We are constantly seeing an unknown peak at m/z 252 when we are performing imaging experiments. We are pretty confident that the interference is arising through the use of an artistic airbrush that we are using to spray coat our samples with, however we have been unable to identify this through MS/MS experiments. MS/MS gives fragments at m/z 116 and m/z 144 so we know it is matrix related with the addition of "something else". Does anyone know what this "something else" could be? This interfering species ionises EXTREMELY well and is tens of orders of magnitude higher than any analytes we are attempting to look for. Any other suggestions would be gratefully received.