Fundamental Studies of Femtosecond Laser Interactions with Solids and Their Applications to Laser Ablation Inductively Coupled Plasma Mass Spectrometry for Environmental Analysis

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been successfully applied in many research areas. Compared to conventional analytical techniques, it has the advantages of minimal sample preparation and high spatial resolution capabilities. Elemental and isotopic fractionation, matrix effects and the lack of matrix-matched standards are problems that limit applications of this technique for routine analysis. The introduction of femtosecond laser (fs) pulses has improved the analytical capabilities of this technique in terms of precision, accuracy and detection limits. However, laser ablation involves complex processes that are not fully understood and requires extensive studies on laser-solid interaction, particle formation, particle transport and ionization of particles in the ICP ion source.;The second objective of my thesis was to validate the application of LA-ICP-MS to the analysis of sediment cores. In Chapter 5, we prepared sediment reference materials using low viscosity epoxy resin to get solid disks suitable for analysis by LA-ICP-MS. The data obtained by fs-LA-ICP-MS agreed well with those obtained by solution based-ICP-MS indicating the applicability of this technique for the analysis of sediment core. The usefulness of core analysis by LA-ICP-MS to interpret variations in element concentrations was demonstrated with a sediment core from Lake Erie.;The first main objective of my Ph.D. is to improve the analytical cababilities of LA-ICP-MS. This was covered in chapters 2, 3, and 4. Chapter 2 focused on understanding the mechanisms of laser-solid interactions and the method of generation of femtosecond laser pulses. The differences between nanosecond and femtosecond laser pulses and optimization of the laser beam for better ablation quality were discussed. In Chapter 3, we evaluated the analytic al capabilities of fs-LA-ICP-MS using different standard reference materials (NIST and Basalt glasses). We showed that femtosecond laser pulses ablates materials with different transparency (NIST 612 vs. NIST 610) in a similar way. Matrix effects and fractionations were shown to be greatly reduced in fs-LA-ICP-MS. In Chapter 4, we investigated the effects of mixed Ar gas plasma by the addition of hydrogen and nitrogen to the central Ar gas before the ablation cell in fs-LA coupled with quadrapole mass spectrometer. Enhancement of sensitivity was observed as a result of increased plasma temperatures due to the high thermal conductivities of these gases. At the same time a slight increase in double charged ions, nitrides and hydrides was observed. Addition of nitrogen to Ar gas before the ablation cell in fs-LA-MC-ICP has improved sensitivity of Tl and Pb and helped to reduce and stabilize the mass bias. This created robust plasma conditions for precise and accurate Pb isotope ratios using Tl as a surrogate to correct for mass bias.