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A highly efficient GCMS method is developed for reliable detection of thebaol, a constituent of opium poppy.A unique behavior for trialkylsilyl derivatives of thebaol derivatives under mass spectrometry conditions is established.Loss of two radicals via violation of the ‘even electron rule’ is specific for trialkylsilyl and perfluoroacyl derivatives of thebaol.A GC–MS method is described for the characterization of thebaol, a component of opium poppy. The method includes preliminary sample derivatization to TMS, TBDMS, TFA, PFP and HFB substituted products. Fragmentation of resulting derivatives is unique under electron ionization, and proceeds via consecutive loss of two radicals that violate the “even-electron rule”. Peaks of [M-2CH3]+. and [M-C4H9-CH3]+. ions show maximum intensities in the spectra of trimethyl- and tert-butyldimethylsilyl-thebaols. Elimination of perfluoroalkyl and methyl radicals from M+. is characteristic for TFA, PFP and HFB thebaols. The same fragmentation peculiarity is characteristic for derivatives prepared from related natural compounds containing vicinal 2-methoxyphenol moieties. The unique fragmentation of trialkylsilyl and perfluoroacyl derivatives of thebaol can be successfully used for thebaol determination within complex mixtures. This is part 4 from the series “Analytical derivatives in mass spectrometry”, parts 1, 2 and 3 see [1–3].