JP2003035671A - Method and apparatus for laser multistage excited emission spectroscopic analysis - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method in which the composition of a solid sample or a liquid sample can be analyzed quickly and with high sensitivity and to provide an analyzer favorable for the method. SOLUTION: The surface of the sample is irradiated with first laser pulses whose power density on the surface is 10<8> to 10<9> W/cm<2> , the same position as the position irradiated with the first laser pulses is irradiated once or twice or more with second and onward laser pulses whose power density on the surface of the sample is 10<9> W/cm<2> or more, and a plasma generated on the surface of the sample is spectroscopically analyzed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser multistage excitation emission spectrometry method and apparatus for the analysis of solid samples such as metals and ceramics and liquid samples such as molten metals and molten salts. It aims to enable rapid, sensitive and accurate analysis.

[0002]

2. Description of the Related Art As a method for rapidly analyzing a solid sample or a liquid sample, there is a laser emission spectroscopic analysis method of irradiating a laser pulse on the sample surface and spectrally analyzing plasma generated thereby. This method does not require sampling because it can be analyzed in a non-contact manner and is excellent in terms of rapidity, but there is a problem that sensitivity and accuracy are particularly low for light elements. The reason for this is that in the emission from the laser plasma, the background due to continuous light is high with respect to the emission spectrum intensity of the element to be analyzed.

On the other hand, since the background has high intensity in the early stage of light emission, a time-resolved measurement method in which the intensity is integrated excluding the early stage of light emission is generally used for the purpose of separating the element emission spectrum from the background. Is used for. Further, as disclosed in, for example, JP-A-62-188919, an attempt has been made to continuously irradiate two or more laser pulses to increase the ratio of the emission spectrum intensity of an analysis element to the background. There is. However, none of the methods has reached the same level of accuracy as the spark emission spectrometry widely used for rapid analysis of solid samples, and attempts to apply it to, for example, process analysis of steel composition in the steelmaking process. In some cases, further improvement in sensitivity and accuracy is required.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above demands, and provides an advantageous analysis method for rapidly and highly sensitively analyzing the composition of a solid sample or a liquid sample. It is intended to be proposed with the device.

[0005]

Means for Solving the Problems Now, in order to achieve the above-mentioned object, the inventors conducted a detailed study on laser pulse irradiation conditions, laser plasma generation behavior and sample vaporization behavior. It has been found that it is effective to continuously irradiate two or more laser pulses having different characteristics at appropriate intervals in order to efficiently generate and increase the emission intensity of the element to be analyzed. The present invention is based on the above findings.

That is, the structure of the present invention is as follows. 1. The power density on the surface of the sample is 10 ⁸ W
/ Cm ² or more and 10 ⁹ W / cm ² or less, the first laser pulse is irradiated, and within 100 μs after the irradiation of the first laser pulse,
Plasma generated by irradiating the same position as the irradiation position of the first laser pulse with one or two or more second and subsequent laser pulses having a power density on the sample surface of 10 ⁹ W / cm ² or more. A multistage laser-excited emission spectroscopic analysis method, comprising:

[0007] 2. Two or more laser oscillators having different laser pulse power densities, an electrical delay device for setting the laser pulse irradiation interval, and an optical device for matching the irradiation positions of the pulses emitted from the two or more laser oscillators. Laser multi-stage excitation emission spectroscopic analysis apparatus, characterized in that the laser multi-stage excitation emission spectroscopic analysis apparatus has an optical means and a spectroscopic analysis apparatus for transmitting an emission spectrum from the sample to the spectroscopic analysis apparatus.

3. One laser oscillator, an electrical delay device for setting the laser pulse irradiation interval, an optical means for transmitting the emission spectrum from the sample to the spectroscopic analysis device, and the spectroscopic analysis device, A laser multistage excitation emission spectroscopic analyzer, wherein the laser oscillator has optical and electro-optical means for changing the power density of the laser pulse.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The analysis method and the analysis apparatus of the present invention will be specifically described below. In the conventional laser emission spectroscopy, in order to increase the emission intensity, a method of setting the focus of the laser pulse on the sample surface or somewhere inside the sample surface is generally adopted, and the characteristic of the laser pulse is the laser oscillator itself. In most cases, the output characteristics were specified.

However, in reality, not only the output characteristics of the laser pulse but also the plasma generation behavior and the sample vaporization behavior change due to the deviation from the focal position of the condenser lens, etc., so that the characteristics of the laser pulse on the sample surface are changed. Can be said to be the most important. Even in the multi-stage excitation method, for example, in the above-mentioned JP-A-62-188919, the half-width at half maximum is 1 μm or less and energy of 10 mJ / pulse or more is required, but the characteristics of the laser pulse on the sample surface are changed. The effects of the impact have not been scrutinized.

Therefore, the inventors conducted a detailed examination of the power density of the laser pulse on the surface of the sample (steel material), the plasma emission intensity, and the vaporization amount of the sample, and obtained the results shown in FIG. The power density on the surface of the sample means a value obtained by dividing the energy of the laser pulse by twice the half-value width of the pulse and further dividing it by the irradiation area of the laser beam.

As shown in the figure, the power density at which the plasma emission intensity is maximum and the power density at which the vaporization amount of the sample is maximum are different. Therefore, in the present invention, the power densities of the first laser pulse for the purpose of vaporizing the sample and the second and subsequent laser pulses for the purpose of plasma spectroscopy are changed and each power density is set within a predetermined range. As a result, the plasma emission intensity by the second and subsequent laser pulses is dramatically increased.

That is, in the present invention, the power density of the first laser pulse is 10 ⁸ W / cm ² or more and 10 ⁹ W
/ Cm ² or less. Because the power density is 10 ⁸ / cm ²
If it is less than 10%, it is too low to obtain a sufficient vaporization amount.
^If it exceeds ⁹ W / cm ² , the plasma on the sample surface becomes high density in the early stage of laser pulse irradiation, and the laser pulse is not absorbed by this plasma and does not reach the sample surface, so that a sufficient amount of vaporization cannot be obtained. Is. The power density of the second and subsequent laser pulses was set to 10 ⁹ W / cm ² or more. This is because the emission intensity of plasma becomes insufficient at less than 10 ⁹ W / cm ² .

Further, according to the present invention, the interval from the irradiation of the first laser pulse to the irradiation of the second and subsequent laser pulses is 10 times.
It is important to set it within 0 μs. That is, immediately after the irradiation with the first laser pulse, the vaporized sample exists in high density on the sample surface in the form of atoms or fine clusters.
When the sample in the vaporized state is irradiated with the second and subsequent laser pulses, the emission intensity of plasma increases, but the sample in the vaporized state diffuses with time and the density on the sample surface becomes low. It is important to irradiate the laser pulse within 100 μs after the irradiation of the first laser pulse. In order to obtain a higher vaporized sample density, it is more preferable to irradiate the second and subsequent laser pulses within 20 μs.

The preferred laser multi-stage excitation emission spectroscopic analysis method according to the present invention has been described above. Next, a suitable laser multi-stage excitation emission spectroscopic analysis device will be described for carrying out such a laser multi-stage excitation emission spectroscopic analysis method. To do.
As a device for performing multi-stage excitation of two or more stages, two or more laser oscillators are installed in parallel and used as a laser oscillator that oscillates one and the first laser pulse, and oscillates the second and subsequent laser pulses. There is a device used as a laser oscillator or a device for obtaining a plurality of laser pulses with a single laser oscillator, but according to the present invention, the power density on the sample surface,
Any of the above devices may be used as long as the interval from the irradiation of the first laser pulse to the irradiation of the second and subsequent laser pulses can be obtained. Especially, when a plurality of laser oscillators are used, pumping or Q
-It is advantageous to set the irradiation interval of each laser pulse using an electrical delay circuit triggered by a signal such as switching.

In order to change the power density of the laser pulse on the sample surface, a method of changing the output of the laser oscillator itself is generally used. However, a method of controlling the deviation from the focal position of the condenser lens is used. Good. When the latter method is adopted, the power density can be controlled in a wider range by changing the focal length of the condenser lens for the first laser pulse and the second and subsequent laser pulses.
It can be realized by providing an optical element such as a lens for changing the opening angle of the laser beam on the optical path of the laser pulse.

The type of laser oscillator is not particularly limited as long as it has a predetermined power density, for example, N
d: YAG laser (wavelength 1.06 μm), CO ₂ laser (wavelength 9 to 11)
μm) or the like can be used. 10 ⁸ in this invention
A Q-switched pulsed laser with a high peak output is suitable for obtaining a power density of W / cm ² . Further, since the accuracy of the oscillation frequency is improved by integrating the emission intensity of each pulse, it is preferable that the oscillation frequency is about 10 Hz or higher from the viewpoint of rapid analysis.

As described above, the laser oscillator
In the case of using a single laser oscillator, and in the case of using only one laser oscillator, as will be described later, the second and subsequent laser pulses are generated by using optical means such as a mirror, a wave plate, and a polarization beam splitter. It is irradiated at the same position as the laser pulse of 1. The same position here means that most of the irradiation areas by the respective laser pulses overlap each other, and the deviation of the center of each laser beam on the sample surface is 1 / the diameter of the laser beam having the maximum irradiation area.
If it is 4 or less, the desired purpose can be achieved.

FIG. 2 schematically shows a laser multi-stage excitation emission spectroscopic analysis apparatus having two laser oscillators. In the figure, number 1 is a first laser oscillator, 2 is a second laser oscillator,
3 is a reflection mirror, 4 is a polarization beam splitter, 5 is a condenser lens, 6 is a plano-concave lens, 7 is a wave plate, 8 is a sample, 9 is an optical fiber, 10 is a spectroscopic analyzer, and 11 is a laser pulse irradiation interval. Is an electrical delay device for setting.

Of these, the polarization beam splitter 4 is the first
This is an optical means for matching the irradiation positions of the laser pulse by matching the respective optical paths of the laser pulse and the second laser pulse. At this time, the second wave plate 7
By changing the polarization direction of the second laser pulse, only the optical path of the second laser pulse is changed by the beam splitter 4. However, these two laser pulses do not necessarily have to have the same optical path, and the optical paths may be slightly changed so that they are irradiated to the same position by different condenser lenses. Further, the plano-concave lens 6 was installed for the purpose of changing the divergence angle of the first laser beam and widening the beam diameter when irradiating the sample surface with the condenser lens 5 to reduce the power density. It is not always necessary to change the output of the laser oscillator 1 itself or to divide the optical path of each laser pulse as described above.

Further, the electrical delay device 11 irradiates a laser pulse at a predetermined time interval to generate plasma on the sample surface. The emission spectrum from this plasma is collected by the optical fiber 9 and the spectroscopic analyzer
It is transmitted to 10, but instead of this optical fiber 9, a lens,
Means combining a concave mirror, a plane mirror, etc. may be used. Further, when the optical fiber 9 is used, the detection sensitivity can be improved by operating a plurality of optical fibers to increase the solid angle at the time of focusing.

The light emitted from the plasma is spectrally analyzed by the spectroscopic analyzer 10 including a multi-channel spectroscope, and the emission intensity of the element to be analyzed is measured. At this time, although it is a generally used method, noise and variations in the emission intensity of each laser pulse are removed by integrating the emission intensity of each laser pulse, and the accuracy is improved. Further, when the time-resolved measurement is performed with the integrated range excluding the time zone where the continuous light intensity at the initial stage of emission is high, the ratio of the emission spectrum intensity to the background is further improved.

The emission condensing section and the spectroscopic analysis section of the laser plasma are well known in the art. That is, the light emission condensing unit is composed of a mirror, a lens, an optical fiber, etc., while the spectroscopic analysis unit is equipped with a multi-channel photodetector such as a photomultiplier tube or a photodiode array. Amplifier that amplifies the signal from the instrument,
And a signal processing device that appropriately processes this signal,
It consists of a computer that calculates data, converts luminescence intensity to concentration, and outputs analysis results.

Next, items peculiar to an apparatus having only one laser oscillator will be described. This laser multistage excitation light emission spectroscopic analyzer has a mechanism for changing the Q-switch stepwise using an electro-optical element in order to obtain a plurality of laser pulses from a single laser oscillator. This mechanism has the advantage that each laser pulse can be emitted to the same position without providing any special optical means, and is also advantageous in terms of device cost. In order to change the characteristics of the first laser pulse and the second and subsequent laser pulses, the generated pulses are further separated by electro-optical means, and different optical elements such as lenses are arranged in the respective optical paths. The controllable range of pulse characteristics on the sample surface can be expanded. The separated second and subsequent laser pulses may be irradiated at the same position as the first laser pulse by using an optical means such as a mirror and a polarization beam splitter.

[0025]

[Examples] Hereinafter, a case where an analysis was carried out using a laser multistage excitation emission spectroscopic analysis apparatus having two laser oscillators shown in FIG. 2 (Examples), a one-stage excitation method (Comparative Example 1) Also, description will be made in comparison with the case where analysis is performed by the conventional two-stage excitation method (Comparative Example 2). In FIG. 3, C is calculated by the above three methods.
Fe emission line (271.44nm) and C emission line (193.09nm) when a laser pulse was irradiated to a steel material sample containing 0.1 mass% of
7 shows a time profile of the emission intensity of the. The laser oscillator used was Nd: YAG for both the first and second laser oscillators.
Laser, pulse energy by Q-switching,
Pulse width of 200m each for the first and second laser oscillators
J, 16ns and 100mJ, 16ns. The irradiation interval of the first and second laser pulses was 10 μs.

In the one-stage excitation method (Comparative Example 1), the second oscillator of the above laser oscillators was used, and the laser pulse irradiation was repeated at 10 Hz for 60 seconds (laser pulse
The emission profile of (irradiated over times) was integrated. The preliminary irradiation was 10 seconds. In the two-stage excitation method (Comparative Example 2 and Example of the present invention), the first and second
The irradiation of the laser pulse was repeated at 10 Hz, and the emission profile for 60 seconds (the first and second laser pulses were irradiated for 600 sets) was integrated. The light emission profile was integrated only by the second laser pulse, and the preliminary irradiation was 10 seconds.

FIG. 3 (a) shows the case where the laser pulse is focused on the sample surface at φ300 μm by the one-step excitation method, and the power density on the sample surface is 4.4 × 10 ⁹ W / cm ² at this time. did. The strong luminescence that appears within a time of 1 μs is the part mainly composed of continuous light, and the region skirting up to around 10 μs is the part mainly composed of luminescence from the element.
As is clear from the figure, the main component Fe emits light up to around 7 μs, but almost no emission of C is detected after 2 μs.

FIG. 3B shows the case where the laser pulse is focused on the sample surface to φ300 μm by the two-stage excitation method. At this time, the power density on the sample surface is measured by the first and second laser oscillators. 8.8 × 10 ⁹ W / cm ² and 4.4 × 10 ⁹ W / c, respectively
m ² According to the figure, similar to FIG. 3A, 1 μs
The intensity of the luminescence inside is strong, but the intensity of the element luminescence that follows is increased, and luminescence is recognized up to about 4 μs even for 0.1 mass% C.

FIG. 3C shows an embodiment of the present invention, in which a plano-concave lens is placed on the optical path of the first laser pulse.
This is a case where the beam diameters of the first and second laser beams on the sample surface are φ2 mm and φ300 μm, respectively. In this case, the power density on the surface of the sample was 2.0 × 10 ⁸ W / cm ² and 4.4 × 10 ⁹ W / cm ² for the first and second laser oscillators. As is clear from the figure, according to the embodiment, as shown in FIG.
Even when compared to (b), the emission of each element was observed for a long time, and the power density of the first laser pulse was changed, whereby the analysis sensitivity can be further improved. In addition,
We also verified the interval between the irradiation of the first laser pulse and the irradiation of the second and subsequent pulses, but the pulse interval was 20 μs.
The emission intensity hardly changed up to the vicinity, but it gradually decreased after that, and when the pulse interval exceeded 100 μs, the emission intensity by the second and subsequent laser pulses decreased significantly, which was satisfactory. The effect of the two-step excitation was not so remarkable.

Next, with respect to the samples with various changes in C content, laser multi-stage excitation emission spectroscopic analysis was carried out by the three kinds of methods shown in FIG. 3, and the intensity ratio of C emission line to Fe emission line (C /
The result of having prepared the calibration curve of C content using Fe) is shown in FIG. Here, the emission intensities of C and Fe were determined by integrating the signal intensities from the detector in the range of 1 to 9 μs after laser pulse irradiation over 600 laser pulse irradiations. As is clear from the figure, in comparison with the one-step excitation method and the conventional two-step excitation method, the two-step excitation method according to the present invention has a larger slope of the calibration curve and a smaller variation. That is, according to the method of the present invention, it is possible to realize the improvement of the inclination, that is, the improvement of the analysis sensitivity, without impairing the advantage of the laser multistage excitation-emission spectroscopic analysis method that is rapidity.

[0031]

As described above, according to the present invention, it is possible to analyze a sample composition rapidly and with good sensitivity, and therefore, a process for producing various metallic materials which requires rapid analysis within a few minutes. It can be used effectively for process control and quality control in.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the emission intensity of laser plasma and the vaporization rate of a sample, and the power density of a laser pulse on the sample surface.

FIG. 2 is a schematic diagram showing an example of the main configuration of a laser multistage excitation emission spectroscopy analyzer according to the present invention.

FIG. 3 shows time profiles of emission intensities of Fe emission line and C emission line in Examples and Comparative Examples of the present invention,
FIG. 5A is a diagram showing a measurement result of a two-stage excitation method according to the present invention, and FIG. 8C is a comparative example of the two-stage excitation method.

FIG. 4 C in steel in Examples and Comparative Examples of the present invention
The calibration curve of the concentration is shown, (a) is the one-stage excitation method,
(B) is a comparative example of the two-stage excitation method, (c) is according to the present invention 2
It is a figure which respectively shows the measurement result of the step excitation method.

[Explanation of symbols]

1 First laser oscillator 2 Second laser oscillator 3 reflection mirror 4 Polarizing beam splitter 5 Condensing lens 6 Plano-concave lens 7 Wave plate 8 samples 9 optical fiber 10 Spectroscopic analyzer 11 Electrical delay device

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Claims (3)

[Claims] 1. A power on a sample surface with respect to the surface
Density is 10⁸W / cm²More than 10 ⁹W / cm²The first ray that is
The pulse is irradiated, and 100 times after the irradiation of the first laser pulse.
Within μs, the same position as the irradiation position of the first laser pulse
The power density on the sample surface is 10⁹W / cm²Above
Irradiating one or more laser pulses after a certain second pulse,
Characterized by spectroscopic analysis of plasma generated on the sample surface
A laser multi-stage excitation emission spectroscopic analysis method. 2. A laser pulse generator having two or more laser oscillators having different laser pulse power densities, an electric delay device for setting a laser pulse irradiation interval, and an irradiation position of each pulse emitted from the two or more laser oscillators. A laser multistage excitation emission spectroscopic analysis apparatus comprising: an optical means for matching, an optical means for transmitting an emission spectrum from a sample to the spectroscopic analysis apparatus, and a spectroscopic analysis apparatus. 3. A laser oscillator, an electrical delay device for setting an irradiation interval of laser pulses, an optical means for transmitting an emission spectrum from a sample to a spectroscopic analysis device, and a spectroscopic analysis device, A laser multistage excitation emission spectroscopic analyzer, wherein the laser oscillator has optical and electro-optical means for changing the power density of the laser pulse.