RANK ANALYSIS OF THE PH-DEPENDENT SYNCHRONOUS FLUORESCENCE-SPECTRA OF SIX STANDARD HUMIC SUBSTANCES

Synchronous fluorescence spectra of standard humic substances (HS) at variable pH can only be adequately explained by positing large (greater than or equal to 7) numbers of fluorescent factors. Although most of the variation (99.9%) can be explained using only two or three major factors, careful representation of the experimental error shows numerous additional minor factors that have been overlooked in previous work. The experimental error in these spectra was measured by repeated spectral acquisition and estimated by a linear regression equation relating the standard deviation, sigma, to fluorescence intensity. Each of the six HS examined gave a unique synchronous spectral shape at pH 7.0, although most showed similar patterns of pH dependence. These results indicate that these synchronous fluorescence measurements have a high information content, suitable for tracer studies of HS.