JP2686698B2 - Isotope ratio analysis method and device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for irradiating a sample with light and analyzing an isotope from its light absorption intensity, and to a method and apparatus for correcting an isotope ratio measurement error due to a temperature change of a sample gas. Is.

[0002]

2. Description of the Related Art Tracers using isotopes are used for diagnosis of diseases in the medical field, and for photosynthesis in the agricultural field.
It is used for studying the metabolism of plants and for tracing ecosystems in the earth science field.

Isotopes used for such purposes include
There are nitrogen, carbon, hydrogen, etc. Particularly in carbon, the mass number of carbon is 12 (hereinafter simply abbreviated as ¹² C) and the mass number of carbon is 13
There is a stable isotope (hereinafter simply referred to as ¹³ C), and this stable isotope is not exposed to radiation like a radioactive isotope, is easy to handle, and its use in the medical field is actively studied. Carbon isotope analysis (sample gas is CO ₂ ) will be described below as a typical example of isotope analysis.

Conventionally, as a carbon isotope ratio analyzer for such an application, a semiconductor laser having an extremely narrow emission spectrum width was used as a wavelength tunable light source, and the laser beam was irradiated to a sample to obtain ¹² CO ₂ and ¹³ CO ₂ light absorption line is measured optical absorption lines not interfere with each other, there is the trace ¹² CO ₂ and than the light absorption intensity of ¹³ CO ₂ ¹² CO ₂ and a change in the abundance of ¹³ CO ₂ with high accuracy. The semiconductor laser easily becomes a wavelength tunable light source by precisely controlling the temperature and drive current of the semiconductor laser.

A schematic block diagram of a conventional carbon isotope ratio analyzer is shown in FIG. 1 is a semiconductor laser, 2 is a sample cell,
Reference numeral 3 is a sample gas inlet, 4 is a sample gas outlet, 5 is a photodetector, 6 is a semiconductor laser temperature control unit, and 7 is a semiconductor laser current control unit.

The semiconductor laser 1 is temperature-reduced so as to retract the vicinity of the light absorption lines of ¹² CO ₂ and ¹³ CO ₂ selected by the temperature control unit 6. Further, the current controller 7 provides an appropriate light output. The laser light from the semiconductor laser 1 thus wavelength-reduced is incident on the sample cell 2.
CO ₂ gas for the purpose of isotope measurement is introduced into the sample cell 2 through the sample gas inlet 3. The laser light incident on the sample cell 2 interacts with the CO ₂ gas and is absorbed. Light emitted from the sample cell 2 is detected by the photodetector 5. By such means, ¹² CO ₂ and ¹³ C as shown in FIG.
The optical absorption line of O ₂ is measured. The light absorption intensity is obtained from the measured absorption line, and the isotope ratio is obtained from the ratio of the two absorption intensities.
The absorption intensity is obtained by the formula lnI / I ₀ from the light intensity I when the absorption is maximum from the absorption line as shown in FIG. 2 and the light intensity I ₀ when there is no absorption at that wavelength. If the absorption is weak, I ₀ -I may be used. Further, the absorption amount (oblique line portion in FIG. 4) may be obtained from the absorption line and used as the absorption intensity. After the measurement, the sample gas is discharged from the sample gas outlet 4.

In the present example, the photodetector 5 detects the transmitted light of the sample cell 2 to obtain the absorption line. However, the semiconductor laser 1 is frequency-modulated by modulating the current of the current control section 7 or the like. By detecting the output signal of the photodetector 5 synchronously with the modulation signal using the lock-in amplifier, the absorption line can be detected with higher sensitivity.

As described above, since both absorption lines of ¹² CO ₂ and ¹³ CO ₂ which do not interfere with each other are selected, and a compact and highly reliable semiconductor laser is used, the isotope ratio can be accurately adjusted. It is a carbon isotope analyzer that can be measured and has high reliability.

[0009]

However, in the conventional carbon isotope ratio analyzer, it was found that the measurement error of the isotope ratio increases when the temperature of the sample gas in the sample cell 12 fluctuates. This is because the absorption intensity changes depending on the temperature variation of the sample gas, but the temperature dependence of the absorption intensity of each of ¹² CO ₂ and ¹³ CO ₂ is different. For applications in the fields mentioned above. The temperature of CO ₂ usually changes depending on the measurement environment, and the measurement error of the isotope ratio increases.

An object of the present invention is to provide an isotope ratio analysis method and apparatus capable of correcting the temperature fluctuation of the measured isotope ratio and measuring and analyzing the isotope ratio with high accuracy.

[0011]

According to the present invention, in the isotope ratio analysis method for analyzing the isotope ratio by the optical absorption intensity of the isotope in the sample gas, the absorption at the first isotope in the sample gas a first temperature change rate of the absorption intensity of the line, the intake
The first isotope in the lower level during the rotational transition of the convergence line
The second temperature change rate of the absorption intensity of the absorption line of the second isotope in the sample gas was found from the calculated value of the change rate of the number of daughters and was present in the lower level during the rotational transition of this absorption line. Second
From the calculated value of the rate of change of the number of isotope molecules , the light absorption intensities of the first and second isotopes in the sample gas are measured to obtain the first and second light absorption intensity measurement values. First of
And at the same time as obtaining the second light absorption intensity measurement value, the temperature of the sample gas is detected to obtain the sample gas temperature measurement value, and based on the sample gas temperature measurement value and the first and second temperature change rates. First and second variation values are obtained, and the first and second variation values are calculated based on these first and second variation values.
An isotope ratio analysis method is provided, which is characterized by correcting the measured light absorption intensity of

Further, according to the present invention, a light source capable of changing the wavelength within a certain range, a sample cell containing a sample gas for receiving a light beam from the light source, and a sample gas in the sample cell are provided. A photodetector that detects the intensity of the light beam from the light source that has passed through, a temperature detection unit that detects the temperature in the sample gas in the sample cell, and an absorption line absorption of the first isotope in the sample gas. The first rate of temperature change of intensity exists in the lower level during the rotational transition of this absorption line
First calculated from the calculated change rate of the number of isotope molecules
And temperature change rate computing means, the second rate of temperature change in the absorption intensity of absorption lines in a second isotope in the sample gas, the absorption
The second isotope in the lower level during the rotational transition of the convergence line
The second temperature change rate calculating means for obtaining the calculated change rate of the number of children and the light absorption intensities of the first and second isotopes of the sample gas are measured based on the output from the photodetector. First
And a light absorption intensity measuring means for obtaining a second light absorption intensity measurement value, a sample gas temperature measurement value from the temperature detecting means, a first temperature change rate from the first temperature change rate calculating means, and the first temperature change rate. Fluctuation value calculation means for obtaining first and second fluctuation values based on the second temperature change rate from the second temperature change rate calculation means, and first and second fluctuation value calculation means from the fluctuation value calculation means.
From the light absorption intensity measuring means based on the variation value of
And light absorption intensity correction calculation means for correcting the second and second light absorption intensity measurement values to obtain the first and second light absorption intensity correction values, and the first and second light absorption intensity correction calculation means.
And an isotope ratio calculating means for obtaining an isotope ratio based on the light absorption intensity correction value of 1.

[0013]

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

FIG. 1 is a block diagram showing an embodiment of the present invention. In the embodiment shown in FIG. 1, the same components as those of the isotope ratio analyzer shown in FIG. 3 are designated by the same reference numerals.

A temperature sensor 8 for detecting the temperature of the sample gas is provided inside the sample cell 2. The temperature measurement value of the temperature sensor 8 is given to the light absorption intensity correction device 9. The light absorption intensity correction device 9 is provided with the outputs of the first temperature change rate calculation device 10 and the second temperature change rate calculation device 11. The first temperature change rate calculation device 10 is
The first temperature change rate of the absorption intensity of the absorption line of the first isotope in the sample cell 2 is obtained and given to the optical absorption intensity correction device 9. The second temperature change rate calculation device 11 obtains the second temperature change rate of the absorption intensity of the absorption line of the second isotope in the sample cell 2 and supplies it to the light absorption intensity correction device 9.

The output of the photodetector 5 is given to the optical absorption intensity measuring device 12. This light absorption intensity measuring device 12 is
Based on the output from the photodetector 5, the light absorption intensity measurement values of the first and second isotopes in the sample gas are obtained and given to the light absorption intensity correction device 9.

The light absorption intensity correction device 9 is based on the temperature measurement value from the temperature sensor 8 and the first and second temperature change rates from the first and second temperature change rate calculation devices 10 and 11. To obtain the first and second fluctuation values, and then correct the first and second light absorption intensity measurement values from the light absorption intensity measuring device 12 based on these first and second fluctuation values. The first and second light absorption intensity correction values are calculated and given to the isotope ratio calculation device 13. This isotope ratio calculation device 13
Calculates the isotope ratio based on the first and second light absorption intensity correction values.

Next, a specific example of the carbon isotope ratio analyzer will be described.

Light is absorbed when a molecule at a lower energy level (hereinafter simply referred to as a lower level) absorbs light and transitions to an upper energy level (hereinafter simply referred to as an upper level). Occurs, the absorption intensity changes as the number of molecules in the lower level changes. The number of molecules existing in this lower level changes depending on the temperature, and its temperature dependence differs depending on the energy level of the lower level of the absorption line to be measured.

The spectral structure of the absorption line measured is CO
_{This is} a vibration / rotational transition spectrum of _two molecules. (Example ¹² C
Vibration and rotation transition of O ₂ spectrum 00001 → 300
12, R (46), vibrational and rotational transition of ¹³ CO ₂ spectrum 00001 → 30011, R (16)) Usually, the lower level of vibrational transition in both absorption lines used for isotope ratio measurement is
Since it is the ground level (00001 in the example), it is not necessary to consider the temperature dependence of the vibrational transition, but it is necessary to consider the temperature dependence because the lower level of the rotational transition is different.

Rate of change in the number of molecules in the lower level of rotational transition ΔNr
/ Nr can be approximated by the following equation (1).

ΔNr / Nr = ΔT · (Q / T−1) ‥‥‥‥‥‥‥‥‥‥‥‥‥ (1) Where, Q = (B ・ J (J + 1) −D ・ J ² (J + 1) )
² ) · h · C / k. However, Nr is the number of CO ₂ molecules existing in the lower level of rotational transition when the temperature of the CO ₂ molecule is the reference temperature T. △ Nr is the temperature of CO ₂ molecule is △
The number of changes in the CO ₂ molecule existing in the lower level when T changed. T
Is the reference temperature (absolute temperature), ΔT is the temperature change from T, B
Rotation constants of CO ₂ molecules, D is distortion constant of CO ₂ molecules, J
Is the rotational quantum number of the CO ₂ molecule, h is Planck's constant, C is the speed of light, and k is the Boltzmann's constant.

The present invention makes good use of this relationship for temperature correction of the measured isotope ratio.

In the embodiment of the present invention, the first temperature change rate calculation device 10 calculates the change rate of the number of molecules of ¹² CO ₂ existing in the lower level to obtain the change rate of the absorption intensity of ¹² CO _2. The second temperature change rate calculation device 11 calculates the change rate of the number of molecules of ¹³ CO ₂ existing in the lower level to obtain the change rate of ¹³ CO ₂ absorption intensity. The light absorption intensity correction device 9 and the absorption intensity ratio detection circuit 9a for detecting the absorption intensity ratio of the ¹³ CO ^_2/12 CO ₂ as shown in FIG. 2, the first
And an adder circuit 9b for adding the output values 10a, 11a from the second temperature change rate computing devices 10, 11, and an output value from the adder circuit 9b and an output value from the absorption intensity ratio detection circuit 9a. It has a multiplication circuit 9c.

In this example, the carbon isotope ratio is determined as follows.

(1) ¹² CO ₂ of the sample gas in the sample cell ₂
And the absorption line of ¹³ CO ₂ are measured, and at the same time, the temperature of the sample gas in the sample cell 2 is measured.

(2) The absorption intensity ratio detection circuit 9a uses ¹² CO ₂
Then, both absorption intensities are obtained from the absorption lines of ¹³ CO ₂ and ¹³ and the ratio of both intensities is obtained.

(3) In the first temperature change rate computing device 10, the number of molecules of ¹² CO ₂ changes according to the above equation (1) from the measured temperature value of the sample gas and the lower energy level. The rate is calculated and used as the rate of change in the absorption intensity of ¹² CO ₂ .
Similarly, in the second temperature change rate computing device 11, the rate of change of the number of molecules of ¹³ CO ₂ is calculated from the measured temperature value and the energy level of the lower level based on the equation (1) to absorb ¹³ CO ₂ . The rate of change in strength.

(4) The first and second temperature change rate calculation devices 10 and 11 may be calculated using a microcomputer, or may be calculated in advance by a computer and stored in a ROM.
It may be stored in (Read Only Memory). Further, although the rate of change is calculated here, the rate of temperature change of the absorption intensity of the vibration / rotational transition spectra of ¹² CO ₂ and ¹³ CO ₂ selected in advance may be experimentally measured, and the value may be used. . (The temperature change rate of the absorption intensity of ¹² CO ₂ is ^{12 at} each temperature by changing the temperature of the sample gas near the reference temperature T.
The absorption intensity of the CO ₂ absorption line is measured, and the temperature change rate based on the absorption intensity at temperature T is determined. The temperature change rate of the absorption intensity of ¹³ CO ₂ is also the same as that of ¹² CO ₂ , the temperature of the sample gas is changed, and the absorption intensity of the absorption line of ¹³ CO _{2 at} each temperature is measured to change the temperature based on the absorption intensity of temperature T. (5) Addition of the change rates obtained by the first and second temperature change rate calculation devices 10 and 11 in the adder circuit 9b to obtain a total change rate.

(6) In the multiplication circuit 9c, the isotope ratio obtained by the absorption intensity ratio detection circuit 9a is multiplied by the change rate of the absorption intensity obtained by the addition circuit 9b to reduce the error in the measured isotope ratio due to temperature.

The absorption intensity ratio may be obtained from the absorption line detected by using the lock-in amplifier described in the description of the conventional carbon isotope analyzer. Further, although the correction method and apparatus for the analysis of carbon isotopes are described here, the present method and apparatus can also be applied to the correction for the analysis of isotopes of carbon, nitrogen, hydrogen and the like.

[0032]

INDUSTRIAL APPLICABILITY Since the present invention can correct the temperature fluctuation of the measured isotope ratio, the isotope ratio can be measured and analyzed with high accuracy.

[Brief description of the drawings]

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

FIG. 2 is a block diagram showing light absorption intensity correction leaving in one embodiment of the present invention.

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

FIG. 4 is a diagram for explaining light absorption intensity in the present invention and the conventional isotope ratio analyzer.

[Explanation of symbols]

 1 semiconductor laser 2 sample cell 5 photodetector 6 temperature control unit 7 current control unit 8 temperature sensor 9 light absorption intensity correction device 10 first temperature change rate calculation device 11 second temperature change rate calculation device 12 light absorption intensity measurement Device 13 Isotope ratio calculator

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-61-11634 (JP, A) JP-A-53-42889 (JP, A) JP-A-51-88291 (JP, A) JP-A-61- 97552 (JP, A) Japanese Patent Application Laid-Open No. 1-501568 (JP, A) Acta Physica Acad emiae Scientific Hungaricae, Vol. 48, No. 1 (1980) p93-102

Claims (2)

(57) [Claims] 1. A method of analyzing an isotope ratio by analyzing an optical absorption intensity of an isotope in a sample gas , wherein a first temperature of an absorption intensity of an absorption line of a first isotope in the sample gas is used . The rate of change can be calculated by the rotational transition of this absorption line.
Calculated rate of change of the number of first isotope molecules existing in the lower level
From obtains, the second temperature change rate of the absorption intensity of absorption lines in a second isotope of the sample gas, at the time of rotational transitions of the absorption line
Calculated rate of change of the number of second isotope molecules existing in the lower level
From seeking to obtain the first and second light absorption intensity measurements by measuring the light absorption intensity of the first and second isotope in the sample gas, the first and second light absorption intensity measurement thereof At the same time as obtaining the value, the temperature of the sample gas is detected to obtain the measured value of the sample gas temperature, and the first and second fluctuation values are obtained based on the measured value of the sample gas temperature and the first and second temperature change rates. And an isotope ratio analysis method which corrects the first and second measured values of optical absorption intensity by the first and second fluctuation values. 2. A light source capable of changing a wavelength within a fixed range, a sample cell containing a sample gas for receiving a light beam from the light source, and a light source which has passed through the sample gas in the sample cell. of a photodetector for detecting the intensity of light, temperature detecting means for detecting the temperature of the sample gas in the sample cell, the first absorption intensity of absorption lines in the first isotope in the sample gas The rate of temperature change is defined as the lower limit of the rotational transition of this absorption line.
Calculated value of the rate of change of the number of the first isotope molecule present in
Et obtains a first temperature change rate calculating means, the second rate of temperature change in the absorption intensity of absorption lines in a second isotope in the sample gas, the lower level during the rotational transitions of the absorption line
The calculated rate of change in the number of second isotope molecules present in
Et obtains a second temperature change rate computing means, said photodetector first and second measuring the light absorption intensity of the first and second isotopes of the sample gas based on the output from the
The light absorption intensity measurement means for obtaining the light absorption intensity measurement value of the sample gas temperature measurement value from the temperature detection means and the first
Change value calculating means for obtaining first and second change values based on the first temperature change rate from the second temperature change rate calculating means and the second temperature change rate from the second temperature change rate calculating means. And the first and second light absorption intensity measurement values from the light absorption intensity measurement means are corrected based on the first and second variation values from the variation value calculation means. The light absorption intensity correction calculation means for obtaining the absorption intensity correction value, and the isotope ratio calculation means for obtaining the isotope ratio based on the first and second light absorption intensity correction values from the light absorption intensity correction calculation means. An isotope ratio calculation device characterized by: