Fluorescence quenching of polycyclic aromatic compounds by humic substances. Part 1. Methodology for the determination of sorption coefficients.

Abstract

An introduction to the methodology of fluorescence quenching measurements as an experimental tool for investigating the sorption of hydrophobic organic substances by dissolved organic matter (OM) is given. Special attention is paid to the implications of inner-filter effects (IFEs) on the determination of sorption coefficients. The essential aspects of the theoretical description of IFEs are outlined and various procedures for the IFE correction of fluorescence quenching measurements in the presence of dissolved OM are presented. Geometric parameters relevant for the quantitative description of IFE applying a 90 degrees arrangement between fluorescence excitation and detection are determined by different methods, newly developed or adapted from the literature, for the fluorescence spectrometer used in this work. Moreover, an experimental validation of the applicability of different IFE correction procedures is performed. In order to evaluate the variations of sorption coefficients that result from the use of different IFE correction procedures, the quenching of the fluorescence of the well-known polycyclic aromatic compound pyrene by a commercially available humic substance in aqueous solution is investigated. Compared to the apparent sorption coefficient, which is obtained without any IFE correction, the sorption coefficients determined with the correction of primary IFE or with the correction of primary and secondary IFE are on average ca. 40 or 60% lower. The application of different IFE correction procedures allows the determination of sorption coefficients with appropriate precision of +/- 15%.