A well-known property of MDZ is that it can undergo a spontaneous ring-opening via hydrolysis in an acidic environment, which enhances its solubility. At physiological pH, the lipid soluble ringclosed form is dominant, allowing penetration of the blood-brain barrier. A study involving MDZ metabolism in mice suggested that its metabolites can undergo a ring-opening as well, although the exact structure of the detected H2O-addition product was not confirmed. Here, we employ IRIS to structurally identify several ring-opened MDZ derivatives. To promote the formation of the ringopened structures, we repeated the incubation of MDZ, but quenched the metabolism using a solution containing 10% formic acid (see method section). This led to the detection of three extra features, listed in Table , in addition to the compounds in Table . Compounds F, G and H are 18 u higher in mass relative to compounds A, B/C and D, respectively. We can therefore hypothesize that they are the ring-opened forms of these compounds. To explore whether IRIS can help identify the full chemical structure of these compounds, we selected compound F and H for further identification using IRIS. for a direct overlay), with additional intense bands around 950 cm -1 and just below 1700 cm -1 in the spectrum of compound F (and some intensity differences). The band around 1700 cm -1 indeed suggests a ring-opened MDZ structure as this band may correspond to a ketone stretching vibration (see molecular structure in Figure ). Indeed, theory predicts a carbonyl stretching band at this frequency for ring-opened MDZ.