Highly Sensitive Analysis Of Heavy Metals In Water By Discharge-Assisted Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy(LIBS)is also known as laser-induced plasma spectroscopy(LIPS).The basic principle is that a high-energy laser is focused on the material to be measured,and a high-temperature and high-density plasma is generated above the surface of the material to be measured,which emits light outward during the cooling.The spectrometer collects the emitted light and determines the element composition and content information of the material to be measured according to the wavelength and intensity information of the light.It has a lot of advantages,for example:a wide range of samples,simplicity of the device,operability,near non-destructive testing,no sample preparation and pre-treatment,wide concentration range,and long-distance detection.Therefore,it has wide applications,such as coal detection,deep sea exploration,food detection,nuclear material detection,water pollution monitoring,and soil monitoring.Although LIBS technology has many advantages and broad application prospects,self-absorption effects,matrix effects,ambient gases and spectral fluctuations have prompted researchers to further improve the LIBS technique..How to better enhance the emission signal of LIBS,improve the analysis sensitivity and signal stability has become a hot and key issue of great interest in the field of LIBS.A large number of works have indicated that spatially confined LIBS,magnetically confined LIBS,pre-heated sample LIBS,double-pulse LIBS,nanoparticle-enhanced LIBS,surface-enhanced LIBS,and changing the distance from focusing lens to sample surface may significantly enhance the emission signal of LIBS,improving the analysis sensitivity and signal stability.In addition,discharge-assisted LIBS can significantly enhance the spectral intensity,analysis sensitivity,and signal stability.In this paper,discharge-assisted LIBS is studied,and the main results are as follows:1.The spark discharge(SD)assisted LIBS(SD-LIBS)technique is used to discuss Al samples,and the effect of SD on the spectral intensity of Al(I)and Al O molecules produced by femtosecond pulse ablation of Al is discussed.The measured neutral atomic lines are Al(I)394.40 nm and 396.15 nm,and the measured Al O radical emission bands are B~2Σ~+-X~2Σ~+(Δν=0)transitions.Under SD-LIBS,the optical emissions of Al atoms and Al O radicals are stronger than that of traditional LIBS.At the same time,when the SD voltage increases,the spectra of Al(I)394.40nm and 396.15 nm gradually become stronger,while the spectra of Al O(0-0)and(1-1)first become stronger and then remain unchanged.In addition,the vibration temperature of Al O radical remains unchanged with increasing the SD voltage.Therefore,SD can enhance the optical emission of Al atoms and Al O radicals,but their sensitivity to SD voltage is different.2.The effect of SD on the self-absorption of Al(I)in femtosecond laser-induced Al plasma is investigated.The Al(I)394.40 nm and 396.15 nm are selected as the analytical lines.The SD can eliminate the self-reversal of Al(I)spectral lines and weaken the self-absorption of Al(I).The theoretical intensity ratio of Al(I)394.40 nm to 396.15 nm is about 1.97.When the delay time is longer than 1.6μs,the measured intensity ratio of the two lines is close to the theoretical value.For a delay time greater than 1.6μs,the plasma can be considered optically thin.Therefore,SD can significantly reduce the self-absorption effect in femtosecond LIBS and improve the sensitivity and precision of spectral analysis.3.Tesla coil discharge(TCD)assisted LIBS(TCD-LIBS)technology is used to improve the optical emission of plasma.The physical mechanism of spectral enhancement is discussed from the discharge distance,laser energy,electron temperature and density.Firstly,the effect of discharge distance(the discharge tip to the sample surface)on the emission intensity is optimized,revealing that the spectral emission of TCD-LIBS is stronger in the distance range from 4 mm to 8 mm.Secondly,the effect of laser energy on spectral intensity is investigated.The spectral intensity can be increased by about 40 times using TCD.Finally,the electron temperature and density are calculated,revealing that the two parameters can be increased by about 1.6 and 1.4 times using TCD.4.The effect of the lens-to-sanple distance(LTSD)on the spectral intensity and detection sensitivity of electro-deposition(ED)LIBS(ED-LIBS)for detecting heavy metal elements is investigated.The focal length of focusing lens is 100 mm.The results show that with decreasing the LTSD,the continuous background intensity and signal intensity of Cr(I)both first increase and then decrease.In addition,the LTSD for the strongest signal intensity of the Cr(I)in the experiment is not near focal point,but at the LTSD of 68 mm.The spectral signal of the substance(to be detected)that only exists on the metal substrate surface is different from the traditional results of the LTSD.Finally,the Cr calibration curve is constructed,finding that the limit of detection(LOD)of the heavy metal Cr element in water at the LTSD of 68 mm is the lowest.The above results provide a basis for the following research.5.The effect of SD on the spectral intensity and analysis sensitivity of trace Pb in water by ED-LIBS is explored.Under SD,the spectral intensity of heavy metal element in LIBS is enhanced.Then,the calibration curve of Pb element is constructed and the LOD is calculated.The LOD at 0,2,and 4 k V discharge voltage is 54,13,and8 ng/m L,respectively.Using SD greatly reduces the LOD analyzed by ED-LIBS.Therefore,the combination of SD and ED-LIBS can achieve high sensitivity analysis of trace Pb in water.6.The effect of SD on the spectral intensity and analysis sensitivity of Cr,Cu,and Pb in water by surface-enhanced LIBS(SE-LIBS)is explored.Six atomic spectral lines of heavy metal Cr,Cu,and Pb elements in aqueous solution at Cr 425.43 nm,Cr427.48 nm,Cr 428.97 nm,Cu 324.75 nm,Cu 327.40 nm,and Pb 405.78 nm are selected as analytical lines.The spectral intensities of heavy metal elements in LIBS are enhanced under the SD.Then,the calibration curves of Cr,Cu,and Pb elements are constructed,and the LODs are calculated.The SD reduce the LODs of Cr,Cu,and Pb elements from 3.33,5.55,and 10.07 ng/m L to 1.19,2.64,and 3.86 ng/m L,respectively.Therefore,combining SD with SE-LIBS may significantly improve the sensitivity for dectecting Cr,Cu,and Pb in water.7.The effect of TCD on the spectral intensity and analysis sensitivity of Cr and Pb in water by SE-LIBS is explored.Four atomic spectral lines of heavy metal Cr and Pb elements in aqueous solution at Cr425.43 nm,Cr 427.48 nm,Cr 428.97 nm,and Pb 405.78 nm are selected as analytical lines.The spectral intensities of heavy metal elements in LIBS are enhanced under the TCD.Then,the calibration curves of Cr and Pb elements are constructed,and the LODs are calculated.The TCD reduce the LODs of Cr and Pb elements from 16.7 and12.2 ng/m L to 7.3 and 5.5 ng/m L,respectively.Therefore,TCD has greatly improved the sensitivity of SE-LIBS to analyze Cr and Pb in water.In conclusion,the discharge(SD and TCD)assisted LIBS can improve the spectral intensity of laser-induced plasmas and the detection sensitivity of heavy metal elements in aqueous solutions.These results provide a certain reference for the highly sensitive detection of heavy metal elements in aqueous solutions,and contribute to the development of LIBS technology.