Microsecond pulsed Grimm glow discharge time-of-flight mass spectrometry study of aerosols generated by nebulization and laser ablation .

摘要

This dissertation discusses the experimental results for interaction of particles with the glow discharge plasma. The objective is to investigate the potential ability of a glow discharge (GD) for ionization of aerosols. In this work, a Grimm-type microsecond pulsed-GD source coupled with a time-of-flight mass spectrometer (TOFMS) were used for producing and detecting the ions formed after the introduction of the aerosol stream into the plasma.;At first, as a source for aerosols, an ultrasonic nebulizer-dehydrator unit was used to generate simple salt particles such as cesium iodide. The geometrical design of choice was found to be a modified Grimm-type source in which particles enter the plasma through an orifice in the middle of the cathode and instead of a conventional cylindrical anode, an anode with four off-centered holes was used. The effect of argon flow, source pressure, pulse potential and repeller delay on signal intensity was studied for NaI and CsI particles.;The optimum condition was used for quantitative studies. The signal had a linear correlation with the concentration of the nebulized salt solution. Heating the cathode was found to improve the signal reproducibility and also sensitivity (approximately 1.3 times). At the optimum condition and heating the cathode to 75 degrees Celsius, a sensitivity of approximately 1.8 mV/mM for cesium was achieved.;Another interesting observation was that the higher-mass ions such as cesium and iodide resulted in larger signals compared to lower-mass ions such as sodium (approximately 10:1 ratio). It was concluded that this was mainly due to the fact that more massive ions experience less scattering at the skimmer orifice. This discrimination between higher-mass and lower-mass ions was reduced by using a larger skimmer orifice (Cs+: Na+ = 2:1).;Finally, an aerosol stream was generated by laser ablation of solids in an external cell. The ablation lasers were a power-chip Nd:YAG laser and a Q-switched, flashlamp-pumped Nd:YAG laser. The samples investigated were aluminum, stainless steel, brass, bismuth alloy, ceramic and pressed pellets of inorganic salts. The effects of laser pulse frequency, energy and discharge potential on particle ionization were studied. Due to better signal stability and intensities, the laser ablation was found to be a more suitable method for aerosol generation when glow discharge is the ion source.