High Sensitivity Detection And Quantitative Analysis Based On Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy（LIBS）is a rapidly developing atomic emission spectroscopy analysis technique,which has advantages of little damage to sample,real-time,in-situ and multi-element detection,and has been widely applied in many fields such as Mars exploration,metallurgy and water quality detection.LIBS spectra contain abundant qualitative and quantitative information of elements,which can be used to detect and analyze elements in samples.However,there are still a lot of works to be done in improving the detection sensitivity and optimizing quantitative analysis.In this work,two samples of Tieguanyin tea and calcium silicate hydrate（C-S-H）cement were studied,and the specific contents are as follows:（1）Lead chrome green（LCG）is an industrial dye containing a large number of heavy metal elements such as Pb and Cr.Many unscrupulous merchants add LCG into old tea to pretend to be new tea.In this paper,the elements of Tieguanyin tea and LCG were analyzed by LIBS technique,and LCG illegally added in tea was detected by the intensity changes of Pb atomic lines and CN（0,0）radical lines.The detection limit（Lo D）of Pb element was calculated as 39 ppm by internal standard method.In addition,we also analyzed the CN,Ca O and C₂ free radical spectral lines in the tea spectra,and established principal component analysis（PCA）models using the characteristic spectral lines of CN and Ca O free radicals,which can effectively identify and classify Tieguanyin tea and tea samples with LCG.（2）The Lo D of 39 ppm of Pb is higher than the requirement that Pb in tea is not higher than 5 ppm stipulated in GB 2762-2017,the national food safety standard of China.Based on this,the ash-assisted LIBS（AA-LIBS）method was proposed to improve the detection sensitivity of Pb elements in tea.The mechanism of signal enhancement of Pb and other metal elements in tea ash was studied by comparing and analyzing the differences of elements in tea ash.This method effectively enriched Pb elements in tea,and the calculated Lo D of Pb was 191.6 ppb,which reached the level of sub-ppm,and the detection sensitivity was increased by about 200 times.The results showed that AA-LIBS was a reliable and highly sensitive method for the detection of Pb in tea,and could also be used for the high sensitivity detection of heavy metal elements in other solid samples.In addition,the generation mechanism of CN,C₂ and Ca O radical emission bands in the spectrum has been studied.（3）Besides that,LIBS also used to analyze the elements of C-S-H cement.Firstly,the time evolution of Ca,Si,O and H elements in the plasma was investigated by controlling the collection and shutoff the laser-induced plasma with an electro-optical crystal.Secondly,the Ca/Si ratio of cement was quantitatively analyzed by calibration-free LIBS（CF-LIBS）.The electron density（n_e）of the plasma was calculated to be2.41×10¹⁷ cm^-3,the plasma temperature（T）was 8114±309 K,and the I_Ca/I_Si spatial distribution maps were obtained.It is found that Ca and Si were randomly distributed in C-S-H cement,which reflects the characteristics of short-range order and long-range disorder of amorphous C-S-H samples.In conclusion,the application of LIBS technology in the detection of illegal LCG has practical application value,and the proposed AA-LIBS method can effectively improve the detection sensitivity of Pb elements.Furthermore,the preliminary quantification results of Ca/Si ratio have important reference significance for the study of mechanical properties and durability of cement-based materials.