A Review of the Auxiliary Measurement of Nobble Gases in LIBS

Laser-induced breakdown spectroscopy (LIBS) is an emission spectrum analysis technology which has great potential in the analysis of high-risk environments and important cultural relics due to the advantages of real-time, multi-element, non-destructive and long-range. Currently, LIBS measurement technology is still affected by experimental equipment and measurement environment, among which the influence of the atmosphere environment on LIBS measurement is particularly significant. Nobble gases are conducive to assisting LIBS measurement, enhancing the spectral signal, improving detection limit, etc., because of their excellent physical properties (specific heat capacity, conductivity, stable chemical properties, large relative molecular mass, etc.), which are increasingly used in laser-induced breakdown spectroscopy. Therefore, based on the detailed investigation of the application of inert gas in the LIBS measurement process, the influencing factors on the spectral signal are analyzed. For example, the very low chemical activity effectively reduces the absorption and annihilation of spectral signals by other gases in the air (oxygen, nitrogen, etc.). The large atomic volume greatly limits the outward expansion of the plasma, causing the temperature per unit volume to rise significantly; The abundance of electrons causes the free electrons in the generated plasma to rise greatly, which increases the electron density; The metastable state promotes the delayed excitation of inert gas to the sample, assists in the excitation of some lighter elements and greatly increases the plasma lifetime; The energy transfer can enhance the emission spectral of some special energy levels to obtain higher enhancement factors; The effect of radon gas on emission line enhancement through decay was also analyzed. However, the emission of inert gases can also be used to calculate the plasma temperature and electron density of samples, which has an important influence on the auxiliary measurement of LIBS. In addition, the application of metastable state in LIBS measurement and enhancement factor of inert gas on the spectral signal in different studies are also listed. Finally, the differences in signal enhancement, ablation quality and time evolution of different inert gases are analyzed and compared. It turns out that helium is special compared to other inert gases because of its physical properties and extra-nuclear structure. However, with the increase of cycle number, the signal enhancement factor of other gases gradually increases. At the same time, the ablation mass gradually decreases, and the time evolution of plasma is also getting longer, which provides theoretical support and reference for the selection of appropriate inert gases in LIBS measurement.