| This thesis describes the experimental work leading to the development of a novel elemental analysis technique based on the coupling of Laser Ablation (LA) with Laser-Enhanced Ionization (LEI), for the direct trace analysis of solid samples. Our approach, consisting of the vaporization of solid samples in an ablation cell followed by the transfer of ejected particles into a miniature concentric burner for LEI detection, benefits from the high sensitivity and high spectral selectivity of LEI as well as from the possibility to directly vaporize small solid samples without any sample preparation. Furthermore, this decoupling of ablation cell and detector allows the independent optimization of vaporization and detection processes. The present document can be divided in three main research topics.; First, we describe the results of our exploration of all possible one-step and two-step laser excitation schemes for the LEI detection of lead (Pb), which was chosen as the target analyte for this project. The choice of excitation wavelengths and excitation energies is presented with respect to signal/noise enhancement. The development of a novel miniature, argon-sheathed concentric burner is also described, and we discuss the benefits obtained with the use of the Ar-O2 mixture, instead of air, which allows a striking reduction in the background signal generated by the multiphoton ionization of NO molecules and enhances LEI sensitivity for Pb.; Secondly, results of the optimisation of the physical coupling of LA with LEI are presented. The effect of the carrier gas composition on the ablation rate and on aerosol transport efficiency is described. The optimisation of the ablation laser beam delivery optics and of the geometry of the ablation cell is also presented.; Finally, the use of LA-LEI for the direct trace analysis of Pb in certified high purity aluminium samples is presented. We show that the high selectivity and high sensitivity provided by LA-LEI, using the optimal experimental conditions determined during this project, allows us to overcome an interference from Mg observed on the first excitation step, by subtracting the first-step LEI signal from the two-step signal. Finally, it is shown that the analytical figures of merit obtained in this work are comparable to those of state-of-the-art techniques such as LA-ICP-MS or LA-LEAFS. |
