| Summary: | Disinfection by-products (DBPs) are important environmental contaminants that have documented human health issues from many epidemiologic studies. Unlike classical contaminants, they are not manufactured, but form during drinking water treatment. As a result, they can be difficult to identify because most are not present in the mass spectral library databases. This perspective article presents lessons learned over the years for tackling unknown DBPs, many of which can also be applied to the identification of other unknown environmental contaminants. These lessons include: the importance of high resolution mass spectrometry, confirming tentative identifications with authentic chemical standards, considering possible isomers with unknown identification, and using multiple analytical techniques to enable a more complete picture of unknowns; understanding that features are not chemical structures; recognizing when the structure you identified in a solvent extract may not be the original form of the chemical in water; how some of our best discoveries are by accident; and finally, the “so what” question—is the compound you identified a concern?
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