Study On Laser-induced Breakdown Spectroscopy Based On Molecular Radicals

Laser-induced breakdown spectroscopy（LIBS）has become"a future super star"in the field of analytical technology because of its unique advantages such as rapidity,simple sample preparation,simultaneous multi-element,in situ and analysis online.However,the poor sensitivity and accuracy restricts the further development of LIBS technology.In recent years,the molecular radical emission spectroscopy as an alternative to the atomic emission spectroscopy was proposed to achieve high sensitivity analysis of hard-to-detect elements,which has become a hotspot in LIBS technology.LIBS technology based on molecular radicals still has problems such as the matrix effect,weak spectral intensity,spectral interference,and hard-to-excite as the traditional LIBS,which limit its further development and application.For this purpose,a systematic study of LIBS based on molecular radicals was carried.The detailed contents and results are as follows:（1）The standard addition method was introduced to eliminate the matrix effect in molecular radicals LIBS.The detection of F element in copper concentrate using Ca F molecule was studied.The effect of different Ca content on the spectral intensity of Ca F molecule caused by the matrix effect was effectively solved by standard addition method.The high-accuracy and high-sensitivity detection of F element in copper concentrate was realized using Ca F molecular band intergral intensity combined with wavelet transform to remove background,the relative error of detection is less than 5%,and the limit of detection is less than 0.01 wt.%,which meets the requirements that the content of F in copper concentrate is not more than 0.1 wt.%.（2）A new method of geometrically constrained nanoparticles enhanced molecular radical emission spectroscopy was adopted to solve the problem of weak spectral intensity in molecular emission spectroscopy.The effect of nanoparticle enhanced Al O and C₂molecular emission spectra was investigated.A geometric constraint method was used to limit the random diffusion of nanoparticle solution,the nano-enhancement factors were increased by 2 times,the relative standard deviations of spectral intensity were improved from more than 10%to less than 7%.Moreover,the conclusion that the laser ablation energy of 450 m J,laser ablation spot diameter of 2000μm and sample pre-ablation are beneficial to the interaction between the laser and the nanoparticles was demonstrated.（3）LIBS assisted with laser induced fluorescence was systematically investigated to suppress the spectral interference in molecular radicals LIBS.The detection of C element in soil was studied,the spectral interference of Fe atomic emission lines on CN molecular emission spectrum was effectively eliminated by using a resonant laser of 421.60 nm,the degree of linear fitting R² of the calibration curve was improved from 0.555 to 0.986,the accurate detection of C element was achieved.The spectral interference of Ca F molecule was also eliminated by LIBS-LIF,which further demonstrated the universality and superiority of this method to reduce the interference of molecule spectrum.（4）Molecule controlled synthesis is proposed to address the problems of hard-to-detect elements and no molecule excitation condition.The selection of supply element for controlled synthesis of molecule was studied,the features of molecule suitable for LIBS analysis were theoretically discussed by the emission intensity of the combination of alkaline earth metal elements（Ca,Sr and Ba）and halogen elements（F,Cl,Br and I）to form molecule.The dissociation energy of molecule of generating strong emission spectra in LIBS is generally larger,and the excited state energy of generating spectral signals should aslo be low.The different controlled synthesis methods were studied:The Sr F molecule was actively constructed by mixing Sr CO₃powder in copper ore,the limit of detection for F element is as low as 5 ppm,and the relative error of prediction is 6.63%.The Ca F and Ca Cl molecule were actively constructed by using calcite matrix,the limits of detection for F and Cl elements in solution can be improved from 22.3 mg/L and 20.9 mg/L detected by traditional LIBS to 0.38 mg/L and 1.03 mg/L,respectively.In conclusion,the LIBS based on molecular radicals was systematically studied in this work,the problems of matrix effect,weak spectral intensity,spectral interference and difficult excitation in molecular emission spectroscopy were all solved by the standard addition method,nanparticle enhanced,resonance excitation and controlled synthesis,respectively.The research in this paper has a positive effect in promoting the development and application of LIBS detection technology based on molecular radical.