JP4455227B2 - Method for measuring gas isotope abundance ratio and system therefor - Google Patents

Description

  The present invention relates to a method for measuring the isotope abundance ratio of a gas and a system therefor, for example, the isotope abundance ratio of methane containing impurities having a mass number close to that of methane isotope, The present invention relates to a method for measuring without requiring an instrument such as a six-way valve or a metering tube in a gas chromatograph apparatus, and a system therefor.

  The mass number of molecules present in the gas can be measured using a mass spectrometer (hereinafter referred to as “MS” as appropriate). Moreover, the composition in the gas can be measured by comparing the intensity of each mass number detected by MS with that of the standard gas. The MS is a device that introduces a measurement gas, ionizes it, separates it into each mass in a vacuum, and performs detection. Some MS can introduce measurement gas at atmospheric pressure. In the present specification and claims, unless otherwise specified, the mass spectrometer is a mass spectrometer whose sample introduction pressure is atmospheric pressure.

By the way, when measuring the composition of ¹³ CH ₄ and ¹² CH ₄ , which are stable isotopes, of methane, for example, among various gases, the mass number of ¹³ CH ₄ detected by MS (hereinafter referred to as “Mass 17” as appropriate). "hereinafter) of the intensity and ¹² that CH ₄ is ionized ⁽¹² CH ₃ ^-) mass number of 15 (hereinafter, as appropriate" the ratio of the intensity of) that mass 15 ", composition of mass 17 of known standard gas The ratio of the strength to the strength of Mass 15 is compared.

Here, when measuring the methane isotope abundance ratio, that is, for measuring the ¹³ CH ₄ concentration in methane, the intensity of ¹² CH _{4 having} a mass number of 16 cannot be used. This is because the mass number of ¹³ CH ₄ ionized ( ¹³ CH ₃ ⁻ ) and the mass number of ¹² CH ₄ are both 16, and they overlap, and cannot be discriminated at the time of measurement.

In addition, when the measurement gas contains impurities close to the mass number of the gas whose isotope abundance ratio is to be measured, it is impossible to measure the isotope abundance ratio with high accuracy. For example, when water is included when measuring the isotope ratio of methane, the mass number of H ₂ O (hereinafter referred to as “Mass 18”) and H ₂ O are ionized by MS due to the presence of water. A mass number of 17 OH ⁻ is detected. For this reason, both OH ^- and ¹³ CH ₄ overlap with 17 and cannot be distinguished in measurement.

For example, Micromass UK Limited “TraceGas Catalog” uses MS for isotope analysis of methane. Due to the above circumstances, methane is combusted and reformed. Carbon dioxide is used and the ratio of ¹³ CO ₂ and ¹² CO ₂ is measured. In Japanese Patent No. 3416226, MS is used for isotope analysis of methane. From the same situation, methane is combusted and reformed to measure carbon monoxide and carbon dioxide.

  Japanese Patent Application Laid-Open No. 6-34616 and Japanese Patent No. 3477606 disclose an atmospheric pressure ionization mass spectrometer (APIMS) for analyzing impurities such as nitrogen, carbon monoxide and hydrocarbons contained in a gas such as oxygen or phosphine. However, after the impurities are removed using a gas chromatograph to separate the impurities and the main component gas, the isotope abundance ratio of the impurity gas is measured. In Japanese Patent Laid-Open No. 8-122314, a gas chromatograph is used to remove impurities and the impurities are removed, and then the target gas is analyzed using MS. It is said that water will be discharged out of the system by switching the valve inside.

Micromass UK Limited “Trace Gas Catalog” Japanese Patent No. 3416226 JP-A-6-34616 Japanese Patent No. 3477606 JP-A-8-122314

  The present invention is a measurement of the isotope abundance ratio of a gas containing an impurity with a mass number close to that of an isotope gas, for example, when measuring the isotope abundance ratio of a methane containing an impurity with a mass number close to that of a methane, A method for measuring the isotope abundance ratio of methane and other gases without the need for methane reforming and without the need for complicated configurations such as general gas chromatographs or valve switching during measurement. And it aims at providing the system for it.

  The present invention relates to (1) a method for measuring the isotope abundance ratio of a gas containing an impurity with a mass number close to that of an isotope gas, wherein an impurity is filled in a conduit extending from a measurement gas container to a mass spectrometer. A removal pipe is arranged, and a pipe that supplies a gas that is not close to the mass number of the measurement gas faces the conduit to the impurity removal pipe so that a gas that is not close to the mass number of the measurement gas is always supplied at atmospheric pressure. Using the system, the gas to be measured is flowed to the mass spectrometer while flowing the gas not close to the mass number of the measurement gas to the impurity removal tube, and the gas pressure flowing into the mass spectrometer is always set to atmospheric pressure. Provided is a method for measuring a gas isotope abundance ratio characterized by measuring a ratio.

  The present invention is (2) a system for measuring the isotope abundance ratio of an impurity containing an impurity having a mass number close to that of an isotope gas, and a conduit extending from a measurement gas container to a mass spectrometer is filled with an adsorbent. In addition, a pipe for supplying a gas that is not close to the mass number of the measurement gas faces the conduit to the impurity removal pipe, and a gas that is not close to the mass number of the measurement gas is always at atmospheric pressure. The system for measuring the isotope abundance ratio of the gas is characterized in that the gas pressure flowing into the mass spectrometer is always set to atmospheric pressure.

  According to the present invention, it is possible to measure the isotope abundance ratio of a gas containing impurities having a mass number close to that of a gas isotope such as methane or ammonia with a simple system with high accuracy. That is, according to the present invention, as in the prior art, for example, no combustion or reforming of methane is required, and a complicated apparatus such as a general gas chromatograph apparatus or a valve switching operation during measurement is required. In this case, the isotope abundance ratio of the gas can be measured with high accuracy.

  FIG. 1 is a diagram for explaining an isotope abundance ratio measuring system and measuring method according to the present invention. In FIG. 1, 1 is a container for a measurement sample (ie, “measurement gas”, referred to as “measurement gas” as appropriate in the present specification and claims), and a sampling back or the like is used. Reference numerals 2 and 3 denote conduits between which a valve V1 is arranged. The conduit 3 is connected to an impurity removing tube (a drying tube when the impurity is moisture) 4, and the impurity removing tube 4 and the mass spectrometer 6 are connected by a conduit 5. The impurity removal tube 4 is for removing impurities from the measurement gas. When the impurity is moisture, it is filled with a desiccant such as silica gel or alumina.

  And the mechanism which flows carrier gas, for example, helium (He), is provided. The mechanism for flowing the carrier gas is to avoid a decrease in the partial pressure of the gas detected by the mass spectrometer 6 due to a decrease in the flow velocity of the measurement gas passing through the impurity removal tube 4. The flow rate of the carrier gas is controlled by an MFC (mass flow controller) 9 from a cylinder 7 through a conduit 8.

  A valve V2 is arranged in the conduit 10 following the MFC 9, the valve V2 is connected to the conduit 11, and the conduit 11 is connected to the conduit 3. The carrier gas whose flow rate is controlled by the MFC 9 is introduced into the conduit 3 through the conduit 10, the valve V2, and the conduit 11, and flows into the mass spectrometer 6.

  A branch pipe 12 is connected to the conduit 10 and arranged. A part of the carrier gas from the conduit 10 flows through the branch pipe 12, and the other end of the branch pipe 12 is open to the atmosphere. This branch pipe 12 is used to constantly set the gas pressure flowing into the mass spectrometer 6 to atmospheric pressure by the carrier gas introduced into the pipe 3 via the pipe 11, and is an important branch pipe in the system of the present invention. .

  In the present invention, the conduit for supplying the gas not close to the mass number of the measurement gas is faced to the conduit to the impurity removal pipe, and the gas not close to the mass number of the measurement gas is always supplied at atmospheric pressure, It is essential to measure the isotope abundance ratio of the gas to be processed with the gas pressure flowing into the mass spectrometer always set to atmospheric pressure. As the mode, in addition to the mode in which the branch pipe 12 is disposed as described above, for example, a pressure reducing valve is disposed between the MFC and V2 in FIG. 1 or a buffer tank is disposed in an appropriate mode. it can.

  FIG. 2 is a system used in the process up to reaching the isotope abundance ratio measurement system according to the present invention of FIG. The difference from FIG. 1 is that the branch pipe 12 is not connected to the conduit 10 and arranged. In the system of FIG. 2, when helium is used as a carrier gas and it is allowed to flow through the mass spectrometer 6 as it is without being opened to the atmosphere, the pressure of the system becomes high and measurement by the mass spectrometer 6 cannot be performed. It was.

  As described above, in the measurement system of the present invention, the flow path from the carrier gas cylinder 7 to the impurity removal pipe 4 and the flow path from the measurement gas container (the container filled with or containing the measurement gas) 1 to the impurity removal pipe 4 are provided. These are provided with a valve V2 and a valve V1, respectively, and a hexagonal valve and a metering pipe which are essential in the prior art are unnecessary. Moreover, there is no need to burn methane to reform it to carbon monoxide or carbon dioxide. Hereinafter, a measurement method using this measurement system will be described.

  At the start of measurement, the carrier gas in the cylinder 7 is flowed in advance with the valve 1 and the valve V2 being closed while controlling the flow rate with the MFC. The carrier gas fills the conduit 10 to the valve V2 and flows to the atmosphere through the branch pipe 12. In this state, after the valve 1 is opened, the valve V2 is opened, and the measurement by the mass spectrometer 6 is started by supplying the carrier gas to the measurement gas and supplying it to the impurity removing tube 4. During the measurement, impurities such as moisture are adsorbed and removed by the impurity removing tube 4.

  At that time, since the end of the branch pipe 12 is opened to the atmosphere to balance the gas pressure, the gas in the measurement gas container 1 does not flow out to the atmosphere side against the flow of the carrier gas. Since the carrier gas always flows to the atmosphere side through the pipe 12, the atmosphere does not flow into the system against the flow of the carrier gas.

  In this system, since impurities such as moisture are adsorbed and removed only by the impurity removal tube 4, the modification as in the prior art (Micromass UK Limited “TraceGas Catalog”, Japanese Patent No. 3416226) is unnecessary. There is no need to use a gas chromatograph having a device configuration such as a six-way valve or a metering tube as in the prior art (Japanese Patent Laid-Open No. 6-34616, Japanese Patent No. 3477606). Thus, it is not necessary to release impurities such as moisture by operating the valve during measurement.

  Moreover, this system can be used for measurement of the isotope abundance ratio of the gas until the adsorption capacity of the adsorbent filled in the impurity removal tube 4 is lowered. When the adsorption capacity of the adsorbent falls, the system can be used repeatedly and continuously by replacing the adsorbent or regenerating the adsorbent by connecting a vacuum pump, for example, to the impurity removal pipe 4. .

  In this way, the system itself is simple, and according to the system, for a gas containing impurities whose mass number is close to that of the isotope to be analyzed, the impurities are, for example, a general mass spectrometer. Even when the concentration is so high that it is detected in the above, the isotope abundance ratio can be measured with high accuracy.

The measurement gas is ionized by the mass spectrometer 6, and the ionized component is separated into each mass in a vacuum to perform detection. Here, when the measurement gas is, for example, methane, the ¹³ CH ₄ concentration can be obtained by the equations (1) to (2) shown in FIG. 3 from the intensity ratio of Mass 17 and Mass 15 measured for the measurement gas. .

The intensity ratio of Mass 17 and Mass 15 of the measurement gas is compared with the intensity ratio of Mass 17 and Mass 15 of the standard gas as shown in Equation (1). Here, for example, when measuring the isotope ratio of methane, as described above, the intensity of ¹² CH _{4 having} a mass number of 16 cannot be used for measuring the ¹³ CH ₄ concentration in methane. This is because the mass number of ¹³ CH ₄ ionized ( ¹³ CH ₃ ⁻ ) and the mass number of ¹² CH ₄ are both 16 and overlap, and cannot be discriminated at the time of measurement.

Strength Mass 17 is the concentration of ¹³ CH ₄ itself, Mass 15 is ¹² CH ₄ is from a mass number of those ionized strength Mass 15 is multiplied by the ionization rate of ¹² CH ₄ to a concentration of ¹² CH ₄ Is. That is, the intensity ratio “Mass 17 / Mass 15” between the two is a ratio between the concentration of ¹³ CH _{4 and} the concentration of ¹² CH ₄ multiplied by the ionization rate of ¹² CH ₄ . Then, β ′ is obtained by dividing the intensity ratio of Mass 17 / Mass 15 of the measurement gas by the intensity ratio of “Mass 17 / Mass 15” of the standard gas whose ¹³ CH ₄ concentration and ¹² CH ₄ concentration are known in advance. It is done.

β ′ is obtained from the measured value by the mass spectrometer, and since ¹³ CH ₄ concentration and ¹² CH ₄ concentration are known in advance for the standard gas, what is unknown in the equation (1) is the ¹³ CH ₄ concentration of the measurement gas. ¹² CH ₄ concentration. Since the stable isotopes of methane are ¹³ CH ₄ and ¹² CH ₄ , the ¹² CH ₄ concentration of the measurement gas is the total 1 minus the ¹³ CH ₄ concentration. Only the ¹³ CH ₄ concentration of the measurement gas is unknown. Therefore, when equation (1) is modified, equation (2) is obtained, whereby the ¹³ CH ₄ concentration of the measurement gas can be obtained.

  EXAMPLES Hereinafter, although this invention is demonstrated in more detail based on an Example, of course, this invention is not limited to these Examples.

In Examples 1 to 3, the system shown in FIG. 1 was used, and helium was used as the carrier gas. Moreover, in the comparative example, it implemented using the apparatus of FIGS. The example using FIG. 1 as a comparative example is an example in which the valve operation or the like is performed differently from the example. The measurement gas from the sampling bag 1 in Examples 1 to 3 and Comparative Examples 1 to 5 is a gas containing CH ₄ and H ₂ O, and the H ₂ O content (capacity) in the gas is about 100 with respect to CH ₄ . 1 / [that is, CH ₄ : H ₂ O≈100: 1 (capacity)].

<Measurement of abundance ratio of ¹² CH ₄ and ¹³ CH _{4 in} methane with natural isotope ratio as standard gas for Example 1 and Comparative Examples 1 to 3>
This measurement uses a system shown in FIG. 4, CH ₄ natural isotopic ratio as a standard gas _{^{(moisture-free) (12 CH 4: 13 CH}} 4 = 98.9: 1.1) of ¹² The abundance ratio of CH ₄ and ¹³ CH ₄ was measured. The system of FIG. 4 is a conventional system that does not have a mixing mechanism such as helium. The valve V3 was opened and the measurement gas from the sampling bag 1 was measured through a mass spectrometer 6 (the same as that used in the system shown in FIG. 1).

<Example 1>
The first embodiment, by using the system shown in FIG. 1, CH ₄ natural isotopic ratio ¹² CH in the gas containing ^{_{(12 CH 4:: 13 CH}} 4 = 98.9 1.1) and H ₂ O _This is an example of measuring the abundance ratio of ₄ and ¹³ CH ₄ . A drying tube filled with silica gel was disposed as the impurity removal tube 4, and a tube (stainless steel tube) having an inner diameter of 1/8 inch (= 3.175 mm) was disposed as the branch tube 12.

Before the start of the test, the valves V1 and V2 are closed. In this way, with the valves V1 and V2 kept closed, helium was controlled to flow at a flow rate of 50 cc / min (3.0 × 10 ⁻³ m ³ / h) by the MFC 9. Next, the valve V1 was opened, and then the valve V2 was opened. Thus, the measurement gas containing CH ₄ and H ₂ O flowing through the conduits 2 and 3 is mixed with helium from the helium cylinder 7 and supplied to the drying tube 4, and measurement by the mass spectrometer 6 is started and measured. did.

  Here, the mass spectrometer 6 used in this example is a normal atmospheric pressure ionization mass spectrometer, and the same mass spectrometer is used in the following examples and comparative examples. According to a conventional method, the measurement gas was ionized, and the ionized component was separated into each mass in vacuum and detected. During the measurement, moisture is absorbed and removed by the drying tube 4.

<Comparative example 1>
In Comparative Example 1, the abundance ratio of ¹² CH ₄ and ¹³ CH ₄ in a gas containing natural isotope ratios of CH ₄ and H ₂ O was measured using the system shown in FIG. As described above, the system of FIG. 4 is a conventional system that does not have a helium mixing mechanism or the like. The valve V3 was opened and the measurement gas from the sampling bag 1 was measured through a mass spectrometer 6 (the same as that used in the system shown in FIG. 1).

6 shows the measurement results of the standard gas, FIG. 7 shows the measurement results of Example 1, and FIG. 8 shows the measurement results of Comparative Example 1. 6 to 8, (a) Mass 17 is due to ¹³ CH ₄ and OH ⁻ , (b) Mass 15 is due to ¹² CH ₃ ⁻ ionized by ¹² CH ₄ , and (c) Mass 18 is due to H ₂ O. Is. Here, when moisture is not contained in the measurement gas as in the standard gas, (a) Mass 17 is only the intensity due to ¹³ CH _{4 and} there is no intensity derived from OH ⁻ , and (c) Mass 18 is measured. Appears as a graph below the limit. In addition, the measurement limit of the mass spectrometer used by the Example and the comparative example is 1 * 10 < ^-10 > (AU).

First, FIG. 6 shows the measurement result of standard gas, that is, methane having a natural abundance ratio that does not contain moisture, so there is no influence of moisture. Thus (a) Mass 17 is due to _{^{13 CH 4, (b) Mass}} 15 is ^{12 12} CH ₄ is ionized CH ₃ ^- is due. In FIG. 6, (c) Mass 18 appears as a graph below the measurement limit.

Next, as shown in FIG. 7, in Example 1, the detection intensity of (a) Mass 17 and (b) Mass 15 is larger than the detection intensity of (c) Mass 18. In Example 1, since moisture by the present invention is significantly removed from the is ¹³ CH ₄ and OH of (a) Mass 17 ^- Of, OH ^- is little, can be measured accurately isotope ing.

On the other hand, as shown in FIG. 8, in Comparative Example 1, (a) Mass 17 and (c) Mass 18 have almost the same detection intensity. In FIG. 8, (a) Mass 17 is due to ¹³ CH ₄ and OH ⁻ , and (c) Mass 18 is due to H ₂ O. However, in Comparative Example 1, moisture was not removed. Since the influence of ¹³ CH ₄ and OH ⁻ which is the origin of 17 becomes large, the isotope ratio cannot be measured accurately.

Table 1, these results of Example 1 according to the results and Comparative Example 1, the natural isotopic ratio containing no standard gas i.e. moisture CH ₄ [i.e. ^{_{CH 4 (12 C: 13 C}} = 98.9: 1.1 ) Only] is shown in comparison with the value of Mass 17 / Mass 15 when measured only by a mass spectrometer. Table 1 also shows the results of Comparative Examples 2 to 3 described later.

  As shown in Table 1, when the standard gas was measured with a mass spectrometer, the error between the value of Mass 17 / Mass 15 and the value of Mass 17 / Mass 15 according to Example 1 was only 0.1%. On the other hand, the error between the value of Mass 17 / Mass 15 when the standard gas was measured with a mass spectrometer and the value of Mass 17 / Mass 15 in Comparative Example 1 was 5.7%. In this way, with the system in which helium is mixed and supplied to the drying tube 4 while flowing the measurement gas, significant isotope analysis can be performed only by removing the moisture by the drying tube 4.

<Comparative example 2>
In Comparative Example 2, in the system shown in FIG. 1, the valve V2 is always closed, the valve V1 is opened at the start of measurement, and the natural isotope ratios of CH ₄ and H ₂ O are obtained from the sampling bag 1 without flowing helium. This is an example in which the gas contained is measured through the drying tube 4 and through the mass spectrometer 6. In the comparative example 2, helium is not flowed at all in the system shown in FIG. 1, and therefore, as shown in FIG. 5, this corresponds to a system having no mechanism from the carrier gas cylinder 7 to the conduit 11 in the system shown in FIG. FIG. 9 is a diagram showing the results.

As shown in FIG. 9, in Comparative Example 2, both the detection intensity of (a) Mass 17 and the detection intensity of (c) Mass 18 are much smaller than (b) Mass 15. In this test, moisture was removed, and among ¹³ CH ₄ and OH ⁻ derived from Mass 17, there was almost no influence of OH ⁻ . However, when the measurement gas is passed through the drying tube 4 but helium is not flowed, the flow rate at which the gas to be processed (that is, the measurement gas that has passed through the drying tube 4) is introduced into the mass spectrometer 6 is the same as in Example 1. Compared to the flow rate, it drops significantly. As a result, the detection intensity of Mass 17 becomes much smaller than the detection intensity of Mass 15, and the isotope ratio cannot be measured with high accuracy.

As shown in Table 1, the value of Mass 17 / Mass 15 and the value of Mass 17 / Mass 15 in Comparative Example 2 when the standard gas, that is, the natural isotope-free CH ₄ containing no moisture is measured with the mass spectrometer 6. The error was 9.7%. On the other hand, in Example 1, the measurement gas is supplied to the drying tube 4 while flowing the helium from the helium cylinder 7, and the flow rate of the measurement gas introduced into the mass spectrometer 6 is the isotope ratio. The speed is such that a detection intensity of Mass 17 sufficient to perform measurement with sufficient accuracy can be provided. It is understood that the difference between the two is due to this point.

<Comparative Example 3>
Comparative Example 3 is an example in which the apparatus of FIG. 1 was used as in Example 1, but the opening and closing of the valve V1 and the valve V2 were reversed from those in Example 1. That is, the valve V2 was opened, and then the valve V1 connected to the sampling bag 1 was opened. Thus, the measurement by the mass spectrometer 6 is started by supplying a measurement gas containing natural isotope ratio CH ₄ and H ₂ O to the drying tube 4 while flowing helium from the helium cylinder 7. Carried out.

  FIG. 10 is a diagram showing the results. As shown in FIG. 10, also in the test of Comparative Example 3, both the detection intensity of (a) Mass 17 and the detection intensity of (c) Mass 18 are much smaller than (b) Mass 15. In the test of Comparative Example 3, although the opening and closing of the valves V1 and V2 is merely reversed from that of Example 1, only the same result as that obtained by the test of Comparative Example 2 is obtained.

  This is because, in the measurement procedure, the valve V2 is opened first, so that helium flows into the mass spectrometer 6 before the measurement gas tries to flow into the mass spectrometer 6, so that the valve V1 is opened later. This is because the measurement gas is difficult to flow to the mass spectrometer 6 because helium has already flowed into the conduit 3 from the conduit 11. As a result, as in Comparative Example 1, the detection intensity of Mass 17 is much smaller than the detection intensity of Mass 15, and the isotope ratio cannot be measured with high accuracy.

And as Table 1 shows, the error between the value of Mass 17 / Mass 15 and the value of Mass 17 / Mass 15 in Comparative Example 3 when CH ₄ of the natural isotope ratio not containing water is measured with a mass spectrometer is It was 6.2%. From this, as in Example 1, in the system shown in FIG. 1, the valve V1 from the sampling bag 1 is opened, and then the valve V2 is opened. It can be seen that this is an important requirement.

<Measurement of the Abundance Ratio of ¹² CH ₄ and ¹³ CH ₄ in ¹² CH ₄ : ¹³ CH ₄ = 60: 40 as Standard Gas for Examples 2-3 and Comparative Examples 4-5>
This measurement uses the system shown in FIG. 4 and the presence of ¹² CH ₄ and ¹³ CH ₄ as standard gases (without water) ¹² CH ₄ : ¹³ CH ₄ = 60.0: 40.0) The ratio was measured. The valve V3 was opened and the measurement gas from the sampling bag 1 was measured through a mass spectrometer 6 (the same as that used in the system shown in FIG. 1).

<Example 2>
The present Example 2 uses a gas containing CH ₄ ( ¹² CH ₄ : ¹³ CH ₄ = 50: 50) and H ₂ O in which ¹² CH ₄ and ¹³ CH ₄ are mixed in equal amounts as a measurement gas. In this example, the abundance ratio of ¹² CH ₄ and ¹³ CH ₄ in the measurement gas was measured in the same manner as in 1. That is, in FIG. 1, the valve V1 is opened, and then the valve V2 is opened. Thus, the measurement gas containing CH ₄ and H ₂ O was flowed first, and helium was flowed to the gas to be mixed and supplied to the drying tube 4, whereby measurement by the mass spectrometer 6 was started and measured. According to a conventional method, the measurement gas was ionized, and the ionized components were separated and detected in vacuum. During the measurement, moisture is adsorbed and removed by the drying tube 4.

<Comparative example 4>
In Comparative Example 4, using the system of FIG. 4, ¹² CH ₄ and ¹³ CH ₄ CH ₄ was mixed with an equal volume _{^{(12 CH 4: 13 CH 4}} = 50: 50) and ¹² in the gas containing H ₂ O Measurement of the abundance ratio of CH ₄ and ¹³ CH ₄ was performed. The valve V3 was opened and the measurement gas from the sampling bag 1 was measured through a mass spectrometer 6 (the same as that used in the system shown in FIG. 1).

11 shows the measurement results of the standard gas, FIG. 12 shows the measurement results of Example 2, and FIG. 13 shows the measurement results of Comparative Example 4. First, FIG. 11 shows the measurement result of methane with a standard gas, that is, ¹³ CH ₄ : ¹² CH ₄ = 50: 50 which does not contain moisture, so there is no influence by moisture. Thus (a) Mass 17 is due to _{^{13 CH 4, (b) Mass}} 15 is ^{12 12} CH ₄ is ionized CH ₃ ^- is due. In addition, the graph of (c) Mass 18 in FIG. 11 shows a measurement limit.

Next, as shown in FIG. 12, in Example 2, the detection intensity of (a) Mass 17 and (b) Mass 15 is higher than the detection intensity of (c) Mass 18. (C) About the graph of Mass 18, the measurement limit appears like the result of the standard gas in FIG. Thus, in Example 2, since water was significantly removed by the present invention, (a) Almost no OH ⁻ was present in ¹³ CH ₄ and OH ⁻ originating from Mass 17, and the isotope ratio was accurately determined. We can measure well.

On the other hand, as shown in FIG. 13, in the test of Comparative Example 4, the detected intensity of (c) Mass 18 is larger than that of FIG. This is because moisture is detected in Comparative Example 4. Since water was not removed in the test of Comparative Example 4, (a) Of ¹³ CH ₄ and OH ⁻ derived from Mass 17, the influence of OH ⁻ is large, so that the isotope ratio can be measured with high accuracy. Absent.

Table 2 shows the results of Example 2 and Comparative Example 4 based on mass analysis of the abundance ratio of ¹² CH ₄ and ¹³ CH ₄ of ¹² CH ₄ : ¹³ CH ₄ = 60: 40 not containing standard gas, that is, moisture. It is shown in comparison with the value of Mass 17 / Mass 15 when measured by the meter alone. Table 2 also shows the results of Example 3 and Comparative Example 5 described later.

As shown in Table 2, it does not include a standard gas i.e. water ^{_{12 CH 4: 13 CH 4 =}} 60: value of Mass 17 / Mass 15 in the case where measurement of CH ₄ in the mass spectrometer 40 According to Example 2 Mass 17 / The error from the value of Mass 15 was only 0.4%. In contrast, moisture-free ^{_{12 CH 4: 13 CH 4 =}} 60: 40 in Comparative Example 4 with the value of the Mass 17 / Mass 15 when measured by the mass spectrometer CH ₄ of Mass 17 / Mass 15 of The error from the value was 3.4%. In this way, with the system in which helium is mixed and supplied to the drying tube 4 while flowing the measurement gas, significant isotope analysis can be performed only by removing the moisture by the drying tube 4.

<Example 3>
In Example 3, a gas containing CH ₂ ( ¹² CH ₄ : ¹³ CH ₄ = 33: 67) in which ¹² CH ₄ and ¹³ CH ₄ are mixed at a ratio of 1: 2 and H ₂ O is used as a measurement gas. In this example, the abundance ratio of ¹² CH ₄ and ¹³ CH ₄ in the measurement gas was measured in the same manner as in Example 1. That is, in FIG. 1, the valve V1 is opened, then the valve V2 is opened, and helium is mixed and supplied to the drying tube 4 while flowing the measurement gas containing CH ₄ and H ₂ O first, Measurement was started by starting measurement with the mass spectrometer 6.

<Comparative Example 5>
In Comparative Example 5, CH ₄ ( ¹² CH ₄ : ¹³ CH ₄ = 33: 67) mixed with ¹² CH ₄ and ¹³ CH ₄ in a ratio of 1: 2 and H ₂ O were used using the system of FIG. Measurement of the abundance ratio of ¹² CH ₄ and ¹³ CH ₄ in the contained gas was performed. That is, in FIG. 4, the valve V3 was opened, and the measurement gas from the sampling bag 1 was passed through the mass spectrometer 6 (the same as that used in the system shown in FIG. 1).

14 shows the measurement results of Example 3, and FIG. 15 shows the measurement results of Comparative Example 5. First, as shown in FIG. 14, in Example 3, the detection intensity of (a) Mass 17 and (b) Mass 15 is larger than the detection intensity of (c) Mass 18. (C) For the Mass 18 graph, the measurement limit appears as in the case of the standard gas in FIG. Thus, in Example 3, since water was significantly removed by the present invention, (a) Almost no OH ⁻ was present in ¹³ CH ₄ and OH ⁻ derived from Mass 17, and the isotope ratio was accurately determined. We can measure well.

On the other hand, as shown in FIG. 15, in the test of Comparative Example 5, the detected intensity of (c) Mass 18 is larger than that of FIG. This is because moisture is detected in Comparative Example 5. Since water was not removed in the test of Comparative Example 5, (a) Of the ¹³ CH ₄ and OH ⁻ derived from Mass 17, the influence of OH ⁻ is large, so that the isotope ratio can be measured with high accuracy. Absent.

Table 2 shows the results of Example 3 and Comparative Example 5 based on mass analysis of the abundance ratio of ¹² CH ₄ and ¹³ CH ₄ of ¹² CH ₄ : ¹³ CH ₄ = 60: 40 not containing a standard gas, that is, moisture. It is shown in comparison with the value of Mass 17 / Mass 15 when measured only with a meter.

As shown in Table 2, moisture-free ^{_{12 CH 4: 13 CH 4 =}} 60: According to Example 2 with respect to the value of the Mass 17 / Mass 15 in the case where the 40 CH ₄ was measured by a mass spectrometer Mass 17 / Mass 15 of The value error was only 0.4%. In contrast, moisture-free ^{_{12 CH 4: 13 CH 4 =}} 60: 40 in Comparative Example 4 with respect to the value of the Mass 17 / Mass 15 in the case where measurement of CH ₄ in the mass spectrometer Mass 17 / Mass 15 of The value error was as much as 3.4%. In this way, with the system in which helium is mixed and supplied to the drying tube 4 while flowing the measurement gas, significant isotope analysis can be performed only by removing the moisture by the drying tube 4.

Moreover, water-free ^{_{12 CH 4: 13 CH 4 =}} 60: According to Example 3 with respect to the value of the Mass 17 / Mass 15 in the case where the 40 CH ₄ was measured by a mass spectrometer Mass 17 / Mass error value of 15 Was only 1.0%. In contrast, moisture-free ^{_{12 CH 4: 13 CH 4 =}} 60: 40 in Comparative Example 5 with respect to the value of the Mass 17 / Mass 15 in the case where measurement of CH ₄ in the mass spectrometer Mass 17 / Mass 15 of The value error was 4.6%. In this way, with the system in which helium is mixed and supplied to the drying tube 4 while flowing the measurement gas, significant isotope analysis can be performed only by removing the moisture by the drying tube 4.

The figure explaining the system and measuring method for isotope abundance ratio based on this invention Diagram showing the system used in the process leading to the present invention It shows the equation for determining the concentration of ¹³ CH ₄ gas measurement by mass spectrometry Diagram showing the system used in Comparative Example 1 Diagram showing the system used in Comparative Example 2 It shows a first embodiment, the measurement result of the existence ratio of ¹² CH ₄ and ¹³ CH ₄ standard gas to Comparative Examples 1 to 3 The figure which shows the result of Example 1 The figure which shows the result of the comparative example 1 The figure which shows the result of the comparative example 2 The figure which shows the result of the comparative example 3 It shows the measurement result of the presence ratios of Example 2 to 3, ¹² CH ₄ and ¹³ CH ₄ standard gas to Comparative Example 4-5 The figure which shows the result of Example 2 The figure which shows the result of the comparative example 4 The figure which shows the result of Example 3 The figure which shows the result of the comparative example 5

Explanation of symbols

1 Container of sample to be measured (sampling bag, etc.)
2, 3 Conduit 4 Impurity removal tube 5 Conduit 6 MS
7 Helium gas cylinder 9 MFC (mass flow controller)
8, 10-11 Conduit 12 Branch pipe V1-V3 Valve

Claims (8)

A method for measuring the isotope abundance ratio of methane containing impurities whose mass number is close to that of an isotope gas,
An impurity removal tube filled with adsorbent is placed in the conduit from the measurement gas methane container to the mass spectrometer, and a gas not close to the mass number of methane as the measurement gas is supplied to the conduit to the impurity removal tube. Facing the conduit
By connecting a branch pipe opened to the atmosphere to the conduit that supplies the gas not close to the mass number of methane as the measurement gas, a gas that is not close to the mass number of methane as the measurement gas is always supplied at atmospheric pressure. Using the system
While the gas which is not close to the mass number of the methane, the measurement gas flowing into the impurity removal tube, measuring gas and a flow of methane, methane is the measurement gas is always atmospheric pressure the gas pressure flowing into the mass spectrometer A method for measuring the isotope abundance ratio of methane , characterized by measuring the isotope abundance ratio. A method of measuring the isotopic ratio of methane according to claim 1, wherein said impurity isotope gas and mass number close to measure the isotopic ratio of methane, which is a water. A method of measuring the isotopic ratio of methane according to claim 1 or 2, a method of gas that does not close to the mass number of the measurement gas is measured isotope abundance ratio of methane, which is a helium. A method of measuring the isotopic ratio of methane according to any one of claims 1 to 3 isotope abundance ratio of methane, wherein the mass spectrometer is an atmospheric pressure ionization mass spectrometer How to measure. A system for measuring the isotope abundance ratio of methane containing impurities with a mass number close to that of an isotope gas,
An impurity removal tube filled with adsorbent is placed in the conduit from the measurement gas methane container to the mass spectrometer, and a gas not close to the mass number of methane as the measurement gas is supplied to the conduit to the impurity removal tube. And face the conduit
By supplying a gas not close to the mass number of methane , which is the measurement gas , by always supplying a gas not close to the mass number of the measurement gas by connecting a branch pipe opened to the atmosphere to the conduit supplying the gas not close to the mass number of the measurement gas, A system for measuring the isotope abundance ratio of methane , characterized in that the gas pressure flowing into the mass spectrometer is always atmospheric pressure. A system for measuring isotope abundance ratio of methane according to claim 5, for the impurity isotope gas and mass number close to measure the isotopic ratio of methane, which is a water system. The system for measuring the methane isotope abundance ratio according to claim 5 or 6 , wherein the gas not close to the mass number of the measurement gas is helium, and the methane isotope abundance ratio is measured. System for. A system for measuring isotope abundance ratio of methane according to any one of claims 5-7, isotopic methane, wherein the mass spectrometer is an atmospheric pressure ionization mass spectrometer A system for measuring ratios.

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