Time-of-Flight Mass Spectrometry with Pulsed Glow Discharge for the Direct Determination of Volatile Organic Compounds in Air, Nitrogen, and Argon: Ionization Processes of Volatile Organic Compounds

Abstract

The determination of volatile organic compounds (VOCs) in various gases, including atmospheric and exhaled air, is necessary to solve a wide range of environmental problems and monitore gas composition, and is increasingly used for diagnosing a variety of diseases. Recently, methods of soft ionization with minimal fragmentation of components have been rapidly developed. Our research group is developing an approach to the direct analysis of mixtures of volatile organic compounds by time-of-flight mass spectrometry with a pulsed glow discharge. Previously, the effects of various gases and gas mixtures on ionization processes have not been compared. This work presents a study of the mechanisms of VOC ionization in argon, nitrogen, and air. Toluene, p-xylene, chlorobenzene, and 1,2,4-trimethylbenzene were selected as model VOCs. The parameters of the microsecond pulsed glow discharge (pulse period and duration, ejection pulse delay, pressure in the discharge cell) have been optimized for each compound and a gas mixture of several volatile organic compounds. The predominant ionization mechanisms are the formation of molecular ions of VOCs through Penning ionization and proton transfer reactions, with their effects varying depending on the gas. The use of argon, even with a small addition of water, leads to the predominance of the proton transfer reaction, while in nitrogen and air mixtures, Penning ionization prevails. Under the optimized conditions in air, for which the highest VOC intensities were attained, the developed approach for analyzing exhaled breaths and samples of atmospheric air was tested.