The precise molecular mass of a compound is found from the m/z value of its molecular ion, M+ (the whole molecule that has lost one electron), in a high resolution mass spectrum. The mass of the lost electron is negligible, so this m/z equals the precise Mr.To use it, you calculate the precise mass of each possible molecular formula by adding the relative isotopic masses of its atoms, then pick the formula that matches the measured value:1H = 1.0078 12C = 12.0000 14N = 14.0031 16O = 15.9949 Worked example 1: a compound has a molecular ion at m/z = 44.0261. Possible formulae with Mr = 44 are:
CO2: 12.0000 + (2 × 15.9949) = 43.9898
N2O: (2 × 14.0031) + 15.9949 = 44.0011
C2H4O: 24.0000 + 4.0312 + 15.9949 = 44.0261
C3H8: 36.0000 + 8.0624 = 44.0624
The measured value matches C2H4O (for example ethanal).Worked example 2: a molecular ion at m/z = 60.0210 could be C3H8O (60.0573), CH4N2O (60.0323) or C2H4O2 (60.0210). Only C2H4O2 matches, so the compound could be ethanoic acid or methyl methanoate.Note that high resolution mass spectrometry gives the molecular formula, not the structure. Other techniques, such as IR and NMR spectroscopy, are needed to decide which isomer is present.
CO2: 12.0000 + (2 × 15.9949) = 43.9898
N2O: (2 × 14.0031) + 15.9949 = 44.0011
C2H4O: 24.0000 + 4.0312 + 15.9949 = 44.0261
C3H8: 36.0000 + 8.0624 = 44.0624
The measured value matches C2H4O (for example ethanal).Worked example 2: a molecular ion at m/z = 60.0210 could be C3H8O (60.0573), CH4N2O (60.0323) or C2H4O2 (60.0210). Only C2H4O2 matches, so the compound could be ethanoic acid or methyl methanoate.Note that high resolution mass spectrometry gives the molecular formula, not the structure. Other techniques, such as IR and NMR spectroscopy, are needed to decide which isomer is present.