Date of Graduation
Eberly College of Arts and Sciences
Fred L. King.
Glow discharge mass spectrometry is a well-developed elemental analysis technique for solid samples. Pulsed glow discharges exhibit advantages over continuous power glow discharges. The brief, high-power pulse yields transient but intense analytical signals, increasing analytical sensitivities; meanwhile, the following power-off period allows samples to be sufficiently cooled to avoid plasma instability. In this work, pulsed glow discharge mass spectrometry has been successfully employed to determine bromine concentrations in a series of thermally labile plastic samples.;Of more analytical interest are the temporal variations in plasma chemistry within the pulsed glow discharges. A plasma perturbation method was employed to investigate the plasma processes after the power termination, i.e., the afterpeak regime. The experimental results strongly indicate that, after power termination, discharge gas ions recombine with thermalized electrons; subsequently, a large population of metastable argon atoms is yielded that plays a central role in excitation and ionization during the afterpeak regime.;Because the dominant ionization mechanisms alternate over a pulse cycle, plasma ionization is temporally dependent. The apparent plasma energy of a millisecond pulsed glow discharge was measured using tungsten hexcarbonyl as a "thermometer molecule". The semi-quantitative results reveal the plasma energies undergo significant temporal and spatial variations, thereby effecting both "soft" and "hard" ionization for a given molecular species. As a result, elemental, structural, and molecular information can be obtained with the same plasma ion source. Its analytical utilities were further demonstrated by analysis of a series of organic and organometallic compounds using pulsed glow discharge time-of-flight mass spectrometry.;To understand the fundamental plasma processes involving molecular species, nitrogen was introduced into pulsed argon and helium glow discharge plasmas as a probe molecule; its interactions with various plasma species were monitored using spectroscopic techniques. The study yielded insights into the excitation and ionization of molecular species in a pulsed glow discharge. The adverse effects of the admixed molecules on plasma ionization were also discussed. These data will be useful in future efforts to optimize the analytical performance of the pulsed glow discharge ion source.
Li, Lei, "Molecular speciation using pulsed glow discharge time -of -flight mass spectrometry" (2004). Graduate Theses, Dissertations, and Problem Reports. 2117.