Methylation artifacts in the gas chromatography of serum extracts. Competitive suppression with trimethyl-d9...

In the course of the identification of drugs in body fluids by gas chromatography-mass spectrometry we have observed the appearance of N-methyl derivatives of certain compounds such as phenobarbital and diphenylhydantoin, especially in extracts of serum. With the aid of deuterium labelled phenobarbital it could, however, be shown that the N-methyl derivatives are not endogenous metabolites but artifacts generated upon injection of the serum extract into the gas chromatograph. Conversely, derivatization with trimethyl-d9 anilinium hydroxide (TMAH-d9) demonstrated the absence of any undeuterated methyl derivative and this reagent should be useful for the detection of endogenous N-methyl derivative which would, of course, remain undetected when using unlabelled TMAH in the course of the conventional methylation technique. There are good indications that lecithin is the methylating agent in serum responsible for the formation of these artifacts.