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.