The results show that isomers resulting from hydroxylation at different sites are effectively differentiated on the basis of IRIS, as was also shown in previous work . Where previous studies focussed on identification using reference standards, we demonstrated here identification in a reference-free manner aided by quantum-chemical computations. Glucuronide metabolites have much higher molecular weights than hydroxylation metabolites, but nonetheless present diagnostic IR spectra with many sharp spectral bands. Nevertheless, due to the large molecular size, the distinction between different glucuronidation-sites on the basis of computed IR spectra is less clear than for hydroxylation isomers. However, in most cases our reference-free approach suggests one candidate that is most likely, which may guide the synthesis of a small number of reference compounds for final confirmation.