The influence of pH on dissolved organic matter fluorescence in inland waters
Résumé
Fluorescence is an easily available analytical technique used to assess the optical characteristics of dissolved
organic matter (DOM). Despite widespread use, there has been some confusion about how robust
fluorescence spectroscopy is to differences in solution pH. Here we assess fluorescence characteristics
of three natural water samples and one commercially available standard (Nordic Reservoir) by modifying
the pH across a range from 3.5 to 9.0 at 0.5 pH increments. We used two statistical approaches to
assess if fluorescence intensity shifted significantly across this pH range. We identified that humic-like
and protein-like fluorescence was largely stable within the pH range of 5.5 to 7.5, which represents 80%
of Swedish lakes and streams. Likewise, we found that the three commonly used fluorescence indices
were robust across the full pH range tested with the exception of the humification index, which had
a narrower range of stability. The commerical humic substance sample was highly unstable with changes
to pH in the regions of protein-like fluorescence being particularly sensitive. One of our conclusions is
that differences in fluorescence intensity in the pH range of 5.5 to 7.5, typical for most inland waters, are
generally minor. We recommend adjusting the pH when samples fall outside this region and to be
especially careful in interpreting results from commercial humic substances.
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