1. A method of tandem mass spectrometry (MS/MS) for use in a mass spectrometer characterized by the steps: (a) providing a sample of precursor ions comprising a plurality of ion types, each ion type comprising a respective mass or range of masses; (b) generating a mass spectrum of the precursor ions using the mass spectrometer so as to determine a respective mass value or mass value range for each of the precursor ion types; (c) estimating an elemental composition for each of the precursor ion types based on the mass value or mass value range determined for each respective ion type; (d) generating a sample of fragment ions comprising plurality of fragment ion types by fragmenting the plurality of precursor ion types within the mass spectrometer; (e) generating a mass spectrum of the fragment ion types so as to determine a respective mass value or mass value range for each respective fragment ion type; (f) estimating an elemental composition for each of the fragment ion types based on the mass value or mass value range determined for each respective fragment ion type; and (g) calculating a set of probability values for each precursor ion type, each probability value representing a probability that a respective fragment ion type or a respective pair of fragment ion types was derived from the precursor ion type.