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
This spectrum is recorded using electron impact. That means that the molecule is positively charged by losing one electron. 
Since that is a very unstable structure, the molecule starts fragmenting losing neutral fragments. 
More processes occur, but these are the ones that we see in the spectrum. 
The peaks are the positively charged fragments. 

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:


Loss of the acetyl group: $180-42=138$
Loss of water from the acid group: $138-18=120$
Loss of $\ce{CO}$ from the acid group: $120-28=92$
Loss of $\ce{CO}$ from the phenolic group: $92-28=64$ 


These are all typical fragmentations. 
The height of the peaks indicate the relative stability of the positively charged fragments.