Protein samples are processed to create a mixture of modified and unmodified or overlapping peptides which are analyzed using mass spectrometry. Correlations between the MS/MS spectra of peptide pairs allow the noise in individual MS/MS spectra to be greatly reduced. A small number of peptide reconstructions can be generated that are likely to contain the correct one. This allows for the de novo reconstruction of protein sequences and peptide and modification identification through a database search using extremely fast pattern matching, rather than time-consuming matching of spectra against databases.