Figure presents a schematic overview of the proposed workflow, where an LC-MS feature with known accurate mass but unknown molecular structure is characterized by recording its IRIS spectrum. This spectrum is then compared to computed spectra of candidate isomers in the library of in silico IR ion spectra. The unknown was annotated with the structure that provides the best IR spectral match in terms of the cosine similarity. As a proof of concept, we demonstrate this workflow using the IRIS spectrum of deprotonated 3,4-dihydroxyphenylacetic acid (DOPAC), a metabolite of dopamine. As previously shown, DOPAC is spectroscopically distinguishable from its structural isomer homogentisic acid (HGA), a biomarker for the genetic disorder alkaptonuria . However, the HMDB contains a total of 18 metabolites that share the elemental composition of DOPAC (C8H8O4), 16 of which can be deprotonated; their computed IR spectra are present in the in silico IR spectral library.