Mass Spectrometry
The full molecular formula can be established by the molecular mass (RMM) and the empirical formula, for a molecular compound. Several physical properties, that is including the vapour density of a gas and so-called colligative properties (like freezing point depression) in solution, can be employed to determine the RMM. Though the most significant method in modern research is mass spectrometry (MS) in which molecular ions are accelerated in an electric field and then pass via a magnetic field in which their paths are bent to an extent that rely on the mass/charge ratio. The conventional MS techniques need a volatile sample, ionized by electron bombardment, but techniques are now presented that overcome the limitations of that method. By a laser beam or by fast atom bombardment (FAB) the direct desorption from solids allow measurement of involatile compounds. Solutions might also be sprayed directly into the spectrometer inlet and the spectrum measured after the solvent has evaporated.
In the example of the compound (X) above, an important peak is found at mass number 214 as supposed for CrC9H6O3 but none at multiples of this value, depicting that the molecular formula in this case is similar as the empirical formula determined from elemental analysis. Though, much more information can be obtained. By MS, Individual isotopes are seen and the model of isotopic distribution should verify the identification. For instance the most abundant isotope of Cr has mass 52 but there are others at 50, 53 and 54. Carbon has 1.1% of 13C along with the main isotope 12C. So, the appearance of the spectrum can be checked against the detailed isotopic distribution supposed from the formula. Compounds also depict fragmentation patterns resulting from decomposition of the ions in the spectrometer. Additionally to the molecular peak at 214 mass units from the X, the appearance of others at 186, 158 and 130 depicts the loss of one, two and three fragments each one of mass 28, which are most expected to be CO units. Even though fragmentation reactions can sometimes be much more complicated, spectrum of the X strongly suggests the presence of three separate CO groups, simply lost from the molecule and in this case possibly bonded to the metal. So, the characteristic nature of the fragmentation process allows some structural information to be obtained, with making MS a powerful fingerprinting method for known compounds.