JP2876267B2 - Carbon 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 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]

BACKGROUND OF THE INVENTION Carbon (C), the mass number other things 12 ⁽¹² C), stable isotope mass number is 13 (hereinafter
¹³ C), 14 radioisotopes, and the like.

[0003] Among them, ¹³ C is different from radioisotopes and is easy to handle because it is not exposed to radiation. Therefore, in the medical field, its use is actively studied, for example, diagnosis is performed by tracing changes in the isotope ratio.

A conventional carbon isotope analyzer for performing such an analysis uses an infrared light absorption spectrum. In general, carbon does not directly resonate with light in the infrared,
It is converted to carbon dioxide (CO ₂ ) and isotope of carbon dioxide ( ¹²
Carbon isotopes ( ¹² C, ¹³ C) are analyzed by measuring the spectrum of CO ₂ , ¹³ CO ₂ ).

Conventionally, a dispersion type spectrometer has been used when measuring the light absorption spectra of ¹² CO ₂ and ¹³ CO ₂ and analyzing carbon isotopes. FIG. 3 shows the configuration of this spectrometer.

In FIG. 3, the emission spectrum is infrared, and light emitted from a lamp 1 having a wide spectrum width is introduced into a sample cell 2. Then, the light interacts with the sample inside the sample cell 2 and is resonantly absorbed. The light from the sample cell 2 enters the diffraction grating 5 via the slit 3a and the mirror 4, and is separated.

At this time, by continuously changing the angle of the diffraction grating 5 and detecting light from the diffraction grating 5 through the slit 3b, a change in light intensity with respect to a continuous wavelength change is measured by the detector 6a. In this way, the light absorption spectrum of the sample is measured. Further, the diffraction grating 5 and the detector 6a
By changing the width of the slit 3b between the two, 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 and the width of the slit of the diffraction grating used.

[0009] Carbon dioxide ¹² CO ₂ and ¹³ to be measured
Since the frequency of each light absorption spectrum is different from that of CO ₂ due to the difference in mass difference, the change in the isotope ratio can be traced by obtaining the light absorption intensity ratio.

[0010]

[Problems to be Solved by the Invention] By the way, ¹² CO ₂ and
The fine structure (vibration / rotation spectrum) of the light absorption spectrum with ¹³ CO ₂ is close and partially overlaps. In the natural state, ¹³ CO ₂ is 1/12 of ¹² CO ₂
There are 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. To such spectra, the spectral resolution of the dispersive spectrometer described above, 1 cm ^-1 degree and for a low, a result of measuring the light absorption spectrum of isotopes of CO ₂ by the dispersive spectrometer, ¹² CO ₂ Separation of the faint ¹³ CO ₂ spectrum is not straightforward, as both the spectra of the ¹³ CO ₂ and ¹³ CO ₂ overlap. Such measurement results affect each other between the two spectra. In addition, since the abundance ratio is largely different as described above, the change of the absorption intensity with respect to the change of the concentration is different 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 to ¹³ CO ₂ cannot be accurately traced without any measurement result.

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

That is, the measurement of carbon isotopes by the conventional dispersive spectrometer has the above-mentioned problems due to the resolution of the apparatus and the combination of light absorption spectra that can be measured by this apparatus. Was.

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

[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 pump having an oscillation wavelength near 2 μm and a variable wavelength is provided. A Tm: YAG solid-state laser, wavelength sweeping means having a wavelength selection element, controlling the wavelength selection element to sweep the oscillation wavelength of the solid-state laser, an output mirror, and a piezoelectric element attached to the output mirror. Frequency modulating means for modulating the frequency of the solid-state laser, and in a state where the output light of the solid-state laser that has been wavelength-swept by the wavelength sweeping means and frequency-modulated by the frequency modulating means has been introduced into the sample cell. and a lock-in amplifier to detect the light absorption characteristics in the sample cell, have, for the detection of ¹³ CO _2, the wave number 4
The light absorption spectrum at 877.57 ± 0.20 cm ⁻¹ was determined, and the wave number 4878.29 ± was used to detect ¹² CO _2.
The light absorption spectrum at 0.20 cm ^-1 was obtained, and based on the intensity ratio between the two light absorption spectra, ¹³ CO ₂ and ¹² CO ₂
A carbon isotope analyzer characterized by detecting CO ₂ is obtained.

Further, according to the present invention, in the above-mentioned carbon isotope analyzer, the detection of ¹³ CO ₂ and ¹² CO ₂ is carried out with a wave number of 4877.57 ± 0.20 cm ⁻¹ and a wave number of 487.
Instead of obtaining the light absorption spectrum at 8.29 ± 0.20 cm ⁻¹ , the detection of ¹³ CO ₂ and ¹² CO ₂ was carried out using the wave number 5007.36 ± 0.20 cm ⁻¹ and the wave number 5.
Light absorption spectrum when the 007.79 ± 0.20cm ^-1, or an optical absorption spectrum when the wavenumber 4978.02 ± 0.20 cm ^-1 and a wavenumber of 4978.61 ± 0.20cm ^-1, or wavenumber 4978.02 ± 0.20cm ^-1
And the light absorption spectrum at a wave number of 4976.27 ± 0.20 cm ⁻¹ or a wave number of 4879.28 ± 0.20
The optical absorption spectrum when the cm ^-1 and a wavenumber of 4879.54 ± 0.20cm ^-1, carbon isotope analyzer and obtaining respectively obtained.

[0017]

That is, the present invention provides a semiconductor laser-pumped Tm: YAG solid-state laser which has a very narrow oscillation spectrum width, a widely variable oscillation wavelength, and oscillates in a wavelength band of 2 μm in which the absorption intensity of CO ₂ is strong. A light source for spectroscopy.

By irradiating this with a sample and measuring the transmitted light, the mutual influence between ¹² CO ₂ and ¹³ CO ₂ and
The method is to measure the light absorption spectrum with high resolution without being affected by H ₂ O, and to measure the change of the carbon isotope with high accuracy and high sensitivity.

With the spread of high-power semiconductor lasers, research and development of solid-state lasers excited by semiconductor lasers have been rapidly expanding. Along with this, we succeeded in realizing oscillation at a temperature or room temperature at which Peltier elements can cool oscillation lines that did not oscillate or oscillation lines that required cooling of liquid nitrogen etc. with conventional lamp excitation. I have. Since these 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.

Among solid-state lasers whose oscillation has been facilitated by such semiconductor laser excitation, there is a 2 μm band laser.

The semiconductor laser-pumped Tm: YAG solid-state laser is a high-power semiconductor laser having an oscillation wavelength of 785 nm as a pumping light source using Tm: YAG obtained by doping Tm ³⁺ ions into YAG, which is a host crystal of the solid-state laser. Used.

The semiconductor laser-pumped Tm: YAG solid-state laser has a center of 2.02 μm (4950 cm ⁻¹ ) and is 1.8.
It is a laser whose wavelength is variable within a 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 a dispersion type spectrometer.

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

However, there is an unsuitable spectrum for the measurement of the isotope ratio, and ¹² CO ₂ and ¹³
A combination of a light absorption spectrum with CO ₂ is required. The conditions are as follows.

As described above, the absorption intensity of the light absorption spectrum of ¹³ CO ₂ is about 1/100 of the absorption intensity of ¹² CO ₂ . Therefore, in order not to be affected by the spectrum of ¹² CO _2, a spectrum of ¹³ CO _{2 having} a strong absorption intensity and a spectrum of ¹² CO _{2 having} a weak absorption intensity are selected,
It is desirable to measure in that combination.

In order to accurately trace the isotope ratio, the intensity of the light absorption spectrum of each of ¹² CO ₂ and ¹³ CO _{2 in} the natural abundance ratio is almost equal.

In addition to the intended vibration and rotation spectra of ¹² CO ₂ and ¹³ CO ₂ , there are many weak vibration and rotation spectra. Unaffected by these.

It is desired to measure and simultaneously measure the light absorption spectra of ¹² CO ₂ and ¹³ CO ₂ under the same conditions. Therefore, both spectra are as close as possible without affecting each other.

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

The combination of the light absorption spectra of ¹² CO ₂ and ¹³ CO ₂ suitable for the measurement of the isotope ratio, which satisfies the above conditions, is in a range that can be swept by the Tm: YAG solid-state laser. 487 of ¹³ CO ₂
4878.29 for 7.57 ± 0.20 cm ⁻¹ and ¹² CO ₂
± 0.20 cm ^-1 pairs and Table 1
It was found that there is a combination shown in FIG.

If a combination of these spectra is used for measurement and the solid-state laser is used as a light source for spectroscopy,
The measurement of the ratio of ¹³ CO ₂ to ¹² CO ₂ can be realized with high accuracy and easily.

[0034]

Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 schematically shows one embodiment of a semiconductor laser-pumped Tm: YAG solid-state laser (13 in FIG. 2 described later) serving as a light source of the present invention. Reference numeral 7 denotes a high-output semiconductor laser. The high-output semiconductor laser 7 has its oscillation wavelength adjusted to 785 nm, which is the absorption wavelength of Tm: YAG, by controlling its temperature. 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 for coupling the oscillation light of the semiconductor laser to the laser resonator. Reference numeral 9 denotes a Tm: YAG rod. The end face a on the semiconductor laser side has a high transmittance for an excitation wavelength of 785 nm, and is coated with an HR for an oscillation wavelength. Further, the end face b on the laser resonator side
Have an AR coating for the oscillation wavelength. The end face b of the rod 9 may have a Brewster angle with respect to the oscillation wavelength. In this case, no coating is needed on end face b. Reference numeral 10 denotes a wavelength selection element, which is a combination of birefringent filters, an etalon, and the like. Reference numeral 11 denotes an output mirror of the laser resonator, and reference numeral 12 denotes a piezoelectric element attached to the output mirror.

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

The pump 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 the laser resonator. As a result, T
The m: YAG rod 9 is excited and laser-oscillates by the resonator, and the oscillation light is extracted from the output mirror 11. The oscillation wavelength is set to 1.87 to 2.16 μm by the wavelength selection element 10 inserted in the laser resonator.
In between, a desired wavelength can be selected and a sweep can be performed. By selecting and inserting an appropriate element, the spectral line width of the laser oscillation light can be controlled.

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

Further, by applying an appropriate bias voltage and an appropriate 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 above-described semiconductor laser excitation Tm: Y
1 shows an embodiment of the present invention using an AG solid-state laser.

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

Now, the solid-state laser 13 has an oscillation frequency of the combination of the above-mentioned ¹³ CO ₂ and ¹² CO ₂ spectra, ie, 4877.57 ± 0.20 cm ^{−1 of 13} CO _2.
It is controlled so as to sweep the range covering both the spectra of ¹² CO ₂ and 4878.29 ± 0.20 cm ⁻¹ .
This is performed by the wavelength selection element 10. At this time, unnecessary portions between the two spectra may be skipped, and it is not necessary to continuously sweep all the portions between the two spectra. The modulation signal from the oscillator 14 is transmitted to the piezoelectric element control unit 15
Is amplified by the solid-state laser 13 together with the bias voltage.
Is applied to the piezoelectric element 12.

Thus, the oscillation spectrum of the solid-state laser 13 is modulated. Alternatively, the oscillation spectrum may be modulated by using the EO modulator 16 shown by a dotted line in the figure without using the piezoelectric element 12 and adding the above signal to the EO modulator 16. The EO modulator 16
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 that has been frequency-modulated and wavelength-swept as described above is introduced into the sample cell 2. The sample is placed in the sample cell 2 at such a pressure that the fine structure of the light absorption spectrum of CO ₂ can be separated. When the wavelength of the laser light is the above-mentioned wave number, the laser light introduced into the cell has an internal sample, ie, CO 2
Interacts with ₂ and resonance absorption occurs. Light exiting the sample cell 2 is detected by the photodetector 6b, and only signals synchronized with the oscillator 14 are detected by the lock-in amplifier 17.

As a result of performing the synchronous detection in this manner, fluctuations in optical output during wavelength sweeping, noise peculiar to a solid-state laser, and the like can be removed, and a signal having a good S / N ratio can be detected. The signal from the detector 6b is supplied by the lock-in amplifier 17 as required.
It is obtained as a form of 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 , Isotope ratio.

[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 selections and the combinations are appropriately separated from each other, and thus are optimal as the measurement targets.
Further, since a semiconductor laser-excited solid-state laser that is tunable in this wavelength band is used for measuring this spectrum combination, the device can be small 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, ¹³ CO ₂ and ¹² CO ₂
The detection of CO _2, is used each light absorption spectrum when the 4877.57 ± 0.20 cm ^-1 and 4878.29 ± 0.20cm ^{-1, 13} CO ₂ and ¹² CO
₂ , the light absorption spectrum at the wave number of 5007.36 ± 0.20 cm ⁻¹ and the wave number of 5007.79 ± 0.20 cm ⁻¹ or the wave number of 4978.02 ± 0.20 cm ⁻¹
Light absorption spectrum at ^-1 and wave number 4978.61 ± 0.20 cm ⁻¹ or wave number 4978.02 ± 0.2
Light absorption spectrum when the 0 cm ^-1 and a wavenumber of 4976.27 ± 0.20cm ^-1, or wavenumber 4,879.28 ±
0.20 cm ^-1 and wave number 4879.54 ± 0.20c
The isotope ratio may be measured using the light absorption spectrum at m ^-1 as a measurement object.

[0048]

As described above, according to the present invention, ¹³
Influence of mutual of CO ₂ and ¹² CO _2, without the influence of strong absorption spectrum of H ₂ O, reasonably close, measured combination of spectral balance of the absorption intensity good ¹³ CO ₂ and ¹² CO ₂ Since a semiconductor laser-excited Tm: YAG solid-state laser is used as the light source for spectroscopy, it is possible to measure the carbon isotope ratio with high accuracy and high sensitivity.

[Brief description of the drawings]

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

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

FIG. 3 is a schematic view of a conventional carbon isotope measuring apparatus using a dispersion type spectrometer.

[Explanation of symbols]

 Reference Signs List 1 lamp 2 sample cell 3a, 3b slit 4 mirror 5 diffraction grating 6a, 6b photodetector 7 high output 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 Section (high voltage power supply) 16 EO modulator 17 Lock-in amplifier

Continuation of the front page (56) References JP-A-4-42041 (JP, A) JP-A-3-170848 (JP, A) JP-A-3-80587 (JP, A) JP-A-59-167080 (JP) , A) Acta Physica Acad emiea Scientiarum Hungaricae, 48 (1), p93-102 (1980) Opt. Lett, Vol. 15, No. 9, (1990) p486-488 (58) Fields investigated (Int. Cl. ⁶ , DB name) G01N 21/35-21/39 WPI (DIALOG) JICST file (JOIS)

Claims (2)

(57) [Claims] 1. An optical absorption spectrum of ¹³ CO ₂ and ¹² CO
_A Tp: YAG solid-state laser having an oscillation wavelength of about 2 μm and variable in wavelength, and a wavelength selection element, and controlling the wavelength selection element to control the solid-state laser. Wavelength sweeping means for sweeping an oscillation wavelength; frequency mirroring means having an output mirror and a piezoelectric element attached to the output mirror for modulating the frequency of the solid-state laser; and In the state where the output light of the solid-state laser frequency-modulated by the frequency modulating means is introduced into the sample cell, a lock-in amplifier that detects a light absorption characteristic in the sample cell, and ¹³ CO ₂ Wave number 4877.57 ± 0.20 cm for detection
The light absorption spectrum at the time of ^-1 was obtained, and the wave number 487.29 ± 0.20 cm was used for the detection of ¹² CO _2.
The light absorption spectrum at the time of ^-1 is obtained, and based on the intensity ratio of the two light absorption spectra, ¹³ CO ₂
And carbon dioxide isotope analyzer for detecting ¹² CO ₂ . 2. The carbon isotope analyzer according to claim 1, wherein a wave number of 4877.57 is used for detecting ¹³ CO ₂ and ¹² CO _2.
± 0.20 cm ⁻¹ and wave number 4878.29 ± 0.20
The optical absorption spectrum when the cm ^-1 instead of obtaining respectively, the detection of the ¹³ CO ₂ and ¹² CO _2, the wave number 5007.
36 ± 0.20 cm ⁻¹ and wave number 5007.79 ± 0.
Light absorption spectrum at 20 cm ^-1 or wave number 49
78.02 ± 0.20 cm ⁻¹ and wave number 4978.61
Light absorption spectrum when the ± 0.20 cm ^-1 or wavenumber 4,978.02 ± 0.20 cm ^-1 and a wavenumber of 497,
Light absorption spectrum at 6.27 ± 0.20 cm ⁻¹ ,
Alternatively, a carbon isotope analyzer characterized by obtaining light absorption spectra at a wave number of 4879.28 ± 0.20 cm ⁻¹ and a wave number of 4879.54 ± 0.20 cm ⁻¹ .