In mass spectrometry, a small sample of a chemical compound is vaporized, bombarded with high energy electrons to ionize the sample, and the ions produced are detected based on the charge to mass ratio of the ions.
Figure 1. Ionization process in mass spectrometry.
Several different types of ions are produced in this process. If the compound loses only one electron, then a molecular ion, having the same mass as the original compound, is produced (see Figure 1). This molecular ion is called the M+ ion (and it gives us the exact molecular weight of the compound). The stream of high energy electrons is sufficiently powerful to break chemical bonds in the molecule, producing a series of molecular fragments. These positively charged fragments are also detected by the instrument, producing the mass spectrum. Organic chemical compounds will fragment in very specific ways depending on what functional groups are present in the molecule (see Figure 2). Analysis of the fragmentation pattern can lead to the determination of the structure of the molecule.
Figure 2. Fragments produced by benzamide.
The complete mass spectrum for benzamide is given in Figure 3. Notice the appearance of the peaks predicted from the fragmentation pattern given in Figure 2.
Figure 3. Mass spectrum for benzamide.
We will learn to deduce the structures of unknown organic compounds from their respective mass spectras. However, as we learn more spectroscopic techniques, we will find that mass spectrometry is most valuable in the determination of molecular weights and formulas.
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