¹²CH₄ and ¹³CH₄ are the two predominant isotopologues of methane. Other isotopologues are too small in quantities to detect. A methane mass spectrum will therefore look like as shown in Figure 13.1

 

Figure 13.1 Electron ionization mass spectrum of methane showing the two predominant isotopologues

Fragmentation: We have already studied that electron ionization imparts large amount of energy to the cations that it generates. The radical cations thus generated undergo extensive fragmentation giving smaller cations. The fragments generated from molecular ions can provide important structural information about the molecules. Let us take ethanol as an example to see how this works: