Charge reduction of electron transfer dissociation product ions

A mass spectrometer is disclosed wherein highly charged fragment ions resulting from the fragmentation of multiply charged parent ions are reduced in charge state within a Proton Transfer Reaction (PTR) cell by reacting the fragment ions with one or more neutral reagent gases. The non-ionic reagent gas may be an uncharged superbase reagent gas or vapour, such as Octahydropyrimidolazepine or Tetramethylguanidine, or a volatile amine, such as trimethyl-amine or triethyl-amine. The parent ions may be fragmented within an Electron Transfer Dissociation (ETD) cell by reacting the parent ions with at least one neutral reagent gas or vapour, e.g. an uncharged alkali metal vapour such as caesium vapor.