The UV fluorescence emission in natural water spectra

The UV peak with maximum position at 300…350 nm under excitation at wavelengths shorter 280 nm is often observed in fluorescence spectra of unfiltered natural water samples, plant origin samples or samples containing living microorganisms. Because the UV emission peak resembles emission of proteins it is called “protein-like fluorescence”. For some samples the UV emission band has complex structure and the peak can be decomposed into two spectral components: so called tyrosine-like (with maximum at 300…320 nm) and tryptophan-like (320…350 nm). Typically after filtration of the samples the UV fluorescence emission disappears. That supports the thesis that in unfiltered water samples the UV fluorescence is caused by proteins. However there are some reproducible experiments when the UV fluorescence was observed for dissolved organic matter (DOM) in the samples of natural water after filtration. This finding has stimulated our research on possible contributors to the UV emission of natural water. Only 3 amino acids (of 20 present in fauna) can fluoresce, they are tyrosine, tryptophan and phenylalanine, which spectra largely overlap. There are other organic compounds in addition to amino acids that can fluoresce in UV range: elementary phenol compounds (phenol, salicylic acid, gallic acid) and monocyclic quinones.

The work had several tasks: to study the features of the UV fluorescence (the position of maximum, spectral width and intensity), to find possible contributors to the UV emission for the samples of different origin, and to study the relationship between the UV fluorescence and “humic-type” fluorescence the visible range. We describe fluorescence measurements performed on the following samples: (1) solutions of aromatic amino acids, monocyclic phenolic compounds and quinones in water; (2) riverine, marine and lake water samples before and after microfiltration (pore size of 0.2 micrometer); (3) low-molecular weight and colloidal fractions of natural water samples; (4) living cells of microorganisms in cultures (bacteria, algae, micromycetes) and their cultural liquid. Absorption and fluorescence spectra with excitation wavelength were measured in laboratory conditions.

We found that such factors like temperature, pH, UV irradiation, addition of hydrogen peroxide affect the intensity of the UV fluorescence band. The UV fluorescence observed for natural waters samples, living cells of microorganisms and cultural liquid disappeared after sample filtration. For molecular DOM fractions we did not observed the UV fluorescence. There were just few exceptions. The intensive UV band appeared after ultrafiltration (with the pore size of 5 nm) of several marine samples, while the same sample before filtration demonstrated low UV signal. It is possible to make the assumption that some small compounds (e.g. phenols) separates from macromolecules of humic substances during ultrafiltration. Thus, these compounds in the monomer form fluoresce in the UV, but associated to macromolecule they contribute to visible humic-type fluorescence.