Supersonic jet spectroscopy for environmental analysis

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Abstract
Dioxins and their precursors such as halogenated benzenes and phenols are suspected to be carcinogenic, and therefore they are required to be measured at ultratrace levels. Supersonic jet spectrometry combined with multiphoton ionization mass spectrometry is developed in this study for solving this problem. In order to improve the ionization yield, either 150-fs or 500-fs laser pulse is used as an excitation/ionization source, and the results are compared with those obtained using a 15-ns laser pulse. The ionization yield is substantially improved by decreasing the laser pulsewidth, which is explained by the short lifetime of the excited state for halogenated benzenes, which is arising from spin-orbit coupling and fast relaxation to the triplet state. In order to improve the sensitivity, i.e., to increase the number of ions induced by laser irradiation, a collinear ionization scheme is studied, in which the axis of jet expansion is designed to be identical to the direction of the ion drifting in the time-of-flight tube. The enhancement, by a factor of 50, in the ionization signal is achieved, which coincides with an expected value of 36 from a geometry. Supersonic jet spectrometry is also used for the detection of halogenated aromatic hydrocarbons formed by thermal decomposition of polyvinyl chloride and chlorinated polyvinyl chloride. Interestingly, chlorinated aromatic hydrocarbons are not detected from polyvinyl chloride but detected only from chlorinated polyvinyl chloride. Isomer specific determination of dichlorobenzene is also demonstrated in this study.

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