High resolution mass spectrometry can be used to determine the molecular formula of a compound from the accurate mass of the molecular ion Mass Spectrometry The peak with the highest mass/charge ratio will be normally due to the original molecule that hasn’t fragmented (called the molecular ion) . As the charge of the ion is +1 the mass/ charge ratio is equal to Mr. When organic molecules are passed through a mass spectrometer, it detects both the whole molecule and fragments of the molecule C4H10 = 58 Molecular ion formed: M → [M]+. + e– Definition: molecular ion is the molecule with one electron knocked off (It is both an ion and a free radical) High resolution mass spectroscopy can measure the mass to 5 d.p. This can help differentiate between compounds that appear to have similar Mr (to the nearest whole number) Accurate masses of atoms: H = 1.0078 C = 12.0000 O = 15.9949 N = 14.0031 For example, the following molecular formulas all have a rough Mr of 60, but a more precise Mr can give the molecular formula. e.g. Mr = 60.02112 molecular formula = C2H4O2 Mr = 60.05751 molecular formula = C3H8O Mr = 60.03235 molecular formula = CH4N2O Example 1 A compound is found to have an accurate relative formula mass of 46.0417. It is thought to be either CH3CH2OH or H2NCH2NH2 . Calculate the Mr of each compound to 4 decimal places to work out which one it is. CH3CH2OH = (12.0000 x 2) + (15.9949 x 1) + (1.0078 x6) = 46.0417 H2NCH2NH2 . = (12.0000 x 1) + (14.0031 x 2) + (1.0078 x6) = 46.0530
22.214.171.124 Mass spectrometry
Mass spectrometry can be used to determine the molecular formula of a compound.
Students should be able to use precise atomic masses and the precise molecular mass to determine the molecular formula of a compound.