Fulvic acid fluorescence, quenching

If one wants to understand why such changes occur, one can look at a few of the basic equilibrium properties of such complexes. Figure 1 illustrates the trends which occur when a sample is titrated with copper, monitoring three different parameters. The black dots indicate the relative amount of bound copper as indicated by free copper ions sensed with an ion-selective electrode (Xc of left ordinate). The triangles represent the change of the absorbance of the solution at 465 nm (right ordinate). The curve with the open circles is the relative quenching of the fulvic acid fluorescence (Q of left ordinate). We see that we are able to probe several different types of sites with different types of probes for this multidentate system. 

Figure 1. Trends in three experimentally observed parameters during a titration of fulvic acid with Cu(II) ion. Key , relative amount of bound Cu ion (Xc of left ordinate) A, absorbance of the solution at 465 nm (right ordinate) and O, relative quenching of the fulvic acid fluorescence (Q of left ordinate).

The fluorescence properties of two fulvic acids, one derived from the soil and the other from river water, were studied. The maximum emission intensity occurred at 445-450 nm upon excitation at 350 nm, and the intensity varied with pH, reaching a maximum at pH 5.0 and decreasing rapidly as the pH dropped below 4. Neither oxygen nor electrolyte concentration affected the fluorescence of the fulvic acid derived from the soil. Complexes of fulvic acid with copper, lead, cobalt, nickel and manganese were examined and it was found that bound copper II ions quench fulvic acid fluorescence. Ion-selective electrode potentiometry was used to demonstrate the close relationship between fluorescence quenching and fulvic acid complexation of cupric ions. It is suggested that fluorescence and ion-selective electrode analysis may not be measuring the same complexation phenomenon in the cases of nickel and cobalt complexes with fulvic acid. 

Figure 7.3. The effect of pH and Cu metal on DOM fluorescence. (A) The DOM fluorescence from the mouth of the Ems-Dollart estuary as a function of pH. (Reprinted from Laane, R.W.P.M. Influence of pH on the fluorescence of dissolved organic matter. Mar. Chem., 11, 395-401, 1982, with permission from Elsevier.). (B) The enhancement of soil fulvic acid fluorescence increases with pH, but the effect is mitigated by the addition of a transition metal such as Cu, which induces fluorescence quenching. (Adapted with permission from Saar, R.A. and Weber, J.H. Comparison of spectrofluor-ometry and ion-selective electrode potentiometry for determination of complexes between fulvic acid and heavy-metal ions. Anal. Chem., 52, 2095-2100. Copyright 1982 American Chemical Society.)...

Ryan, D.K. and Weber, 3.H., 1982. Fluorescence quenching titration for determination of complexing capacities and stability constants of fulvic acids. Anal. Chem., 54 986-990. 

Waite, T. D., and F. M. M. Morel. 1984b. Ligand exchange and fluorescence quenching studies of the fulvic acid-iron interaction. Analytica Chemica Acta 162 263-274. 

Cabaniss, S.E. and Shuman, M.S. (1988) Fluorescence quenching measurements of copper-fulvic acid binding. Anal. Chem., 60, 2418-2421. 

The presence of at least two fluorophores, and possibly a third, associated with metal ion binding in fulvic acid strongly suggests the need for multiple binding site models. Existing linear and nonlinear models will be reviewed for both fluorescence quenching and enhancement. A new modified 1 1 Stem - Volmer model will be introduced as well as two site and multiple site models. Application of the models to Cu binding by fulvic acid and certain well defined model systems are discussed. 

Fluorescence Quenching of Soil Fulvic Acid Emission Scans... 

Figure 1. Fluorescence quenching of emission spectra due to Cu(II) ion titration of 15 ppm soil fulvic acid at 0.1 M ionic strength and pH 5. The FA sample was excited at 335 nm and emission wavelengths were scanned from 300 to 600 nm. Cu(n) concentrations of each emission spectra are (a) 0 uM, (b) 4.9 uM, (c) 9.8 uM, (d) 14.7 uM, and (e) 22 uM.

Figure 2. Fluorescence quenching of 15 ppm soil fulvic acid at 0.1 molar ionic strength titrated with Cu(II) ion (O) pH 5, (V) pH 6, and ( ) pH 7. The solid lines (—) illustrate the calculated intensity values from the nonlinear data treatment approach.

Investigation of Fulvic Acid—Cu Complexation by Ion-Pair Reversed-Phase High-Performance Liquid Chromatography with Post-Column Fluorescence Quenching Titration... 

Conclusions Concerning Band Assignments. Two aspects of the photophysics arc clear electrons and radical cations are formed promptly. The third well defined component, the "black residue", arises with a time constant similar to the decay of the fluorescence of fulvic acid and has a range of energies, plus lifetime and quenching patterns, that suggest the triplets that sensitization experiments show to be present. We can... 

Al(III) appears both to decrease and increase DOM fluorescence when forming a complex with fulvic acid (FA), isolated from soil (Ryan et al., 1996), river (Elkins and Nelson, 2002), and marine (da Silva and Machado, 1996) environments. The effect is presented in Figure 7.4 (redrawn from Elkins and Nelson, 2002), in which the change to relative fluorescence intensity of a river FA after addition of Al(ni) is compared to additions of Cd(II) and of Cu(II). Immediately after addition of the Al(lll) metal, fluorescence (ex = 344/an = 424) increased and reached a stable intensity at 5.0 x 10 M Al(IIl). By contrast, additions of up to 2 X 10 M Cd(II) did not change river FA fluorescence intensity. The addition of Cu(II) to the river FA solution produced fluorescence quenching, as shown in Figure 3B. 

---