JPH05340872A - Carbon isotope analyzing instrument - Google Patents

Abstract

PURPOSE:To obtain an isotopic CO2 analyzing instrument which can analyze a light absorption spectrum for a carbon isotope. CONSTITUTION:The title analyzing instrument is provided with a semiconductor laser-excited Tm: YAG solid-state laser 13 having an oscillation wavelength of about 2mum and a variable wavelength, piezo-electric element controlling section 15 which sweeps the oscillation frequency of the laser 13, oscillator 14 which modulates the frequency of the laser 13, and a lock-in amplifier 17 which detects light absorption characteristics in a sample cell 2 and analyzes a light absorption spectrum for isotopic CO2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon isotope analyzer for irradiating a sample with light and analyzing ¹³ CO ₂ and ¹² CO ₂ from its light absorption spectrum.

[0002]

2. Description of the Related Art Carbon (C) has a mass number of 12 ( ¹² C) as well as stable isotopes having a mass number of 13 (hereinafter
¹³ C) and radioactive isotopes such as 14 exist.

Of these, ¹³ C, unlike radioactive isotopes, is not exposed to radiation and is easy to handle. Therefore, in the medical field, its use is actively studied, for example, by making a diagnosis by tracing the change of isotope ratio.

[0004] As a conventional carbon isotope analyzer for performing such an analysis, there is one using an optical absorption spectrum in the infrared region. In general, carbon does not directly resonate with light in the infrared region, so
It is converted to carbon dioxide (CO ₂ ) and isotope of carbon dioxide ( ¹²
Carbon isotopes ( ¹² C, ¹³ C) are analyzed by measuring spectra of CO ₂ , ¹³ CO ₂ .

Conventionally, a dispersion type spectrometer has been used when measuring the optical absorption spectra of ¹² CO ₂ and ¹³ CO ₂ and analyzing carbon isotopes. The structure of this spectrometer is shown in FIG.

In FIG. 3, the light emitted from the lamp 1 whose emission spectrum region is infrared and has a wide spectrum width is introduced into the sample cell 2. There, the light interacts with the sample inside the sample cell 2 and is resonantly absorbed. The light emitted from the sample cell 2 passes through the slit 3a and the mirror 4 and enters the diffraction grating 5 to be dispersed.

At this time, the angle of the diffraction grating 5 is continuously changed, and the light from the diffraction grating 5 is detected through the slit 3b, whereby the intensity change of the light with respect to the continuous wavelength change is measured by the detector 6a. In this way, the light absorption spectrum of the sample is measured. In addition, the diffraction grating 5 and the detector 6a
By changing the width of the slit 3b between them, the spectral resolution of this device can be changed.

The limit of the resolution of the above-mentioned conventional carbon isotope analyzer is determined by the number of grooves of the diffraction grating used and the width of the slit.

Carbon dioxide ¹² CO ₂ and ¹³ to be measured
Since the frequency of each light absorption spectrum differs from that of CO ₂ due to the difference in mass difference, the change in the isotope ratio can be traced by determining the light absorption intensity ratio.

[0010]

By the way, with ¹² CO ₂
The fine structures (vibration / rotation spectra) of the light absorption spectrum with ¹³ CO ₂ are close to each other and partially overlap each other. In the natural state, ¹³ CO ₂ is 1/12 of ¹² CO ₂ .
There is only about 100, and the light absorption intensity is ¹² CO
It is about 1/100 of ₂ .

Further, the spectrum of each fine structure has a full width at half maximum of about 0.1 cm ^{-1 in} the atmosphere. In contrast to such a spectrum, the spectral resolution of the above-mentioned dispersive spectrometer is as low as about 1 cm ^−1, so the result of measuring the optical absorption spectrum of the isotope of CO ₂ by the above-mentioned dispersive spectrometer is ¹² CO ₂ And ¹³ CO ₂ spectra overlap, and separation of the weak ¹³ CO ₂ spectrum is not easy. Such measurement results influence each other between the two spectra. Further, since the abundance ratios differ greatly as described above, the change in absorption intensity with respect to the change in concentration differs between ¹² CO ₂ and ¹³ CO ₂ . That is, ¹³ CO ₂ changes almost linearly, while ¹² CO ₂ changes logarithmically. Therefore, the trace of the ratio of ¹² CO ₂ and ¹³ CO ₂ cannot be accurately performed with the measurement result as it is.

Although such a measurement result is corrected by calculation during analysis, there is a limit to accurate measurement of change in isotope ratio. Further, the influence of the light absorption spectrum of impurities contained in the sample and moisture (H ₂ O) existing on the optical path and having high absorption intensity cannot be ignored because the spectral resolution is low as described above. Furthermore, when detecting a trace amount of isotope change, it is necessary to widen the width of the slit 3b in order to increase the sensitivity. This leads to a further reduction in resolution.

That is, the measurement of carbon isotopes by the conventional dispersive spectrometer has the above problems due to the resolution of the device and the combination of the optical absorption spectra measurable by this device. It was

Therefore, in view of the above-mentioned drawbacks, a technical object of the present invention is to provide a carbon isotope analyzer for analyzing carbon isotopes with high accuracy from an optical absorption spectrum.

[0015]

According to the present invention, in a carbon isotope analyzer for analyzing ¹³ CO ₂ and ¹² CO ₂ from an optical absorption spectrum, a semiconductor laser excitation having an oscillation wavelength in the vicinity of 2 μm and a variable wavelength is used. tm: a YAG solid state laser, means for sweeping the oscillation wavelength of the solid-state laser, means for modulating the frequency of the solid-state laser, and a lock-in amplifier to detect the light absorption characteristics in the sample cell, have, ¹³
Wave number 4877.57 ± 0.20 cm ^{−1 for} CO ₂ detection
The light absorption spectrum at the time of is obtained, and for the detection of ¹² CO ₂ ,
A carbon isotope analysis characterized by obtaining an optical absorption spectrum at a wave number of 4878.29 ± 0.20 cm ⁻¹ and detecting ¹³ CO ₂ and ¹² CO ₂ based on the intensity ratio of the both optical absorption spectra. The device is obtained.

Further, according to the present invention, in the carbon isotope analyzer, the combination of spectra shown in Table 2 below is used for detection of ¹³ CO ₂ and ¹² CO ₂ , respectively. An isotope analyzer is obtained.

[0017]

[Table 2]

That is, the present invention provides a semiconductor laser-excited Tm: YAG solid-state laser which has an extremely narrow oscillation spectrum width, a wide range of oscillation wavelengths, and oscillates in a wavelength band of 2 μm where CO ₂ absorption intensity is strong. Use as a light source for spectroscopy.

By irradiating the sample with this and measuring the transmitted light, the mutual influence of ¹² CO ₂ and ¹³ CO _2, and
It is a method for measuring a light absorption spectrum with high resolution without being affected by H ₂ O and for measuring changes in carbon isotopes with high accuracy and high sensitivity.

With the widespread use of high-power semiconductor lasers, research and development of solid-state lasers excited by semiconductor lasers are rapidly expanding. Along with this, in the conventional lamp excitation, we succeeded in oscillating an oscillation line that did not oscillate or an oscillation line that needed cooling such as liquid nitrogen at a temperature or room temperature that could be cooled by the Peltier element. There is. Since they use a semiconductor laser as an excitation source, they have many advantages such as low power consumption, high efficiency, small size and light weight, long life and high reliability.

A 2 μm band laser is one of the solid-state lasers which can be easily oscillated by such semiconductor laser excitation.

The semiconductor laser pumped Tm: YAG solid-state laser is a high-power semiconductor laser having an oscillation wavelength of 785 nm, which is obtained by using Tm: YAG doped with Tm ³⁺ ions in YAG which is a host crystal of the solid-state laser. It was used.

The semiconductor laser pumped Tm: YAG solid state laser has a center of 2.02 μm (4950 cm ⁻¹ ) and 1.8.
The wavelength tunable laser is in the range of 7 to 2.16 μm (5350 to 4630 cm ⁻¹ ).

The oscillation spectrum width of the solid-state laser is 3 × 1.
It is 0 ⁻⁶ cm ^{−1 to} 3 × 10 ⁻⁴ cm ^−1, which is much narrower than that of a dispersion spectrometer.

The Tm: YAG solid-state laser has an oscillation wavelength of 2 μm band (3000 to 5) in which CO ₂ absorption intensity is strong.
Since it is 000 cm ⁻¹ , each spectrum of vibration and rotation of CO ₂ can be measured at high density.

However, there is an unsuitable spectrum when measuring the isotope ratio, and ¹² CO ₂ and ¹³ which are suitable for measuring the isotope ratio are used.
A combination of light absorption spectra with CO ₂ is required. The conditions are as follows.

As described above, the absorption intensity of the optical absorption spectrum of ¹³ CO ₂ is about 1/100 of the absorption intensity of ¹² CO ₂ . Thus, ¹² CO ₂ in the manner which is not affected by spectrum, the spectrum of strong absorption intensity ¹³ CO _2, whereas, select the spectra of the weak ¹² CO ₂ absorption strength,
It is desirable to measure in that combination.

In addition, in order that the isotope ratio can be traced accurately, the intensities of the light absorption spectra of ¹² CO ₂ and ¹³ CO _{2 in} the natural abundance ratio are almost equal.

In addition to the desired vibration / rotation spectrum of ¹² CO ₂ and ¹³ CO ₂ , there are many weak vibration / rotation spectra. Not affected by these.

It is desired to measure the light absorption spectra of ¹² CO ₂ and ¹³ CO ₂ under the same conditions, and to measure them simultaneously. Therefore, the two spectra are as close as possible so that they do not affect each other.

It is not affected by the light absorption spectrum of impurities having a high absorption intensity contained in the sample gas, particularly H ₂ O.

In the combination of the optical absorption spectra of ¹² CO ₂ and ¹³ CO ₂ which is suitable for the isotope ratio measurement, it is within the range that can be swept by the above Tm: YAG solid state laser. As a whole, ¹³ CO ₂ 487
7.57 ± 0.20 cm ⁻¹ and ¹² CO ₂ 4878.29
Combination of ± 0.20 cm ^-1 as a pair, and Table 1
It turns out that there are combinations shown in.

If a combination of these spectra is used for measurement and the solid-state laser is used as a light source for spectroscopy,
Highly accurate and easy measurement of the ratio of ¹³ CO ₂ and ¹² CO ₂ can be realized.

[0034]

Embodiments of the present invention will now be described with reference to the drawings.

FIG. 1 schematically shows one embodiment of a semiconductor laser pumped Tm: YAG solid state laser 13 which is a light source of the present invention. Reference numeral 7 denotes a high-power semiconductor laser, and the high-power semiconductor laser 7 has an oscillation wavelength of Tm: Y by controlling its temperature.
It is adjusted to the absorption wavelength of 785 nm of AG. At the same time, the drive current is also controlled, and as a result, the optical output of the semiconductor laser 7 is controlled to be constant. Reference numeral 8 denotes an optical system that couples the oscillation light of the semiconductor laser to the laser resonator.
Reference numeral 9 is a Tm: YAG rod, and the end face a on the semiconductor laser side has a high transmittance with respect to the excitation wavelength of 785 nm, and is HR coated with respect to the oscillation wavelength.
Further, the end face b on the laser resonator side is A with respect to the oscillation wavelength.
R coating is applied. The end face b of the rod 9 may have a Brewster angle with respect to the oscillation wavelength. In this case, the end surface b does not need to be coated. Reference numeral 10 denotes a wavelength selection element, which is a combination of birefringent filters, an etalon, and the like. Reference numeral 11 is an output mirror of the laser resonator, and 12 is a piezoelectric element attached to the output mirror.

A laser resonator is formed by the end face a of the Tm: YAG rod 9 and the output mirror 11.

The pumping light output from the high-power semiconductor laser 7 is coupled by the optical system 8 so as to match the fundamental mode of oscillation of this laser resonator. As a result, T is highly efficient.
The m: YAG rod 9 is excited and oscillates by the resonator, and the oscillation light is extracted from the output mirror 11. With the wavelength selection element 10 inserted in the laser resonator, its oscillation wavelength is 1.87 to 2.16 μm.
In between, it is possible to select and sweep to an arbitrary wavelength. The spectral line width of the laser oscillation light can also be controlled by selecting and inserting an appropriate element.

Excitation power, that is, high-power semiconductor laser 7
Since the optical output of the laser is controlled to be constant as described above, the optical output of the oscillation light is controlled to be constant when controlled to a certain wavelength.

Further, by applying an appropriate bias voltage and modulation voltage to the piezoelectric element 12 attached to the output mirror 11, the laser resonator length can be slightly changed and the oscillation frequency can be modulated.

FIG. 2 shows the semiconductor laser excitation Tm: Y described above.
An embodiment of the present invention using an AG solid state laser will be shown.

Reference numeral 13 denotes the semiconductor laser excitation Tm: Y
AG solid-state laser, 2 is a sample cell, and 6b is a photodetector. Reference numeral 14 is an oscillator used when frequency-modulating the oscillation light of the solid-state laser 13, reference numeral 15 is a piezoelectric element control unit for controlling the piezoelectric element 12 shown in FIG.
O (Electro-Optic) modulator, 17
Is a lock-in amplifier.

Now, in the solid-state laser 13, the oscillation frequency thereof is a combination of the spectra of ¹³ CO ₂ and ¹² CO ₂ , that is, 4877.57 ± 0.20 cm ^{−1 of 13} CO _2.
And ¹² CO ₂ at 4878.29 ± 0.20 cm ⁻¹ are controlled to sweep over a range covering both spectra.
This is done by the wavelength selection element 10. At this time, unnecessary portions between both spectra may be skipped, and it is not necessary to continuously sweep all over both spectra. The modulation signal from the oscillator 14 is supplied to the piezoelectric element controller 15
Is amplified by the solid-state laser 13 together with the bias voltage.
Is applied to the piezoelectric element 12.

In this way, the oscillation spectrum of the solid-state laser 13 is modulated. Alternatively, instead of using the piezoelectric element 12, the EO modulator 16 shown by the dotted line in the figure may be used, and the oscillation spectrum may be modulated by adding the above signal thereto. In addition, the EO modulator 16
In addition to being installed outside the solid-state laser 13, it can be installed inside the laser resonator, that is, between the rod 9 and the output mirror 11 in FIG.

The output light from the solid-state laser 13 which has been frequency-modulated and wavelength-swept in this way is introduced into the sample cell 2. The sample cell 2 is filled with the sample at a pressure at which the fine structure of the optical absorption spectrum of CO ₂ can be separated. When the wavelength of the laser light is the above wave number, the laser light introduced into the cell is a sample inside, that is, CO
Reacts with ₂ , causing resonance absorption. The light emitted from the sample cell 2 is detected by the photodetector 6b, and only the signal synchronized with the oscillator 14 is detected by the lock-in amplifier 17.

As a result of performing the synchronous detection in this way, it is possible to remove the fluctuation of the optical output during the wavelength sweep, the noise peculiar to the solid-state laser, etc., and it is possible to detect a signal with a good S / N ratio. The signal from the detector 6b is supplied by the lock-in amplifier 17 as necessary.
It is determined as the form of the first derivative and second derivative of the shape of the light absorption spectrum. The resulting ¹² CO ₂ and ¹³ peak values of each of the light absorption spectrum signal with CO _2, or the shape and measuring the area, by obtaining a ratio of the absorption intensity than the ratio ¹² CO ₂ and ¹³ CO ₂ , The isotope ratio is obtained.

[0046] As described above, with a strong waveband of light absorption intensity of CO _2, well-balanced in absorption intensity of ¹² CO ₂ and ¹³ CO _2, the combination of the light absorption spectrum that is not affected by other impurities The selection and the combination thereof are suitable for measurement because they are appropriately separated from each other.
Furthermore, since a semiconductor laser pumped solid-state laser that is tunable in this wavelength band is used for measuring this combination of spectra, the device can be made compact and lightweight, and the isotope ratio can be measured with high accuracy and high sensitivity. And a highly reliable carbon isotope analyzer can be realized.

In the above example, 487 of ¹³ CO ₂ was used.
7.57 ± 0.20 cm ⁻¹ and ¹² CO ₂ 4878.29
Although the combination of the light absorption spectra of ± 0.20 cm ⁻¹ is used, the isotope ratio may be measured by using other combinations of the light absorption spectra shown in Table 1 as the measurement targets.

[0048]

As described above, according to the present invention, ¹³
Measures a combination of spectra of ¹³ CO ₂ and ¹² CO _{2 that} are reasonably close to each other and have a well-balanced absorption intensity without being affected by the mutual absorption of CO ₂ and ¹² CO ₂ or the strong absorption spectrum of H ₂ O. Since a semiconductor laser-excited Tm: YAG solid-state laser is used as a light source for spectroscopy as a target, it is possible to measure the carbon isotope ratio with high accuracy and high sensitivity.

[Brief description of drawings]

FIG. 1 is an example of a semiconductor laser pumped solid state laser which is a part of the present invention.

FIG. 2 is an embodiment of the device of the present invention.

FIG. 3 is a schematic diagram of a carbon isotope measurement device using a conventional dispersion type spectrometer.

[Explanation of symbols]

 1 Lamp 2 Sample cell 3a, 3b Slit 4 Mirror 5 Diffraction grating 6a, 6b Photodetector 7 High-power semiconductor laser 8 Optical system 9 Rod 10 Wavelength selection element 11 Output mirror 12 Piezoelectric element 13 Solid-state laser 14 Oscillator 15 Piezoelectric element control Part (high voltage power supply) 16 EO modulator 17 Lock-in amplifier

Claims (2)

[Claims] 1. A light absorption spectrum of ¹³ CO ₂ and ¹² CO
_In the carbon isotope analyzer for analyzing ₂ , a semiconductor laser pumped Tm: YAG solid-state laser having an oscillation wavelength in the vicinity of 2 μm and a variable wavelength, means for sweeping the oscillation wavelength of the solid-state laser, and a frequency of the solid-state laser are set. It has a means for modulating and a lock-in amplifier for detecting the light absorption characteristics in the sample cell, and has a wave number of 4877.57 ± 0.20 cm for detecting ¹³ CO _2.
The light absorption spectrum at ^-1 was obtained, and the wave number was 4878.29 ± 0.20 cm for the detection of ¹² CO _2.
The optical absorption spectrum at ^-1 is obtained, and based on the intensity ratio of the two optical absorption spectra, ¹³ CO ₂
And a carbon isotope analyzer characterized by detecting ¹² CO ₂ . 2. The carbon isotope analyzer according to claim 1, wherein a combination of spectra shown in Table 1 is used for detecting ¹³ CO ₂ and ¹² CO ₂ , respectively. .. [Table 1]