JPS636827B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a laser emission spectroscopic analysis method, and is particularly suitable for direct analysis of hot metal, molten steel, slag, etc. This invention relates to improvements in a laser emission spectroscopic analysis method for spectroscopically analyzing light.

As laser technology progresses, attempts are being made in various fields to utilize it as an excitation source to perform emission spectroscopic analysis. That is, when the sample surface is irradiated with a powerful laser beam focused by a condensing lens with an appropriate focal length, the surface layer is rapidly heated. In particular, when laser light is pulsed for several tens of nanoseconds, energy is locally injected before heat diffuses into the sample, causing melting and evaporation. The vapor is further excited by the laser, turning into plasma and emitting light.
This light is transmitted to a spectrometer using an appropriate light introduction system, separated into spectra by a diffraction grating, and then detected using a photographic film, photomultiplier tube, photodiode, etc. Laser emission spectrometry is a method for investigating the content of target elements. Although this method has excellent features such as being applicable to non-conductive substances and being able to be analyzed in the atmosphere, it has large fluctuations in data and problems with analysis accuracy. It is generally believed that the biggest cause of this is fluctuations in the laser output intensity, and the usual method is to constantly monitor the laser intensity using an appropriate method and collect data only when it is within a predetermined range. ing. However, this method cannot cope with changes in the intensity distribution of the laser beam, nor can it eliminate the effects of fluctuations in the evaporative excitation process due to the shape of the sample surface, dirt, the presence of oxide films, etc. It had drawbacks.

The present invention was made to eliminate the above-mentioned conventional drawbacks, and regardless of fluctuations in laser output intensity, etc.
An object of the present invention is to provide a laser emission spectroscopic analysis method that allows highly accurate analysis.

The present invention provides a method for spectroscopically analyzing light emitted from a sample surface when irradiated with a laser beam, in which the emission mode of the laser beam is fixed to a TEM ₀₀ mode with a Gaussian distribution. At the same time, the analysis value is obtained from the spectral line intensity of the element to be measured when the intensity of a predetermined spectral line in the sample emitted light or the intensity ratio of a predetermined pair of spectral lines is within a predetermined range, The purpose has been achieved.

The present invention relates to a laser emission spectroscopic analysis method, and the present invention solves fluctuations in spectral line intensity, which are considered to be disadvantageous.
This was done based on the results of research conducted by the inventors. That is, the inventors' investigation revealed the following.

(1) When the output from the laser oscillator is increased, the coefficient of variation of the spectral line intensity (standard deviation divided by the average value) decreases, but if an amplifier is placed after the laser oscillator and the output is further increased,
Conversely, the coefficient of variation of the spectral line intensity increases.

(2) Even when laser light is obtained using only a laser oscillator without using an amplifier, the fluctuations in the spectral line intensity are larger than the fluctuations in the laser output.

Experimental results are shown in FIGS. 1 and 2. The sample used in the experiment was an iron alloy, and the laser beam used was an infrared laser with a wavelength of 1.06 μm and a pulse width of 15 nsec. As is clear from FIG. 1, when the laser output exceeds 2J, which uses an amplifier, the coefficient of variation suddenly increases. This is thought to be caused by the intensity distribution of the laser pulse. In other words, in general, the intensity distribution (mode) in the vertical cross-sectional direction of the laser beam emitted from a laser oscillator has various symmetries, and when high output is required as used for emission spectroscopy, the intensity distribution (mode) in the vertical cross-sectional direction is This is a multi-mode oscillation in which the mode changes. However, when the amplifier is operated when several intensity distribution peaks exist in one beam, a specific peak will be preferentially amplified. Therefore, when a laser beam having such an intensity distribution is focused on the sample surface, a region with high laser irradiation density will be generated locally within the spot diameter. Such fluctuations in the irradiation density change from pulse to pulse, resulting in fluctuations in the intensity of the emission spectrum. Therefore,
Regarding the laser used for emission spectroscopy, the inventors considered fixing the laser to the TEM ₀₀ mode, which provides a Gaussian distribution output, by utilizing the mode-locking technique used in holography and the like. In addition to this mode, a circular distribution can be obtained.
Although TEM ₀₁ mode etc. can also be used, Gaussian distribution is considered to be optimal for emission spectroscopic analysis. still,
Modelocking reduces laser power, but this can be compensated for by increasing the laser oscillator power and adding an amplifier.
Of course, if the TEM ₀₀ mode is fixed, the mode will not change even if an amplifier is used.

On the other hand, Figure 2 shows the coefficient of variation of the 271.4 nm Fe spectral line intensity (solid line A) and the coefficient of variation of the laser output (solid line B) when an iron alloy is irradiated with the same laser light as in Figure 1. This is a comparison.
Even though no amplifier is used, the intensity variation of the spectral lines is larger than the variation range of the laser output. Such intensity changes in the emission spectrum can be considerably improved by the mode lock described above, but the effects of evaporation occurring on the sample surface during laser irradiation and fluctuations in the excitation process cannot be ignored. The latter cannot be removed simply by controlling the laser output or mode, so the predetermined spectral lines obtained during laser irradiation, for example, in the case of iron alloys, one or two Fe spectral lines of appropriate wavelengths are selected. The idea was to constantly monitor the intensity or intensity ratio and read the spectral line intensity of the element to be measured only when it is within a predetermined fluctuation range. In particular, the latter method of regulating the intensity ratio limits the temperature of the plasma generated by laser irradiation, and is effective in improving analysis accuracy.

The present invention has been made based on the above findings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a laser emission spectrometer employing a laser emission spectrometry method according to the present invention will be described in detail with reference to the drawings.

As shown in FIG. 3, this embodiment includes a laser oscillator 10 incorporating a mode-lock mechanism (not shown) for oscillating laser light, and an oscillator power supply 11 for driving the laser oscillator 10.
, a high-speed switching element power supply 12 for operating a high-speed switching element (not shown) incorporated in the laser oscillator 10 , and an up-collimator for enlarging the beam diameter of the laser beam oscillated from the laser oscillator 10 . 13, an amplifier 14 for amplifying the laser beam emitted from the up-collimator 13, an amplifier power supply 15 for driving the amplifier 14, and a device for changing the irradiation direction of the laser beam emitted from the amplifier 14. a right-angle prism 16, a condensing lens 18 for converging the laser beam onto the surface of the sample 20, and a sample 20 arranged to face a predetermined direction with respect to the laser beam.
a spectrometer optical system 24 consisting of, for example, a concave mirror, for imaging the light emitted by laser irradiation from the surface of the spectrometer 22 onto an entrance slit 26 of the spectrometer 22;
A spectroscope 22 for separating the sample emitted light imaged on the entrance slit 26 into spectral lines using a diffraction grating or the like, and a spectral line having a monitoring wavelength among the spectral lines separated by the spectroscope 22. one or two monitoring photodetectors 28 for detecting the intensity of the spectral line of the element to be measured and converting it into an electrical signal; and a measuring light for detecting the intensity of the spectral line of the element to be measured and converting it into an electrical signal. Detects whether the intensity of a predetermined spectral line in the sample emitted light or the intensity ratio of a predetermined pair of spectral lines is within a predetermined range according to the outputs of the detector 30 and the monitoring photodetector 28. and a gate circuit 34 that is opened by the spectral line intensity monitor 32 when the intensity of a predetermined spectral line in the sample emitted light or the intensity ratio of a predetermined pair of spectral lines is within a predetermined range. and obtain a measurement value from the output of the measurement photodetector 30 when the gate circuit 34 is open,
It is composed of a display section 36 for displaying information.

As the laser oscillator 10, for example, the wavelength
0.69μm ruby laser or wavelength 1.05~
A 1.06 μm infrared laser can be used.

The laser material used in the amplifier 14 is made thicker than the laser oscillator 10 in order to suppress the laser density below a certain value. The up collimator 13 is for canceling out the difference in diameter of the laser materials.

The right-angle prism 16 is used to irradiate the surface of the sample 20 with laser light at a predetermined angle, and when the laser emitting section including the laser oscillator 10 is installed in a predetermined direction from the beginning. can be omitted.

The action will be explained below.

First, the oscillator power supply 11 and the amplifier power supply 15
The electrical energy stored in the laser oscillator 10
and transmits it to the laser material in the amplifier 14 to excite it. When a predetermined amount of energy is stored in the laser substance in the laser oscillator 10, a signal is sent from the oscillator power supply 11 to the high-speed switching element power supply 12 to activate the high-speed switching element, and the energy stored in the laser oscillator 10 is activated. released all at once. A mode-lock mechanism is incorporated in the laser oscillator 10, and the emitted pulsed laser light is in the TEM ₀₀ mode of Gaussian distribution. The laser beam emitted from the laser oscillator 10 passes through an up-collimator 13 to an amplifier 14.
incident on . Then, the energy stored in this amplifier 14 is also instantaneously released, further intensifying the laser beam. The intense laser pulse thus obtained is focused onto the surface of the sample 20 via the right angle prism 16 and the condensing lens 18.
Then, the surface of the sample 20 is locally heated within a short time and turns into plasma. At this time, the light emitted from the plasma is guided to the spectrometer 22 by the spectrometer optical system 24, and is dispersed by a diffraction grating or the like of the spectrometer 22. Among the dispersed lights, a spectral line of a specific wavelength for monitoring is detected by the monitoring photodetector 28, and the spectral line intensity monitor 32 detects the intensity of a predetermined spectral line or the intensity ratio of a predetermined pair of spectral lines. It is detected whether or not it is within the range of .
The gate circuit 34 is opened only when the intensity or intensity ratio of the spectral line is within a predetermined range, and the measuring photodetector 3 receives the spectral line of the element to be measured.
The output of 0 is transmitted to the display section 36, and the measurement results are displayed. The spectral line intensities of the elements to be measured thus obtained are data processed using a conventional method.

As explained above, according to the present invention, there is an excellent effect that highly accurate analysis can be performed regardless of fluctuations in laser output intensity and the like.

When measuring the Si spectral line intensity at 288.2 nm in iron alloys, the coefficient of variation of the spectral line intensity measured using the conventional method was 13.0%, but when the laser output was fixed to TEM ₀₀ mode, , improved to 9.0%, and further, according to the present invention, the laser output is fixed to TEM ₀₀ mode, and
When the intensity of the Fe spectrum line at 271.4nm was monitored and the intensity of the Si spectrum line at 288.2nm was read only when it was within ±5% of the predetermined value, it was 7.3%.
Also, according to the present invention, the laser output can be increased.
Fixed to TEM ₀₀ mode and set to 271.4nm.
When two Fe spectral lines at 273.1 nm were monitored and the Si spectral line intensity was read when the intensity ratio was within ±5% of the predetermined value, it was 5.9%. laser output
Compared to just fixing to TEM ₀₀ mode,
It was confirmed that the coefficient of variation of the spectral line intensity was becoming smaller.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the laser output and the variation coefficient of the 271.4 nm Fe spectral line intensity based on the inventors' experiments, and Figure 2 is a diagram showing the relationship between the laser output and the variation coefficient of the laser output and the 271.4 nm Fe spectral line intensity. FIG. 3 is a diagram showing a comparison of the relationship between coefficients of variation of Fe spectral line intensities in nm. FIG. It is a line diagram. DESCRIPTION OF SYMBOLS 10... Laser oscillator, 14... Amplifier, 18... Condensing lens, 20... Sample, 22... Spectrometer, 28... Photodetector for monitoring, 30... Photodetector for measurement, 32... Spectral line intensity monitor, 34... Gate circuit, 36
...Display section.

Claims (1)

[Claims] 1. In a laser emission spectroscopic analysis method in which light emitted from a sample surface when irradiated with a laser beam is spectrally analyzed, the emission mode of the laser beam is fixed to a Gaussian distribution type TEM ₀₀ mode, and the sample emission is A laser characterized in that an analysis value is obtained from the spectral line intensity of the element to be measured when the intensity of a predetermined spectral line in light or the intensity ratio of a predetermined pair of spectral lines is within a predetermined range. Emission spectroscopy method.