Study On Elemental Detection Of Aqueous Solution Using Surface-enhanced Laser-induced Breakdown Spectroscopy

As a new spectroscopic technique,laser-induced breakdown spectroscopy(LIBS),has been applied in many fields due to its attractive features,such as rapid,simultaneous multielement detection,and in situ,real-time analysis capabilities.However,the detection performance of LIBS is still unsatisfactory when directly analyzing liquid samples because it suffers from several problems,e.g.,water splashing,surface ripples,and plasma quenching.To address these problems,the analysis of heavy metal elements(Cu,Pb,Cd,and Cr)in aqueous solution using LIBS was systematically studied and new assisted approaches were proposed for improving the spectral intensity and the performance of quantitative analysis.Finally,the proposed method was applied to rapidly analyze of rare earth elements in wastewater.The concrete research contents are as follows:To investigate the infulence of the sample preparation on LIBS detection sensitivity of heavy metal elements in aqueous solution,liquid surface and liquid aerosol methods were compared.The results showed that the liquid surface had a higher spectral intensity and detection sensitivity due to its lower plasma threshold.With the aid of liquid surface,the limit of detections(LoDs)of Cu,Pb,Cd,and Cr elements were 0.871,16.084,71.984,and 1.787 ?g/mL,respectively.However,the LoDs were not achieved the level of the discharge standard of pollutants for municipal wastewater treatment plant.Chemical replacement combined with surface-enhanced LIBS(CR-SENLIBS)was proposed to improve the detection sensitivities of trace heavy metal elements in aqueous solutions.Compared to conventional SENLIBS,the preparation time was reduced from 15 min to 1 min,and the matrix effect could also be mitigated.The results showed that the LoDs of heavy metal elements(Cu,Pb,Cd,and Cr)were 0.200,0.136,0.386,and 0.016 ?g/m L,respectively,which were improved by 18~441 times compared with conventional LIBS.Except Cd element,the LoDs of other elements have reached the level of the discharge standard of pollutants for municipal wastewater treatment plant.The enhancement mechanism of SENLIBS was studied for further improving the detection sensitivity.Five different metallic substrates(Zn,Al,Mg,Cu,and Co)were chosen as the substrates for SENLIBS to study the effect on the analytical sensitivity.The results showed that the average LoD of SENLIBS under the assistance of different metal shows the following trends: Zn < Mg < Al < Cu < Co.Further investigation showed that the lower ablation threshold fluence and laser-induced breakdown thresholds of the metallic substrates are,the higher detection sensitivity of SENLIBS is.Filter paper was introduced as a transmission medium between the liquid droplet and the metallic substrate to improve the accuracy of SENLIBS.The results showed that the analytical accuracy of SENLIBS can be improved by about 50%.With preparing liquid samples by several times in the same place on metal-zinc surface,the LoDs for Cu,Pb,Cd,and Cr elements were reached 0.033,0.027,0.018,and 0.019 ?g/m L,respectively,which were basically achieved the level of the discharge standard of pollutants for municipal wastewater treatment plant.Finally,the SENLIBS was applied to determinate rare earth elements(lanthanum,cerium,praseodymium,and neodymium)in an aqueous solution.Compared with conventional LIBS analysis,the intensity RSDs and LoDs of rare earth elements detection by using SENLIBS were improved about 45% and 64%,respectively.These above new methods of SENLIBS proposed in this thesis,have greatly improved the accuracy and sensitivity of SENLIBS analysis.Therefore,this study provides technical and theoretical support for wider application of LIBS in waste water analysis and monitoring.