Fluorescence Imaging of Analyte Profiles in an Inductively Coupled Plasma with Laser Ablation as a Sample Introduction Source

Fluorescence Imaging of Analyte Profiles in an Inductively Coupled Plasma with Laser Ablation as a Sample Introduction Source Lance Moses Department of Chemistry and Biochemistry, BYU Doctor of Philosophy Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has risen to among the top tier techniques for the direct analysis of solid samples. However, significant problems remain that must be solved to achieve the full analytical potential of LA-ICP-MS. Inefficient conversion of aerosol to ions within the ICP or transmission through the MS interface may decrease precision, sensitivity, and/or accuracy. Although fundamental mechanisms that govern ion production and transmission have been studied extensively in solution-nebulization (SN) ICP-MS instruments, significant gaps in our understanding remain. Furthermore, it is unclear to what extent differences between the aerosols generated during SN and LA influence either ion production or transmission. In this work, I initially investigated differences in the spatial distributions of Ca, Ba, and Sc ions generated by LA and SN using high-resolution LIF imaging. Ions formed from aerosol generated by LA at low fluence were distributed over much greater axial and narrower radial distances than SN aerosol. Additionally, I investigated the effects of solvent, laser fluence, and ablation atmosphere (He vs Ar) on ion distributions in the ICP. Unlike solvent, changing laser fluence and ablation atmosphere produced considerable changes in the ion signal intensity and spatial distribution during LA. At greater laser fluence, the radial distance over which ions were distributed dramatically increased. Surprisingly, when helium was mixed with argon as carrier gas, ion signals decreased. Many of these effects were assumed to be related to changes in the number and size of particles generated during LA. In a follow-up study, relative contributions to ion densities in the ICP from particles of different sizes were investigated. LIF images were recorded while filtering particles above a threshold size online. Micron-sized particles contributed the majority of ions formed in the ICP. For Ba, Ca, and Sc, differences in the axial position where nanometerand micron-sized particles vaporized were 2, 1, and less than 1 mm, respectively. I also performed experiments to identify changes in the ion signal related to changing ablation conditions vs. changing ICP conditions associated with helium additions to the carrier gas. LIF images were recorded during different combinations of He/Ar added upstream and/or downstream of the ablation cell. Changes in the ion signal during ablation in helium vs argon did not always match expectations based on changes in particle numbers and sizes measured with SEM. The results force re-examination of some of the fundamental assumptions about the effect of carrier gas composition on the performance of LAICP-MS. The research described in this dissertation provides valuable insight into fundamental aspects of key ICP processes related to LA generated aerosol.