Doctor of Philosophy (PhD)
The fundamental physical processes of mid infrared laser ablation in the context of laser desorption mass spectrometry were investigated. Understanding the mechanisms of infrared laser desorption and ablation can lead to improvements in these techniques and expand their applications. Particles were generated from glycerol irradiated at atmospheric pressure using a tunable infrared laser at wavelengths between 2.6 and 3.8 µm. The wavelength dependence of size distributions of ablated particles was measured. The particle concentration roughly tracked the infrared absorption spectrum of glycerol and the mean particle size tracked the inverse of the IR absorption. A novel approach, post ablation particle irradiation, was developed to manipulate the size of ejected particles. Particles generated by a 2.94 µm laser ablation of 3-nitrobenzyl alcohol were irradiated with a 351 nm ultraviolet laser at different time delays. A reduction in the average particle size and an increase in the total particle concentration were recorded. In addition to particle sizing, fast photography was used to study the dynamics of mid infrared laser desorption and ablation. Images of scattered light from glycerol ablation at atmospheric pressure were recorded using a CMOS camera from 25 ns to 1000 µs delay. The velocity of the expanding plume ranged from greater than 300 m/s near the 3.0 µm OH stretch absorption of glycerol to less than 100 m/s near the 3.4 µm CH stretch. Modeling calculations of these experiments suggest that the vigorous ablation is driven by phase explosion in the stress confinement regime near the wavelength of OH stretch absorption followed by a plume evolution for milliseconds. The wavelength and energy of the infrared laser can be used to effectively “tune” the composition of the desorption plume. This ability to control the composition of the plume will be used in the development of IR laser ambient ionization mass spectrometry techniques such as atmospheric pressure matrix-assisted laser desorption ionization (AP-MALDI) and matrix-assisted laser desorption electrospray ionization (MALDESI) where the ability to control material removal is critical to efficient ionization.
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Fan, Xing, "Particle analysis in infrared laser desorption and ablation" (2010). LSU Doctoral Dissertations. 1685.