Electrospray ionization mass spectrometry of stable nitroxide free radicals and two isoindoline nitroxide dimers

Measurement of 4-Hydroxy-2,2,6,6-Tetramethyl-Piperidine-N-Oxyl by high Performance Liquid Chromatography–Electrospray Ionisation Mass Spectrometry

“…In accordance with previous observations, a major ion at m/z 172 was found in the ESI mass spectrum, attributed to the molecular ion (M + , B, Fig. Not observed previously, 6 it is believed to correspond to net loss of two methane molecules from M + (i.e. [6][7][8] In addition, both singly and doubly protonated species were also detected, m/z 173 (MH +. )…”

“…4 EPR measurements provide useful information about the environment of the nitroxide, more specifically the extent of the hydration of the N-O group, and correlation times (in the region 10 --6 -10 -11 s) particularly relevant to many biological systems. 6 In the present study, 4-hydroxy-tempo has been measured quantitatively by both ESI-MS and ESI tandem mass spectrometry (MS/MS) in multiple reaction monitoring (MRM) mode, the latter technique demonstrating particularly high sensitivity for the compound. In a recent report, a range of stable nitroxide derivatives was characterised by electrospray-ionisation mass spectrometry (ESI-MS).…”

Electrospray mass spectrometry of stable iminyl nitroxide and nitronyl nitroxide free radicals

“…However, the more prominent molecular species observed in the ESI mass spectrum of iminyl nitroxide derivative 4 is the MH 2 +· ion ( m / z 331). Fragmentation of MH 2 +· occurs primarily at the isoindoline nitroxide group (Scheme ) according to mechanisms we have described previously 2. However, the ion at m / z 315 in the spectrum of 4 again arises not from fragmentation but from the secondary amine formed in the source.…”

“…The sensitivities of the nitroxides in the ESI mode are low by normal standards, with most compounds requiring concentrations of at least 50 µ M in acetonitrile in order to achieve a reasonable signal‐to‐noise ratio. We have previously reported that this was the case with isoindoline nitroxides 2. Spectra obtained form methanol solutions gave even lower sensitivities and these spectra exhibited extensive fragmentation.…”

“…Mass spectrometry provides an avenue for the rapid characterization of nitroxide radicals. We have recently reported on electrospray ionization mass spectrometry (ESI‐MS) of substituted isoindoline nitroxide radicals2 and have continued the study with nitronyl nitroxides (compounds 1–3 ) and iminyl nitroxides (compounds 5–7 ) in order to determine their mass spectral properties and facilitate the use of mass spectrometry as a means of structure elucidation.…”