the ionization can either be carried out in positive or negative ionization mode. in the positive mode, the relative proton affinities of the reactant ions and the gaseous analyte molecules allow either proton transfer or adduction of reactant gas ions to produce the ions [m+h] of the molecular species. in the negative mode, [m−h] ions are produced by either proton abstraction, or [m+x] ions are produced by anion attachment. most work on the apci-ms analysis has been in positive mode.