Compound H has a mass difference of 176 u (corresponding to glucuronidation) with compound F, and a mass difference of 18 u with compound D and is therefore likely the result from ring-opening of MDZ-N-glucuronide 1 (compound D). In that case, we may expect the aglycone of compound H produced in a CID MS/MS experiment to correspond to ring-opened MDZ. Figure compares the IR spectrum of the glucuronide-loss CID fragment ion (m/z 344) of compound H to the IR spectrum of compound F (m/z 344), confirming that these spectra are indeed highly similar, if not identical. This is another example of the ability of IRIS to link phase I and phase II metabolites. In contrast, the IR spectrum of compound H itself (m/z 520) and the theoretical IR spectrum for the lowest-energy conformer of protonated ring-opened MDZ-N-glucuronide 1 (Figure ) show subtle deviations. The ring-opened MDZ-N-glucuronide 2 isomer (Figure ) can not be excluded based on