JP3304846B2 - Gas measuring device and isotope concentration ratio measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas measuring apparatus and an isotope concentration ratio measuring apparatus, and more particularly to an apparatus for analyzing a concentration ratio of a plurality of isotopes in a gas to be measured by utilizing absorption of a laser beam.

[0002]

2. Description of the Related Art Analytical methods utilizing isotopes as tracers are known in the fields of chemistry and medicine. Conventionally, radioisotopes are often used as the tracer. This is because the behavior of the tracer can be easily detected by radiation detection. However, non-radioactive isotopes have been used as tracers in recent years because radioisotopes have problems in exposure and management.

[0003] Recently, it has been reported that "Helicobacter pylori", a bacterium that can be present on the stomach wall, is a causative agent that causes gastric ulcer, gastric cancer and the like. Helicobacter pylori decomposes urea to carbon dioxide (C
O ₂ ). Therefore, there has been proposed a method for examining the presence or absence and abundance of Helicobacter pylori by utilizing its properties. Specifically, after a certain period of time after oral administration of the urea-based reagent to the subject, the breath of the subject is collected, and whether the breath contains carbon dioxide generated by Helicobacter pylori or not. , The presence or absence of the presence is determined.

In this case, the above-mentioned urea-based reagent is labeled with ¹³ C, which is a non-radioactive isotope, so that ¹³ CO ₂ in the exhaled air and
¹² CO ₂ ratio and its abundance in nature (1:99)
By measuring the slight deviation between the two, Helicobacter pylori can be quantified.

[0005]

As methods for measuring the concentration ratio of isotopes, various methods such as a method by mass spectrometry and a method by spectral analysis are known. However, it is difficult to analyze the isotope concentration ratio simply and with high sensitivity by any of the conventional methods. In particular, the conventional Helicobacter pylori test method requires a large amount of exhaled air in order to ensure the resolution, and it has been pointed out that the handling is inconvenient and the burden on the subject.

It is to be noted that there is a demand for a device capable of simply and sensitively quantifying a specific substance not only in the above-described analysis of bacteria but also in general gas analysis.

US Pat. No. 4,684,805 describes an apparatus for measuring an isotope ratio by passing a laser beam through a sample gas and detecting light absorption by a photogalvanic effect at that time. However, the absorption of the laser light when the laser light passes through the gas once is very small, and it is difficult to increase the detection sensitivity.

US Pat. No. 5,394,236 (JP-A-7-20054)
JP-A-4-364442 and JP-A-6-18411 disclose an isotope ratio measuring apparatus, which can analyze the isotope ratio with high accuracy while having a simple configuration. A device that can be used has not been realized.

The present invention has been made in view of the above-mentioned conventional problems, and has as its object to provide an apparatus capable of analyzing and measuring a gas to be measured at high sensitivity and at low cost by utilizing the absorption of laser light. It is in.

Another object of the present invention is to provide an apparatus which can obtain sufficient sensitivity with a small amount of exhalation when performing exhalation analysis.

[0011]

(1) In order to achieve the above object, the present invention provides a laser oscillator for outputting a laser beam, an absorption cell into which a gas to be measured is introduced, and both ends thereof. Having a pair of mirrors, a measurement resonator that resonates the incident laser light, light absorption detection means for detecting absorption of the laser light by the gas to be measured , and gas discharge in the absorption cell. Discharging means to be performed,
It is characterized by including.

According to the above configuration, when the laser light from the laser oscillator is incident on the measuring resonator, the laser light resonates while being reflected back and forth many times in the resonator. Since the laser beam is absorbed by the gas to be measured for each reflection, a discharge means is provided.
Thus, according to the present invention, as a result, the degree of light absorption can be increased as compared with the related art, and a high detection sensitivity can be obtained.

As described above, according to the present invention, a resonator similar to a resonator provided in a laser device or the like is used as an external resonator for measurement. Highly sensitive detection of molecules can be achieved, and the required amount of gas introduced into the cell can be reduced as compared with the conventional case.

The pair of mirrors described above are desirably arranged to face each other. For example, one of the mirrors is an incident mirror and the other is an output mirror. In that case, it is desirable that the entrance mirror is configured to completely reflect the laser light from inside the cell,
It is desirable that the emission mirror be configured to reflect the laser light almost and transmit only a part (for example, several%) of the light. The distance between the pair of mirrors is basically set based on resonance conditions.

In a preferred aspect of the present invention, an optical path length sweeping means for periodically varying an optical path length between a pair of mirrors in the measurement resonator is provided. Here, preferably,
The optical path length sweeping means is a piezoelectric element. According to such a sweep of the optical path length, even if there is some error in the distance between the mirrors (manufacturing error, aging, etc.), the laser beam can be tuned at any point during the sweep. That is, a standing wave can be formed between the mirrors, and gas analysis or the like can be performed with high sensitivity by utilizing light absorption at that time. If a piezoelectric element is used as the optical path length sweeping means, sweeping can be realized with a simple configuration, and the sweep cycle can be adjusted freely by adjusting the frequency of an electric signal supplied to the piezoelectric element.

In a preferred aspect of the present invention, the discharge means
Is an electrode provided in the absorption cell, and a voltage is applied to the electrode.
Means for applying In this case, for example, the discharge is performed using a high frequency (RF), or the discharge is performed by intermittent application of a DC high voltage.

In a preferred aspect of the present invention, the light absorption detection means includes a photodetector for detecting laser light emitted from the measurement resonator.

(2) In order to achieve the above object, the present invention provides a first laser oscillator for outputting a first laser beam having a wavelength corresponding to a first isotope, and a second isotope. A second laser oscillator that outputs a second laser beam having a wavelength corresponding to a, an absorption cell into which the sample gas is put, and a pair of mirrors provided at both ends thereof,
A first measurement resonator that resonates the first and second laser beams that are simultaneously or separately incident, an absorption cell into which a reference gas is put, and a pair of mirrors provided at both ends thereof The first and second incident simultaneously or separately
A second measurement resonator for resonating the laser light, light absorption detection means for detecting the absorption of each laser light by the sample gas and the reference gas, and each laser light by the sample gas and the reference gas Concentration ratio calculating means for calculating the isotope concentration ratio based on the absorption of the isotope.

According to the above configuration, the sample gas and the reference gas are introduced into the first measurement resonator and the second measurement resonator, respectively. Alternatively, the concentration ratio of the isotope of the molecule is calculated.

In the first measurement resonator, light absorption of the first laser beam is observed by the first isotope in the sample gas, while second absorption is observed by the second isotope component in the sample gas. Is observed for the laser light. The concentration ratio is calculated from the difference or ratio of the light absorption. At this time, the second measurement resonator is used for so-called reference value calibration or reference value reference. For example, the difference between the light absorption of the first laser light by the first isotope in the reference gas and the light absorption of the second laser light by the second isotope in the reference gas in the second measurement resonator is The sensitivity is adjusted so as to be zero, or the ratio is used as a reference when calculating the concentration ratio.

The first laser beam and the second laser beam are generated so as to have wavelengths respectively corresponding to the resonance absorption lines of the isotope to be measured, for example, the first isotope and the second isotope When measuring the body concentration ratio, the first
In the laser oscillator of the first embodiment, a first isotope is used as a laser medium, and in the second laser oscillator, a second isotope is used as a laser medium.

According to the present invention, since the laser light is reciprocally reflected in the measurement resonator, a high light absorption effect can be obtained. That is, highly sensitive detection can be performed with a small amount of gas.

In a preferred aspect of the present invention, the light absorption detection means includes first and second photodetectors provided corresponding to the first laser light and the second laser light, respectively. A first optical path switch for guiding the first and second laser beams emitted from the first and second laser oscillators to the first or second measurement resonator, and the first or second measurement A second optical path switch for guiding the first and second laser beams emitted from the resonator for use to the first and second photodetectors.

In a preferred aspect of the present invention, the image processing apparatus includes a difference calculator for calculating a difference between output signals from the first and second photodetectors, wherein the density ratio calculator is configured to calculate a difference based on an output of the difference calculator. To calculate the density ratio.

In a preferred aspect of the present invention, a lock-in amplifier for synchronously detecting an output of the difference calculator according to a switching cycle of the first and second optical path switches is included. Also,
In a preferred embodiment of the present invention, the first isotope is ¹³ CO
₂ and the second isotope is ¹² CO ₂ . In a desirable mode of the present invention, the sample gas is exhaled air.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an overall configuration of an isotope concentration ratio measuring apparatus which is a gas measuring apparatus according to the present embodiment. This device measures the concentration ratio of isotopes using the absorption of laser light. In particular, for the above-mentioned test of Helicobacter pylori, the isotope in the breath of the subject after administration of the reagent is used. Concentration ratio ( ¹² CO
₂ : A breath analyzer that analyzes ¹³ CO ₂ ).

In FIG. 1, a laser device 10 and a laser device 12 are so-called carbon dioxide laser devices, each having the same gas composition (eg, CO ₂ , N ₂ , He) and partial pressure ratio as a general carbon dioxide laser. . However, in the laser device 12, ordinary C ₂ gas is used as CO ₂ gas.
O ₂ gas is used. This is a gas whose isotope abundance is the natural isotope abundance ( ¹² CO ₂ : ¹³ CO ₂ is 99: 1). On the other hand, in the laser device 10, ¹³ CO
₂ is CO ₂ gas is utilized in which the initiative. Therefore,
The laser device 12 generates laser light having a wavelength centered at 10.6 μm, and the laser device 10 generates 11.0 μm
Is generated.

The apparatus according to the present embodiment is capable of measuring ¹² C
Since it measures the concentration ratio between O ₂ and ¹³ CO ₂ , a ¹² CO ₂ laser device and a ¹³ CO ₂ laser device are used as described above. Of course, when measuring another isotope concentration ratio, a laser device that generates a laser beam having a specific wavelength that causes light absorption for each isotope to be measured is prepared.

The laser devices 10 and 12 have the same structure as a general carbon dioxide laser device. Laser device 1
0 is a laser medium cell 14 in which ¹³ CO ₂ gas is sealed.
a and mirrors 14b and 14c provided at both ends thereof
And a laser resonator 14 comprising: The laser device 12 includes a laser medium cell 16a in which ¹² CO ₂ gas is sealed, and mirrors 16b and 1 provided at both ends thereof.
6c. In each of the laser resonators (internal resonators) 14 and 16, laser light is enhanced and generated by optical resonance. Mirror 1
The mirror 4b and the mirror 16b are mirrors that generate perfect reflection, and the mirrors 14c and 16c are partially (for example, 1%).
(Amount of light). That is, laser light is emitted through the mirrors 14c and 16c.

Each of the laser resonators 14 and 16 is provided with a discharge electrode (or discharge coil) for ionizing the internal gas, but is not shown. Incidentally, AC or DC high-voltage power from the discharge power supply 72 is supplied to those discharge devices.

In the apparatus of the present embodiment, the external resonators (measuring resonators) 20 and 22 are mechanisms for measuring light absorption by gas. External resonator 2 for sample gas
0: Sample gas (sample gas) as the exhalation of the subject
Is enclosed. The exhaled breath was collected from the subject a certain time after orally administering the ^13C- labeled urea-based reagent to the subject. On the other hand, the external resonator 22 for the reference gas is used as a reference, and the external resonator 22 may be a CO ₂ gas having a natural isotope abundance ratio or a CO ₂ gas before the above-described urea-based reagent administration. The subject's breath is enclosed.

The external resonators 20 and 22 are basically
It has the same configuration as the internal resonator in the laser device. More specifically, the external resonator 20 includes an absorption cell 20a in which a gas is filled in a hollow container, and a pair of mirrors 20 provided at both ends thereof for reflecting laser light.
b, 20c. Similarly, the measurement resonator 22 includes an absorption cell 22a and a pair of mirrors 22b,
2c. Incidentally, mirrors 20b and 22
An incident mirror b allows laser light to enter the cell and reflects the laser light from inside the cell. The mirrors 20c and 22c are emission mirrors that almost reflect the laser light from inside the cell and pass a part of the laser light. These mirrors are arranged perpendicular to the optical axis, and are made of, for example, a known dielectric multilayer film or a gold vapor deposition film. The surface of each mirror may be flat, but may be curved to enhance the light collecting property. Each of the absorption cells 20a and 22a is formed, for example, as a columnar or columnar glass bulb, and has an internal gas pressure of, for example, several Torr-100 Torr.

The external resonators 20 and 22 have their optical path lengths and the like set so as to resonate with the incident laser light. The incident laser light is repeatedly reflected back and forth between the two mirrors, and is absorbed by, for example, gas molecules excited by electric discharge. Therefore, high sensitivity can be obtained as in the case where a long measurement cell is configured. Further, there is an advantage that the apparatus can be downsized and the amount of gas used can be reduced. In the external resonators 20 and 22, it is necessary to observe light absorption to be observed multiple times. Therefore, the mirrors having a high reflection coefficient with respect to any of the laser beams are used for the external resonators 20 and 22. , 22 should be sufficiently reduced. When a high-reflection mirror is used, the power of the light emitted from the external resonator is reduced. However, since a CO ₂ laser generally has a higher output than other laser devices, it does not pose a problem.

In FIG. 1, the external resonator 20
The electrode for gas ionization provided at 22 is omitted in the figure. Such electrodes are supplied with power from a discharge power supply 72, which causes a discharge within the cell. Specific examples of the external resonators 20 and 22 will be described later with reference to FIG.

An optical path switch 24 is provided at a stage before the external resonators 20 and 22, and an optical path switch 26 is provided at a stage after the resonators 20 and 22 for measurement. The optical path switch 24 is
Fixed mirrors 30, 32, 34, 36 and switching mirror 38,
40. The optical path switch 26 includes fixed mirrors 42, 44, 46, 48 and switching mirrors 50, 5
And 2. When the switching mirrors 38, 40, 50, and 52 are on the A side in the figure by an interlocking mechanism (not shown), optical paths L1 and L2 are formed, and two laser beams (first beams) from the laser devices 10 and 12 are formed. Both the laser light and the second laser light) are guided to the external resonator 20 for the analyte gas. Further, two laser beams from the external resonator 20 are guided to the photodetectors 54 and 56, respectively. On the other hand, switching mirrors 38, 40,
When 50 and 52 are on the B side in the figure, the optical paths L3 and L4 are formed,
External laser 22 for the reference gas
It is led to. Further, the two laser beams from the external resonator 22 are guided to the photodetectors 54 and 56, respectively.

Each of the switching mirrors 38, 40, 50, 5
2 operates according to the synchronization signal generated by the synchronization signal generator 70. This synchronization signal is, for example, a signal of 10 Hz, and the optical path is alternately switched at such timing. The synchronization signal is also supplied to a lock-in amplifier 64 described later for synchronous detection.

The photodetectors 54 and 56 are optical sensors that output a signal of a level according to the amount of light, and their output signals are supplied to a differential amplifier via level adjusters 58 and 60 including variable resistors and the like. 62 is input. The differential amplifier 6
2 outputs a difference signal representing the difference between the two signals. The difference signal is synchronously detected by the lock-in amplifier 64 at the timing of the optical path switching (selection of the external resonator).
As a result, a signal depending on the difference in light absorption between the two isotopes is obtained. The signal is input to the signal processing circuit 66, and the isotope concentration ratio is calculated based on the magnitude of the signal. Then, the display 68 displays the density ratio as a numerical value. In this case, the presence or absence of Helicobacter pylori may be determined and quantified from the numerical value, and the result of the determination may be displayed.

When the concentration ratio is measured by the apparatus according to the present embodiment, first, the laser beams from the laser apparatuses 10 and 12 are introduced into the external resonator 22 for the reference gas,
At that time, the output of the differential amplifier 62 becomes zero,
Level adjusters 58 and 60 are adjusted. Then, two laser beams are introduced into the measurement external resonator 20 for the sample gas. In this case, in the external resonator 20 for sample gas, ¹² for different CO ₂ and ^{1 3} concentration ratio of CO ₂ concentration ratio in reference gas, results in the signal appearing at the output of the differential amplifier 62, the The concentration ratio of ¹² CO ₂ and ¹³ CO _{2 in} the sample gas can be determined based on the signal level. In addition,
In the above embodiment, two laser beams are incident on the external resonators for measurement 20 and 22 at the same time, but they may be incident separately. However, according to the simultaneous incidence, the measurement conditions can be matched, and the measurement time can be shortened.

Next, an example of an external resonator 74 suitable for the external resonators 20 and 22 shown in FIG. 1 will be described with reference to FIG.

In FIG. 2, the absorption cell 76 has a cylindrical shape. The length of the absorption cell 76 is, for example, 10 to
It is about 20 cm. Mirror plates 78 and 80 are provided at both ends of the cell 76 so as to seal both ends, and mirrors 78A and 80A are formed on one surface of the mirror plates 78 and 80 perpendicularly to the optical axis. Port 76A and 76B are formed in cell 76, and port 76A
And gas is exhausted through port 76B. A cock 82 is provided for each port 76A and 76B.
And 84 are provided. Gas introduction and discharge may be automated.

Ring-shaped electrodes 86 and 88 are provided near both ends of the absorption cell 76, and a high-frequency voltage, for example, a DC voltage, for example, a pulse voltage is applied between them for gas discharge. Any other electrode may be used as the discharge electrode. For example, a discharge coil may be provided so as to surround the entire cell 76. Further, an electrode for discharge may be provided inside the cell 76. In this example, each of the mirrors 78A and 80A constitutes an end wall of the cell 76, but such a mirror may be provided outside the cell 76.

Between the mirror plate 80 and the absorption cell 76,
In this embodiment, a piezoelectric element 90 made of PZT or the like is provided as a sweeping unit. The piezoelectric element 90 has a ring shape. When an electric signal is supplied between both surfaces, ultrasonic vibration is generated, and the piezoelectric element 90 expands and contracts. The optical path length between the mirrors 78A and 80A can be periodically swept by the expansion and contraction movement. Of course, the sweep may be performed using members other than the piezoelectric element 90. In this example, the piezoelectric element 90 is provided on the mirror 80A side, but it may be provided on the mirror 78A side.

The frequency of the signal supplied to the piezoelectric element 90 is
The switching frequency of the optical path switching devices 24 and 26 (for example, 10
Hz), it is desirable to set the frequency sufficiently higher than that and not to be in an integral multiple of the switching frequency. For example, a frequency that is not an integral multiple of 10 Hz near 20 KHz is set.

According to such a sweeping means, since the resonance frequency of the external resonator can be repeatedly swept, the frequency can be repeatedly tuned to the two laser beams. Therefore, even if the optical path length between the mirrors is deviated from the resonance length of the laser light, it is possible to repeatedly and automatically obtain a tuning result at the time of sweeping, which eliminates the need for extremely strict processing and assembly of the resonator. There are advantages. Incidentally, a similar optical path length sweeping means can be incorporated in the laser device.

Next, the operation of the apparatus of this embodiment will be described with reference to FIGS. In FIG. 3A, the switching mirrors 38, 40, 50, and 52 shown in FIG. 1 are switched to the A side, whereby the laser devices 10 and 12 are switched.
2 shows a state in which the two laser beams from are both introduced into the external resonator 20 for the analyte gas. In this state, the two laser beams are detected by the two photodetectors 54 and 56 via the external resonator 20. In FIG. 3B, the switching mirrors 38, 40, 50, and 52 shown in FIG. 1 are switched to the B side, whereby the two laser beams from the laser devices 10 and 12 are both coupled to the external resonance for the reference gas. The state in which it is introduced into the vessel 22 is shown. In this state, the two laser beams are detected by the two photodetectors 54 and 56 via the external resonator 22.

As described above, first, in the state shown in FIG. 3B, the differential amplifier 62 using the level adjusters 58 and 60 is used.
The sensitivity is adjusted so that the output becomes zero. Thereafter, the switching mirrors 38, 40, 50, and 52 are alternately switched at the cycle of the synchronization signal.

FIG. 4 shows the waveform of each signal at the time of the switching. FIG. 4 does not show the fluctuation due to the sweep by the piezoelectric element 90, and shows the envelope of the signal. Here, FIG.
(B) shows the output of the detector 56, (C) shows the output of the differential amplifier 62,
(D) shows the output of the lock-in amplifier 64.

As shown in the figure, when the switching mirror is switched to the B side, that is, when both laser lights are introduced into the external resonator 22 for the reference gas, the detector 54
And the output of the detector 56 are basically the same. However, when the switching mirror is switched to the A side and the two laser beams are introduced into the external resonator 20 for the sample gas, the concentration ratio between ¹² CO ₂ and ¹³ CO ₂ of the sample gas becomes the reference gas. (That is, if the concentration of ¹³ CO ₂ is increased), the output level of the differential amplifier 62 is increased in accordance with the difference. The output signal of the differential amplifier 62 is controlled by the lock-in amplifier 64 at the switching frequency of the switching mirror.
And the lock-in amplifier 6
The output of 4 corresponds to the increase in the concentration of ¹³ CO ₂ . That is, based on the concentration ratio between ¹² CO ₂ and ¹³ CO _{2 in} the reference gas, from the output level of the lock-in amplifier 64,
The concentration ratio between ¹² CO ₂ and ¹³ CO _{2 in} the sample gas can be converted.
Consequently, the presence or absence of Helicobacter pylori present on the stomach wall of the patient can be determined and quantified from the concentration ratio.

Incidentally, in order to reduce errors due to thermal characteristic fluctuations of the laser devices 10 and 12 and the external resonators 20 and 22, they are integrated, and they are further cooled by a common cooling system. It is desirable to do.

In the above embodiment, the sensitivity is adjusted so that the output of the differential amplifier 62 becomes zero at the time of measuring the reference gas.
The concentration ratio of the isotope of the sample gas can be calculated using the ratio of the signals.

[0052]

As described above, according to the present invention,
The analysis and measurement of the gas to be measured can be performed at high sensitivity and at low cost by utilizing the absorption of laser light. Also, when performing breath analysis, sufficient sensitivity can be obtained with a small amount of breath.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of an isotope concentration ratio measuring device according to the present invention.

FIG. 2 is a diagram showing a specific example of an external resonator.

FIG. 3 is a diagram showing the operation of the isotope concentration ratio measuring device according to the present invention.

FIG. 4 is a diagram showing waveforms of respective signals.

[Explanation of symbols]

10 ¹³ CO ₂ laser device, 12 ¹² CO ₂ laser device, 14,16 laser resonator (internal resonator), 20
External resonator for analyte gas, 22 External resonator for reference gas, 24, 26 Optical path switch, 54, 56 Photodetector, 5
8,60 level adjuster, 62 differential amplifier, 64 lock-in amplifier, 70 synchronization signal generator, 72 power supply for discharge.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Junichi Kawanabe 6-22-1, Mure, Mitaka-shi, Tokyo Aloka Co., Ltd. (56) References JP-A-8-261922 (JP, A) JP-A JP-A-11-94737 (JP, A) JP-A-11-94739 (JP, A) JP-A-11-94739 (JP, A) JP-A-11-94739 (JP, A) A) JP-A-7-294418 (JP, A) JP-A-52-114390 (JP, A) JP-A-52-100276 (JP, A) JP-A-52-98578 (JP, A) (58) Survey Field (Int.Cl. ⁷ , DB name) G01N 21/00-21/61 WPI / L (QUESTEL) Practical file (PATOLIS) Patent file (PATOLIS)

Claims (11)

(57) [Claims] 1. A laser resonator for outputting a laser beam, an absorption cell into which a gas to be measured is introduced, and a pair of mirrors provided at both ends of the absorption cell. A gas measuring apparatus, comprising: a detector; a light absorption detecting means for detecting absorption of laser light by the gas to be measured; and a discharging means for performing gas discharge in the absorption cell . 2. The gas measuring apparatus according to claim 1, further comprising an optical path length sweeping means for periodically varying an optical path length between a pair of mirrors in the measurement resonator. 3. A gas measuring apparatus according to claim 2, wherein said optical path length sweeping means is a piezoelectric element. 4. A device according to claim 1, wherein the discharge means is a gas measuring device which comprises an electrode provided on the absorption cell, means for applying a voltage to the electrode. 5. The gas measuring apparatus according to claim 1, wherein said light absorption detecting means includes a light detector for detecting a laser beam emitted from said measuring resonator. 6. A first laser oscillator for outputting a first laser beam having a wavelength corresponding to a first isotope, and a second laser beam having a wavelength corresponding to a second isotope. A second laser oscillator for outputting, an absorption cell into which a sample gas is put, and a pair of mirrors provided at both ends of the absorption cell. The first and second laser beams incident simultaneously or separately are resonated. A first measuring resonator to be caused, an absorption cell into which a reference gas is put, and a pair of mirrors provided at both ends of the first and second laser beams. A second measurement resonator that resonates; a light absorption detection unit that detects absorption of each laser beam by the sample gas and the reference gas; and a light absorption detection unit that detects absorption of each laser beam by the sample gas and the reference gas. Same Isotope concentration ratio measuring apparatus characterized by comprising a concentration ratio calculating means for performing calculation of body density ratio, the. 7. The apparatus according to claim 6, wherein the light absorption detecting means includes first and second photodetectors provided corresponding to the first laser light and the second laser light. And the first emitted from the first and second laser oscillators
And a first optical path switch that guides the second laser beam to the first or second measurement resonator; and a first and second laser emitted from the first or second measurement resonator. And a second optical path switch that guides light to the first and second photodetectors. 8. The apparatus according to claim 7, further comprising a difference calculator for calculating a difference between the output signals from the first and second photodetectors, wherein the density ratio calculator is configured to operate the difference calculator. An isotope concentration ratio measuring device for calculating a concentration ratio based on an output. 9. The device according to claim 8, further comprising a lock-in amplifier that performs synchronous detection on an output of the difference calculator according to a switching cycle of the first and second optical path switches. Isotope concentration ratio measuring device. 10. The isotope concentration ratio measuring apparatus according to claim 6, wherein the first isotope is ¹³ CO ₂ and the second isotope is ¹² CO ₂ . 11. The apparatus according to claim 6, wherein the sample gas is exhaled air.