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 MS5. 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 MS6. 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 MSn 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]-.
AstraZeneca, Process R&D, Analytical Chemistry, 151 85 Södertälje, Sweden. hefeng.pan@astrazeneca.com
Hefeng
H
Pan
D019281:Dimerization; D007477:Ions / Q000737:chemistry*; D008958:Models, Molecular; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*
D007477:Ions