Electrospray ionisation (ESI) mass spectrometry combined with collision-induced decomposition of ions given by captopril, lisinopril and enalaprilat, angiotensin-converting enzyme (ACE) inhibitors, has been used to characterise these compounds. As expected, the ESI mass spectra are characterised by the presence of abundant [M H] ions with very little fragmentation at low cone voltages. Fragment ions are produced by collision-induced decomposition of these ions at higher cone voltages, and in MS/MS experiments. Fragmentation pathways and structures of fragment ions observed have been proposed. # 1998 John Wiley & Sons, Ltd. Received 26 August 1998; Revised 29 September 1998; Accepted 4 October 1998 Captopril, lisinopril and enalaprilat, angiotensin-converting enzyme (ACE) inhibitors, have been used as therapeutic agents because, in addition to their specific enzyme inhibition, they possess anti-inflamatory properties and protect the ischemic myocardium against reperfusion injury. These various actions of ACE inhibitors may be related to their ability to scavenge deleterious free radicals [1][2][3][4] . It has been shown that captopril, lisinopril and enalaprilat ACE inhibitors are effective OH . radical scavengers. 4 This was established by the deoxyribose assay in the presence of iron complexed with EDTA.5 ACE inhibitors also prevent deoxyribose degradation, in reaction mixtures which do not contain EDTA, suggesting that they exhibit some degree of metal-binding capabilities. 6,7 Furthermore, evidence of the formation of complexes between ions of metals, copper in particular, and ACE inhibitors, especially captopril and lisinopril, has recently been reported. 8 Because of the importance of these inhibitors, we have undertaken a study of their electrospray ionisation (ESI) mass spectra and their low energy MS/MS spectra with a view to establishing a simple method of characterising these compounds.
EXPERIMENTAL
Instrumentation and proceduresThe experiments were performed using a "Quattro II" QhQ tandem mass spectrometer (Micromass, Altrincham, UK) equipped with an atmospheric pressure ionisation source operated in the nebuliser-assisted electrospray mode. The potential of the electrospray needle was set at 3.5 kV and the extraction cone voltage was varied between À30 and À60 V and was optimised for each MS/MS spectrum. The source temperature was held at 70°C and samples were introduced into the mass spectrometer by means of a fused silica capillary tube and a Harvard syringe pump. Mass spectra were acquired over the range m/z 100-1000 during 10 s scans, operating the data system in the multichannel acquisition (MCA) mode. Calibration was performed using NaI.For low energy collision-induced decomposition (CID) experiments, preliminary work was carried out in conventional MS mode on the [M H] ions at higher cone extraction voltages. More detailed information was obtained, using MS/MS, by selecting the precursor ion in the first quadrupole mass analyser, Q1, subjecting this to CID in the hexapole collision cell...
