Solid-phase microextraction of volatile organic compounds - Estimation of the sorption equilibrium from the Kovats index, effect of salinity and humic acids and the study of the kinetics by the development of an ''agitated/static layer'' model

The sorption equilibrium and kinetics of 11 chlorinated C-1- and C-2-hydrocarbons and monocyclic aromatic hydrocarbons on the poly(dimethylsiloxane) solid-phase microextraction coating was studied. A linear regression between the logarithm of the partitioning coefficients, corrected by Henry's law constant, and the Kovats indexes on a 100% poly(dimethylsiloxane) GC stationary phase was shown (r=0.997, n=11). The effect of salinity in artificial seawater on the sorption equilibrium was studied and compared to the salt effect on the solubility and on the air-water equilibrium partitioning. The effect of humic acids on the equilibrium partitioning in SPME was examined and compared to predictions from mass balances estimating the importance of the mass sorbed on the humic acid fraction. Kinetics of the mass transfer of the volatile organic compounds from the stirred aqueous phase into the organic phase were investigated. Equilibration times, i.e., the time at which 95% of the final sorbed mass is preconcentrated, varied between 0.77 (1,2-dichloroethane) and 35.0 min (tetrachloroethylene). These equilibration times were linearly related to the partitioning coefficients (r=0.993, n=11). A new model was developed in order to simulate the sorption process. In the model an agitated water body and an aqueous static layer at the water-polymer interface were considered. It was assumed that the mass transfer was limited by Fickian diffusion in the aqueous static layer and in the organic phase. Experimental results confirmed this approach and revealed a static water layer with a thickness between 3.0 and 5.5 mu m for benzene, trichloroethylene and tetrachloromethane, indicating that the aqueous static layer is dependent on the sorption equilibrium coefficient.