CN115541563A - Element quantitative analysis method based on laser-induced breakdown spectroscopy - Google Patents
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Abstract
The invention relates to the technical field of laser-induced breakdown spectroscopy, in particular to a quantitative element analysis method based on laser-induced breakdown spectroscopy, which comprises the following steps: inputting elements needing to draw a Saha-Boltzmann diagram; reading matched corresponding element atom and ion information; presetting plasma temperature; updating the plasma temperature, and drawing a Saha-Boltzmann diagram; calculating the plasma temperature by utilizing a Saha-Boltzmann diagram curve; judging whether the calculated plasma temperature is close to the preset plasma temperature or not; the partition function was calculated and the CF method was used to derive the concentrations of each component. The invention has ingenious design and improves the working efficiency; reading a spectrum, removing background noise, calibrating the intensity of the spectrum, searching a spectral line corresponding to a peak value, and calculating a distribution function to obtain the concentration of each component; the calculation amount is greatly reduced, and the possibility of human errors is reduced; the accuracy of quantitative analysis is improved.
Description
Technical Field
The invention relates to the technical field of laser-induced breakdown spectroscopy, in particular to a quantitative element analysis method based on laser-induced breakdown spectroscopy.
Background
Laser Induced Breakdown Spectroscopy (LIBS) is a research topic that is always at the forefront of the field of analytical science, and is commonly used for elemental analysis of solid samples. The LIBS technology has the advantages of no need of a complex sample preparation process, high analysis speed, real-time online or remote analysis, and analysis of any form of sample and any element, and thus has wide application in the fields of material science, biomedicine, agriculture, environmental science, archaeology, space exploration and the like. In 1999, ciucci et al proposed a free-scaling laser-induced breakdown spectroscopy (CF-LIBS) technique that utilizes detected atomic spectra of elements to achieve quantitative elemental analysis of a single sample. In 2007, tognoni et al subsequently improved the CF-LIBS technique, increased the ion spectra using the elements on the basis of the original, and greatly improved the accuracy of the analysis. To this end, the CF-LIBS technology is beginning to be widely used. However, because processing the spectrum of the sample is a very complicated matter, a large number of operations are involved, a large amount of time is consumed by manual calculation from beginning to end, and with the increase of the types of the identification elements, the calculation amount is multiplied, and the probability of errors is greatly improved.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides an element quantitative analysis method based on laser-induced breakdown spectroscopy, which is ingenious in design, comprises all processes required by spectrum analysis, and greatly improves the working efficiency; reading a spectrum, removing background noise, calibrating the intensity of the spectrum, searching a spectral line corresponding to a peak value, drawing a Saha-Boltzmann oblique line, and calculating a distribution function to obtain the concentration of each component; the calculation amount is greatly reduced, and the possibility of human errors is reduced; and the spectral line intensity is calibrated, so that more accurate spectral line intensity is obtained, and the accuracy of quantitative analysis is improved.
In order to solve the technical problems, the invention adopts the following technical scheme:
the invention provides an element quantitative analysis method based on laser-induced breakdown spectroscopy, which comprises the following steps:
s1, inputting elements needing to be drawn into a Saha-Boltzmann diagram;
s2, judging whether corresponding element atom and ion information needing to be matched exist or not, if not, reading a spectrum file, and generating corresponding element atom and ion information; if yes, performing step S3;
s3, reading matched corresponding element atom and ion information;
s4, presetting plasma temperature;
s5, updating the plasma temperature, and drawing a Saha-Boltzmann diagram;
s6, fitting a Saha-Boltzmann diagram;
s7, calculating the plasma temperature by utilizing a Saha-Boltzmann diagram curve;
s8, judging whether the calculated plasma temperature is close to the preset plasma temperature or not, and if not, executing the step S5; if yes, executing step S9;
s9, calculating a distribution function, and obtaining the concentration of each component by using a CF method;
and S10, ending.
In step S2, the method for reading the spectrum file and generating the information of the corresponding element atoms and ions includes the following steps:
s21, reading a spectrum file;
s22, median filtering is carried out, and the filtered file is stored;
s23, judging whether a file of a point needing to be calibrated exists or not, and if so, executing a step S24;
s24, reading a file of points needing to be calibrated;
s25, reading the next point;
s26, extracting points on two sides of the wavelength of the central point by using the filtered file;
s27, judging whether the spectral line intensity of a point close to the central point is larger, if so, executing a step S28, and if not, executing a step S29;
s28, substituting into formula
Calculating to obtain a fitting value, assigning the fitting value to a central point, and updating the fitting value to the filtered file;
s29, judging whether uncalibrated points remain in the file or not, and if yes, executing a step S25; if not, executing step S30;
s30, searching a peak value, and generating corresponding element atom and ion information.
In step S28, when the wavelengths and the spectral intensities corresponding to the left and right sides of the calibration point x are x1, x2, y1, and y2, respectively, the spectral intensity of the calibration point x may be calculated as:
in step S8 and step S9, the method for calculating the distribution function includes: the partition function Us (T) will use the solution of F values in the CF method, which is normalized, i.e. the concentrations of all elements add up to 1, as follows:
wherein qs is the intercept of the fitted oblique line, the F value is obtained by solving the formula, and the concentration of each component can be theoretically calculated, and the formula is as follows:
in step S7, the method for calculating the plasma temperature using the Saha curve includes: calculating points on a Saha-Boltzmann plane corresponding to each spectral line according to a preset plasma temperature; the calculation formula is as follows:
in the above formula, m e As electron mass, k B Boltzmann constant, h planck constant, n e Is the electron density;
fitting a Saha-Boltzmann slope through the obtained points on the plane, calculating the plasma temperature through the slope of the slope, and gradually and iteratively calculating the actual plasma temperature by comparing the calculated plasma temperature with the preset plasma temperature.
The invention has the beneficial effects that:
the invention has ingenious design, contains all the procedures required by spectrum analysis, and greatly improves the working efficiency; reading a spectrum, removing background noise, calibrating the intensity of the spectrum, searching a spectral line corresponding to a peak value, drawing a Saha-Boltzmann oblique line, and calculating a distribution function to obtain the concentration of each component; the calculation amount is greatly reduced, and the possibility of human errors is reduced; and spectral line intensity calibration is carried out, so that more accurate spectral line intensity is obtained, and the accuracy of quantitative analysis is improved.
Drawings
Fig. 1 is a flow chart of an element quantitative analysis method based on laser-induced breakdown spectroscopy according to the present invention.
Fig. 2 is a spectrum diagram of the continuous background noise of this embodiment with a maximum value of 2000 and a sampling window size of 49.
FIG. 3 is a schematic diagram of the Saha-Boltzmann diagonal of this example.
Detailed Description
In order to facilitate understanding of those skilled in the art, the present invention will be further described with reference to the following examples and drawings, which are not intended to limit the present invention. The present invention is described in detail below with reference to the attached drawings.
An element quantitative analysis method based on laser-induced breakdown spectroscopy, as shown in fig. 1, comprises the following steps:
s1, inputting elements needing to be drawn into a Saha-Boltzmann diagram;
s2, judging whether corresponding element atom and ion information needing to be matched exist or not, if not, reading a spectrum file, and generating corresponding element atom and ion information; if yes, performing step S3;
s3, reading matched corresponding element atom and ion information;
s4, presetting plasma temperature;
s5, updating the plasma temperature, and drawing a Saha-Boltzmann diagram;
s6, fitting a Saha-Boltzmann diagram;
s7, calculating the plasma temperature by utilizing a Saha-Boltzmann diagram curve; the method for calculating the plasma temperature by using the Saha curve comprises the following steps: calculating points on a Saha-Boltzmann plane corresponding to each spectral line according to a preset plasma temperature; the calculation formula is as follows:
in the above formula, the degree of degeneracy g of k level k A is the transition probability, E is the energy level, I is the relative spectral intensity, m e As electron mass, k B Is Boltzmann constant, h is Planckian constant, n e Is the electron density;
fitting a Saha-Boltzmann slope through the obtained points on the plane, as shown in fig. 3, calculating plasma temperature by the slope of the slope, and calculating actual plasma temperature step by iteration by comparing the calculated plasma temperature with a preset plasma temperature;
s8, judging whether the calculated plasma temperature is close to the preset plasma temperature or not, and if not, executing the step S5; if yes, executing step S9;
s9, calculating a distribution function, and obtaining the concentration of each component by using a CF method; the method for calculating the distribution function comprises the following steps: the partition function Us (T) will use the solution of F values in the CF method, which is normalized, i.e. the concentrations of all elements add up to 1, as follows:
wherein qs is the intercept of the fitted oblique line, the F value is obtained by solving the formula, and the concentration of each component can be theoretically calculated, and the formula is as follows:
and S10, ending.
Particularly, the invention has ingenious design, contains all the procedures required by spectrum analysis, and greatly improves the working efficiency; reading a spectrum, removing background noise, calibrating the intensity of the spectrum, searching a spectral line corresponding to a peak value, drawing a Saha-Boltzmann oblique line, and calculating a distribution function to obtain the concentration of each component; the calculation amount is greatly reduced, and the possibility of human errors is reduced; the debugging function is added, and part of spectral lines can be identified wrongly due to the matrix effect or the self-absorption effect, so that when the corresponding element atom and ion information files needing to be matched exist, the spectral lines cannot be automatically updated according to the spectral data.
In this embodiment, in step S2, the method for reading the spectrum file and generating the information of the corresponding element atom and ion includes the following steps:
s21, reading a spectrum file by using a spectrometer; the spectrum measured by the spectrometer contains two pieces of information, namely wavelength and spectral intensity (namely spectral line intensity) corresponding to the wavelength;
s22, median filtering is carried out, and the filtered file is stored; the median filtering is a nonlinear signal processing technology which is based on a sequencing statistic theory and can effectively inhibit noise, and the basic principle of the median filtering is to replace the value of one point in a digital image or a digital sequence by the median of all point values in a neighborhood of the point, so that the surrounding pixel values are close to the true values, and isolated noise points are eliminated; the part needs to set two parameters, namely the maximum value of noise and the size of a sampling window; the maximum value of the noise is set based on the continuous background noise in the measured spectrum, the larger the continuous background noise in the spectrum, the larger the value; the size of the sampling window is different according to different measurement samples, and the value is smaller when the types of elements in the samples are more; when the types of the sample elements are less, the values are larger; as shown in fig. 2, the spectrum is obtained when the maximum value of the continuous background noise is set to 2000 and the sampling window size is set to 49; the bottom-most line represents the noise to be filtered;
s23, judging whether a file of a point needing to be calibrated exists or not, and if so, executing a step S24;
s24, reading a file of points needing calibration (hereinafter referred to as central points);
s25, reading the next point;
s26, extracting points on two sides of the wavelength of the central point by using the filtered file;
s27, judging whether the spectral line intensity of a point close to the central point is larger, if so, executing a step S28, and if not, executing a step S29;
and S28, the calibration module is used for determining the positions of the spectral lines based on the initial identification. Since the data measured by the spectrometer are discrete points, the actual spectral peak is in the two-point range, i.e., the peak must be greater than or equal to the measured value between the two points. The spectral line shape of the actual spectrum can be reduced by Gaussian fitting, so a first-order Gaussian fitting method can be adopted for calibration, and the formula is as follows:
the formula is a first-order Gaussian function, a, b and c all represent a constant, and e is used as a mathematical constant and is the base number of a natural logarithm function; obtaining a fitting value, assigning the fitting value to a central point, and updating the fitting value into the filtered file; when the wavelengths and the spectral intensities corresponding to the left side and the right side of the calibration point x are x1, x2, y1 and y2 respectively, the spectral intensity of the calibration point x can be calculated as follows:
s29, judging whether uncalibrated points remain in the file, and if so, executing a step S25; if not, executing step S30;
s30, searching a peak value, comparing the peak value with an NIST library corresponding to the input element, and generating atom and ion information of the corresponding element; the part adopts a built-in peak searching function findpeaks of the matlab, and the function needs to set the minimum peak intensity and the minimum distance between two peaks; for minimum peak intensity, since the denoising process has been performed, it is possible to reduce the peak intensityThe minimum peak intensity can be set smaller, so that more spectral lines can be identified; the minimum distance between the two peaks depends on the element to be measured, and the denser the theoretical spectral line of the element to be measured is, the smaller the minimum distance between the two peaks should be; identifying a spectral line after finding a peak value in a spectrum, identifying element information corresponding to the spectral line, setting an allowable error value, wherein the value depends on measured spectral data, and generally takes the maximum value of two adjacent points, which can also be called the resolution of a spectrometer; NIST is a shorthand of national institute of standards and technology, and is used for basic and application research in the aspects of physics, biology and engineering, research in the aspects of measurement technology and test methods, and standard, standard reference data and related services; here, the degeneracy g of the energy level of the atom line, ion line and their corresponding transition probability A, k of the measured element is extracted from NIST database in advance k Energy level E and relative spectral intensity I.
Specifically, under the above setting, in the spectral line intensity calibration module, the spectral line intensity is calibrated by a first-order gaussian fitting method, so that more accurate spectral line intensity is obtained, which is beneficial to improving the accuracy of quantitative analysis.
Although the present invention has been described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present invention.
Claims (5)
1. An element quantitative analysis method based on laser-induced breakdown spectroscopy is characterized by comprising the following steps:
s1, inputting elements needing to be drawn into a Saha-Boltzmann diagram;
s2, judging whether corresponding element atom and ion information needing to be matched exist or not, if not, reading a spectrum file, and generating corresponding element atom and ion information; if yes, performing step S3;
s3, reading matched corresponding element atom and ion information;
s4, presetting plasma temperature;
s5, updating the plasma temperature, and drawing a Saha-Boltzmann diagram;
s6, fitting a Saha-Boltzmann diagram;
s7, calculating the plasma temperature by utilizing a Saha-Boltzmann diagram curve;
s8, judging whether the calculated plasma temperature is close to the preset plasma temperature or not, and if not, executing the step S5; if yes, executing step S9;
s9, calculating a distribution function, and obtaining the concentration of each component by using a CF method;
and S10, ending.
2. The method for quantitative analysis of elements based on laser-induced breakdown spectroscopy of claim 1, wherein in the step S2, the method for reading the spectrum file and generating the information of corresponding element atoms and ions comprises the following steps:
s21, reading a spectrum file;
s22, median filtering is carried out, and the filtered file is stored;
s23, judging whether a file of a point needing to be calibrated exists or not, and if so, executing a step S24;
s24, reading a file of points needing to be calibrated;
s25, reading the next point;
s26, extracting points on two sides of the wavelength of the central point by using the filtered file;
s27, judging whether the spectral line intensity of a point close to the central point is larger, if so, executing a step S28, and if not, executing a step S29;
s28, substitution formula
Calculating to obtain a fitting value, assigning the fitting value to a central point, and updating the fitting value to the filtered file;
s29, judging whether uncalibrated points remain in the file, and if so, executing a step S25; if not, executing the step S30;
s30, searching a peak value, and generating corresponding element atom and ion information.
3. The method of claim 2, wherein in step S28, when the wavelengths and the spectral intensities corresponding to the left and right sides of the calibration point x are x1, x2, y1 and y2, respectively, the spectral intensity of the calibration point x can be calculated as:
4. the method for quantitative analysis of elements based on laser-induced breakdown spectroscopy as claimed in claim 1, wherein in the steps S8 and S9, the method for calculating the distribution function is: the distribution function Us (T) is to use the solution of F value in CF method, solving F value uses normalization method, i.e. the concentrations of all elements are added to 1, the formula is as follows:
wherein qs is the intercept of the fitted oblique line, the F value is obtained by solving the formula, and the concentration of each component can be theoretically calculated, and the formula is as follows:
5. the method for quantitative elemental analysis based on laser-induced breakdown spectroscopy of claim 1, wherein in the step S7, the method for calculating the plasma temperature by using the Saha curve comprises: calculating points on a Saha-Boltzmann plane corresponding to each spectral line according to a preset plasma temperature; the calculation formula is as follows:
in the above formula, m e As electron mass, k B Boltzmann constant, h planck constant, n e Is the electron density;
fitting a Saha-Boltzmann slope through the obtained points on the plane, calculating the plasma temperature through the slope of the slope, and gradually and iteratively calculating the actual plasma temperature by comparing the calculated plasma temperature with the preset plasma temperature.
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