Using mass spectra to distinguish between compounds

Suppose you had to suggest a way of distinguishing between pentan-2-one and pentan-3-one using their mass spectra.

pentan-2-one		CH3COCH2CH2CH3
pentan-3-one		CH3CH2COCH2CH3

Each of these is likely to split to produce ions with a positive charge on the CO group. In the pentan-2-one case, there are two different ions like this:

• [CH₃CO]⁺

• [COCH₂CH₂CH₃]⁺

That would give you strong lines at m/z = 43 and 71.

With pentan-3-one, you would only get one ion of this kind:

• [CH₃CH₂CO]⁺

In that case, you would get a strong line at 57. You don't need to worry about the other lines in the spectra - the 43, 57 and 71 lines give you plenty of difference between the two. The 43 and 71 lines are missing from the pentan-3-one spectrum, and the 57 line is missing from the pentan-2-one one.

The two mass spectra look like this: