A non‐covalent dimer formed in electrospray ionisation mass spectrometry behaving as a precursor for fragmentations

A non‐covalent‐bonded dimer was detected in the positive ion electrospray ionisation (ESI) mass spectra of a synthetic impurity. In tandem mass spectrometry (MS/MS) experiments using collision‐induced dissociation (CID), the ion was found to behave as a [M+H] + ‐type precursor ion for fragmentation until MS 5 . The dimer was probably formed through multi‐hydrogen bonds over a proton bridge. When the fragmentation occurred at the center of the bridge, the dimer was broken apart to give monomer fragments at MS 6 . However, no corresponding deprotonated dimer [2M–H] − was found in the negative ion ESI spectra. The dimer was extremely stable, and it could still be observed when a fragmentation voltage of up to 50 V was applied in the ionisation source. The formation of the non‐covalent dimer was also found to be instrument‐dependent, but independent of sample concentration. Accurate mass measurements of the [2M+H] + and [M+H] + ions, and their MS n product ions, provided the basis for assessing the fragmentation mechanism proposed for [2M+H] + . The fragmentation pathway was also illustrated for the deprotonated molecule [M–H] − . Copyright © 2008 John Wiley & Sons, Ltd.