Fluorescence fingerprint of fulvic and humic acids from varied origins as viewed by single-scan and excitation/emission matrix techniques
Résumé
Excitation/emission matrix (EEM), single-scan excitation and synchronous fluorescence spectra of a series of FA and HA from distinct environments are presented. The EEM plots show at least four spectral features whose corresponding Ex/Em pairs relate to the α', α, β and γ (or δ) fluorophores previously found in natural waters spectra. The α' and α peaks, which identify typical humic-like components, are present in all samples, independently of the organic matter (OM) source. In FA, their Ex/Em pairs are ~260 nm/460 nm and ~310 nm/440 nm, respectively. In HA their excitation and emission maxima are red-shifted, the corresponding Ex/Em pairs being located at ~265 nm/525 nm and ~360 nm/520 nm, respectively. The appearance of β and γ (or δ) peaks is dependent both on the OM origin and on HS aging. The former (Ex/Em ~ 320 nm/430 nm), that has been associated with the incidence of marine humic-like material, is present only in a few marine and estuarine HA. It emerges as a shoulder on the α peak and its detection is dependent on a balance between its magnitude and the magnitude and emission maxima location of the α peak. The γ (or δ) peak (Ex/Em ~ 275 nm/315 nm in FA, and ~275 nm/330 nm in HA), on the other hand, is better visualized in FA than in HA diagrams. It has typical protein-, mainly tryptophan-like, fluorescence properties and appears with varied significance in a few marine and estuarine samples being hardly detected in samples from exclusively terrestrial environments. It is also shown in this study that with selected λex, λem and Δλ values, regular emission, excitation and synchronous spectra can, together, provide a good picture of the OM sources and aging for extracted HS. \textcopyright 2004 Elsevier Ltd. All rights reserved.