A bit of clarification to the question would help. Are you intending to measure hydroben peroxide directly (in which case GC/MS may not be a good idea)? Are you intending to measure fish that have been treated or waters that the fish have been in? If fish, are these live fish that have been treated; dead, whole fish; or specific parts of the fish? If you are not planning to measure hydrogen peroxide directly, then you are looking to detect some compund that results from the presence of peroxide. Do you have such a compund in mind or is discovery of the product part of this project?

Actually i should be clearer.

I have to determine Hp on fish surface in order to prove the illegal use of Hydrogen peroxide. Hp used on fish surface give it back a better look but it's an unlegal procedure in Italy. So that i have to know if fish has been trated with Hp. All I've found was a Japanese article based on GC-ECD measurment of hydroxyanisole as its PFB (pentafluorobenzoyl) derivatives, which is a product of the hydroxylation of anisole by reacting with Hp and hexacyanoferrate (III). I'm not so confident with this method.

The problem is that i have to see traces of Hp to demostrate the absence otherwise the minimum presence of Hp.