A method to determine the model-of-composition of a heavy petroleum or hydrocarbon samples comprising: (a) separating the heavy petroleum or hydrocarbon samples into asphaltenes and deasphalted oils, DAO; (b) obtaining molecular ions or pseudo molecular ions of parent molecules of said heavy petroleum or hydrocarbon sample by soft ionization methods; and (c) determining elemental formulae of said molecular ions or pseudo molecular ions and quantifying corresponding concentrations using Fourier Transform Ion Cyclotron Resonance Mass spectrometry, FTICR MS; CHARACTERIZED IN THAT: - said DAO are separated into chemical meaningful fractions including saturates, aromatics, sulfides and polars to improve certainty in assigning chemical structures to the molecular formulas obtained by mass spectrometry and said aromatics are separated into aromatic ring class fractions including 1-ring aromatics, ARC1, 2-ring aromatics, ARC2, 3-ring aromatics, ARC3, 4-ring+ aromatics, ARC4+, to improve certainty in assigning chemical structures to the molecular formulas obtained by mass spectrometry; - the soft ionization methods are field desorption/field ionization to ionize saturate molecules, Atmospheric Pressure Photoionization, APPI/Atmospheric Pressure Chemical Ionization, APCI, to ionize aromatic petroleum molecules, positive ion ESI (PESI) to ionize basic nitrogen molecules, negative ion ESI (NESI) to ionize acidic molecules, and laser desorption ionization or matrix assisted laser desorption to ionize high boiling molecules, that boil above 704°C (1300°F); AND BY THE STEPS OF: (d) assembling a full composition by combining compositions from various molecular fractions and ionization methods; and (e) reconciling the output from steps (c)-(d) with other analytical measurements that determine hydrocarbon and petroleum properties, including Field desorption MS for molecular weight distribution, bulk properties, average structures by NMR, and heteroatom types by X-Ray Photoelectron Spectroscopy, using a constrained entropy maximization procedure to obtain a model-of-composition. (a) separating the heavy petroleum or hydrocarbon samples into asphaltenes and deasphalted oils, DAO; (b) obtaining molecular ions or pseudo molecular ions of parent molecules of said heavy petroleum or hydrocarbon sample by soft ionization methods; and (c) determining elemental formulae of said molecular ions or pseudo molecular ions and quantifying corresponding concentrations using Fourier Transform Ion Cyclotron Resonance Mass spectrometry, FTICR MS; - said DAO are separated into chemical meaningful fractions including saturates, aromatics, sulfides and polars to improve certainty in assigning chemical structures to the molecular formulas obtained by mass spectrometry and said aromatics are separated into aromatic ring class fractions including 1-ring aromatics, ARC1, 2-ring aromatics, ARC2, 3-ring aromatics, ARC3, 4-ring+ aromatics, ARC4+, to improve certainty in assigning chemical structures to the molecular formulas obtained by mass spectrometry; - the soft ionization methods are field desorption/field ionization to ionize saturate molecules, Atmospheric Pressure Photoionization, APPI/Atmospheric Pressure Chemical Ionization, APCI, to ionize aromatic petroleum molecules, positive ion ESI (PESI) to ionize basic nitrogen molecules, negative ion ESI (NESI) to ionize acidic molecules, and laser desorption ionization or matrix assisted laser desorption to ionize high boiling molecules, that boil above 704°C (1300°F); (d) assembling a full composition by combining compositions from various molecular fractions and ionization methods; and (e) reconciling the output from steps (c)-(d) with other analytical measurements that determine hydrocarbon and petroleum properties, including Field desorption MS for molecular weight distribution, bulk properties, average structures by NMR, and heteroatom types by X-Ray Photoelectron Spectroscopy, using a constrained entropy maximization procedure to obtain a model-of-composition.