Analysis of mass spectrum:

To demonstrate the analysis of a mass spectrum, we shall look at the simple alkane nonane shown in figure.  Nonane encompass a molecular formula of C₉H₂₀ and a molecular weight of 128. The parent ion is the molecular ion at 128.

Figure: Mass spectrum for nonane.

There is a small peak at m/e 129 that is due to a molecule of nonane consisting of one ¹³C isotope (i.e.¹²C₈ ¹³CH₂₀). The natural abundance of ¹³C is 1.1%. Hence the chances of a ¹³C isotope being exist in nonane are 9 × 1.1% = 9.9%.

The base peak is at m/e 43. This is most probably a propyl ion [C₃H₇] ⁺. There are peaks at m/e 29, 43, 57, 71, 85 and 99. These types of peaks are all 14 mass units apart that correspond to a CH₂ group. The existence of a straight chain alkane is frequently pointed out by peaks that are 14 mass units apart.