Fluorescence studies of inner filter effects, oxygen quenching, and the phenolic content of dissolved organic carbon.

摘要

Fluorescence spectroscopy is a popular analytical technique, and when combined with multidimensional data collection and analyzed with advanced chemometric methods, it can be an effective tool in the analysis of convoluted samples. Despite its advantages, several photophysical processes can prevent the proper qualitative and quantitative analysis of fluorescence spectra. Inner filter effects (IFE), dynamic and static quenching, and energy transfer are some of the phenomena that impact interpretation of fluorescence data. The dual-pathlength method for correcting spectra for IFE was developed, applied to the study of a jet fuel (JP-4), and compared to "standard" correction procedures. It was found that the method was adequate in compensations for IFE in JP-4 at lower concentrations and that IFE dominated, while resonance energy transfer was negligible, in this concentration regime.;Methods of purging solutions with an inert gas are typically applied in fluorescence experimentation in order to avoid quenching by oxygen, but details of the purging procedures and extent of quenching are often only superficially addressed. Through the study of four polyaromatic hydrocarbons (PAHs), useful guidelines for the deoxygenation of solutions were provided- specifically addressing purge times for solutions of fluorophores with various lifetimes. Quantitative analyses of fluorescence intensity (and enhancement factors), solvent loss during purging, oxygen removal rates, and values for diffusion-controlled bimolecular rate constants for an experimental setup of interest were provided along with a more rigorously examined methodology that may be adapted to similar purging situations. In order to better investigate the roles of dynamic and static oxygen quenching of PAHs, a novel fiber optic-based system that simultaneously measures fluorescence intensity, absorbance, fluorescence lifetime, and oxygen concentration was developed and employed on a system of 1-aminoanthracene in cyclohexane. Dissolved oxygen was allowed to diffuse from the system for ∼6.5 hours under an atmosphere of nitrogen while measurements were made. Solvent loss during deoxygenation was found to be slightly faster than expected for cyclohexane diffusion into nitrogen, but via absorbance measurements, the change was quantified and IFE corrections were found to be vital in compensating data for concentration changes. Stern-Volmer plots revealed that dynamic quenching represents ∼95% of total quenching in the system and the remaining ∼5% may be equally split between potential complexation and "sphere of action" static quenching mechanisms.;Parallel factor analysis (PARAFAC) was applied to multidimensional fluorescence measurements of humic samples in an attempt to analyze phenolic content. Through this exploratory study, fluorescence-based PARAFAC results were correlated with phenol concentrations derived from the Folin-Ciocalteau reagent-based method. Three components of a five-component PARAFAC fit of the humic samples showed some correlation with phenol concentrations. A method of analysis based on standard additions of phenols to a humic sample was also investigated. When combined with PARAFAC, the standard addition method showed a limited ability to quantify "phenol-like" components in the humic sample, however, additions of tannic acid to the sample uncovered a possible energy transfer mechanism. The potential impact of Climate Change on humic material, its phenolic content, and the role of these phenols in drinking water treatment are discussed.