Fluorescence Spectroscopy of Aqueous Leaf Litter Extracts and Their Complexes with Aluminum
- William Shotyk and
- Garrison Sposito
Fluorescence spectroscopy is a sensitive method for discriminating among humic substances both in structure and in provenance. This study was conducted to determine whether the same is true for an important nonhumic organic material, the water-soluble fraction of forest leaf litter. Aqueous extracts of leaf litter from three tree species [chestnut (Castanea sativa L. Miller), western white pine (Pinus monticola Douglas ex. D. Don), and willow (Salix spp.)] were examined by conventional excitation and synchronous-scan excitation fluorescence spectroscopy. The spectra obtained varied with the plant source of the leaf litter extract (LLE) as well as with the LLE concentration. This sensitivity to organic-matter origin indicates that fluorescence spectroscopy will be useful to distinguish among LLE of differing provenance. The synchronous-scan spectra of the LLE showed some peaks that decreased in relative intensity with increasing LLE dilution, and some that increased with increasing dilution. This behavior was attributed to the presence of both Type I fluorophores, which are not quenched because they do not form charge-transfer complexes with nonfluorescent constituents of the LLE, and Type II fluorophores, which are quenched by the formation of charge-transfer complexes and self-quenching processes, leading to intensity increases at high dilution. Synchronous-scan excitation spectra also were obtained for Al/LLE mixtures at pH 4.5, with the total Al concentration varying from 0 to 60 mmol m−3. The spectra showed that Al-complex formation may result either in increases of relative fluorescence intensity or in fluorescence quenching, depending both on the source of LLE and on its concentration. This result may reflect a concentration dependence of charge-transfer complex formation and the possibility that excited-state complexes of Al are less fluorescent than ground-state Al complexes.
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