JPH06129983A - Method for correcting isotope ratio and device for correcting isotope ratio - Google Patents

Abstract

(57) [Summary] [Purpose] The isotope ratio can be measured with high accuracy in a wide pressure range. [Configuration] Absorption intensity detection circuits 8a and 8b for absorption lines of a large amount and a small amount of isotopes, a means for obtaining a pressure width parameter 9a of a large amount isotope and a pressure width parameter 9b of a minute isotope, and a sample gas pressure 10 An absorption coefficient calculation circuit 11 for obtaining each absorption coefficient at the gas pressure using the measuring means and the pressure parameters 9a and 9b and the gas pressure 10.
a, 11b, and division circuits 12a, 12b for dividing the absorption intensity of each isotope by each absorption coefficient.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Isotope tracers are used in the medical field for diagnostic applications of diseases, in the agricultural field for photosynthetic research and in the metabolism of plants, and in the earth science field for tracing ecosystems.

Isotopes used for such purposes include, for example, nitrogen, carbon, hydrogen and the like. In particular, in carbon, there are stable isotopes having a carbon mass number of 12 (hereinafter simply abbreviated as ¹² C) and a carbon mass number of 13 (hereinafter simply abbreviated as ¹³ C), and the stable isotopes are radioactive isotopes. Since it is not exposed to radiation and is easy to handle, its use is actively studied in the medical field.

Carbon isotope analysis (sample gas is CO ₂ ) will be described below as a typical example of isotope analysis.

Conventionally, as a carbon isotope 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 sample was irradiated with the laser light to obtain ¹² CO ₂ and ¹³ CO ₂ . There is a method of measuring a light absorption line of which the light absorption lines do not interfere with each other and tracing the change in the ratio of ¹² CO ₂ and ¹³ CO ₂ with high accuracy.

The semiconductor laser can easily be a wavelength tunable light source by precisely controlling the temperature and driving current of the semiconductor laser. Further, AlGaAs and InGaAsP semiconductor lasers have been vigorously researched and developed for optical communication and optical information processing in recent years, and have become compact, highly efficient and highly reliable.
If a semiconductor laser having such practically excellent characteristics is used, the entire device can be made extremely small, easy to handle and highly reliable.

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

The semiconductor laser 1 is temperature-swept so as to sweep in the vicinity of the light absorption lines of ¹² CO ₂ and ¹³ CO ₂ selected by the temperature control section 6 of the semiconductor laser. Further, the current controller 7 provides an appropriate light output. The laser light from the semiconductor laser 1 thus wavelength-swept 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, and the laser light incident on the sample cell 2 interacts with the introduced CO ₂ gas. Be absorbed. The light emitted from the sample cell 2 is detected by the photodetector 5.

The light absorption lines of ¹² CO ₂ and ¹³ CO ₂ are measured by such a procedure, the light absorption intensity is obtained from the measured absorption line, and the isotope ratio is obtained from the ratio of both absorption intensities.

The absorption intensity is determined by the formula -ln I / I ₀ from the light intensity I when the absorption is maximum from the absorption line as shown in FIG. 3 and the light intensity I ₀ when there is no absorption at that wavelength.

When the absorption is weak, it may be approximated to I ₀ -I. After the measurement, the sample gas is discharged from the sample gas discharge port 4.

In this example, the photodetector 5 detects the transmitted light of the sample cell 2 to obtain the absorption line. However, the current of the current control section 7 is modulated, and the semiconductor laser 1 is frequency-modulated. 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 the absorption lines of ¹² CO ₂ and ¹³ CO ₂ do not interfere with each other, and a compact and highly reliable semiconductor laser is used, the isotope ratio is high. It is a highly reliable carbon isotope analyzer that can measure with high accuracy.

In such a carbon isotope analyzer, it is usually ¹²
In order to prevent the absorption lines of CO ₂ and ¹³ CO _{2 from} interfering with each other as much as possible and not to be influenced by the absorption of other gases, only CO ₂ of the collected gas is extracted and then introduced into the sample cell 2. Then, the pressure of the CO ₂ gas in the sample cell 2 is set to a low pressure (50 Torr
The absorption line is measured under the condition.

[0015]

However, it has been found that in such a measuring method, the measurement error increases when the pressure of the sample gas in the sample cell 2 fluctuates. This is because the absorption line width changes due to the pressure fluctuation of the sample gas, and as a result, the peak value of the absorption line (I in Fig. 3) changes, but the way it spreads is ¹²
This is because measurement errors increase because CO ₂ and ¹³ CO ₂ are different from each other.

An example of the measurement is shown in FIG. FIG. 4 is a graph in which the variation rate of the measured isotope ratio at each pressure is plotted while changing the natural abundance ratio CO ₂ pressure from 8 Torr to 46 Torr. However, the measurement value at a sample gas pressure of 46 Torr is used as a reference. In this way, the isotope ratio fluctuates greatly depending on the pressure. In the applications in each field as described above, the CO ₂ pressure usually changes greatly depending on the measurement conditions, and the measurement error of the isotope ratio increases.

In view of the above circumstances, it is an object of the present invention to provide an isotope ratio correction method and a correction device capable of accurately measuring an isotope ratio in a wide pressure range.

[0018]

A method for correcting an isotope ratio according to the present invention which achieves the above-mentioned object, when analyzing the isotope ratio from the optical absorption intensity of the isotope, is a pressure width parameter of an absorption line of a trace isotope. As the pressure range parameter of the trace isotope with the collision partner molecule as the major isotope, and the pressure range parameter of the multiple isotope with the collision partner molecule as the major isotope as the pressure range parameter of the absorption line of the major isotope, The pressure of the sample gas is measured at the same time as the absorption line measurement of the isotope, the measured pressure and the absorption intensity of both absorption lines measured from both pressure width parameters are corrected, and the isotope ratio that increases due to the pressure fluctuation of the sample gas is measured. It is characterized by reducing the error.

One of the isotope ratio correction devices is an absorption intensity detection circuit for each absorption line of a large amount and a small amount of isotopes, a means for obtaining a pressure width parameter of a large amount isotope and a pressure width parameter of a minute isotope, A means for measuring the sample gas pressure, an absorption coefficient calculation circuit for obtaining each absorption coefficient at the gas pressure using each pressure width parameter and gas pressure, and the absorption intensity of each isotope is divided by each absorption coefficient. It is characterized by having a division circuit.

[0020]

[Function] When analyzing the isotope ratio from the optical absorption intensity of the isotope, the pressure dependence of the absorption line widths of ¹² CO ₂ and ¹³ CO ₂ is obtained in advance, and the absorption lines of ¹² CO ₂ and ¹³ CO ₂ are obtained. At the time of measurement, the pressure of CO ₂ gas is measured to correct the pressure fluctuation of the isotope ratio, and the isotope ratio is measured with high accuracy.

The contents of the present invention will be described in detail below.

The light absorption intensity of the sample gas, where the optical path length of the laser light passing through the sample cell 2 is L and the pressure of the sample gas is P, is expressed by the equation (1).

[0023]

-Ln I / I ₀ = α ₀ · L · P (1) α ₀ is the absorption coefficient per unit optical path length and unit pressure.
Further, the relationship between the absorption coefficient α ₀ and the absorption line width can be approximated by the equation (2).

[0024]

[Equation 2] C ₀ is a coefficient depending on the absorption line of each vibration / rotation mode, Δ
Is the absorption line Doppler width (half-width at half maximum), Γ is the absorption line collision width (half-width at half maximum), and a = (ln 2) ^1/2 · Γ / Δ.

C ₀ and Δ have no pressure dependence, but the collision width Γ
Is pressure dependent. The dependence depends on the vibration / rotation mode of each absorption line and the molecule that absorbs light and transits to the upper level and the collision partner molecule. If the constant indicating the pressure dependence of the collision width Γ (hereinafter simply referred to as the pressure width parameter) is γ, the collision width Γ is

[Expression 3] Γ = γ × P (3)

Since the absorption intensity and the pressure have such a relationship, for example, when the pressure width parameter γ of ¹² CO ₂ and ¹³ CO ₂ is different, the collision width Γ is different, and as a result, the pressure dependence of the absorption coefficient α ₀ is different. , The pressure dependence of absorption intensity is different.

The present invention utilizes such a relationship between pressure and absorption intensity to correct fluctuations in the measured isotope ratio due to pressure fluctuations. Both pressure width parameters used here are not easily found because they depend on the composition of the sample gas, and therefore both pressure width parameters must be well approximated to easily realize the present invention.

[0028]

FIG. 1 shows an embodiment of the present invention. In Figure 1, 8a is proportional gas having the higher natural abundance (e.g., ¹²
Absorption intensity detection circuit for absorption line of CO ₂ (hereinafter, referred to as major isotope), 8b is absorption intensity of absorption line for isotope gas with lower natural abundance (eg, ¹³ CO ₂ , hereinafter referred to as trace isotope) The detection circuit, 9a is the pressure width parameter of the major isotope (the pressure width parameter of the major isotope whose collision partner molecule is the major isotope at the selected absorption line), and 9b is the pressure width parameter of the minor isotope (selected. The pressure width parameter of the trace isotope whose collision partner molecule is a major isotope in the absorption line), 10 is the pressure of the sample gas in the cell, 11a is the pressure width parameter 9
an absorption coefficient calculation circuit for obtaining an absorption coefficient at a pressure of the sample gas of the multi-isotope using a and the pressure 10 of the sample gas,
Reference numeral 11b is an absorption coefficient calculation circuit for obtaining the absorption coefficient at the pressure of the trace isotope by using the pressure width parameter 9b and the pressure 10 of the sample gas, and 12a is the absorption intensity of the large isotope detected at 8a, which is obtained at 11a. A division circuit for dividing by a coefficient, and 12b shows a division circuit for dividing the absorption intensity of the trace isotope detected in 8b by the absorption coefficient obtained in 11b.

In principle, the pressure range parameter 9a must consider a trace isotope as a collision partner molecule,
In the present invention, the collision partner molecule is approximated by using only multiple isotopes. Similarly, the pressure range parameter 9b is approximated to the collision partner molecule with only a large number of isotopes.

For the pressure width parameter 9a, first, the absorption line width is measured by inserting a high pressure isotope into the sample cell at a low pressure so that the collision width is negligible. The line width is measured, and the increased line width for each unit pressure increase is obtained and used as the pressure width parameter. For the pressure width parameter 9b, first, the absorption line width is measured by inserting a trace amount of isotope into the sample cell at a low pressure so that the collision width is negligible, and thereafter, the pressure of several isotopes is increased to increase the absorption line width. Is measured and the increasing line width for each unit pressure increase is obtained and used as the pressure width parameter.

The operation of this structure will be described below.

1) At the same time as measuring the absorption lines of ¹² CO ₂ and ¹³ CO _{2 in} the sample cell 2, the pressure 1 of the sample gas in the sample cell 2
Measure 0.

2) The absorption intensity detection circuits 8a and 8b determine both absorption intensities from the absorption lines of the major and minor isotopes.

3) The absorption coefficient calculation circuit 11a uses the measured sample gas pressure 10 and the pressure width parameter 9a to calculate the absorption coefficient of the major isotope by the equations (2) and (3). Similarly, in the absorption coefficient calculation circuit 11b, the sample gas pressure 10
And the pressure width parameter 9b are used to obtain the absorption coefficient of the trace isotope by the equations (2) and (3). However, the Doppler width and pressure width parameters of the major and minor isotopes are obtained in advance.

The absorption coefficient calculation circuits 11a and 11b may be calculated by using a logic circuit or a microcomputer, or may be calculated in advance by a computer and read by a ROM (Read Only).
Memory) may be stored.

4) In the division circuit 12a, the absorption intensity obtained by the absorption intensity detection circuit 8a is divided by the absorption coefficient value obtained at 11a to correct the change in the absorption intensity due to the pressure of the major isotope. Similarly, in the division circuit 12b, the absorption intensity obtained by the absorption intensity detection circuit 8b is divided by the absorption coefficient value obtained at 11b to correct the change in the absorption intensity due to the pressure of the trace isotope.

As described above, the isotope ratio can be obtained by comparing the corrected absorption intensities of the trace isotopes and the corrected absorptions of the large isotopes.

The integral term of the above equation (2) is a series expansion equation.

[Equation 4] May be solved using.

The absorption line 8 detected by using the lock-in amplifier described in the explanation of the conventional carbon isotope analyzer.
The absorption intensity may be determined by a and 8b. Further, although the method and apparatus for correction of analysis of carbon isotopes are described here, the present method and apparatus can be applied to correction of analysis of other isotopes.

[0040]

As described above, ¹² CO ₂ and ¹³ CO ₂
At the same time as measuring the absorption line with and measuring the CO ₂ gas pressure,
^Since the isotope ratio is calculated by correcting the absorption intensity of ¹² CO ₂ and ¹³ CO ₂ , the isotope ratio can be measured with high accuracy in a wide pressure range. Further, due to such characteristics, it can be applied to applications in many fields.

[Brief description of drawings]

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

FIG. 2 is a schematic block diagram showing a conventional CO ₂ isotope analyzer.

FIG. 3 is a graph showing an example of CO ₂ absorption line measurement.

FIG. 4 is a graph showing an example of isotope ratio measurement at each pressure.

[Explanation of symbols]

1 semiconductor laser 2 sample cell 3 sample gas inlet 4 sample gas outlet 5 photodetector 6 semiconductor laser temperature controller 7 semiconductor laser current controller 8a ¹² CO ₂ absorption intensity detection circuit 8b ¹³ CO ₂ absorption intensity Detection circuit 9a ¹² CO ₂ pressure width parameter 9b ¹³ CO ₂ pressure width parameter 10 Sample gas pressure 11a ¹² CO ₂ absorption coefficient calculation circuit 11b ¹³ CO ₂ absorption coefficient calculation circuit 12a, 12b Subdivision circuit

Claims (2)

[Claims] 1. When analyzing the isotope ratio from the optical absorption intensity of the isotope, the pressure width parameter of the trace isotope in which the collision partner molecule is the majority isotope as the pressure width parameter of the absorption line of the trace isotope, and the Using the pressure width parameter of the isotope absorption line and the pressure width parameter of the multiple isotope with the collision partner molecule as the isotope, the pressure of the sample gas is measured at the same time as the absorption line measurement of the isotope. A method for correcting an isotope ratio, which comprises correcting the absorption intensities of both absorption lines measured from the both pressure width parameters. 2. A circuit for detecting absorption intensity of each absorption line of a large amount and a small amount of isotopes, a means for obtaining a pressure width parameter of a large isotope and a pressure width parameter of a minute isotope, and a means for measuring a sample gas pressure. , An absorption coefficient calculation circuit for obtaining each absorption coefficient at the gas pressure using each pressure width parameter and the gas pressure, and a division circuit for dividing the absorption intensity of each isotope by each absorption coefficient. Isotope ratio correction device.