120th General Meeting of the KCS

Type Symposium
Area [Analytical Chemistry Division - JASIS (Japan) Joint Symposium] Elemental Analysis Using Plasma Spectroscopy and Mass Spectrometry
Room No. Room C308+C309
Time FRI 15:30-:
Code ANAL-4
Subject Analysis of chemical reactions in the plasma for quantitative underwater laser-induced breakdown spectroscopy
Authors Tetsuo Sakka
Department of Energy and Hydrocarbon Chemistry, Kyoto University, Japan
Abstract 　Laser induced breakdown spectroscopy (LIBS) is a method for spectrochemical analysis of the target materials on the basis of the plasma generation by a pulsed laser irradiation. The method does not require pretreatment of the sample, and can realize elemental analysis of a solid target submerged in water without taking it out from the water. We can expect ranges of applications, in which on-site and in-situ analysis is crucial. Especially, we are interested in the application to deep-sea mineral explorations. In general, the density of atomic species in the plasma is very high due to the confinement effect of water. Therefore, the atomic emission lines are usually very broad, and continuum dominates the spectra, leading to the difficulty in elemental analysis. We have found that a longer pulse, such as the pulse width of 100-ns or so, can avoid the high density of the plasma at the delay time of several hundred nanoseconds to one microsecond from the pulse irradiation, and are successful in observing narrow atomic lines from the laser-induced plasma in water [1]. 　For LIBS we observe the laser-ablated atomic species in the plasma. For the spectral line intensity to be quantitative as the elemental analysis of the target materials, the elemental composition in the plasma should preserve the target composition. However, after a certain delay from the irradiation, the temperature of the plasma decreases, and this causes the formation of diatomic or larger molecules, which alter the composition of free atomic species. This obviously results in an unwanted alteration of the relative spectral intensities. Therefore, to obtain quantitative results, we have to investigate not only the stoichiometry of the ablation process, but also chemical reactions in the plasma. 　In the present work, we investigate the time evolution of atomic species in the plasma and in the subsequent bubble. To obtain information of the population densities of atomic species after the plasma quenching, we developed an instrumentation to measure absorption spectra of the species in the bubble after the plasma quenching. It is highly probable that the plasma in water is in the local thermal equilibrium (LTE) in the sense that electronic excitation temperatures, vibrational and rotational temperatures of diatomic molecules, etc., all agree with each other. This suggests that the chemical reactions are also close to equilibrium. We simulated the densities of chemical species by assuming that they are in the equilibrium, and that the equilibrium changes rapidly by following the rapid decrease of the temperature. Such simulation results explained well the experimental observation of the behavior of the atomic densities in the plasma. [1] T. Sakka et al., Appl. Phys. Lett., 88, 061120 (2006).
E-mail sakka.tetsuo.2a@kyoto-u.ac.jp