1. A system for determining corresponding mass peaks in experimental and library product ion spectra using a mass-to-charge ratio (m/z) tolerance probability function with values between 1 and 0, comprising: an ion source that ionizes one or more known compounds of a sample, producing an ion beam of precursor ions; a tandem mass spectrometer that receives the ion beam from the ion source and that selects at least one precursor ion from the ion beam corresponding to at least one compound of the one or more known compounds and fragments the at least one precursor ion, producing a product ion mass spectrum for the at least one precursor ion; and a processor in communication with the tandem mass spectrometer that receives the product ion mass spectrum from the tandem mass spectrometer, receives an m/z tolerance probability function that varies from 1 to 0 with increasing values of an m/z difference between two mass peaks and that includes one or more values between 1 and 0, retrieves from a memory a library product ion mass spectrum for the at least one compound, calculates an m/z difference between at least one experimental product ion mass peak in the product ion mass spectrum and at least one library product ion mass peak in the library product ion mass spectrum, calculates an m/z tolerance probability, (p m/z ) 1 , from the m/z difference using the m/z tolerance probability function, and determines if the at least one experimental product ion mass peak and the at least one library product ion mass peak are corresponding peaks based on the m/z tolerance probability, (p m/z ) 1 .