Systems and methods for identifying precursor ions of product ions from combined product ion spectra are provided. N precursor ions are selected. N groups of the N precursor ions are created. The tandem mass spectrometer is instructed to perform multiplexed precursor ion selection on the continuous beam of ions, fragment each of the N−1 precursor ions, and measure the intensities of the product ions, producing N product ion spectra. A heat map is plotted, producing N heat maps. The N product ion spectra are combined into a combined product ion spectrum. A corresponding precursor ion of a peak is identified by finding a heat map of the N heat maps that does not have data for the mass of the peak and determining that a precursor ion of the N precursor ions that is not included in a group that produced the heat map is the corresponding precursor ion.