Study On LIBS Measurement Technology Based On Plasma Self-confinement

The direct discharge of industrial waste water and domestic sewage seriously endangers the safety of human life and the protection of ecological environment.It is of great significance to detect the composition and content of waste water.Most of the commonly used methods for detection of heavy metal elements in water require complex pretreatment of samples,which has low detection efficiency and cannot be used for simultaneous analysis of multiple elements.Therefore,it is urgent to develop a real-time,in-situ method for multi-element analysis.Laser-induced breakdown spectroscopy(LIBS)is a fast,real-time,non-sample preparation and multi-element simultaneous detection technology,which has been widely used in the detection of different forms of samples.However,when LIBS is directly used in liquid detection,it faces many technical difficulties.Liquid sputtering will pollute optical components and accelerate the quenching of the plasma.The fluctuation of liquid will affect the excitation of sample by laser pulse,the stability of plasma and the acquisition of spectral signal.These issues lead to low sensitivity and poor repeatability of detection eventually.Based on the above difficulties,this paper carried out three aspects of research.1.Study on the method of direct measurement of liquid sheet by LIBS.The stable liquid sheet by means of the collision of two liquid jets was used for sampling way of liquid measurement,which has many advantages such as weak sputtering and fluctuation,high stability,high repeatability.Since the heavy metal element Cr has strong hazard,the different concentrations of Cr solution were prepared.2.A new method for plasma self-confinement mode was proposed.Compared with the unilateral excitation mode,we proposed a plasma self-confinement mode for liquid measurement by LIBS.The experimental parameters were optimized and quantitatively measured were carried out.The limit of detection for Cr I 357.8 nm and Na I 588.9 nm were51.26 ppm and 2.02 ppm,which was better than the results of 71.56 ppm and 2.52 ppm obtained by unilateral excitation mode.At the same time,the electron density of the plasma was calculated.It was found that the electron density of the plasma confinement mode is higher than that of the unilateral excitation.The dynamic process of interaction between laser pulses and liquid sheet was studied,it was found that the sputtering and fluctuation of liquid sheet was effectively inhibited.The dynamic process of plasma shock wave was studied,it was found tha the stability of plasma improved and the expansion rate of plasma slowed down.3.Study on plasma self-confinement combined with double pulse LIBS(DP-LIBS)method.Based on the plasma self-confinement mode,collinear double pulse was used to enhance the spectral intensity.Firstly,the quantitative measurement of unilateral DP-LIBS was carried out,and the interval time between the two pulses was optimized.It was found that the coupling effect of the two pulses was the best when the interval time was 0.7 μs,and the best signal enhancement could be obtained.The limit of detection for Cr and Na were 49.31 ppm and 0.71 ppm,respectively.Furthermore,the detection limits of Cr and Na were 47.73 ppm and 2.01 ppm using plasma self-confinement combined with DP-LIBS.The results of this study showed that the plasma self-consfinement mode was an effective method for liquid measurement by LIBS.This method can be used for rapid and efficient quantitative detection of elements in aqueous solution.