Fluorescence Inner-Filtering Correction for Determining the Humification Index of Dissolved Organic Matter

Abstract

The use of fluorescence spectrometry has been suggested as a simple method to determine the extent of natural organic matter humification by quantifying the red-shifting of fluorescence emission that occurs with increasing humification. Humification indices are calculated by dividing fluorescence intensity at longer wavelengths by intensity at shorter wavelengths. These indices calculated without any specific efforts to standardize dissolved organic matter (DOM) concentration will result in index values thatvary with DOM concentration due to fluorescence innerfiltering effects. This study critically evaluated the effect of DOM concentration on humification index determination using organic matter isolated from field corn extract, soil: water extract, and soil fulvic acid. The results show that humification index values are sensitive to DOM concentration of the solution and are linear with respect to transmittance of the solution at the 254 nm used as the excitation wavelength. An approximate correction for DOM is to exploit the linear nature of the regression fit and to determine index values at the extrapolated 100% transmittance value. An exact correction using explicit correction factors for both primary and secondary innerfiltration effects was shown to give humification index values that are concentration invariant when absorbance of the solution at 254 nm was less than approximately 0.3 unit. Defining the humification index as the fluorescence intensity in the 300-345 nm region divided by the sum of intensity in the 300-345 nm and 435-480 nm regions was statistically advantageous. This study suggests that for quantitative results which can be used to compare humification of natural organic matter across different studies, correction of the fluorescence emission spectra for innerfiltration effects is needed.

Calibration, standardization, and quantitative analysis of multidimensional fluorescence (MDF) measurements on complex mixtures (IUPAC Technical Report)

“…It is again clear that this correction method is suitable for minimizing the effect of IFE. This equates to an A of approximately 0.09 cm −1 , which agrees well with threshold values reported in the literature [43,48,49].…”

Impact of Photooxidation and Biodegradation on the Fate of Oil Spilled During the Deepwater Horizon Incident: Advanced Stages of Weathering

“…435-480 by the peak area 300-435 nm + 435-480 nm, with excitation at 254 nm. 34 The humification index of the DOM, was nearly equivalent at the beginning of the biodegradation experiment in both the endogenous and sterile incubations that were not amended with a sand patty (Fig 5E). The same index of the DOM leached form the sand patties were significantly lower and different at T 0 between the dark and irradiated treatments.…”

“…All samples were dilution corrected (absorbance at 254 = 0.1) with MilliQ water to reduce inner-filter effects. 34 Excitation-emission matrix fluorescence intensities were corrected for Rayleigh and Raman scattering by using a Milli-Q water blank-subtracted and instrument bias in excitation and emission prior to correction for inner-filter effects. 35 Fluorescence intensity was normalized to quinine sulfate units as previously described.…”

Characteristics of dissolved organic matter in the Upper Klamath River, Lost River, and Klamath Straits Drain, Oregon and California

“…(2) Fluorescent dissolved organic matter (fDOM): The fraction of DOM that emits a fluorescent signal, measured at ex370 em460. (3) Humification index (HIX): Ratio of the emission intensity in the 435-480 nm region to the emission intensity in the 300-345 nm region, with an excitation at 254 nm (Ohno, 2002)…”

“…Lower values indicate DOM sourced from labile material such as animal manure or algal exudates. Higher values indicate an increasing degree of humification (Ohno, 2002).…”