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Detection of salt in soil by employing the unique sub-target effect in a pulsed carbon dioxide (CO2) laser-induced breakdown spectroscopy

An experimental study of laser-induced breakdown spectroscopy using a transversely excited at atmospheric pressure (TEA) CO2 laser, also called pulsed CO2 laser, for detection of salt in soil sample has been carried out. The soil sample was mixed with grease to allow attachment soil sample to the surface of a nickel plate acting as sub-target. LIBS equipment used in this work consist of the TEA CO2 laser operated at a wavelength of 10.6 μm with pulse energy and pulse duration of 3 J and 200 ns, respectively. The laser beam was focused onto the soil sample attached to the nickel plate using a Zinc Selenium (ZnSe) lens (f = 200 mm) through a ZnSe window. The emission spectrum from the generated plasma was detected using an optical multichannel analyzer (OMA) system consisting of a 0.32-m-focal length spectrograph with a grating of 1200 grooves/mm and a 1024-channel photodiode detector array equipped with a microchannel plate intensifier. At this stage, the soil sample used this work is a standard soil produced by Japan Powder Standard Testing (JIS Test Powder 1, Class 7) with a given salt concentration. The experimental data acquisition was made under different kinds of surrounding gas at a pressure of 1 atm. As the result, the soil samples can be attached well to the nickel plate sub-target, hindering sample blowing off during laser irradiation. The emission lines from salts, such as Ca II 393.36 nm, Ca II 396.84 nm, and Ca I 422.67 nm can clearly be detected. It was found that other salts such Na, and Mg can also be detected. This result implies that TEA CO2 LIBS with the aid of a metal sub-target and grease as a binder can be used for direct detection of salts, namely Ca, Mg, and Na in soil.