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 for on‐site monitoring of apolar hydrocarbons (HCs) in aqueous samples. Typically, the time to reach analyte partition equilibrium between sample matrix and fiber cladding for well‐agitated solutions and low‐molecular‐weight HC species is in the range of 2–20 min. Thus, spectral absorbance data measured in extraction equilibrium are used for calibration and evaluation. However, for samples with no or only low agitation (e.g., in situ measurement in a monitoring well) and even for well‐stirred samples containing HCs with higher molecular weight (>C8–C10), it may take several hours to reach equilibrium, which is by far too long for applications in field analysis. In this context, a calibration and evaluation algorithm based on HC extraction kinetics has been developed, which allows performing quantitative measurements 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‐NIR instrument, which is linearly dependent on analyte concentration. The algorithm has been implemented in the control and evaluation software of an SPME‐NIR photometer system and tested with different aqueous HC samples.