Temporal changes in purity and specific activity of tritium-labeled 2,3,7,8-tetrachlorodibenzo-p-dioxin: Radiopurity model for toxicology

The specific activity (S) and radiopurity (R) of tritium labeled 2,3,7,8-te trachlorodibenzo-p-dioxin, [H-3]TCDD, were measured by gas chromatography/m ass spectrometry (GC/MS) while attempting to accurately characterize TCDD d oses received by invertebrates, fish, and fish embryos during several toxic ology studies conducted over a 3 year period. The [H-3]TCDD sample was foun d to consist of six TCDD analogues involving hydrogen, deuterium, and triti um substitution at the 1,6-dibenzo-p-dioxin carbon positions and a complex mixture of impurities (with and without tritium labels). Planar aromatic im purities were identified as tolyl-TCDD adducts and appeared to result from the decay of H-3 radiolabels to produce TCDD carbocations which reacted wit h the solvent, toluene. Formation of TriCDD and tolyl-TriCDDs, from both TC DD and tolyl-TCDDs, probably resulted from radiolysis-induced loss of a chl orine to form TriCDD free radicals which reacted with toluene. The measurem ent of S for [H-3]TCDD by GC/MS was accurate and precise (+/-3%) because re lative, rather than absolute, amounts of the analogues were determined. Cha nges in S over time were accurately modeled as a function of the conversion of each [H-3]TCDD analogue to a solvent-TCDD analogue at a rate determined by H-3 loss due to decay. Storage, purification, and use of tritiated chem icals for toxicology studies requires consideration of the H-3 decay-relate d phenomena. For example, hydroxylated TCDD is an expected decay reaction p roduct of [H-3]TCDD in tissues and may be misidentified as a metabolite.