JP4483492B2 - Laser emission spectroscopy - Google Patents

Description

  The present invention relates to a laser emission spectroscopic analysis method for analyzing a component of a sample from the intensity of an excitation emission line from plasma generated by laser pulse irradiation, and particularly to enable determination of elements in a sample with high accuracy. To do.

  Analytical methods for quantifying the concentration of component elements in a sample include spark discharge optical emission spectrometry that directly analyzes a solid sample, fluorescent X-ray analysis, and chemical dissolution of the sample to determine the concentration of the element in the solution. ICP emission analysis (ICP) is known.

  Among the analysis methods described above, chemical analysis methods such as the ICP emission analysis method can be analyzed with very high accuracy, but they require sample solution processing, interference element removal processing, etc. There is a disadvantage that it takes several hours to get the result.

  In this regard, spark discharge emission analysis and fluorescent X-ray analysis are less accurate than chemical analysis methods such as ICP emission analysis, but can analyze solid samples directly within 5 minutes. There is an advantage that an analysis result can be obtained in a short time.

In addition, in the analysis method of such a component element, various examinations have conventionally been conducted on measures for improving analysis accuracy.
For example, in Patent Document 1, in spark discharge analysis, the threshold value of emission intensity is determined from the average value and standard deviation of the emission intensity distribution of each element, and the emission data within the range is adopted to improve the analysis accuracy. ing.
However, this method cannot eliminate abnormal data caused by selective evaporation.
Japanese Patent Laid-Open No. 8-1455891

By the way, in recent years, in addition to the analysis methods described above, laser emission spectroscopy has attracted attention. This laser emission spectroscopic analysis method is an analysis method for quantifying elements in a sample by irradiating the surface of the sample with a laser pulse and performing spectroscopic analysis of excitation light from plasma generated thereby.
This method is an analysis method that can directly analyze a solid sample, like the spark discharge emission spectrometry method and the fluorescent X-ray analysis method. Furthermore, compared to these analysis methods, there are fewer restrictions on the analysis sample, such as cutting and polishing. There is an advantage that sample preparation can be simplified.

  However, in laser emission spectroscopic analysis, the number of laser pulses that can be irradiated per unit time per second is as low as 100 or less due to the limitations of the laser pulse that is the excitation source of the sample, and 300 to 1000 times per second. Compared with a spark discharge analysis method capable of discharging and a fluorescent X-ray analysis method capable of continuously irradiating a wide range of X-rays and receiving signals from a wide range and averaging them, there is a problem that the analysis accuracy is slightly inferior.

  The present invention has been developed in view of the above circumstances, and an object of the present invention is to propose a laser emission spectroscopic analysis method in which the analysis accuracy is greatly improved as compared with the prior art.

That is, the gist configuration of the present invention is as follows.
( 1 ) When performing component analysis of the sample by dispersing the wavelength of excitation light from the plasma generated on the sample by laser pulse irradiation and measuring the emission intensity of the dispersed excitation light,
Measure the emission intensity for each laser pulse, extract the emission intensity data of a certain ratio from the measured total emission intensity data in ascending order of selective evaporation, and obtain the emission intensity ratio of the element to be analyzed and the main constituent elements of the sample. A laser emission spectroscopic analysis method characterized by quantitatively analyzing the concentration of an element to be analyzed based on the emission intensity ratio.
(2) In performing the component analysis of the sample in (1),
Measure the emission intensity of the element to be analyzed and the main constituent element of the sample for each laser pulse. From the measured total emission intensity data, the emission intensity distribution with the target element to be analyzed as the vertical axis and the main constituent element of the sample as the horizontal axis The emission intensity data of a certain ratio is extracted from the created emission intensity distribution in ascending order of occurrence of selective evaporation, an approximate line is drawn from the extracted data by the method of least squares, and the slope of this approximate line is analyzed. A laser emission spectroscopic analysis method characterized in that the concentration of an analysis target element is quantitatively analyzed based on the emission intensity ratio of the main constituent elements of the sample and the sample.
(3) The laser emission spectroscopy according to (2), wherein, when the boiling point of the analysis target element is lower than the boiling point of the main constituent element of the sample, the approximate straight line is a lower tangent line of the emission intensity distribution. Analytical method.
(4) In the above (2), when the boiling point of the element to be analyzed is higher than the boiling point of the main constituent element of the sample, the approximate straight line is an upper tangent line of the emission intensity distribution. Analytical method.

  According to the laser emission spectroscopic analysis method of the present invention, it is possible to perform quantitative analysis of sample component elements with higher accuracy than the conventional method in which all the emission intensities by a plurality of laser pulses are integrated and quantitatively calculated.

The present invention will be specifically described below.
Now, when the composition (concentration) of an analysis sample is quantified by laser emission spectroscopy, the emission intensity of each element obtained by one laser pulse irradiation is extremely weak. By integrating the obtained emission intensity, the signal intensity is stabilized and the analysis accuracy is improved.

In addition, in order to reduce the effects of laser pulse energy variations and the time-dependent changes in the condensing optical system and detection system, the emission intensity of the main elements that compose the sample is measured together with the emission intensity of the target element. In addition, the intensity ratio between the emission intensity of the analysis target element and the emission intensity of the main constituent element of the sample is obtained and standardized to reduce variation for each analysis opportunity. The relationship between the element to be analyzed and the main constituent elements in the sample is typically found in the composition of the alloy. For example, when the chromium concentration in a stainless alloy, which is an iron-chromium alloy, is quantified, the main constituent element is iron and the analysis target element is chromium. The main constituent element is generally a component having the maximum concentration, but is not necessarily limited to this as long as the emission intensity can be stably and accurately obtained.
Then, the concentration of the element in the sample is quantified by comparing the obtained emission intensity or emission intensity ratio with a calibration curve prepared in advance using a standard sample with a known concentration.

  As one of the causes of the degradation of analysis accuracy in such laser emission spectroscopy, since the variation in energy and power of the laser pulse irradiated on the sample is considered, the inventors checked the emission intensity for each laser pulse, We thought that the analysis accuracy could be improved by the verification, and conducted a thorough investigation on the behavior of the emission intensity of the analysis target element and the main constituent elements of the sample for the standardization and their correlation.

  As a result, in the past, analysis was performed based on data obtained by integrating all of the obtained luminescence intensities. However, some of the luminescence intensities obtained were selective evaporation, that is, the boiling point of the constituent elements of the sample. As a result, the plasma having a composition different from the composition of the sample was generated by the evaporation of the low element first, and it was found that the data derived from the selective evaporation greatly deteriorated the analysis accuracy.

In the method described in Patent Document 1 described above, abnormal data generated by this selective evaporation cannot be excluded, and therefore, even if these data extraction methods are applied to laser emission spectroscopy, sufficient improvement in analysis accuracy is expected. Can not.
That is, the method described in Patent Document 1 attempts to improve analysis accuracy by determining a threshold value of emission intensity from the average value and standard deviation of the emission intensity distribution of each element and adopting emission data within the range. However, abnormal data due to selective evaporation is characterized by a high strength of elements having a low boiling point, and there is no correlation with the average or median of the distribution.

  Thus, the inventors have intensively studied a data extraction method capable of effectively eliminating data derived from such selective evaporation, and as a result, arrived at the present invention.

First, a method (hereinafter referred to as a reference method) using the average value of the emission intensity ratio of the element to be analyzed and the main constituent elements of the sample and the standard deviation of the intensity distribution, which is the basis of the present invention, will be described .

This reference method calculates the emission intensity ratio of the element to be analyzed and the main constituent elements of the sample for each laser pulse irradiation, calculates the average value and standard deviation (σ), and then emits light. Only data in which the intensity ratio is in the range of (average value of emission intensity ratio−α ₁ × σ) to (average value of emission intensity ratio + α ₂ × σ), that is, shows abnormal emission on the emission intensity distribution. Analysis accuracy is improved by removing the data.
Here, α ₁ and α ₂ are coefficients for optimizing the amount of data to be extracted, have a correlation with the number of measured and recorded data, and can be obtained in advance. When α ₁ and α ₂ are less than 0.2, the data range to be extracted becomes too narrow, and as a result, there arises a problem that data reproducibility at the time of repeated analysis is poor. On the other hand, when α ₁ and α ₂ exceed 2.0, most of the data falls within the extraction range, and the extraction effect is lost.
Therefore, in this reference method , the coefficients α ₁ and α ₂ are limited to the range of 0.2 to 2.0, respectively.
When the boiling point of the element to be analyzed is lower than the boiling point of the main constituent element, α ₁ > α ₂ is better, and α ₁ <α ₂ is better in the opposite case. improves.
For example, when the analysis target element is Cr and the sample constituent main element is Fe, α ₁ = 0.8 to 1.5, α ₂ =
It is preferable to set it to about 0.2 to 0.5.

  FIG. 1 shows an emission intensity distribution chart in which Cr is determined as an analysis target element and Fe is selected as a sample constituent main element for determination of Cr in stainless steel, and emission intensity for each laser pulse is plotted for these elements. The horizontal axis is the emission intensity of Fe, the vertical axis is the emission intensity of Cr, and each plot is the emission intensity data for each laser pulse. In addition, the measurement was implemented by irradiating the laser pulse to the cross section which cut | disconnected the stainless steel ingot with the band saw.

  As shown in FIG. 1, the emission intensity distribution is concentrated around a specific emission intensity corresponding to the Cr / Fe concentration ratio of the analyzed sample, and the individual emission intensity is widely distributed in the range of several hundred to several tens of thousands of counts. It was confirmed that Thus, the data is distributed over a wide range because the evaporation amount of the sample is changed due to variations in energy density and power density of each laser pulse on the sample. Also, the reason why the distribution spreads to the side where the Cr emission intensity is high with respect to the Fe emission intensity is considered to be because selective evaporation occurs in Cr having a lower boiling point than that of Fe.

  In this way, the emission intensity data obtained from each laser pulse is considered to reflect the effect of selective evaporation of the sample. Therefore, extracting the data by eliminating this influence, that is, the average value of the emission intensity ratio It is considered that the emission intensity ratio reflecting the sample composition can be obtained by extracting the data on the side without selective evaporation.

The reference method described above was developed based on this idea, and this method can improve the analysis accuracy.
The data extraction procedure according to this reference method is illustrated in FIG. In this example, data is extracted with α ₁ = 1.0 and α ₂ = 0.3. Each data is the same as in FIG. 1, and the extracted data is indicated by white circles. From the reference method , it can be seen that the data of the region considered that the occurrence of the selective evaporation is small is efficiently extracted.

Next, the present invention will be described. This onset Ming method extracts the luminous intensity data of the fixed ratio in order generation is less selective evaporation from the total emission intensity data measured is a method for determining the light emission intensity ratio, specifically, the analyte element A method using the lower tangent of the emission intensity distribution of the main constituent elements of the sample can be exemplified.

As described above, since the emission intensity distribution created from the data for each laser pulse is affected by the selective evaporation of the sample, it is necessary to eliminate this influence.
As shown in FIG. 1, when the boiling point of the element on the vertical axis in the emission intensity distribution is lower than the boiling point of the element on the horizontal axis, the data reflecting the influence of selective evaporation appears on the upper side of the distribution. The smaller the emission intensity data, the smaller the occurrence of selective evaporation. For this reason, emission intensity data with small occurrence of selective evaporation can be obtained by extracting data with a small emission intensity ratio. When a straight line is drawn on the data extracted in this way by the method of least squares, for example, as shown in FIG. 3, the lower tangent of the intensity distribution is obtained, and the slope is used as the emission intensity ratio of the analysis sample. The accuracy can be improved. Each data is the same as in FIG. 1, and the extracted data is indicated by white circles. Further, the lower tangent shown in FIG. 3 was calculated by extracting 10% of the individual emission data having the lower intensity ratio.

Here, in order to obtain the lower tangent line, focus on only the region where the intensity ratio is low (about 5 to 20% of the whole) in the obtained intensity distribution, and obtain an approximate straight line for these data by the method of least squares. good.
Needless to say, when the boiling point of the element on the vertical axis in the emission intensity distribution is higher than the boiling point of the element on the horizontal axis, the upper tangent should be used. However, within the scope of the inventors' investigation, the data for obtaining the upper tangent tends to include a lot of abnormal data due to selective evaporation, so it is better to obtain the lower tangent in order to obtain an accurate quantitative result. .

Next, the configuration of the laser emission spectroscopic analyzer used for carrying out the present invention will be specifically described.
FIG. 4 shows an example of a laser emission spectrometer suitable for use in the practice of the present invention. In the figure, reference numeral 1 denotes a laser oscillator, 2 denotes a reflection mirror, 3 denotes a condenser lens, 4 denotes a sample, 5 denotes Excitation light reflecting / condensing mirror, 6 is a spectroscope, 7 is a photodetector, 8 is a memory for storing data for each pulse, and 9 is a control computer.

The laser pulse emitted from the laser oscillator 1 is condensed and irradiated on the surface of the sample 4 through the reflecting mirror 2 and the condenser lens 3 to generate plasma on the sample. The excitation light from the plasma is condensed by the excitation light reflecting / condensing mirror 5 and guided to the spectrometer 6. The excitation light incident on the spectroscope 6 is wavelength-dispersed, and the emission intensity is detected by the photodetector 7 installed at a position corresponding to the emission line wavelength of the analysis element. The detected light emission intensity is converted into digital data, and then stored in the data storage memory 8. The control computer 9 performs data extraction processing and quantitative calculation.
If the control computer 9 is provided with a sufficiently high speed analog / digital conversion circuit and data for each laser pulse can be measured and recorded in real time, the data storage memory 8 can be omitted.
The control computer 9 controls the detection timing of the photodetector 8 in synchronization with the oscillation of the laser oscillator 1.

  As the photodetector in the present invention, it is preferable to use a detector capable of high-speed gate in a time shorter than 2.0 μs in order to separate continuous light emitted from plasma immediately after laser irradiation and elemental light. For example, a photomultiplier tube with a high-speed amplifier, a CCD detector with an intensifier, etc. are advantageously suitable.

  The spectroscope needs only to have a wavelength resolution capable of separating the emission lines of the element to be analyzed and the main constituent elements of the sample from the emission lines of the interfering elements in the vicinity thereof, and is used in conventional spectroscopic methods. A wavelength dispersion type spectroscope can be used.

  As the laser oscillator, a pulse laser such as an Nd: YAG laser used in a conventional laser emission spectroscopic analysis method may be used.

The Cr concentrations in various concentrations of Fe—Cr alloys (ferritic stainless steel) shown in Table 1 were measured according to the conventional method, Invention Method 1 and Invention Method 2.
The obtained results are shown in comparison with Table 1.
As the laser pulse, an Nd: YAG laser having a wavelength of 1064 nm, an energy of 100 mJ, and a pulse width of 10 ns was used. For focusing the laser pulse, a condenser lens with a focal length of 250 mm was used, a 1000 mm vacuum spectrometer was used for the spectrometer, and a photomultiplier tube was used for the photodetector.
Then, each stainless steel sample was irradiated with 2000 shots of laser pulses, emission intensity data was measured and recorded, and data was appropriately extracted from the obtained emission intensity distribution to quantify Cr elements.

The conventional method is a quantitative result when the emission intensity ratio is obtained after integrating the emission intensity data of all laser pulses, that is, when the emission intensity ratio R is obtained by the following equation.
R = Σ (ICr ₁ + ICr ₂ + ICr ₃ +... + ICr ₁₉₉₈ + ICr ₁₉₉₉ + ICr ₂₀₀₀ )
/ Σ (IFe ₁ + IFe ₂ + IFe ₃ +... + IFe ₁₉₉₈ + IFe ₁₉₉₉ + IFe ₂₀₀₀ )
However, ICr and IFe are emission intensity (Intencity) of Cr and Fe, respectively.

The reference method is a result of analysis by the above-described reference method. When data is extracted with α ₁ = 1.0 and α ₂ = 0.3 using the average value and standard deviation of the emission intensity ratio, that is, (average value−1.0 It is the result of extracting data in the range of (* standard deviation) to (average value + 0.3 * standard deviation). The above α ₁ and α ₂ are numerical values determined in advance so as to obtain the highest accuracy by trying to quantify each sample shown in Table 1 using the same device as that used in the examples. .

Invention method is a result of analyzing by the onset Ming method is an example using the lower tangent line of the luminous intensity distribution. To calculate the lower tangent line, 200 points with low Cr / Fe emission intensity ratio (equivalent to 10%) are used, an approximate straight line is obtained by the method of least squares, it is set as the lower tangent line, and the slope is the emission intensity ratio of the sample. Quantitative calculation was performed.

  As is clear from Table 1, by extracting data according to the present invention and performing component analysis, a more accurate value close to the Cr concentration by chemical analysis can be obtained, and the analysis accuracy is significantly improved compared to the conventional method. is doing.

It is the figure which showed an example of the light emission intensity distribution for every laser pulse. It is the figure which showed the extraction point of the emitted light intensity data according to a reference method . It is a diagram showing a calculation procedure of the lower tangential line according to the present onset Akinori. It is the figure which showed the main structural examples of the laser emission spectroscopy analyzer concerning this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser oscillator 2 Reflecting mirror 3 Condensing lens 4 Sample 5 Excitation light reflecting condensing mirror 6 Spectrometer 7 Photodetector 8 Pulse data storage memory 9 Control computer

Claims (4)

In performing component analysis of the sample by dispersing the wavelength of the excitation light from the plasma generated on the sample by laser pulse irradiation and measuring the emission intensity of the dispersed excitation light,
Measure the emission intensity for each laser pulse, extract the emission intensity data of a certain ratio from the measured total emission intensity data in ascending order of selective evaporation, and obtain the emission intensity ratio of the element to be analyzed and the main constituent elements of the sample. A laser emission spectroscopic analysis method characterized by quantitatively analyzing the concentration of an element to be analyzed based on the emission intensity ratio. In performing the component analysis of the sample according to claim 1,
Measure the emission intensity of the element to be analyzed and the main constituent element of the sample for each laser pulse. From the measured total light emission intensity data, the emission intensity distribution with the vertical axis of the target element and the horizontal axis of the main constituent element of the sample The emission intensity data of a certain ratio is extracted from the created emission intensity distribution in ascending order of occurrence of selective evaporation, an approximate line is drawn from the extracted data by the method of least squares, and the slope of this approximate line is analyzed. A laser emission spectroscopic analysis method characterized in that the concentration of an analysis target element is quantitatively analyzed based on the emission intensity ratio of the main constituent elements of the sample and the sample. 3. The laser emission spectroscopic analysis method according to claim 2, wherein when the boiling point of the analysis target element is lower than the boiling point of the main constituent element of the sample, the approximate straight line is a lower tangent of the emission intensity distribution. 3. The laser emission spectroscopic analysis method according to claim 2, wherein when the boiling point of the analysis target element is higher than the boiling point of the main constituent element of the sample, the approximate straight line is an upper tangent line of the emission intensity distribution.

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