APPLICATION OF A 2-DIMENSIONAL CHROMATOGRAPHY SYSTEM FOR GAS-PHASE PHOTODEGRADATION STUDIES OF POLYCHLORINATED DIBENZO-P-DIOXINS

Gas-phase photodegradation of polychlorinated dibenzo-p-dioxins was studied with a two-dimensional gas chromatographic system consisting of two independently operated gas chromatographs interfaced to a photoreactor. Gas flow through the chromatographs and photoreactor were directed with two switching valves. This arrangement permitted isolation and irradiation of solute in the photoreactor for varied periods and under different atmospheres. The irradiation experiments revealed that different dioxin congeners degrade at different rates; e.g., a 20-min irradiation resulted in the degradation of approximately 80% of 2,3,7-trichlorodibenzo-p-dioxin, while only 30% of the 2,3,7,8-tetrachlorodibenzo-p-dioxin was found to degrade under the same conditions. As expected, photodegradation rates decreased with an increase in the number of chlorines. Degradation rates were also influenced by the position of chlorine substitutions. The results showed that, in contrast to solution-phase studies, congeners with peri chlorines photodegrade more rapidly than congeners with laterally substituted chlorines. These results indicate that, in the gas phase, more toxic (laterally substituted) congeners are more persistent than less toxic (peri substituted) analogues.