Phase-Resolved Fluorescence Spectral and Lifetime Characterization of Commercial Humic Substances

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Abstract
Phase-resolved excitation-emission matrices (PREEMs) are shown to provide a unique visual representation of the intrinsic fluorescence properties of humic acids under a variety of solution conditions. The calculation of spectral peak ratios in PREEMs as well as steady-state excitation-emission matrices provides a convenient means for quantitating differences between the spectra with good precision. Absorbance correction is shown to be essential for accurate comparison among spectral features. Increased detail is available from PREEMs at various modulation frequencies that reveal the distribution of fluorescence lifetime contributions across the spectral surface. Direct measurement of fluorescence lifetime recovered three ranges of lifetime components in the humic substances, <1 ns, 2–5 ns, and 8–14 ns, that are consistent with previously reported lifetimes. PREEMs, which provide a concise “survey” of how the lifetimes change across the spectrum, may aid in pinpointing spectral regions that provide the best lifetime discrimination among samples.