To demonstrate the applicability of the in silico IR spectral library to actual biofluids, we elucidated the molecular structure of an LC-MS feature (-ESI, m/z 131.0713, RT=6.27 min.) elevated in a patient's plasma sample (see SI for experimental details). The measured m/z suggests a chemical formula of [C6H12O3-H] -(Δm = +7.6 ppm), which corresponds with 10 entries in our spectral library. Figure shows the experimental IR spectrum of the LC-MS feature and the three best matching computed IR spectra. All ten spectral comparisons are shown in Figure . The three top-ranked structures all possess a carboxylate moiety an a nearby hydroxyl group, giving rise to three main spectral bands between 1200 and 1700 cm -1 that are also observed in the IRIS spectrum. Specifically, these bands originate from symmetric and antisymmetric carboxylate O-C-O stretching (1300 and 1650 cm -1 ) and O-H bending (1450 cm -1 ). The 3-hydroxyhexanoate anion is ranked highest, as the experimental band at 850 cm -1 is also reproduced, corresponding to the O-H out-of-plane bending vibration (Figure ). For the 2hydroxy isomers, this O-H bending vibration is computed at 700 cm -1 (Figure ). This assignment is further corroborated by inspecting the top-25 matches of an unsupervised search, which yields the 3hydroxycarboxylate motif in 13 out of 25 structures, while the 2-hydroxycarboxylate substructure occurs only once (see Figure ).