Mass Spec - What could a loss of 28amu as the first fragmentation be?

I guess 28 could be CO, C2H4 or N2. However I can't work out a mechanism by which CO could be the first fragmentation of any common organic compound. It seems like a diazonium salt could lose N2 as the first fragmentation but I've reason to believe this is not the unknown compound I'm trying to work out.


