# What is the structure of 92 m/z fragment for aspirin

Aspirin has the following structure:

What would the fragment structure be for m/z = 92?

Even though $\ce{C6H4O}$ is a molecular structure with mass 92, I don't think this will be formed since two bonds needs to be broken. Also the 92 peak is a major peak and the probability of two bonds breaking to lead to major peak is unlikely. The mass spectrum is as follows

• C6H4O has appropriate mass not C4 BTW there's no problem at all with losing two groups, there are further losses too, and peak 43 is even higher. – Mithoron Jun 2 '18 at 17:11
• @Mithoron I have edited the question to change the C4H4O to $\ce{C6H4O}$ – Raoul Kessels Jun 4 '18 at 21:18

A fragment with m/z of 92, 93 or 94 is typical for phenolic compounds. The aromatic $\ce{C-O}$ bond is very strong and therefore first all the rest is lost. In this case the fragmentations are:
1. Loss of the acetyl group: $180-42=138$
2. Loss of water from the acid group: $138-18=120$
3. Loss of $\ce{CO}$ from the acid group: $120-28=92$
4. Loss of $\ce{CO}$ from the phenolic group: $92-28=64$