Study On The Trace Elemental Detection Of Aqueous Solution Using Laser-induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy(LIBS)is a laser-excited atomic emission spectroscopy technology,which uses a focused laser to interact with the sample surface to generate plasma.The elemental information can be deduced by analyzing spectra.In recent years,LIBS has been widely applied in rapid soil detection,industrial detection,space exploration,and food inspection.However,in the water detection of LIBS,the weak spectral intensity and poor stability are caused because of the liquid level fluctuations,liquid splashing,and plasma quenching,which restricts the development and application of LIBS in element detection of water.Based on the liquid-solid conversion assisted LIBS,this thesis realizes the morphological conversion of water,which effectively avoids the problems of liquid splashing and plasma quenching.First,the enhancement mechanism of the liquid-solid conversion assisted LIBS is studied.On this basis,some new methods are proposed to improve the detection stability of liquid-solid conversion assisted LIBS and to improve the detection sensitivity of nonmetallic elements in water.The detailed contents are as follows:The enhancement mechanism of liquid-solid conversion assisted LIBS was investigated.Four substrates of Zn,Mg,Ni,and Si were selected to study the relationship between the detection sensitivity obtained and substrate physical properties.Taking Cr and Pb as examples,the studies have shown that there is a negative correction between detection sensitivity and the boiling point of four substrates.The limits of detection(Lo D)of Cr and Pb obtained on the optimal were 0.0011 mg/L and 0.004 mg/L,respectively,which was lower than the national drinking water detection limits.A geometric constraint liquid-solid conversion(GCLSC)method was proposed to improve the detection stability and quantitative accuracy of the liquid-solid conversion assisted LIBS.The repeatability of sample preparation,the stability of spectral intensity,and the accuracy of quantitative analysis of three methods of unconstrained direct liquid-solid conversion (UDLSC),filter paper assisted liquid-to-solid conversion(FPLSC),and GCLSC were evaluated.The results showed that the repeatability,the stability,and the accuracy could be improved greatly using GCLSC.Compared with UDLSC,the average relative error(ARE)of quantitative analysis using GCLSC could be improved by 70% and the root mean square error of cross-validation(RMSECV)could be improved by 60% for B and Cr.An indirect LIBS method was proposed to improve the detection sensitivity of difficultto-excitable non-metallic elements Cl and S.The results showed that compared with direct LIBS,the detection sensitivity of Cl and S could be improved from 1567 mg/L and 1448 mg/L to 2 mg/L and 5 mg/L,respectively,which could meet the detection limits of Cl and S in groundwater.Molecular LIBS was proposed to improve the detection sensitivity of N in water.The CN emission spectra in the plasma were analyzed to deduce the content of N in water.The results showed that the detection sensitivity of N could be improved from 107 mg/L to 0.6 mg/L using CN spectral line 388.4 nm compared with using N atomic line 746.83 nm,which could meet the environmental quality standard of national surface water.In summary,the liquid-solid conversion assisted LIBS was investigated in this thesis.The enhancement mechanism of liquid-solid conversion assisted LIBS was investigated.A geometric constraint liquid-solid conversion method was proposed to improve the stability of liquid-solid conversion.Methods of indirect LIBS and molecular LIBS were proposed to improve the detection sensitivity and accuracy of difficult-to-excitable elements Cl,S,and N in water,by which analytical sensitivity was significantly improved.This thesis provides a theoretical and experimental guide for the development and application of liquid-solid conversion assisted LIBS in elemental detection in water.