Mass Spectometry

Mass Spectometry

1. Mass Spectometry involves the ionisation of gas-phase particles to produce species carrying a positive charge - e.g. M+, the molecular ion

   • The mass of the molecular ion will give the molar mass of the molecule
   • The M+ ion can also be broken into fragments - those carrying a positive charge will be deflected by a magnetic field, while uncharged radical fragments will not
     • Following acceleration and deflection by a magnetic field, the relative abundance of the different ions is detected and a mass spectrum produced
       • This spectrum shows abundance plotted against the mass to charge (i.e. m/z) ration
         • The more stable the ion, the more likely it is to appear in the spectrum - e.g. tertiary carbocations will have a higher abundance than primary carbocations
           • The peak with the highest abundance is referred to as the base peak

2. In considering a mass spectrum, it is important to look at both the positions of the peaks and also the differences between the m/z values of the major peaks

   • These differences show the mass of fragments broken off
     • The presence of either chlorine or bromine can be identified by the presence of both M+ and (M+2)+ peaks due to the presence of isotopes - in the case of chlorine, 35Cl and 37Cl are present in a 3:1 ratio; whereas bromine, 79Br and 81Br are present in a 1:1 ratio
       • Some common fragments are above

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