Abstract: The use of mass defect signatures to impart milliDalton mass differences between isotopically labeled peptides at the MS1-level allows multiplex quantification without the increased mass spectral complexity that occurs with mass difference approaches. Provided herein is a mass defect-based chemical tag, dimethyl pyrimidinyl ornithine (DiPyrO), that is compact and easy to synthesize at high purity in few steps using commercially available starting materials. The multiplex DiPyrO tags are amine-reactive and can impart a mass difference onto labeled peptides and through calculated substitution of heavy isotopes. DiPyrO offers up to 10-plex quantification on current Orbitrap or FT-ICR platforms without increasing mass spectral complexity. The synthesis of the DiPyrO tag is provided along with viability of the DiPyrO tag for labeling complex proteomics samples using yeast extract digests and its effect on labeled peptides during LC-MS2 analysis.