Arginine side chains play critical roles in many protein-ligand interactions and enzyme catalysis. Unambiguous resonance assignment is a prerequisite for the nuclear magnetic resonance (NMR) spectroscopy studies of arginine side chains dynamics and hydrogen exchange properties from which one can expect to elucidate in more detail the roles of arginine residues in protein structure and function. Here we present a new mass spectrometry (MS)-based method for assigning the side-chain resonances of arginine residues in 2D H-N NMR spectra. The method requires no additional isotopic labeling, and relies on knowledge of the amino acid sequence, the modification of the guanidino groups and liquid chromatography-mass spectrometry rather than the protein's structure or properties. Correlating the modification rates can connect cross-peak positions from NMR data with MS data to support resonances assignments. In the present work, we have extended our original application to natural abundance human ubiquitin to provide ε-NH assessments of three arginine for this well-studied protein.