Extracted ion chromatograms of the doubly charged 9-17 peptide (KSTGGKAPR) on histone H3 unmodified on K9 (A), trimethylated on K9 (B), and monoacetylated on K9 (C). Differences in retention time under reversed phase chromatography facilitate PTM assignment of propionylated histone peptides. In general, the acetylated peptide elutes earlier than the unmodified peptide, which elutes earlier than the monomethylated peptide. Inserts depict the MS of the respective trimethylated and monoacetylated histone peptides acquired in the Orbitrap. Note that the spacing of the isotopes (~0.5 m/z) indicates the histone peptide as doubly charged. Abbreviations: pr = propionyl, me3 = trimethyl, ac1 = acetyl.