Retention model for sorptive extraction-thermal desorption of aqueous samples : application to the automated analysis of pesticides and polyaromatic hydrocarbons in water samples

In this report, an automated method for sorptive enrichment of aqueous samples is presented. It is based on sorption of the analytes of interest into a packed bed containing 100% polydimethylsiloxane (PDMS) particles followed by thermal desorption for complete transfer of the enriched solutes onto the GC column. Compared to other solvent-less sample preparation techniques for water samples, several improvements can be noted of which the most obvious are an enhanced sensitivity and improved blanks. Moreover, degradation products formed from the PDMS material can easily be identified with the use of a mass spectrometric detector. As these products contain silicone, they do not interfere with the target solutes (pesticides, PAHs). In this report a theory model is derived which allows calculation of breakthrough volumes from octanol–water partitioning coefficients (KO/W). Alternatively, the KO/W value required for complete retention can be calculated using only the sample volume and trap specific parameters. For a sample volume of 10 ml, theory predicts a required log KO/W of 1.77 for the trap used here which was found to be in good agreement with experimental results. For the most apolar solutes, with a log KO/W in excess of seven, poor recoveries were found. This is most likely due to adsorption of these apolar solutes in the system. With the current set-up, detection limits are in the order of 10 ng/l using mass spectrometric detection in the full scan mode.