Resonance-enhanced multiphoton ionization and VUV-single photon ionization as soft and selective laser ionization methods for on-line time-of-flight mass spectrometry:: Investigation of the pyrolysis of typical organic contaminants in the steel recycling process

A newly conceived compact and mobile time-of flight mass spectrometer (TOFMS) for real-time monitoring of highly complex gas mixtures is presented. The device utilizes two selective and sensitive soft ionization techniques, viz., resonance-enhanced multiphoton ionization (REMPI) and single-photon ionization (SPI) in a (quasi)-simultaneous mode. Both methods allow a fragmentationless ionization. The REMPI method selectively addresses aromatic species, while with SPI applying vacuum ultaviolet light (118 nm) in principle all compounds with an ionization potential below 10.5 eV are accessible. This provides comprehensive information of the chemical composition of complex matrixes. The combustion and pyrolysis behavior of five organic materials typically used in steel processing in China was studied. The trace amounts of organic compounds in the gas phase during combustion and pyrolysis were monitored selectively and sensitively by real-time SPI/REMPI-TOFMS. The measurements were carried out at several constant temperatures in the range from 300 to 1190 degreesC in both synthetic air and nitrogen. Timely resolved mass spectra reveal the formation and subsequent growth of aromatic molecules. At lower temperatures, highly alkylated PAHs predominate, while at temperatures above 800 degreesC, the more stable benzene and PAHs without side chains prevail. Potential hyphenation of SPI/REMPI-TOFMS to methods of thermal analysis is discussed.