A unique technique of laser-induced breakdown spectroscopy using transversely excited atmospheric CO2 laser for the sensitive analysis of powder samples
Introduction
It is known that some powder samples such as food materials and health supplements contain minerals, which are necessary for human health. Thus, the analysis of these powders during the production process is urgently necessary. Also, it was found that recently, the herb medicine sample contains poisonous heavy metals such as chromium (Cr), which might be contained in the sample during the growth of the plant due to the contamination of environmental pollutants in the soils such as industrial and traffic emission [1]. Therefore, the development of a technique for realizing a rapid quantitative analysis with high precision and high sensitivity on the powder samples is urgently needed.
The feasibility of laser-induced breakdown spectroscopy (LIBS) for quantitative analysis has directed this technique to be applied to various types of samples including solids, liquids, and gases [2]. However, a little works have been endeavored to the quantitative analysis of powder samples using standard LIBS technique. This is due to the blowing-off of the sample when the Nd-YAG laser, which is commonly employed as an energy source, was directly focused on the sample. In order to solve the problem, the sample is usually pressed into a pellet. However, although the powder samples are prepared in the form of pellet, the sample is still not as hard as the other solid samples such as ceramic. Thus, a strong shock wave, which is necessary to induce high-temperature plasma [3], [4], cannot be produced due to the lack of repulsion forces on the sample surface, resulting in low intensity of plasma emission.
In contrast to the technique described above, we have successfully employed a transversely excited atmospheric (TEA) CO2 laser to conduct a direct analysis on organic powder sample [5]. The TEA CO2 laser is suitable to the analysis of powder samples because the wavelength and the pulse duration of the TEA CO2 laser are longer compared to that of the Nd:YAG laser. In the study, the milk powder sample was placed into a small hole of the metal plate and covered by a metal mesh. However, we noticed that the sensitivity of elemental result was still low, namely the emission line of Ca was rather weak relative to the line of C–H molecular band. It means that the atomization of the sample still does not proceed well. Thus, this technique cannot be applied to the quantitative analysis of poisonous materials containing at low concentration in the organic powder samples. In order to overcome the problem above, we propose a unique technique utilizing a strong and high-temperature gas breakdown plasma induced by a metal mesh placed in tight contact on the powder sample and enhanced by a metal subtarget effect, which specially takes place only for the case of the TEA CO2 laser.
Section snippets
Experimental setup
The schematic diagram of experimental procedure used in this study is shown in Fig. 1(a). A TEA CO2 laser (Shibuya SQ 2000, energy of 3 J, wavelength of 10.6 μm, pulse duration of 200 ns) was employed as an energy source. The laser beam was focused by a ZnSe lens (f = 200 mm) onto a sample surface through ZnSe window. The laser energy was fixed at 1.5 J by setting an aperture in the path of laser beam. The spot size of laser beam in the sample surface was 2 mm × 2 mm, which results in a power
Results and discussion
A comparative study of organic powder sample using a standard LIBS technique and a new present technique, namely TEA CO2 laser-induced gas breakdown plasma, has been conducted. As mentioned in the introduction, in standard LIBS using the Nd-YAG laser, the shock wave plays important role to produce a strong plasma. Thus, the hardness of the sample is strongly required. When the sample is not so hard, the strong shock wave cannot effectively be produced, resulting in weak intensity of plasma
Conclusions
It was demonstrated in this study that the analysis of powder samples can successfully be made using a TEA CO2 laser-induced gas breakdown plasma technique. In this study, the samples were placed on a metal surface and covered by a metal mesh. The metal mesh serves to suppress the blow-off of the sample and to assist the generation of gas breakdown plasma, while the metal subtarget put on the back side of the sample is used to enhance the gas breakdown plasma induced by the metal mesh. When a
Acknowledgement
This paper was supported by research funds of Chonbuk National University in 2009.
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