Library and validation set IR spectra were calculated for the protonated ([M+H] + ), deprotonated ([M-H] -) and sodiated ([M+Na] + ) forms of all entries in the HMDB 4.0 with a molecular weight lower than 210 Da. This amounts to a total of 11823 ions generated from 4640 metabolites. Since several tautomers and conformers are included for each ion, the library contains a total of 75941 computed IR spectra. The experimental validation set consisted of 12 IR spectra taken from earlier publications and 177 newly measured IRIS spectra derived from 87 metabolites (Table and Scheme S1). Chemical class information of both the library and validation set was determined by ClassyFire . This assessment showed that all major classes in the library are represented by at least 6 reference metabolites (Figure ) and that the computational library and experimental validation set cover similar distributions in mass and ion type (Figure ). The experimental and computational IR spectra are available through the HMDB website (), where they are interactively viewable (via JSpectraView) in the "spectrum" field of the MetaboCard of the corresponding metabolite .