Declustering and fragmentation of protein ions from an electrospray ion source

A triple quadrupole mass spectrometer with a high pressure collision cell has been used to explore the declustering and fragmentation processes that may occur in the vacuum interface region of an electrospray or ionspray ion source. Using apomyoglobin as a model protein compound, collisional processes in Q2 were used to elucidate possible mechanisms which could occur in the orifice-skimmer region to affect the observed charge state distribution. The results indicate that charge loss or gain through collisional loss of a proton or electron does not occur; rather, higher collision energy results in better declustering of lower charge state ions, and fragmentation of higher charge state ions. The net result is an apparent shift toward lower charge state as the collision energy in the free jet region is increased. In addition, the data suggest that a mixture of heavily clustered monomers and possibly dimers and multimers are present in the expansion from ion source into vacuum, and it is this mixture which is acted on by the declustering field to produce the observed mass spectrum. The presence of these “superclusters” needs to be considered in any theory of ion desorption and transport processes in the source and interface region.