Emission spectra humic fractions

Figures 3a-f show the emission and excitation spectra for all six humic fractions. The excitation and emission maxima are listed in Table III along with the maxima of the phase-resolved emission spectra. In each case the emission spectrum was scanned with the excitation maximum wavelength held constant, and the excitation spectrum was scanned with the emission maximum wavelength held constant. Several interesting features are noted. The two humic samples ( Figures 3a,b) each have two excitation maxima and it appears that a double peak has been merged into the emission scan as evidenced by the shoulder on the high energy side of the emission peak. Similarly it seems evident that the exaggerated shoulders in the emission spectra of all the fractions point to the inclusion of two emission peaks in each spectrum. This evidence suggests the presence of two chromophores in each humic fraction.

Humic materials fractionated on the basis of hydrophobicity and proton affinity continue to exhibit two fluorophores as discussed in the section "Exciation-Emission Spectra. Strong evidence to establish the existence of at least two chromophores is seen in the phase-resolved spectra. These spectra are shown in Figures 4 a-f. They consist of the phase-resolved emission spectrum of each of the two fluorophores plotted separately and the normal emission spectrum of the humic fraction. If the nulling out of one fluorophore is exact then the sum of the two separate phase resolved spectra should be additive to equal the normal spectrum. In these figures the normal emission spectrum was measured separately from the two phase resolved emision spectra. The phase resolved spectra were then superimposed onto the scan of the normal emission spectrum. 

Figure 4. Phase-resolved plots of the six humic fractions superimposed on the normal emission scan for each fraction. The emission spectrum of the first fluorophore was suppressed and a scan was made of the second fluorophore then the second fluorophore was suppressed and an emission scan was made of the first fluorophore. Fractions hydrophobic humic weak (a) and strong (b) acids.

Figure 13.2 Emission fluoresoence speotrum of the fraction containing bulk organic matter in tap water from the Skagen aquifer, excited with monochromatic light at 280nm, while an emission spectrum from 330 to 500 nm was recorded. The 310nm Raman scatter is accompanied by a single broad band peak around 460 nm characteristic of humic substances. Source-. Andersen etal., (2002), reproduced with permission.

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