In this study, a combination of double bond equivalence (DBE) distributions and hydrogen-deuterium exchange mass spectrometry (HDX-MS) was applied to elucidate the chemical structures of nitrogen containing compounds in polar fractions prepared by medium pressure liquid chromatography (MPLC). The NI and N1S1 class compounds in the fraction eluted with 100% toluene (Polar1) were determined to have pyrrole as a core structure. In contrast, the N-1 and N1S1 class compounds in the fraction eluted with ethyl acetate and methanol (Polar2) were shown to have either pyridine or N-alkylated pyrrole as a core structure. Theoretical calculations proved that N-alkylated pyrroles have higher polarizability and/or dipole moments than pyrroles, and hence, N-alkylated pyrroles are eluted with more polar solvents. It was also concluded that the thiophene group is included in the N1S1 class in the polar fractions. The N1O1 class compounds exhibited similar HDX results, suggesting that they contained pyrroles in the Polarl fraction and pyridines and N-alkylated pyrroles in the Polar2 fraction. However, the lower DBE distribution of the N1O1 class compounds indicated that this class included non-aromatic structures that were not observed in other fractions. The results presented in this study show that previously reported MPLC methods can effectively separate basic and non-basic nitrogen-containing compounds. Moreover, the chemical structures of compounds in heavy oils can be successfully elucidated by using a combination of DBE distribution and HDX-MS. (C) 2017 Elsevier Ltd. All rights reserved.