Kinetic evaluation method for SPME-NIR measurements of analytes with long equilibration time

In recent years, it has been demonstrated that SPME in polymer-clad optical fibers can be efficiently combined with NIR evanescent-wave spectroscopy f or on-site monitoring of apolar hydrocarbons (HCs) in aqueous samples. Typi cally, the time to reach analyte partition equilibrium between sample matri x and fiber cladding for well-agitated solutions and low-molecular-weight H o species is in the range of 2-20 min, Thus, spectral absorbance data measu red in extraction equilibrium are used for calibration and evaluation, Howe ver, for samples with no or only low agitation (e,g., in situ measurement i n a monitoring well) and even for well-stirred samples containing HCs with higher molecular weight (> C8-C10), it may take several hours to reach equi librium, which is by far too long for applications in field analysis, In th is context, a calibration and evaluation algorithm based on HC extraction k inetics has been developed, which allows performing quantitative measuremen ts of such samples within 20 min, This method uses the initial gradient of the response curve of a given HC compound or mixture recorded by the SPME-N IR instrument, which is linearly dependent on analyte concentration, The al gorithm has been implemented in the control and evaluation software of an S PME-NIR photometer system and tested with different aqueous HC samples. (C) 2001 John Wiley & Sons, Inc.