"Dueling" ESI: instrumentation to study ion/ion reactions of electrospray-generated cations and anions

Novel instrumentation has been developed which allows for the sequential injection and subsequent reaction of oppositely-charged ions generated via electrospray ionization (ESI) in a quadrupole ion trap mass spectrometer. The instrument uses a DC turning quadrupole to sequentially direct the two ion polarities into the ion trap from ESI sources which are situated 90 degrees from the axial (z) dimension of the trap, and 180 degrees from one another. This arrangement significantly expands the range of ionic reactants amenable to study over previously-used instrumentation. For example, ion/ion reactions of multiply-charged positive ions with multiply-charged negative ions can be studied. Also, reactions of multiply-charged ions with singly-charged ions of opposite polarity that could not be generated by previously used ionization methods, or that could not be efficiently injected through the ion trap ring electrode, can be studied with the new instrument. This capability allows, for example, the charge state manipulation of negatively-charged precursor and product ions derived from proteins and oligonucleotides via proton transfer reactions with singly-charged cations generated by ESI.