We will consider a couple of examples of mass spectra.  Mass spectrum of methane is expected to give peaks corresponding to CH₄⁺, CH₃⁺,CH₂⁺ and CH⁺.    All peaks do not have equal intensities.  The stablest ion has the largest intensity and for comparison with other peaks, this intensity is set to 100%.  Ethane will show peaks at C₂H₆⁺, C₂H₅⁺, CH₃⁺ (intense).  CCl₄ shows corresponding to CCl₄⁺, CCl₃⁺, CCl₂⁺ and CCl⁺  . The major peaks in the dissociation of n-decane are at 142 (I=0.05), 113 (I=0.05), 99 (I = 0.08) 85 (I = 0.2), 71 (I = 0.35) 57 (I = 0.85), 43 (I = 0.1) and 29 (I = 0.22).  Here, I corresponds to the relative intensity. A “fine structure” corresponding to the loss of one or two hydrogen atoms is also observed.  A sketch of the mass spectrum is given in Fig. 15.11.

The peak at 142 corresponds to  C₁₀ H₂₂⁺. A loss of C₂H₅ gives a peak at 113 (C₈H₁₇⁺).  Stepwise losses of CH₂ (14 atomic mass units, u) groups give peaks at 99, 85, 71, 57, 43 and 29.  The last one corresponds to C₂H₅⁺.  Other peaks at 27, 41, 55, 56, 69, 70, 84, 98 and 112 are also seen (not shown in Fig 15.)  The intensity of the peaks is difficult to predict a-priori and these depend on the stabilities of the structures.  In the above spectrum, C₃H₈⁺ is the most stable and long lived ion.  Analysing mass spectra can actually be whole lot of fun.

Example 15.2  Rationalize the peak structure of the mass spectrum of  CH₃ CH₂ CH₂ Br   (1-bromopropane).
Solution.  Br has two isotopes of mass 79 and 81 and both have nearly the same abundance.  Parent molecular ions are observed at m/q of 122 and 124. The loss of ⁷⁹Br from the molar mass of 122u gives a base peak at 43 for the ion CH₃ CH₂ CH₂⁺ . The sketch of the spectrum is shown in Fig 15.12.