Analyzing the M Peak

In the spectrum of methane, the peaks below 16 are formed by fragmentation of the molecular ion. Methane is a small molecule, and there are very few ways for the molecular ion to fragment. There are only C—H bonds, so fragmentation of methane simply involves the loss of hydrogen atoms. 

We have explained all but one of the peaks in the mass spectrum of methane. Notice that there is a peakwfe= 17. This peak, called the (M+1) peak, will be discussed in more detail in Section 15-10. 

In some cases, it is possible for the (M) peak to be entirely absent, if it is particularly susceptible to fragmentation. In such cases, there are more gentle methods of ionization (other than El) that allow the parent ion to survive long enough to pass through the spectrometer. We will briefly review one of those methods later in this chapter. 

When analyzing a mass spectrum, the first step is to look for the (M) peak, because it indicates the molecular weight of the molecule. This technique can be used to distinguish compounds. For example, compare the molecular weights of pentane and of 1-pentene. 

Pentane has 5 carbon atoms (5 X 12 = 60) and 12 hydrogen atoms (12 X 1 = 12) and therefore a molecular weight of 72. In contrast, 1-pentene only has 10 hydrogen atoms and therefore has a molecular weight of 70. As a result, we expect the mass spectrum of pentane to exhibit a molecular ion peak at 72, while the mass spectrum of 1-pentene should exhibit a molecular ion peak at 70. 

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