JP2561210B2 - Isotope analyzer - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an isotope analyzer for irradiating a sample containing an isotope with a laser beam and obtaining the abundance ratio of the isotope from the optical absorption spectrum of the isotope.

[0002]

2. Description of the Related Art Conventionally, a semiconductor laser having an extremely narrow emission spectrum width is used as a wavelength tunable light source, and its laser light is irradiated to a sample containing isotopes so that the spectra of the isotopes do not interfere with each other and water content is high. There is known an isotope analyzer that measures an optical absorption spectrum that is not affected by the above and traces changes in the isotope ratio with high accuracy and high sensitivity.

FIG. 3 is a block diagram showing a conventional isotope analyzer. 1 is a semiconductor laser, 2 is a sample cell, 3
Is a sample gas inlet, 4 is a sample gas outlet, 5 is a photodetector, 6 is a lock-in amplifier, 7 is a semiconductor laser temperature controller, 8 is a semiconductor laser current controller, and 9 is a semiconductor laser current modulator. The oscillator 10 for multiplying the signal is an AD for analog / digital converting the output signal of the lock-in amplifier 6.
It is a converter.

The semiconductor laser 1 easily becomes a wavelength tunable light source by sweeping the temperature or driving current of the semiconductor laser 1. The semiconductor laser 1 sweeps around the absorption spectrum of an isotope having a low abundance ratio (hereinafter simply referred to as a trace isotope) and the absorption spectrum of an isotope having a high abundance ratio (hereinafter simply referred to as a major isotope) by the temperature control unit 7. So that the temperature is swept. Further, the drive current of the semiconductor laser 1 is controlled by the current control unit 8 so that an appropriate light output is obtained. Further, it is current-modulated by the signal of the oscillator 9 and slightly frequency-modulated.

The laser light from the semiconductor laser 1 thus wavelength-swept and frequency-modulated is incident on the sample cell 2. The laser light incident on the sample cell 2 interacts with the isotope in the cell, is absorbed, and is detected by the photodetector 5.
Of the signals detected by the photodetector 5, only the signal synchronized with the oscillator 9 is detected by the lock-in amplifier 6, and as a result,
The drift of the light intensity of the semiconductor laser 1 can be removed and a signal with a good S / N ratio can be detected. The output signal of the lock-in amplifier 6 is converted into a digital value by the AD conversion circuit 10 and then sent to the computer.

The signal thus detected has the first-order differential shape of the absorption spectrum (such a detection method is simply referred to as 1f detection hereinafter). Therefore, the isotope ratio can be obtained by obtaining the peak values of the detection signals of both the major isotope and the trace isotope and determining the ratio of both absorptions (Fig. 4, ¹² CO ₂ and
¹³ CO ₂ spectrum measurement example). Furthermore, by synchronously detecting a signal having a frequency twice that of the oscillator 9 by the lock-in amplifier 6, a second-order differential shape of the absorption spectrum (such a detection method is simply referred to as 2f detection hereinafter) can be detected. 2f
In the detection, the drift of the light intensity can be further removed and a signal with a good S / N ratio can be detected.

In this way, both absorption spectra of isotopes do not interfere with each other and are not affected by water, and both spectra are measured by a lock-in amplifier, and a laser beam having an extremely narrow oscillation spectrum width is used as a wavelength tunable light source. Since a semiconductor laser having a smaller size and higher reliability is used, the isotope ratio can be measured with high accuracy, and a highly reliable isotope analyzer can be obtained.

[0008]

However, the natural abundance ratios of isotopes are usually very different (for example, the natural abundance ratios of carbon isotopes are ¹² C: 98.9%, ¹³ C: 1.1%, and nitrogen isotopes). Abundance ratios are ¹⁴ N: 99.6%, ¹⁵ N: 0.4%), and as a result, major isotopes ( ¹² C, ¹⁴ N) and trace isotopes ( ¹³ C, ¹⁵ N).
The absorption strengths of are greatly different. Furthermore, the isotope ratio measurement accuracy is required to be as high as 0.1 to 1 permil. In order to meet these requirements, a lock-in amplifier with a very wide dynamic range and an AD converter with ultrahigh precision are required. As a result, an ordinary lock-in amplifier cannot measure the isotope ratio with high accuracy, and a lock-in amplifier having a wide dynamic range is expensive, which increases the cost of the device. Further, there is a problem that it is difficult to realize an AD converter of 15 bits or more.

To address these problems, the absorption intensity difference can be reduced by using an absorption cell with a long optical path length and an absorption cell with a short optical path length, but the optical path length changes due to temperature, vibration, etc., and the isotope ratio is stable. Cannot be measured. Also, it is possible to reduce the difference in absorption intensity by selecting a spectrum with a large absorption coefficient for measuring trace isotopes and a small absorption coefficient for measuring large amounts of isotopes.
The problem is that there are very few suitable spectra to choose from.

In view of the above drawbacks, the technical problem of the present invention is to provide a lock-in amplifier having a wide dynamic range,
It is an object of the present invention to provide an inexpensive isotope analysis device that does not require an ultra-high-precision AD converter and can easily measure the isotope ratio with high precision.

[0011]

According to the present invention, the oscillation wavelength of a semiconductor laser is swept to measure the optical absorption spectrum of the isotope gas in the sample cell, and the isotope is determined from the optical absorption amount of the isotope. In the isotope analyzer that determines the abundance ratio of the isotopes, the laser light intensities of each of the laser beams irradiated at the time of measuring the absorption amount of the trace isotopes and the abundance of the isotopes are determined by the natural abundance ratio of the isotopes and the absorption coefficient of the abundance of the isotopes. Obtained from the absorption coefficient of each isotope, the laser light intensity at the time of the spectrum measurement of the trace isotope and the spectrum of the trace isotope, and the difference in the absorption amount between the trace isotope and the isotope is substantially the same. As described above, an isotope analysis device is obtained which has a controlling light intensity controller.

According to the present invention, the oscillation wavelength of the semiconductor laser is swept, and the optical absorption spectrum of the isotope gas in the sample cell is measured by this, while the semiconductor laser is swept simultaneously with the wavelength sweep of the semiconductor laser. Frequency modulation,
An isotope analyzer that synchronously detects the optical absorption spectrum of an isotope using a lock-in amplifier and determines the abundance ratio of the isotope from the optical absorption amount of the isotope. The modulation amplitudes of the isotope natural abundance, the absorption coefficient of the trace isotope, and the absorption coefficient of the major isotope, respectively, are calculated. An isotope analysis device is provided which has a modulation amplitude controller for controlling so that the difference in absorption amount between the body and the abundant isotope is substantially the same.

Further, according to the present invention, a semiconductor laser driven by a drive current, a temperature control section for controlling the temperature of the semiconductor laser, a current control section for controlling the drive current, and an output to the current control section. An oscillator that frequency-modulates the semiconductor laser by outputting a signal, a sample cell for accommodating a measurement sample, and a photodetector that detects laser light incident from the semiconductor laser through the sample cell. In the isotope analysis device having a lock-in amplifier for inputting a detection signal of the photodetector and detecting only a synchronized signal by inputting an output signal from the oscillator, the semiconductor laser and the sample The transmitted light intensity variable optical element provided on the optical path between the cell and the laser light intensity irradiated at the time of spectrum measurement of the absorption amount of the trace isotope and the isotope The transmitted light intensity so that the difference between the absorption amount of the trace isotope and that of the trace isotope is substantially the same. An isotope analyzer characterized by having a light intensity controller for outputting a control signal to a variable optical element, and controlling laser light intensity during spectrum measurement of a trace isotope and spectrum measurement of a large amount of isotopes. can get.

According to the present invention, a semiconductor laser,
A temperature control unit for controlling the temperature of the semiconductor laser; a current control unit for controlling the driving current of the semiconductor laser; and an oscillator for outputting a signal to the current control unit to frequency-modulate the semiconductor laser. A sample cell for accommodating a measurement sample, a photodetector for detecting a laser beam emitted from the semiconductor laser through the sample cell, and a detection signal of the photodetector are input, while the oscillator from the oscillator is input. In an isotope analyzer having a lock-in amplifier that inputs a signal and detects only a synchronized signal, an amplifier that amplifies the signal from the oscillator and outputs the amplified signal to the current control unit, and a large amount of trace isotopes. When measuring the spectrum of isotopes, the modulation amplitude of frequency modulation is obtained from the natural abundance ratio of isotopes, the absorption coefficient of trace isotopes, and the absorption coefficient of high-volume isotopes. A modulation amplitude controller that outputs a control signal to the amplifier so that the difference between the absorption amount of the major isotope and the absorption amount of the major isotope is substantially the same. An isotope analysis device is obtained which is characterized by controlling the modulation amplitude over time.

[0015]

According to the above construction, by controlling the laser light intensity or the frequency modulation amplitude of the laser light, the light absorption amounts of the spectra of the major isotope and the minor isotope are made to be approximately the same, and with high accuracy and ease. It becomes possible to measure the isotope ratio, and it becomes possible to provide an apparatus of lower cost.

[0016]

EXAMPLES Examples of the isotope analyzer according to the present invention will be described below with reference to the drawings.

First Embodiment In FIG. 1, 1 is a semiconductor laser, 2 is a sample cell, 3 is a sample gas inlet, 4 is a sample gas outlet, 5 is a photodetector,
6 is a lock-in amplifier, 7 is a temperature controller of the semiconductor laser, 8 is a current controller of the semiconductor laser, 9 is an oscillator for current-modulating the semiconductor laser, 10 is an analog / digital output signal of the lock-in amplifier 6. A / D converter for conversion, 11 is a light intensity controller for controlling light intensity, 12 is an optical element for varying transmitted light intensity, 17 is a natural abundance ratio of isotopes, and absorption coefficients of trace isotopes and macro isotopes are stored. The storage unit 18 for storing signals is a signal for instructing measurement of a trace isotope and a macro isotope.

The semiconductor laser 1 becomes a variable wavelength light source easily by sweeping the temperature or driving current of the semiconductor laser 1. The semiconductor laser 1 sweeps around the absorption spectrum of an isotope having a low abundance ratio (hereinafter simply referred to as a trace isotope) and the absorption spectrum of an isotope having a high abundance ratio (hereinafter simply referred to as a major isotope) by the temperature control unit 7. So that the temperature is swept. Further, the drive current of the semiconductor laser 1 is controlled by the current control unit 8 so that an appropriate light output is obtained. Further, it is current-modulated by the signal of the oscillator 9 and slightly frequency-modulated.

The laser light from the semiconductor laser 1 thus wavelength-swept and frequency-modulated is incident on the sample cell 2 through the optical element 12. The laser light incident on the sample cell 2 interacts with the isotope in the cell, is absorbed, and is detected by the photodetector 5. Of the signals detected by the photodetector 5, only the signal synchronized with the oscillator 9 is detected by the lock-in amplifier 6, and as a result, the drift of the light intensity of the semiconductor laser 1 can be eliminated and a signal with a good S / N ratio can be obtained. Can be detected. The output signal of the lock-in amplifier 6 is converted into a digital value by the AD conversion circuit 10 and then sent to the computer.

On the other hand, the natural abundance ratios of the measured isotopes and the absorption coefficients of the trace isotopes and the major isotopes are input to the light intensity controller 11 from the storage unit 17, and the trace isotopes and the major isotopes of the natural abundance ratios are calculated. The amount of absorption is required. The absorption amount ΔI can be approximated by the equation (1) when the absorption is weak.

ΔI = I ₀ · α · L · P (1) where I ₀ is the incident light intensity, α is the absorption coefficient of the isotope, L
Is the optical path length of the sample cell 2, and P is the gas pressure in the sample cell 2. Here, the gas pressure in the sample cell is constant, and the partial pressure of each isotope, that is, the natural abundance ratio is P. The light intensity controller 11 adjusts the I
Determine the ratio of ₀ .

Next, the light intensity controller 11 controls the transmitted light intensity variable optical element 1 so that the incident intensity becomes a ratio of I ₀ determined according to the input trace isotope / major isotope measurement signal 18.
2 is controlled to change the incident intensity.

Although the transmitted light intensity variable optical element 12 is used here, the light intensity may be changed by controlling the current of the current controller 8.

According to the above embodiment, the semiconductor laser 1 having a very narrow spectral width is used as the wavelength variable light source, the mutual absorption of isotopes does not occur, and the optical absorption spectrum of the lock-in amplifier 6 is not affected by the spectrum of water. The isotope ratio can be measured with high accuracy and easily, and the apparatus is low-priced because the light absorption amount of the spectra of the major isotope and the trace isotope is made approximately the same by the light intensity controller.

Next, a second embodiment of the present invention will be described with reference to FIG.

In FIG. 2, 13 is a modulation amplitude controller for controlling the modulation amplitude of frequency modulation, and 14 is an amplifier for modulation amplitude for frequency modulation in the current control section 8. In FIG. 2, the same parts as those in FIG. 1 are designated by the same reference numerals, and duplicate description will be omitted.

The natural abundance ratio of measured isotopes and absorption coefficients of trace isotopes and major isotopes are input from the storage unit 17 to the modulation amplitude controller 13, and the incident light intensity I ₀ is kept constant as in the first embodiment. Then, the absorption ratio of the trace isotope and the major isotope in the natural abundance ratio is obtained. Based on this absorption amount ratio, the modulation amplitude a of the frequency modulation is controlled so that the absorption amounts of the trace isotope and the macro isotope detected by the lock-in amplifier 6 become approximately the same.

Paper: Second-Harmonic Detection with T
unable Diode Laser-Comparison of Experiment and The
As described in ory (Appl.Phys.B26, 203-210 (1981)), the peak value of the absorption spectrum in 2f detection is a / Δν = 2.2 (Δν: half-width at half maximum of absorption spectrum). It becomes maximum and becomes smaller as a / Δν becomes smaller. In 1f detection, it becomes maximum at a / Δν = 1,
It can be seen that the smaller / Δν, the smaller.

By making good use of this relationship, the modulation amplitude a is controlled so that the absorption peak values of the trace isotope and the isotope are the same. The method is Y = (a / Δ for 2f detection.
ν) /2.2, if 1f detection, the relative intensity Y of the peak value of the absorption spectrum depending on a by linearly approximating Y = a / Δν
Is determined, and the value of a is determined so that the absorption ratio of the previously obtained trace isotope and the isotope of large amount is the same as the peak value of the isotope of large amount and that of the trace isotope.

Next, the modulation amplitude controller 13 is a trace isotope
The amplification factor of the amplifier 14 for the modulation signal is controlled so that the modulation amplitude a becomes the value obtained by the above according to the abundance isotope measurement signal 18.

The second embodiment has the same effect as that of the first embodiment.

[0032]

As described above, according to the present invention, a semiconductor laser having a very narrow spectral width is used as a wavelength tunable light source, the isotope mutual interference does not occur, and the optical absorption spectrum is locked in without being affected by the spectrum of water. Measured with an amplifier,
By controlling the light intensity and the modulation amplitude, the amount of light absorption in the spectra of the major isotope and the trace isotope is set to be approximately the same, so that the isotope ratio can be measured with high accuracy and easily, and the device is inexpensive.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of an isotope analyzer according to the present invention.

FIG. 2 is a block diagram showing a second embodiment of the present invention.

FIG. 3 is a block diagram showing a conventional isotope analyzer.

FIG. 4 is an infrared absorption spectrum diagram of CO ₂ by a conventional CO ₂ isotope analyzer.

[Explanation of symbols]

 1 Semiconductor Laser 2 Sample Cell 3 Sample Gas Inlet 4 Sample Gas Outlet 5 Photodetector 6 Lock-in Amplifier 7 Semiconductor Laser Temperature Controller 8 Semiconductor Laser Current Controller 9 Oscillator 10 AD Converter 11 Light Intensity Controller 12 Transmitted light intensity variable element 13 Modulation amplitude controller 14 Amplifier 17 Storage section 18 Trace isotope / major isotope measurement signal

Claims (4)

(57) [Claims] 1. An isotope analyzer in which the oscillation wavelength of a semiconductor laser is swept, the optical absorption spectrum of the isotope gas in the sample cell is measured, and the abundance ratio of the isotope is determined from the optical absorption amount of the isotope. , The laser light intensities to be irradiated when measuring absorption spectra of trace isotopes and macro isotopes are obtained from the natural abundance ratio of isotopes, the absorption coefficient of trace isotopes, and the absorption coefficient of macro isotopes. A light intensity controller that controls the laser light intensity at the time of measuring the spectrum of the isotope and that at the time of measuring the spectrum of the major isotope so that the difference in absorption amount between the trace isotope and the major isotope is substantially the same. An isotope analysis device comprising: 2. The oscillation wavelength of the semiconductor laser is swept, and thereby the optical absorption spectrum of the isotope gas in the sample cell is measured, while the semiconductor laser is frequency-modulated at the same time as the wavelength sweep of the semiconductor laser. In the isotope analyzer that detects the optical absorption spectrum of the isotope by using a lock-in amplifier and obtains the abundance ratio of the isotope from the optical absorption amount of the isotope, the modulation of the frequency modulation when measuring the spectra of trace isotopes and abundant isotopes. The amplitude is calculated from the natural abundance ratio of isotopes, the absorption coefficient of trace isotopes, and the absorption coefficient of major isotopes, and the modulation amplitude at the time of measuring the spectra of the minor isotopes and the spectra of the major isotopes is calculated as the minor isotopes. An isotope analyzer having a modulation amplitude controller for controlling so that the difference in absorption amount of a large amount of isotopes is substantially the same. 3. A semiconductor laser driven by a drive current, a temperature controller for controlling the temperature of the semiconductor laser, a current controller for controlling the drive current, and an output signal output to the current controller. An oscillator for frequency-modulating the semiconductor laser, a sample cell for accommodating a measurement sample, a photodetector for detecting laser light incident from the semiconductor laser through the sample cell, and a photodetector of the photodetector. While inputting a detection signal, in an isotope analysis device having a lock-in amplifier for inputting an output signal from the oscillator and detecting only a synchronized signal, the light between the semiconductor laser and the sample cell is detected. The transmitted light intensity variable optical element installed on the road and the laser light intensity irradiated at the time of the spectrum measurement of the absorption amount of the trace isotope and the isotope are set to the natural abundance ratio of the isotope. Obtained from the absorption coefficient of the isotope and the absorption coefficient of the high-volume isotope, respectively, a control signal to the transmitted light intensity variable optical element so that the difference in absorption amount between the trace isotope and the high-volume isotope becomes substantially the same. An isotope analyzer having a light intensity controller for outputting and controlling the laser light intensity during spectrum measurement of a trace isotope and spectrum measurement of a large amount of isotopes. 4. A semiconductor laser, a temperature control section for controlling the temperature of the semiconductor laser, a current control section for controlling a drive current of the semiconductor laser, and an output signal to the current control section to output the output signal. An oscillator for frequency-modulating a semiconductor laser, a sample cell for containing a measurement sample,
A photodetector that detects laser light emitted from the semiconductor laser through the sample cell, and a detection signal of the photodetector are input, while a signal from the oscillator is input to obtain only a synchronized signal. In an isotope analyzer having a lock-in amplifier for detecting, an amplifier that amplifies a signal from the oscillator and outputs the amplified signal to the current control unit, and a modulation amplitude of frequency modulation during spectrum measurement of a trace isotope and a large amount of isotope. , The natural abundance ratio of isotopes, the absorption coefficient of a trace isotope, and the absorption coefficient of a macro-isotope, respectively, so that the difference between the absorption amounts of the trace isotope and the macro-isotope becomes substantially the same. An isotope analysis device having a modulation amplitude controller for outputting a control signal to control the modulation amplitude at the time of spectrum measurement of a trace isotope and at the time of spectrum measurement of a large amount of isotopes.