described as a model. Experimentally, an equilibrated and disconnected headspace was used as a reference for the freely dissolved concentration. T he expected contribution of desorbed analytes to the uptake flux was measur ed for PCB no. 52 in a protein-rich solution, while it was not measured in a matrix containing artificial soil. The latter was possibly due to slow de sorption of the analyte from the artificial soil. On the basis of the prese nt study, a contribution of desorbed analytes to the uptake nux is expected only if (1) the rate-limiting step of the uptake process is diffusion thro ugh the UWL, (2) the concentration of the sorbed analyte is high, and (3) d esorption from the matrix is fast.