| With the development of global industrialization,the problem of heavy metal pollution in water is becoming more and more serious in our world,which poses great threat to human health and ecological environment.So it is urgent to find a method that can quickly and sensitively determine the components of metal elements in bulk water.Now,there mainly exist several analytical techniques for the detection of trace metal elements in aqueous samples,such as ICP-OES,AAS,AFS and electrochemical methods,but these techniques require time-consuming and complex sample preparation,which is not suitable for fast,real-time or in situ monitoring.Laser induced breakdown spectroscopy(LIBS),first proposed by Brech and Cross in 1962,is a valuable spectroscopic technique can be used for qualitative and quantitative determining elemental composition in bulk aqueous solutions.Besides possessing the advantage of fast response with very little or no sample preparation,LIBS is one of the few techniques that have the potential to provide real-time,in situ,noncontact detection,which makes it especially suitable to monitor the toxic metal pollution in industrial wastewater.Despite having the obvious superiority when it compared with other analytical techniques,however,the conventional single pulse LIBS(SP-LIBS)faced many challenges such as the poor sensitivity and repeatability for metallic elements analysis in bulk aqueous solutions.It can be partly attributed to the unique nature of water,as we know,the water is characterized by high thermal conductivity,but hard to be compressed.Therefore,the generated plasma suffers strong compression and quenching effects,which could be responsible for the decreased emission intensity and the increased broadening of spectral lines.Then the fast cooling of plasma will accelerate electron-ion recombination and shorten the lifetime of the plasma.Additionally,a large amount of the input laser energy is consumed for the vaporization of liquid and the production of shockwaves and bubbles,only a very small part of energy is absorbed by the plasma to produce characteristic emission,thus reduce the excitation efficiency and restrict its further development of underwater applications.In this study,two Q-switched Nd:YAG lasers operating at 1064 nm were used and combined in orthogonal beam geometry to carry out the double pulse LIBS(DP-LIBS)experiment in bulk water.The influence of experimental parameters on the plasma emission of orthogonal DP-LIBS was systematically investigated.The results showed that there is a time window from about 500 ns to 900 ns in DP-LIBS,due to the signal-to-noise ratios are very high,it favors the selection of delay time for spectra acquisition.Besides,the optimized inter-pulse delay time was 40 ?s,and the higher breakdown efficiency was obtained at the relative focus position of 1 mm.Furthermore,with the increase of the first laser energy,the intensity of the spectral line first increased and then decreased rapidly.The dependence of emission intensity of DP-LIBS on the second laser energy was observed,which shows an exponential relationship,it might be related to the gaseous environment in which the second laser induced plasma was produced and confined.Under the optimized experimental condition,a 20-times enhancement of the Ba II 455.4 nm line intensity of DP-LIBS over the maximal intensity of SP-LIBS was obtained.The relative standard deviation(RSD)of DP-LIBS was derived to be 2.97%,and the value was about 4-times better than that of SP-LIBS.The detection performance between DP-LIBS and SP-LIBS was compared,the detection sensitivity of DP-LIBS was improved by a factor of 37,and the limit of detection(LOD)of Ba in bulk water was reduced from 31.35 ppm in SP-LIBS to 1.78 ppm in DP-LIBS. |
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