Abstract: Peptide mass mapping is a technique whereby masses determined from mass spectrometry of a protein digest are compared to the masses of theoretical peptides derived from a reference protein, specified as an amino-acid sequence. In some cases differences between experimental and theoretical masses can be accounted for by chemical modifications of the actual protein with respect to the reference, often as a result of post-translational modification (PTM). Typically such modifications are applicable to specific sets of amino-acid residues. Analysis of these mass differences can therefore lead to identification of PTMs. In various cases, it is desirable that such analysis in general allow for the possibility of a peptide having several different PTMs, and furthermore it is desirable in various cases that the chemical compatibility of a putative combination of PTMs with the peptide sequence be verified. Embodiments are described herein wherein compatibility verification is formulated as a problem in graph theory.