Use of soft plasma ionization source at evacuated air atmospheres in time-of-flight mass spectrometry to suppress fragmentation of volatile organic compounds

This study describes a measuring system for mass spectrometry, consisting of a glow discharge ionization source for soft plasma ionization and a time-of-flight mass spectrometer, to detect toxic volatile organic compounds rapidly and easily. It is the most important to determine how the complicated fragmentation of such compounds can be suppressed to occur so as to recognize the mass spectra of the volatile organic compounds as their fingerprints. The novelty of this work is that the optimal discharge condition for the soft plasma ionization-time-of-flight mass spectrometer system could be selected, so that the parent mass peak of analyte molecules could be observed both with high sensitivity and with little or no fragmentation of them. Use of air gas at a pressure of 1000 Pa provided the most favorable result for these criteria, whereas, in a previous report, the soft plasma ionization source operating with argon at a pressure of 346 Pa had yielded additional mass peaks of the fragmented species. The reason for this would be explained by the fact that energetic electrons in the plasma, which principally cause the fragmentation of the volatile organic compounds, have lower number density at higher gas pressures, through de-accelerated collisions with the plasma gas.