  Event-by-event fluctuations of the chemical composition of the hadronic
system produced in nuclear collisions are believed to be sensitive to
properties of the transition between confined and deconfined strongly
interacting matter. In this paper a new technique for the study of chemical
fluctuation, the identity method, is introduced and its features are discussed.
The method is tested using data on central Pb-Pb collisions at 40$A$~GeV
registered by the NA49 experiment at the CERN SPS.
