Humic-type fluorescence

Am- Taken together, these two peaks comprise humic-like component M. Peak displays variability in peak position in parallel to peak M, such that excitation and emission maxima for both type M humic-like peaks are blue-shifted relative to the analogous maxima for type C humic-like fluorescence (Figure 3.4). 

In this chapter, we present the theory and results of measurements on humic acid fractions using fluorescence techniques. The fluorescence techniques are attractive for this application because of the natural fluorescence of humic materials, the hi sensitivity of fluorescence detection, and the ability to directly observe the morphology of the molecule in aqueous solutions without the need for drying or applying harsh chemical conditions. Several interesting types of information are obtained from fluorescence measurements ... 

In all cases, the fractional decrement of the fluorescent humic acid-type signal at 420 nm is at least as great, within the error, as the change in amine signal. Thus, since the amine signal intensity and the intrinsic fluorescence of the humic acid extract at 420 nm are both hnear in concentration, there would not be a quantitatively comparable loss from the... 

Figure 4.3.4 Stern-Volmer type curve for pyrene fluorescence signal decrease upon increasing the concentration of Suwannee river humic acid in pure water (initial pH 6.5). (From Algarra et al. (2005), with kind permission from Springer Science and Business Media)...

In one study, the decay of fluorescence for DOM in seawater could be reasonably fitted to a single exponential, with a fifetime of 2 ns [51]. Some recent experiments on DOM from the Baltic Sea have suggested that the fluorescence behavior of humic substances at ambient temperatures may best be described by a mixture of chromophores with only one fluorophore, i. e., one type of fluorescence molecule with only one chemical structure [58]. However, other stud-... 

Although time consuming, laboratory-prepared standards should be used when a specific sample type is analysed regularly. Alternatively, the marine standards available fi om the Internatiomd Humic Substances Society (MSS web site http //www.gatech.edu/ihss/) may be used. However, these materials have been obtained from open-ocean waters, and hence might differ in chemical composition and/or fluorescence characteristics from humic compounds present in estuarine and coastal waters. 

The results from this saline lake are in contrast to the results from Provenzano et al. (2008) for hydrophilic (HI) and hydrophobic (HO) humic substances isolated from three increasingly saline soils (Figme 7.6B). This study showed a clear decrease in fluorescence intensity for both DOM fractions, though at different ex/em maxima. Provenzano et al. (2008) indicated that the suppression of ionization was likely responsible for this effect. Exchangeable Na potential increased in each soil type with salinity, but information on Mg + ion was not available in this study. More work is needed on these systems to elucidate the roles of metal-ligand complexation and suppression of ionization in the DOM chemistry of saline lake and salinized soil environments. 

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