How to interprete the molecular mass from a mass spectrum, and then determine the molecular formula?
During mass spectrometry, the compound often fragments into pieces. So usually the highest peak is the molecular mass, but you need to be careful about isotopes such as deuterium (H with a mass of 2). These are usually quite small peaks, however, due to the small proportion of these isoptopes in the sample. The molecular formula – First you would need an empirical formula which you obtain through a combustion analysis. Then you can use the empirical formula and the molecular mass to determine the molecular formula. Eg: Combustion analysis determined that the empirical formula was CH3. The mass spectrum determined a molecular mass of 30. Since CH3 weighs 15, and 15 x 2 = 30, the molecular formula must be C2H6.
