Investigation On The Characteristics And Mechanisms Of The Enhancement Of Laser-induced Breakdown Spectroscopy For Non-flat Samples

Laser-induced breakdown spectroscopy(LIBS),as an elemental analysis technique based on atomic emission spectroscopy,has the advantages of multi-element detection,no vacuum required,little or no sample preparation,real-time analysis,in-situ diagnosis,etc.Therefore,it can be well applied in remote online detection,qualitative identification and quantitative analysis for components of samples.In recent years,LIBS technique is developing to one of the most valuable spectral analysis technique on the basis of the above advantages.And it has a broad application prospect in the field of qualitative and quantitative analysis of trace elements,especially in harsh and extreme environments.Nowadays,successful applications of LIBS have been found in many areas,such as planetary exploration,petroleum,geology,materials,forensics,archaeology,metallurgy,biomedicine,deep-sea exploration,environmental monitoring and other fields.At present,the improvement of detection sensitivity is one of the biggest challenges facing LIBS technology.Although there are various factors causing this poor detection sensitivity,considering the excellent value for LIBS applications in harsh and extreme environments,the non-flat sample surface usually leads to grazing incidence of laser beam,which causes non-negligible impact the LIBS results.Thus,under the extreme conditions of grazing incidence on the non-flat sample surface,how to improve the plasma spectral intensity and the detection sensitivity of LIBS has become an important research subject,and is also the main content of this paper.In this work,the double-pulse method by splitting single beam and the spark and arc discharge synergistically assisted enhancement method based on ion dynamics modulation approach are applied respectively to increase the emission intensity of laser-induced plasma,thereby achieving a substantial increase in the spectral intensity.By comparing the enhancement effect of the spectral intensity,the discharge-assisted LIBS(D-LIBS)based on ion dynamics modulation approach was selected and the detection sensitivity of trace elements was found to be greatly improved.The main contents of this paper are as follows:1)The influence of the non-flat surface on the plasma generation and expansion process was studied.The emission spectra of a series of natural rock samples under the conditions of non-flat and flat sample surfaces were comparatively investigated.In addition,the influence of non-flat sample surface on spectral characteristics of laser-induced plasma is discussed.It was found that the spectral intensities of the atomic lines of the non-flat sample are reduced by nearly 70%compared to those of the flat surface,which indicates that the adverse effect of the non-flat surface on the LIBS results cannot be ignored.2)For non-flat samples,the double-pulse method by splitting single beam was carried out to investigate the corresponding characteristics of plasma and the enhancement effect of the spectral intensities under different shooting modes.When the laser fluence is lower than 96.1 GW/cm~2(wavelength:1064 nm,pulse width:8 ns),the enhancement factor of nearly 2–4 was achieved.Comparing the time evolution characteristics of the plasma in the single-beam and split-beam modes,it was found that the enhancement effect of spectral intensity in the split-beam mode existed in the whole expansion process of plasma,and the plasma generated in the split-beam mode owns a longer lifetime.The mechanisms behind the enhanced optical emission and the temporal evolution of enhancement factors were discussed by using the proposed laser ablation model.3)A novel spark and arc discharge synergistically assisted LIBS enhancement method based on ion dynamics modulation approach was proposed,which was used to reduce the discharge energy of the D-LIBS and further improve its signal intensity.This modulation was first fulfilled through the coaxial arrangement of electric field and laser-plasma shockwave,which confines numerous charged particles in the effective discharge space,and effectively promotes the coupling of electric energy to the laser plasma.Secondly,this modulation enables the significant prolongation of plasma lifetime,from the microsecond to millisecond level,through the combined function of electric source and charging capacitor.In the case of grazing incidence of the laser beam on the non-flat sample surface,comparing to the traditional discharge-enhanced LIBS method,the discharge energy of the D-LIBS in our work is significantly reduced by 1order of magnitude while maintaining over 2 orders-of-magnitude signal enhancement.4)By adopting the D-LIBS combined with the optimized wavelet transform de-noising method,the spectral quality and quantitative analysis capability of LIBS technique were evaluated.It was found that the signal-to-noise ratio,signal-to-background ratio,and stability of emission spectra under the synergistic assistance of spark and arc discharge were improved by about 72.5 times,9.3 times and 42.9%.The Lo D for typical trace elements(Mg)was reduced from hundreds ppm to sub-ppm level(hundreds ppb),and the detection sensitivity of Mg element was approximately 2 orders of magnitude higher than conventional LIBS(C-LIBS)technique.