Excitation-emission fluorescence matrix to study pH influence on organic matter fluorescence in the Amazon basin rivers.

A study of the three-dimensional excitation-emission fluorescence matrix (EEFM) of organic matter from the Amazon basin rivers is reported in this paper. The EEFM, applied to the fractions obtained from sequential tangential ultrafiltration (STUF), give spectroscopic informations on the fluorescent properties of particulate (>0.22 microm), colloidal and dissolved (< 5 kD) organic matter. STUF process does not seem to alter the characteristic peaks position of humic material. Two characteristic excitation-emission peaks were observed in all fractions of the samples investigated, one stimulated by UV excitation (peak A) and one by visible excitation (peak C). Increasing the pH, fluorescence intensities of the fluorophores A and C, respectively, I(A) and I(C), responded in the same manner to pH, increasing with increasing pH. However, the fluorophore A seems more sensitive to this pH variation because its fluorescence intensity enhancement is always higher. Thus the chemical behaviour of the two fluorophores must be somewhat differentiated. The evolution of the I(A)/I(C) ratio with pH is studied for all samples. This investigation showed a linear relation between I(A)/I(C) and pH. Moreover, this behaviour is water type (black or white) and molecular size independent. Indeed, this linear relationship is observed for all samples and all fractions.