A Study of pH Effects on Humic Substances Using Chemometric Analysis of Excitation-Emission Matrices
Keywords:fluorescence, tryptophan, PARAFAC, pH, humic, EEM
Excitation-emission matrices (EEMs) of four humic substances (Aldrich, Fluka, and two humic acids 1S102H and 1S103H from the International Humic Substances Society) were collected as a function of pH over the range pH = 2 to 11 at 20ºC. Parallel factor analysis (PARAFAC) was applied to the 3-D data to separate the overlapping components. The Aldrich and Fluka samples showed only two components, each of which had humic-like spectral features. The IHSS samples were best modeled by three-component PARAFAC models, two of the components being very similar to the components found in the Aldrich and Fluka PARAFAC models. The third component in each of the IHSS cases was a protein-like substance with structured emission around 330 nm, which was assigned to tryptophan. For all the humic substances, the relative concentrations of the humic-related components showed strong pH dependence, but the protein-like components showed essentially no pH dependence in either wavelength or relative concentration. Free tryptophan and a tryptophan-Aldrich humic acid mixture were also studied. Free tryptophan shows strong pH dependence of emission intensity and wavelength (shifting from 350 nm at pH 2 to 361 nm at pH 11) and the emission is featureless within pH 2~11. All evidence suggests that tryptophan in IHSS samples is shielded or protected from the bulk aqueous environment by association with other moieties in the mixture.
Permanent URL: http://hdl.handle.net/2047/d10006059