Extracted amounts by solid-phase microextraction:: A realistic approach to the partition coefficient K

Because of its numerous advantages, the solventless solid-phase microextraction (SPME) sampling method coupled with an efficient chromatographic technique is used more and more to develop new analytical methods pertaining to organic molecules at low concentration in aqueous solutions, especially in the field of environmental chemistry. In a usual analytical procedure, the amount of analyte extracted by the fiber need not be determined, because the quantitation step of the analysis is mainly achieved using SPME external calibration. For some purposes, however, the determination of the partition coefficient K relative to a particular fiber for a specific analyte (for example) has to be calculated with accuracy. The traditional method consists of determining the response coefficient of the detector used for the analyte through a direct-injection calibration curve made from standard solutions in organic solvents and reporting it with the signal observed for the analytical sample. For the same goal, a depletion experiment method is suggested that consists of running several SPMEs from the same standard sample with the same conditions and then fitting the resulting data into an experimental regression curve, the exponential coefficient of which affords an absorption coefficient characteristic of the fiber/analyte system in a defined work-up. This self-calibrating method is revealed to be much more accurate than the previous one. Four pesticides in water solution were chosen to exemplify this study.