Influence of discharge parameters on real-time chemical speciation for gas chromatography pulsed glow discharge plasma time-of-flight mass spectrometry

Abstract

A millisecond pulsed glow discharge ionization source was employed for the determination of aromatic and chlorinated hydrocarbons. Chemical speciation employed four separate dimensions that incorporate analyte volatility, elemental constituents, chemical structure, and molecular weight. Analyses were performed in real time using a gas chromatograph that was interfaced to a pulsed glow discharge time-of-flight mass spectrometer. This instrument allowed for the sequential collection of the complete mass spectrum at three different time regimes occurring during the glow discharge pulse cycle. This report focuses on discharge parameters such as pressure and sampling distance that enhance or suppress different ionization processes. Increasing the ion source pressure resulted in a decrease in analyte ionization and sampling efficiency. Ions sampled in close proximity to the cathode surface demonstrated the best elemental sensitivity. Sampling ions at increasing distance, in the negative glow, produced structural information in the form of fragmentation patterns similar to conventional 70 eV electron impact spectra. Molecular weight information obtained after power termination was ascertained from M⁺ and MH⁺ ions. At very small distances, the most dominant molecular peak was the M⁺ ion produced by the Penning process. At increased sampling distances molecular peaks were dominated by MH⁺ ions; the ionization mechanism is attributed to proton transfer reactions from background species such as ArH⁺.