Using fragmentation patterns

This section will ignore the information you can get from the molecular ion (or ions). That is covered in three other pages which you can get at via the mass spectrometry menu. You will find a link at the bottom of the page.

Let's have another look at the mass spectrum for pentane:

What causes the line at m/z = 57?

Solution

How many carbon atoms are there in this ion? There can't be 5 because 5 x 12 = 60. What about 4? 4 x 12 = 48. That leaves 9 to make up a total of 57. How about C₄H₉⁺ then?

C₄H₉⁺ would be [CH₃CH₂CH₂CH₂]⁺, and this would be produced by the following fragmentation:

The methyl radical produced will simply get lost in the machine.

The line at m/z = 43 can be worked out similarly. If you play around with the numbers, you will find that this corresponds to a break producing a 3-carbon ion:

The line at m/z = 29 is typical of an ethyl ion, [CH₃CH₂]⁺:

The other lines in the mass spectrum are more difficult to explain. For example, lines with m/z values 1 or 2 less than one of the easy lines are often due to loss of one or more hydrogen atoms during the fragmentation process.

Example 2