New methods of identifying organic compounds

Abstract

A 'reversed' geometry mass spectrometer, in which the ion beam passes through the magnetic sector before the electric sector, offers several advantages for the study of large organic molecules. The method used is to select individual ionic species formed in the ion source in turn by using the magnet and to study the fragmentation of these species in the field-free region in front of the electric sector. Either unimolecular or collision-induced fragmentations can be investigated, the masses of the daughter species being determined by scanning the electric sector. By selecting a variety of individual ions, a comprehensive fragmentation ‘ map' of the molecular species can be constructed. Because it is a voltage that is scanned, the instrument can readily be computer controlled which gives improved reproducibility of scanning, together with other advantages. The several pathways that often link a particular fragment ion with the molecular ion provide complementary information concerning ion structure. The fragmentation pattern of any ion is often sufficiently characteristic of the ion structure to allow direct identification of structural features present to be made by comparing the pattern from the relevant ion with that of an ion formed from a known reference compound. By using these methods the molecular structure of large organic molecules can often be deduced. Large isomeric molecules such as steroids, differing only in the structure of a side chain, can be distinguished by selecting only ions containing the side chain for study. The new methods also offer advantages for the detection and identification of individual components in mixtures.