Resonant laser ablation of metals for trace metal analysis, by plasma optical and inductively coupled plasma mass spectrometries

The goal of traditional laser ablation solid sampling is to produce a gaseous plume of material with the same stoichiometric ratio of constituents as found in the bulk solid material. However, it has been shown that an increased number of analyte atoms can be selectively ablated from the solid surface by simultaneously tuning the ablation laser wavelength to match a gas-phase transition of the analyte, and by reducing the power density of the incident laser. Tuning the ablation laser to such a wavelength has been termed resonant laser ablation solid sampling (RLA). This dissertation addresses the two major aspects of RLA that have remained relatively unexplored: the determination of the mechanism of resonant enhancement, and the analysis of the overall analytical utility of RLA. A tunable optical parametric oscillator laser system was used to study RLA in a variety of matrices, including pure metals, metal alloys, thin films, sol-gels, and agarose gels. ^ Chapter 1 reviews the advantages, literature applications, and existing theories of resonant laser ablation. ^ Chapter 2 examines the analytical characteristics of RLA for trace metal analysis. Various types of data, including ablation wavelength scans, mass scans, and observation of the number of laser shots required to penetrate thin films, are presented. ^ In Chapter 3, a new mechanism for the RLA phenomenon is proposed to be based on radiation trapping, and supplemented by desorption induced by electronic transitions (DIET) processes. ^ Chapter 4 explores the analytical utility of RLA for quantitative analysis. Calibration curves were constructed to quantify zinc in soil and lead in house paint. Also, RLA was considered as a method to compensate for nonstoichiometric ablation in difficult samples. ^ Near-field laser ablation solid sampling is described as a promising application of resonant laser ablation in Appendix A. ^ Appendix B reviews applications of laser ablation sampling for the speciation of metalloproteins that had been separated by gel electrophoresis. Preliminary data are presented that suggest that RLA might improve the selectivity and sensitivity of metalloprotein measurements. ^ Appendix C suggests several modulation techniques that might be applied to improve quantitative analyses using resonant laser ablation.