A method of identifying precursor ion species from their fragments comprises obtaining mass spectra of a plurality of precursor ion species and their fragments to high mass accuracy. The fragment mass spectrum, obtained from fragmentation of multiple precursor ion species, is then scanned it identify pairs of fragments whose combined mass matches the mass of one of the precursor ion species. Once pairs of fragment ion shave been matched to precursor ions, the composite fragment ion spectrum is broken down into portions, one per fragment pair. Analysis continues until no further pairs are identified. A simplified fragment ion spectrum is then reconstructed for each precursor sample ion by stitching together the broken down sections of the composite fragment spectrum. The resultant reconstructed, simplified fragment spectra are sent to a search engine which returns a score-sorted list of likely candidates for each synthetic fragment ion spectrum.