I am doing a whole bunch of pesticides by LC-MS/MS and bumped into a totally coincidental isobaric interference.

Bromophos-methyl (M+H = 365) can fragment 365->125, where the 125 is thiophosphate portion of the molecule. I decided to choose the isotopic peak 367-> 125 due to its greater intensity.

But when I ran a big pesticide mix I ran into a surprising collision from fenpropathrin (M+H = 350). The NH4 adduct is very sensitive at 367, and it too has a 125 fragment. For some reason I found it very amusing that, although I'm running all sorts of other thiophosphates that also have the 125 fragment, the one that has an isobaric interference is actually a pyrethroid. The 125 is actually a substituted cyclopropyl ring. And the peak was so huge that it dwarfed the real Bromophos-Me peak. So I had to set Bromophos-Me back down to 365.

I would imagine this is a common story... you think you are looking at the right peak and it's actually something totally different. Any stories to share? :)