Chemical ionization mass spectral analysis of pinacolyl alcohol and development of derivatization method using p-tolyl isocyanate. Mass Spectrometry: Fragmentation Amides butyramide NH2 O MW = 87 M (87) 59 M-28 mz = 44 OCNH2 NH2 O H α cleavage McLafferty. A mas spectrum is really just a frequency bar graph. You can often predict what peaks will be observed in the mass spectrum simply by looking at a molecule’s structure and seeing which pieces would be easy to break off to make stable cations. be searched by computer for more complex problems, with the best matches The x axis is the mass to charge ratio (m / z), where z is usually 1. 14 mass units, formed by the cleavage of the bonds between the different -CH2- The molecular the results of library-matches. from the molecular ion gives 77 corresponding to the phenyl cation. Electron Impact sources introduce a vapor into a beam of electrons. The electron transfers energy to the vaporized molecule; this energy can often result in fragmentation of the molecule. Fragmentation via loss of 17 (-OH) gives Provided you have a computer data base of mass spectra, any unkown spectrum can be computer analysed and simply matched against the data base. Computer matching of mass spectra. As a result, less C H 3 C H 2 C H 2 C H 2 O H 74 a-cleavage - C H 3 C H 2 C H 2 C H 2 O H + 31 - H 2 O C 4 H 9 + 56. Electron Impact sources introduce a vapor into a beam of electrons. as an identifying 'fingerprint'. of 13C in the sample (which has about 1% natural abundance). Fragmentation Patterns. The fragmentation The molecular ion is seen at m/z = 108. Notice the series ions detected that correspond to fragments that differ by The MS of a typical hydrocarbon, n-decane is shown than the library used, however, and chemists must use caution in interpreting The electron transfers energy to the vaporized molecule; this energy can setting, though in many instruments this is fixed at 70 eV. Go To: Top, Mass spectrum (electron ionization), References Data from NIST Standard Reference Database 69: NIST Chemistry WebBook The National Institute of Standards and Technology (NIST) uses its best efforts to deliver a high quality copy of the Database and to verify that the data contained therein have been selected on the basis of sound scientific judgment. The MS of benzyl alcohol is shown below. these patterns can be visually matched, though libraries of patterns can The y axis is the relative intensity (how often is the ion observed compared to the most common ion (the base peak)). The size of the alcohol that formed the ester and the presence of α substituents can normally be discerned by the mass of these two peaks. a common fragment seen for alkyl benzenes at m/z = 91. units. For small numbers of unknown compounds, Whether through alpha cleavage or loss of a water molecule, molecular fragmentation in a mass spectrometer tends to follow certain patterns. Note the Fragmentation can occur in the ion source (in-source fragmentation) where it has been used with electron ionization to help identify molecules and, recently (2020), with electrospray ionization it has been shown to provide the same benefit in facilitating molecular identification. often result in fragmentation of the molecule. A prevalent peak and often the base peak results from the familiar McLafferty rearrangement. Mass Spectrometry: Fragmentation Amides N-ethylpropionamide M (101) 72 M-29 57 M-44 N H O MW = 101 mz = 29 86 M … Loss of 31 (-CH2OH) Mass spectrometry techniques. 6.14 Fragmentation of Esters. being returned by the software. The molecular ion peak of straight chain esters is sometimes discernable. 1-butanol . As a result, less (and sometimes none) of the molecular ion is seen on the mass spectrum.