| Laser-induced breakdown spectroscopy(LIBS)is an atomic emission spectroscopy technique that is considered to be the "new star in the field of chemical analysis" It does not require complex sample pre-treatment,causes little damage to the sample,and allows real-time,rapid,and in-situ analysis of a wide range of elements compared to other spectroscopic techniques.However,the application of LIBS technology to complex compositions is still a challenging,mainly because complex compositions are usually composed of a large number of elements,and the concentration of trace and main elements varies widely,which makes quantitative analysis results vulnerable to matrix effects and self-absorption of elemental spectral lines.In addition,due to the dispersion of complex compositions,standard samples are often difficult to obtain,and the conventional calibration methods and calibration free are no longer applicable,which are the key scientific and technical problems encountered when LIBS technology is applied to the quantitative analysis of complex compositions.In this paper,we establish a quantitative CF-LIBS analysis method for the detection of complex compositions based on the existing calibration free laser induced breakdown spectroscopy(CF-LIBS)method,which is based on the characteristics of laser plasma,to achieve rapid and accurate detection of complex compositions.The details of the study and the results are as follows.(1)The basic principles of the method of calibration free and quantitative analysis of complex compositions of substances are presented.Firstly,the theoretical calculation method of laser-induced plasma composition(developed by Matlab2016a)was obtained based on the characteristics of laser-induced plasma in local thermal equilibrium,which is the basis for the application of CF-LIBS technique to the quantitative analysis of the substances of complex compositions.This theoretical calculation method was then applied to theoretically calculate the trends of relative particle number density with plasma temperature for different compositions of each element in complex composition samples such as potash and cement clinker,respectively.The results show that the relative particle number densities of different ionization levels of each element are in a dynamic equilibrium,and the total particle number density of each element is constant regardless of the change of plasma electron temperature.Based on the above theoretical calculation,the laser plasma spectral radiation characteristics in local thermal equilibrium,and considering the self-absorption effect of spectral lines and the Stark broadening,Doppler broadening and instrumental broadening,the plasma electron density,electron temperature,relative parts of each element,total particle number and plasma size as variables,the calculation of the emission spectral line patterns(referred to as laser plasma theoretical spectra)for each element in complex composition samples is presented.The method was applied to calculate the laser plasma theoretical spectra of potash and cement clinker samples,respectively.The results show that instrumental broadening is often not negligible for the calculation of theoretical spectra,and that for different elemental spectral lines the decision to use Gauss,Lorentz or Voigt profiles for the calculation of normalised line shapes is based on the relative values of Doppler broadening,Stark broadening and instrumental broadening of the spectral lines.The theoretical spectra were obtained to provide the basis for the final quantitative CF-LIBS analysis of complex composition samples.Based on the above work,a method and procedure for the quantitative CF-LIBS analysis of complex composition sample components was established.The main idea is to use the plasma electron density,electron temperature,relative number of each element and laser plasma size in the complex composition sample as cyclic iteration variables,respectively,to select the corresponding elemental analysis spectra,and make all the variables by cyclic iteration so that the obtained theoretical spectra and experimental spectra consistent with each other through iteration.Then the absolute concentrations of the various components in the sample are obtained.(2)An experimental LIBS measurement system was built for the quantitative analysis of complex composition samples and the LIBS spectra of two complex composition samples,potash and cements clinker,was measured.The proposed CF-LIBS analysis method was applied to the quantitative analysis of sample composition,and the validity of the quantitative analysis method was verified.The main work includes 1)Using the maximum signal-to-noise ratio of the Mg II 279.83 nm LIBS signal in potash as the optimization criterion,the optimal defocusing amount of-0.5 mm,the optimal ICCD signal acquisition delay of 1.3 ?s,the optimal sample movement speed of 0.5 mm/s,and the optimal laser pulse energy of 15 m J were determined.2)The LIBS spectra of potash and cement clinker samples were measured under the optimal experimental parameters.3)The established CF-LIBS quantitative analysis calculation method was used to quantify the composition of potash and cement clinker samples,respectively,and the results showed that the relative error of the analysis method was less than 0.2% for the prediction of the concentration of major elements in potash,and less than 7% for the prediction of trace elements.For cement clinker samples,the relative errors in the prediction of each element obtained by this analytical method were below 12% except for Si and Na.The possible causes of the large prediction errors between the predicted and the factory tested values of some elements were analyzed and discussed.The results of the study show that each parameter should be determined by choosing the analytical line with a high correlation,such as the determination of the electron density should try to choose the analytical line with a large Stark broadening,to determine the relative number of elements should try to choose the analytical line with a small optical thickness,etc.,the appropriate degree of selection of the spectral line and the accuracy of the Stark spread parameters will directly affect the final quantitative analysis results The results of the quantitative analysis of the components of two complex composition samples show that the quantitative analysis method can better predict the absolute content of each element in the complex samples,which proves that the method is a well-established calibration free quantitative analysis method that can be applied to the rapid,online and accurate quantitative determination of complex composition substances without standard samples,and has an extremely strong application background in the field of spectral quantitative analysis technology. |
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