JP2006118999A - Gas detection device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To operate accurately a gas concentration even if water adheres to the inside of a chamber of a mass spectrometer. <P>SOLUTION: This gas detection device is equipped with: a sampling tube for sampling the air from a monitoring zone; and the mass spectrometer provided on the base end side of the sampling tube, for performing mass spectrometry by ionizing the sampled air. The spectrum intensity wherein a mass charge ratio (m/z) is 1(H+) and the spectrum intensity wherein the mass charge ratio (m/z) is 2(H2) are detected, and the spectrum intensity wherein the mass charge ratio (m/z) is 1(H+) is subtracted from the spectrum intensity wherein the mass charge ratio (m/z) is 2(H2), to thereby operate the hydrogen gas concentration. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a gas detection device.

As means for detecting hydrogen gas, conventionally, a semiconductor type gas sensor has been well known. In recent years, there is also a hydrogen sensor using a mass spectrometer (see Patent Document 1).
JP-A-9-128992

  A mass spectrometer is an excellent instrument that can detect the gas to be detected in the range of one molecule to 100%, but it is a precision analytical instrument that is mainly used indoors. Not suitable for leak detection. Further, when water adheres to the chamber of the mass spectrometer, there is a problem that accurate gas concentration cannot be measured in a low concentration gas region. Accordingly, an object of the present invention is to make it possible to accurately calculate a gas concentration even if water adheres to a chamber of a mass spectrometer.

  The present invention has been made to solve the above-described problems. A sampling tube that samples air from a monitoring area, and a mass that is provided on the proximal end side of the sampling tube and ionizes the sampled air to perform mass analysis. A gas detection apparatus comprising an analyzer detects a spectral intensity having a mass-to-charge ratio of 1 and a spectral intensity having a mass-to-charge ratio of 2, and calculates a hydrogen gas concentration in the air. Is. Further, the hydrogen gas concentration is calculated by subtracting the spectrum intensity having the mass to charge ratio of 1 from the spectrum intensity having the mass to charge ratio of 2.

  The present invention detects a spectral intensity having a mass-to-charge ratio of 1), a spectral intensity having a mass-to-charge ratio of 2, and a spectral intensity in a mass spectrometer. The hydrogen gas concentration is calculated by subtracting the spectral intensities.

  For this reason, even if hydrogen molecules are generated by hydrogen ions generated from water molecules due to the influence of water adhering to the inside of the chamber, the generated hydrogen molecules are subtracted from the hydrogen molecules detected in the air, so that accurate hydrogen It becomes possible to calculate the gas concentration.

  FIG. 1 is a system block diagram for explaining a gas detector G of the present invention. The gas detection device G according to the present invention includes a mass spectrometer MS and a sampling portion including a sampling tube 10 and a dry pump.

  In the figure, MS is a mass spectrometer. The mass spectrometer MS includes an ionization unit 2, a mass separation unit 4 (also called an analysis unit), and a detection unit 6. The mass spectrometer MS is provided on the base end side of the sampling tube 10 and ionizes sampled air. Perform mass analysis.

  As the ionization unit 2 of the mass spectrometer MS, the most common electron impact ionization (EI) method is used as an example. This is a method in which accelerated electrons collide with a sample (neutral) molecule to be ionized.

  For the mass separator 4, for example, the most popular quadrupole type is used. In the quadrupole type, the same voltage of the same polarity is applied to each of the two diagonal lines of the four pole-shaped electrodes, and the mass number of ions that can pass through the pole when this polarity is switched at high speed is proportional to the voltage applied to the pole. By utilizing this, only ions having a specific mass-to-charge ratio (m / z) (hydrogen ions in the present embodiment) are allowed to pass through and separation is performed. Here, m is the molecular weight and z is the number of charges.

  The detection unit 6 includes a photomultiplier tube and the like, and detects one separated ion. The detection unit 6 is connected to an external data analysis processing device 8 and is configured to send a detection signal to the data analysis processing device. Then, by separating and detecting ions for each mass, a mass spectrum composed of a horizontal axis (ion mass number) / vertical axis (ion detection intensity) is obtained. The data analysis processing device 8 processes the spectrum data from the mass spectrometer MS, selects only necessary peak values, and analyzes the data.

  More specifically, the data analysis processing device 8 will be described later. The mass intensity-to-charge ratio (m / z) is 1 (H +) and the mass-to-charge ratio (m / z) is 2 (H2). The spectrum intensity is detected, and the hydrogen gas concentration is obtained by subtracting the spectrum intensity of the mass to charge ratio (m / z) of 1 (H +) from the spectrum intensity of the mass to charge ratio (m / z) of 2 (H2). Is calculated.

  Since the ionization unit 2, the mass separation unit 4, and the detection unit 6 constituting the mass spectrometer MS require a high degree of vacuum for gas analysis, two turbo molecular pumps TMP and two dry pumps 9 are used as vacuum pumps. The table is directly connected, and the pressure in the chamber of the mass spectrometer MS is set to a pressure of about 10 ↑ -5 Torr. This pressure is adjusted in the range of 10 ↑ -4 to 10 ↑ -6 Torr, and a preferable value is 10 ↑ -5 Torr. In addition, you may make it comprise the system of the differential exhaust system which increases the number of pumps and reduces the pressure in the chamber of the ionization part 2, the mass separation part 4, and the detection part 6 one by one.

  The part related to the mass spectrometer MS described above can be replaced with another already known mass spectrometer MS. For example, other mass spectrometers such as a soft ionization method, an ion trap type, a magnetic field type, and a TOF type, which have different ionization and mass separation principles, may be used.

  Next, the sampling part will be described. Reference numeral 10 denotes a sampling pipe, the tip of which is installed in the monitoring area, and the dry pump 12 is connected to the base end to sample air from the monitoring area. Here, the monitoring area is, for example, a hydrogen storage facility such as a hydrogen station or an area where a fuel cell is provided.

  In this embodiment, the “dry pump” that does not use the oil working liquid is used as the vacuum pump, without using the “wet pump” that uses the oil working liquid. This is because when the “wet pump” is used, oil, which is the working fluid necessary to perform the exhaust operation, causes oil contamination in the mass spectrometer MS, causing interference peaks due to oil components in the spectrum. This is because. There is also a problem that the sample gas is dissolved in the oil and the sample peak does not disappear as a residual peak indefinitely.

  It is desirable to make the inner diameter of the sampling tube 10 as thin as possible so that the flow rate does not decrease. The inner diameter is adjusted according to the distance from the monitoring area to the dry pump 12. For example, if the distance is about 1 m, a sampling tube having an inner diameter of about 0.3 mm or less is used. If the distance is about 10 m, the inner diameter of the sampling tube is adjusted to about 0.52 mm, and the tube diameter is adjusted so that a constant flow rate of air can be sampled within a predetermined time.

  A first membrane filter 13 is provided at the tip of the sampling tube 10, and a second membrane filter 15 is provided in the middle of the sampling tube 10. As the membrane filter, a polymer film such as rubber, a ceramic porous body, a metal sintered body, a liquid film, or the like is used. The first membrane filter 13 is formed of, for example, a polymer film and has a pore diameter of 0.01 to 1 μm, preferably 0.05 μm. The pore diameter of the second membrane filter 15 is 1 μm to 0.3 mm, preferably about 0.5 μm. As the second membrane filter 15, a metal sintered body or a wire mesh is used in particular.

  The membrane filters 13 and 15 have three functions (effects) depending on the pore diameters: “drip-proof”, “hydrogen selectivity”, and “extinguishing (preventing explosion caused by hydrogen gas)”. If the hole diameter is less than 0.6 mm, there is an effect of extinguishing the flame. When the pore size is 10 μm or less, a membrane (filter) that cuts oxygen molecules and efficiently selects and transmits only hydrogen molecules having a small molecular diameter.

  In the sampling tube 10, the primary side of the dry pump 12 and the mass spectrometer MS are connected by a gas introduction tube 20 having a thin tube diameter. A needle valve 22 for controlling the flow rate is provided in the middle of the gas introduction pipe 20 to restrict the flow path of the gas introduction pipe 20. In place of the needle valve 22, an orifice may be used. Here, the pressure on the dry pump side 12 of the sampling tube 10 is 760 torr (approximately 10 ↑ 3 torr), the pressure of the mass spectrometer MS is 10 ↑ −5 torr, and is in a high vacuum state, and the pressure difference between the two is large. . For this reason, the sampling tube 10 and the mass spectrometer MS are connected via the gas introduction tube 20, and the flow path is narrowed by the needle valve 22. By doing so, it becomes possible to maintain the vacuum state of the mass spectrometer MS and also allow gas components to pass through, so that a so-called differential exhaust system is configured. Since sampled air is introduced into the mass spectrometer MS through such a differential exhaust system, impurities are unlikely to enter the mass spectrometer MS, and long-term monitoring is possible.

  In the figure, reference numeral 30 denotes a heater as a heating means. The heater 30 heats the entire mass spectrometer MS and the gas introduction tube 20 at 100 ° C. This is because the pressure inside the mass spectrometer MS is low, and if water drops adhere to the gas pipe (capillary) in the mass spectrometer MS, it is more likely to freeze than under atmospheric pressure. It is for preventing. In particular, since the portion narrowed and narrowed by the needle valve 22 in the gas introduction pipe 20 has a high possibility of freezing, heating by the heater 30 is desired.

  Next, a case where hydrogen leaks from a cracked part such as a pipe in a facility such as a hydrogen station (not shown) in an outdoor monitoring area will be described. Although hydrogen gas is diffused into the air, a part of the hydrogen gas is sampled through a membrane filter at the tip of the sampling tube 10.

  At this time, since the membrane filter 13 is provided at the tip of the sampling tube 10, raindrops do not enter the sampling tube 10, and only gas is allowed to pass into the sampling tube 10, so that moisture in the air (liquid ) Can be prevented from entering. In other words, the size of water droplets such as raindrops is 20-50 μm larger than the membrane filter 13, so that the drip-proof effect is improved by providing the membrane filter 13, and even if the gas detection device G is installed outdoors. There is no hindrance.

  Further, by using a membrane filter 13 having a small pore diameter of 0.05 μm, it is possible to prevent only oxygen molecules having a small molecular diameter to be detected from permeating and flowing oxygen molecules into the sampling tube 10. By selecting only hydrogen molecules in this way, the hydrogen gas entering the sampling tube 10 can be concentrated.

  Most of the hydrogen gas sucked in by the sampling pipe 10 is led to the dry pump 12 side, exhausted and released into the atmosphere, and part of the hydrogen gas passes through the gas introduction pipe 20 and the needle valve 22, Introduced into the mass spectrometer MS.

  By the way, “the inner diameter of the sampling tube 10 is desirably as thin as possible so as not to reduce the flow velocity”. One reason is that the flow rate is not reduced, and at the time of gas leakage, the gas is detected at an early stage, but there is another reason. This is because the use of a large diameter tube may cause an explosion when sampling hydrogen gas.

  The flame extinguishing distance of hydrogen gas, that is, the minimum distance necessary for generating a flame by explosion is 0.6 mm. That is, if the inner diameter of the sampling tube 10 is less than 0.6 mm, it is possible to prevent an explosion from occurring in the middle of the sampling tube 10 even if the distance of the sampling tube 10 is increased. In this embodiment, since the sampling tube 10 having an inner diameter of less than 0.6 mm is used, there is no particular problem of explosion, but the second membrane filter (metal) having a pore diameter smaller than the flame extinguishing distance of hydrogen gas. By providing the (manufactured filter) 15, it becomes possible to more reliably prevent explosion due to the sucked hydrogen gas.

  When the tube diameter is small and the pressure in the tube is lower, the flow of gas molecules flowing through the sampling tube 10 changes from a viscous flow to a molecular flow, and the mean free path in which molecules do not collide with each other increases. Becomes a concentrated state.

  When the air containing the sampled hydrogen gas is introduced into the mass spectrometer MS, it is ionized by the ionization unit 2, and the mass separation unit 4 performs separation by passing only hydrogen ions having a specific mass-to-charge ratio. The detection unit 6 detects the separated ions. Then, the detection unit 6 sends a detection signal to the data analysis processing device 8. Here, since a mass spectrum having a peak at a mass to charge ratio of 2 is obtained, the peak is recognized as a hydrogen molecule and detected. In this manner, the hydrogen gas leaked into the air is detected outdoors (monitoring area) by the mass spectrometer MS.

  Next, a method for calculating the concentration of hydrogen gas in the data analysis processing apparatus 8 will be described with reference to FIGS. First, a calibration curve of hydrogen gas prepared as a preparation for calculating the concentration is created, and calibration curve data is registered in the data analysis processing device 8. In preparing the calibration curve, several sample gases of hydrogen gas having different concentrations are prepared, sample gas is sampled from the sampling tube 10, and the sample gas is detected by the mass spectrometer MS. Then, a mass spectrum is obtained, and a peak intensity (also referred to as spectrum intensity) of mass to charge ratio (m / z) 2 is obtained. For the prepared sample gases having different concentrations, the peak intensities are sequentially obtained, and a logarithmic graph showing the relationship between the gas concentration and the peak intensity as shown in FIG. 2 is created and registered in the data analysis processing device 8. By creating this calibration curve data, the peak intensity can be obtained from the mass spectrum and the concentration of hydrogen gas can be calculated. In the calibration curve of FIG. 2, although the hydrogen concentration is 0, there is a slight peak intensity because water molecules adhering to the chamber become hydrogen ions, and the hydrogen ions are combined with other hydrogen ions. This is because it becomes a hydrogen molecule.

    Next, the effect of water adhering to the chamber constituting the detection unit 6 of the mass spectrometer MS will be described based on the mass spectra shown in FIGS. FIG. 3 is a mass spectrum of the mass spectrometer MS, and shows a case where the hydrogen gas concentration as a detection target is high, and FIG. 4 shows a case where the hydrogen gas concentration as a detection target is low. In the mass spectra of FIGS. 3 and 4, what appears at the mass-to-charge ratio (m / z) 2 of the X axis is the spectrum of the hydrogen molecule (H2). On the other hand, what appears in the mass-to-charge ratio (m / z) 1 on the X axis is a spectrum of hydrogen ions (H +).

  When water is attached in the chamber, the water molecule is detected as a water molecule (H 2 O) by the mass spectrometer MS, resulting in a mass-to-charge ratio 18 peak in the mass spectrum, but only slightly. However, water molecules are separated into hydrogen ions and hydroxyl ions, and a peak with a mass-to-charge ratio of 1 also occurs. The hydrogen ions separated and generated generate hydrogen molecules by further adhering to each other. That is, as a result, a small amount of hydrogen molecules are generated due to the presence of water molecules.

  Therefore, in the peak of mass-to-charge ratio 2, the majority is due to the hydrogen molecules of the detected hydrogen gas, but the influence of the hydrogen molecules generated based on the water molecules is very slight. It will appear at the peak. For this reason, when the hydrogen gas concentration to be detected is high, there is no particular effect. However, when the hydrogen gas concentration is somewhat low, for example, when the gas concentration is 1% or less, the gas is simply determined using the calibration curve from the peak intensity. If the concentration is calculated, an accurate gas concentration cannot be obtained.

  Therefore, in the present embodiment, when the concentration calculation is performed in the data analysis processing device 8, the spectrum intensity with the mass to charge ratio (m / z) of 1 (H +) and the mass to charge ratio (m / z). Detects the spectral intensity of 2 (H2) and subtracts the spectral intensity of the mass to charge ratio (m / z) of 1 (H +) from the spectral intensity of the mass to charge ratio (m / z) of 2 (H2). The hydrogen gas concentration is calculated.

    Assuming that the size of the peak of mass-to-charge ratio 1 generated from water molecules and the size of the peak of hydrogen molecules generated by water molecules appearing in the peak of mass-to-charge ratio 2 are almost equal, this calculation method is used. Thus, it becomes possible to eliminate the influence of hydrogen molecules caused by water molecules in the peak of mass to charge ratio 2, and to calculate a more accurate gas concentration.

  Note that there is humidity in the atmosphere, and this humidity varies depending on time and place. For this reason, when air is sampled in a high humidity area in the mass spectrometer MS of the present embodiment, the peak of the mass-to-charge ratio 1 becomes large and also affects the peak 2. Therefore, the influence of water molecules as described above A method of calculating the concentration that can eliminate the above is desired.

  By the way, when measuring a dilute concentration of hydrogen gas using a mass spectrometer as in this embodiment, the influence of fluctuations in atmospheric pressure appears, so accurate measurement is performed except for the influence of fluctuations in atmospheric pressure. It is necessary to carry out calibration for that.

  Here, an example of calibration will be described. In the atmosphere, argon is contained at an average concentration of about 9300 ppm, and since argon is hardly emitted from social industrial activities and human daily activities, these concentrations in the atmosphere are kept almost constant. It is less likely to fluctuate under the influence of the above activities. Therefore, the hydrogen gas in the atmosphere can be measured by correcting the change in concentration due to the fluctuation of atmospheric pressure using this argon as an index.

  In the present embodiment, when the hydrogen gas B is desired to be monitored, the argon concentration A0 in the atmosphere is first measured in advance in a standard state. The argon concentration At at an arbitrary time t during monitoring is measured, and the change in At with respect to the initial concentration A0 is calculated as a correction coefficient (A0 / At).

  When the atmospheric pressure varies, the corrected hydrogen gas concentration Bt at time t is measured by multiplying the hydrogen gas concentration Bt measured at a certain time t by the correction coefficient (A0 / At). is there.

It is a system block diagram for demonstrating the gas detection apparatus G of this invention. It is a graph of a calibration curve showing the relationship between hydrogen gas concentration and peak intensity. The case where high concentration hydrogen gas is detected in the mass spectrum of the mass spectrometer is shown. The case where low concentration hydrogen gas is detected in the mass spectrum of the mass spectrometer is shown.

Explanation of symbols

2 ionization section, 4 mass separation section, 6 detection section,
8 Data analysis processor, 10 Sampling tube, 12 Dry pump,
13 membrane filter, 15 membrane filter, 20 gas inlet tube,
22 needle valve, 30 heater,
MS mass spectrometer, TMP turbo molecular pump,

Claims (2)

In a gas detection apparatus comprising: a sampling tube that samples air from a monitoring area; and a mass spectrometer that is provided on a proximal end side of the sampling tube and ionizes the sampled air to perform mass analysis.
A gas detection apparatus that detects a spectral intensity having a mass-to-charge ratio of 1 and a spectral intensity having a mass-to-charge ratio of 2, and calculating a hydrogen gas concentration in the air. The gas detection apparatus according to claim 1, wherein a hydrogen gas concentration is calculated by subtracting a spectral intensity having a mass-to-charge ratio of 1 from a spectral intensity having a mass-to-charge ratio of 2. 5.

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