Thermal decomposition curves of dibenzo-p-dioxins by a scanned flow tube pyrolysis method with on-line ion trap MS detection

Abstract

A screening method was developed for the rapid recording of thermal decomposition curves. The apparatus is made up of a miniature oven for sample evaporation and a flow tube pyrolyzer which is coupled directly to the open split interface of an ion trap mass spectrometer (ITMS). The sample oven is maintained at a fixed temperature while the pyrolyzer temperature is scanned from 100 to 1200°C when recording a decomposition run. Only helium or hydrogen can be used as the flow gas and flow rates are set to obtain residence times of 0.1 to 1 s in the hot zone. Full scan mass spectra are recorded on the reactor effluent with a typical sampling rate of 2 s per spectrum. Thermal decomposition curves are finally obtained by plotting single ion traces of characteristic reactant ions. The buildup and disappearance of major decomposition products can be followed in the same way. This technique permits the determination of A) the decomposition temperature of the reactant, B) the identity of the major stable decomposition products and C) the decomposition temperatures of these products. The method is useful for rapid evaluations of thermal decomposition temperatures in conjunction with hazardous waste incineration and chemical storehouse fires. Test measurements included a number of halogenated dibenzo-p-dioxins, a trichloro-dibenzofuran and the pesticide linuron.