| Laser induced breakdown spectroscopy(LIBS)is a kind of emission spectrum analysis method.The principle of this method is to produce plasma through the breakdown of the sample by the intense laser beam,and analyze the emission spectrum of the plasma when it is cooled to achieve the purpose of element composition analysis.The main detection object of LIBS is metal elements,and its advantages of rapid in situ,micro damage and simultaneous detection of multiple elements make LIBS technology more and more attention by researchers in various fields.However,when LIBS is applied to the detection of liquid samples such as ocean exploration,it faces the difficulty of low sensitivity.In order to improve the signal quality and detection sensitivity of LIBS in underwater detection,this paper proposed a micro-gas column assisted LIBS(MGC-LIBS)method for the application of in situ detection of marine metal elements.The liquid LIBS signal is enhanced by creating a micro gas column in situ underwater.The main research contents of this paper are as follows:(1)A set of auxiliary hardware system based on peristaltic pump and needle tube is designed and implemented,which can form a small section of micro-gas column in the liquid to assist LIBS detection.The optical path setting scheme of MGC-LIBS system was determined,and the experimental parameters of the system,such as laser energy,needle diameter and gas velocity,were selected through experiments.(2)The physical characteristics of plasma generated by MGC-LIBS system were studied,and compared with plasma generated by LIBS directly breakdown of bulk water,it was found that the electron temperature of plasma generated by MGC-LIBS system was higher and the electron density was lower.The influence of ambient gas on the spectral intensity and evolution of the plasma produced by MGC-LIBS system was studied.The results show that the spectral signal of Mg in argon is nearly 4.7 times stronger than that in air,and the electron temperature and density of the plasma are higher than those in air.(3)The detection ability of MGC-LIBS system is evaluated.It has been found that MGC-LIBS can enhance the spectral signals of Ca,Mg and Sr which are difficult to be observed when LIBS directly breaks through seawater.Compared with the direct breakdown of seawater,the spectral intensities of the above elements are enhanced by an order of magnitude.The detection limits of Mg,Ca,Sr,Na,K and Li were 0.88 mg/L,0.35 mg/L,0.48 mg/L,0.18 mg/L,0.36 mg/L and 0.13 mg/L,respectively.The results of quantitative analysis show that the inversion error of various metal elements is between-11.51% and 9.27% due to the large range of Na concentration in the sea water concentration range,and the minimum error of other elements is0.18%.(4)The quantitative analysis technology of seawater salinity based on spectral detection is explored.The OUN-LIBS technology applied in field seawater detection was introduced,and in order to measure the salinity of seawater more accurately according to the definition of salinity,LIBS technology combined with Raman detection method is proposed to detect the salinity of solutions containing different elements.The effective measurement of salinity of solutions containing different elements has been achieved initially. |
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