4-phenylbenzoic acid fragmentation

Oct 2018
Hi everyone,

I’ve attached a mass spectrum of 4-phenylbenzoic acid that I’ve made recently, and I’m struggling to assign the major peaks.

I know the peak at 198 is due to the molecule itself as an ion (EI is used to ionise the molecule). I also know that the peak at 181 is due to a molecular ion of the compound without the hydroxyl part which is lost as a fragment.

But I’m struggling with the one at 152 - at first I thought it would be the biphenyl ion fragment, but that gives an m/z of 153, and not 152. Likewise I thought the one at 76 would be the phenyl ion fragment, but shouldn’t that be at 77, and not 76.

I’ve looked at other mass spectra of the same compound and they also have major peaks at 152 and 76. I was wondering what they were actually due to?