Identifying the molecular structure of compound E (m/z 518), which is the product of both glucuronidation and hydroxylation of MDZ, requires determination of both the site of hydroxylation and glucuronidation. Many structural isomers can therefore be considered as candidate, but we can expedite identification by making use of the fact that glucuronides generally expel the glucuronide moiety upon collision induced dissociation (CID). The resulting aglycone fragment ion retains the OH-group, so that an IR spectrum of the aglycone can reveal the position of the hydroxyl moiety, narrowing down the list of candidate structures for the glucuronide. Figure compares the IR spectrum of the glucuronide-loss CID fragment ion (m/z 342) of compound E with the IR spectrum of compound B (m/z 342) from Figure . The two spectra are near-identical. Small differences in band intensities can be attributed to day-to-day fluctuations in IR laser power. Compound B was identified above as 1'-OH-MDZ, so that we can assign this structure to the aglycone of compound E as well.