High Resolution Mass Spectrometry

In a mass spectrum, the peak with the highest m/z value (ignoring small isotope peaks) is usually the molecular ion peak, M⁺. Its m/z value gives the relative molecular mass, M_r, of the compound.A low resolution mass spectrometer measures m/z to the nearest whole number. This is not enough to identify a molecule, because many different molecular formulae share the same whole-number M_r. For example, C₃H₈, CO₂ and C₂H₄O all have M_r = 44.High resolution mass spectrometry measures m/z to four or more decimal places. The precise masses of atoms are not whole numbers (except ¹²C, which is exactly 12 by definition), so each molecular formula has a slightly different precise mass. Comparing the measured value with calculated values identifies the molecular formula.Precise relative atomic masses:
¹H = 1.0078
¹²C = 12.0000
¹⁴N = 14.0031
¹⁶O = 15.9949Worked example: a compound gives a molecular ion peak at m/z = 44.0261. Is it C₃H₈, CO₂ or C₂H₄O?
C₃H₈: (3 × 12.0000) + (8 × 1.0078) = 44.0624
CO₂: 12.0000 + (2 × 15.9949) = 43.9898
C₂H₄O: (2 × 12.0000) + (4 × 1.0078) + 15.9949 = 44.0261
The compound is C₂H₄O.The molecular formula found this way can then be combined with test tube results and IR spectra to work out the structure.