GAS MEASUREMENT DEVICE AND GAS MEASUREMENT METHOD

Abstract

After a measurement-target gas has been introduced into a measurement cell (40) to a predetermined pressure, a measurement by CRDS at a predetermined wavenumber is performed using a laser source unit (1), optical switch (3), optical resonator (4) and photodetector (5). A portion of the measurement-target gas is subsequently discharged from the measurement cell (40) to lower the pressure, and a measurement at the wavenumber of an absorption peak of the target component .sup.14CO.sub.2 is performed. Since the influence of the absorption by .sup.14CO.sub.2 in the measurement at high pressure is negligible, the concentration of the background, including .sup.12CO.sub.2, can be determined from a ring-down time determined in this measurement. An absorption coefficient calculated from a ring-down time determined from measurement data acquired at low pressure contains an influence of the background, while the absorption coefficient of the background at low pressure can be determined from the concentration of the background determined at high pressure. Using this absorption coefficient, a concentration-computing operator (73) determines the absorption coefficient of only .sup.14CO.sub.2 which is free from the influence of the background, and calculates the concentration of only .sup.14CO.sub.2. Thus, based on the results of the two measurements performed at different pressures, an accurate absolute concentration of a target component, such as .sup.14CO.sub.2, can be obtained. The measurement time can be shortened as compared to a conventional case.

Claims

1. A gas measurement method for determining a concentration of a target component contained in a measurement-target gas by cavity ring-down absorption spectroscopy, comprising: a first measurement step for performing a measurement by cavity ring-down absorption spectroscopy for a wavelength of an absorption peak of the target component under a first pressure by irradiating the measurement-target gas with laser light; a second measurement step for performing a measurement by cavity ring-down absorption spectroscopy by irradiating the measurement-target gas under a second pressure different from the first pressure with laser light; and a calculation step for calculating the concentration of the target component by performing a calculation on a measurement result of the first measurement step and a measurement result of the second measurement step.

2. The gas measurement method according to claim 1, wherein: the second measurement step is for performing the measurement by cavity ring-down absorption spectroscopy for a wavelength at which an influence of an absorption by the target component is negligible, the wavelength being different from the wavelength of the absorption peak of the target component; and the calculation step is for estimating a concentration of non-targeted components in the measurement-target gas under the second pressure based on the measurement result of the second measurement step, then estimate, from that concentration, a contribution of an absorption by the non-targeted components to an absorption coefficient determined from the measurement result of the first measurement step, and perform a calculation which removes an influence of the absorption by the non-targeted components.

3. The gas measurement method according to claim 1, wherein: the second measurement step is for performing the measurement by cavity ring-down absorption spectroscopy for the wavelength of the absorption peak of the target component; and the calculation step is for preparing a system of equations based on the measurement result of the first measurement step and the measurement result of the second measurement step, and to solve the system of equations to calculate the concentration of the target component from or in which an influence of the non-targeted components in the measurement-target gas is removed or reduced.

4. The gas measurement method according to claim 1, wherein: the measurement-target gas contains CO.sub.2 gas; and the target component is .sup.14CO.sub.2 which is one of isotopes in the CO.sub.2.

5. A gas measurement device configured to determine a concentration of a target component contained in a measurement-target gas by cavity ring-down absorption spectroscopy, comprising: a laser light emitter; an optical resonator including a measurement cell configured to contain a measurement-target gas, the optical resonator configured to produce oscillations of laser light emitted from the laser light emitter and introduced into the measurement cell; a photodetector configured to detect laser light extracted from the optical resonator; a pressure regulator configured to regulate a pressure of the measurement-target gas in the measurement cell; a controller configured to control the pressure regulator when performing a measurement for the measurement-target gas in the measurement cell by cavity ring-down absorption spectroscopy; and a calculation processor configured to calculate the concentration of the target component by performing a calculation on a plurality of measurement results respectively obtained at different pressures under a control of the controller.

6. The gas measurement device according to claim 5, wherein the controller is configured to control the laser emitter and the photodetector in addition to the pressure regulator so as to perform: a first measurement step for performing a measurement by cavity ring-down absorption spectroscopy for a wavelength of an absorption peak of the target component under a first pressure by irradiating the measurement-target gas with laser light; and a second measurement step for performing a measurement by cavity ring-down absorption spectroscopy by irradiating the measurement-target gas under a second pressure different from the first pressure with laser light.

7. The gas measurement device according to claim 6, wherein: the controller is configured to perform, in the measurement under the second pressure, the measurement by cavity ring-down absorption spectroscopy for a wavelength at which an influence of an absorption by the target component is negligible, the wavelength being different from the wavelength of the absorption peak of the target component; and the calculation processor is configured to estimate, based on a measurement result obtained under the second pressure, a concentration of non-targeted components in the measurement-target gas under the second pressure, then estimate, from that concentration, a contribution of an absorption by the non-targeted components to an absorption coefficient determined from a measurement result obtained under the first pressure, and perform a calculation which removes an influence of the absorption by the non-targeted components.

8. The gas measurement device according to claim 6, wherein: the controller is configured to perform the measurement by cavity ring-down absorption spectroscopy for the wavelength of the absorption peak of the target component in the measurement under the second pressure; and the calculation processor is configured to prepare a system of equations based on a measurement result obtained under the first pressure and a measurement result obtained under the second pressure, and to solve the system of equations to calculate the concentration of the target component from or in which an influence of the non-targeted components in the measurement-target gas is removed or reduced.

9. The gas measurement device according to claim 6, wherein the pressure regulator is configured to regulate the pressure of the measurement-target gas in the measurement cell to the first pressure by compulsorily discharging a portion of the measurement-target gas from the measurement cell to an outside, starting from a state in which the measurement cell contains the measurement-target gas under the second pressure.

10. The gas measurement device according to claim 6, wherein the pressure regulator is configured to regulate the pressure of the measurement-target gas in the measurement cell to the second pressure by additionally supplying the measurement cell with the measurement-target gas which remains unsupplied, starting from a state in which the measurement cell is filled with the measurement-target gas supplied beforehand and contains the measurement-target gas under the first pressure.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0074] FIG. 1 is a configuration diagram of the main components of a CRDS device as one embodiment of the present invention.

[0075] FIGS. 2A and 2B are graphs showing calculation results of the characteristics of the absorption by CO.sub.2 isotope gases at a pressure of 1013.25 Pa.

[0076] FIGS. 3A-3C are graphs showing calculation results of the characteristics of the absorption by CO.sub.2 isotope gases at a pressure of 10132.5 Pa.

[0077] FIG. 4 is a flowchart showing one example of the procedure of the measurement and processing in the case of determining the concentration of a target component in the CRDS device according to the present embodiment.

[0078] FIG. 5 is a flowchart showing another example of the procedure of the measurement and processing in the case of determining the concentration of a target component in the CRDS device according to the present embodiment.

[0079] FIGS. 6A and 6B are diagrams showing calculation results of the characteristics of the absorption by H.sub.2O isotope gases at a pressure of 1013.25 Pa.

[0080] FIGS. 7A and 7B are diagrams showing calculation results of the characteristics of the absorption by H.sub.2O isotope gases at a pressure of 101325 Pa.

[0081] FIG. 8 is a schematic configuration diagram of a commonly used CRDS device.

[0082] FIG. 9 is a schematic diagram showing a relationship between mode frequency and laser oscillation frequency in an optical resonator.

[0083] FIG. 10 is a schematic diagram of an absorption spectrum of a measurement-target gas around a peak wavelength of an absorption line of .sup.14CO.sub.2.

[0084] FIG. 11A is a graph showing a calculation result of an absorption spectrum of CO.sub.2 isotope gases, and FIG. 11B is a chart showing a calculation result of the degrees of contribution of the absorption by .sup.12CO.sub.2, .sup.13CO.sub.2 and .sup.14CO.sub.2 at the position of the absorption peak of .sup.14CO.sub.2.

[0085] FIG. 12A is a graph showing a calculation result of an absorption spectrum of CO.sub.2 isotope gases under the condition that the pressure of the measurement-target gas is increased, and FIG. 12B is a chart showing a calculation result of the degrees of contribution of the absorption by .sup.12CO.sub.2, .sup.13CO.sub.2 and .sup.14CO.sub.2 at the position of the absorption peak of .sup.14CO.sub.2.

DESCRIPTION OF EMBODIMENTS

[0086] A CRDS device as one embodiment of the gas measurement device according to the present invention, and a gas measurement method using the same device, are hereinafter described with reference to the attached drawings.

[0087] Initially, using FIGS. 11A-12B, the previously described problem to be solved by the present invention will be summarized, and the principle of the background removal in the present invention will be described.

[0088] FIGS. 11A and 12A each show a calculation result of an absorption spectrum of .sup.12CO.sub.2, .sup.13CO.sub.2 and .sup.14CO.sub.2 around the position of an absorption peak of .sup.14CO.sub.2. The only difference between FIGS. 11A and 12A is the assumed pressure of the measurement-target gas. Specifically, when the pressure is relatively low (in the present case, 0.03 atm), the absorption peak of .sup.14CO.sub.2 can be clearly observed, whereas the absorption peak of .sup.13CO.sub.2 whose base portion overlaps the absorption peak of .sup.14CO.sub.2 cannot be recognized in an isolated form. Therefore, spectrum fitting for the absorption by .sup.13CO.sub.2 cannot be correctly performed, and it is difficult to accurately estimate the background (i.e., the baseline of the peak).

[0089] A gas molecule normally has a plurality of absorption peaks corresponding to its rotation, translation and vibration. Even when signals overlap each other in an absorption peak at one wavelength in the previously described manner, the absorption coefficient may possibly be determined by using another absorption peak at another wavelength. However, the light source and mirrors used in a CRDS device each have a limited wavelength range within which it can work properly. It is often the case that the device cannot deal with the wavelengths of a plurality of absorption peaks of a target component in a measurement-target gas. There has practically been no effective method for measuring the absorption coefficient in such cases.

[0090] In a conventional CRDS measurement, it is assumed that the pressure and temperature of the measurement-target gas are constantly maintained. When the concentration of .sup.14CO.sub.2 needs to be determined, it is normal to set the pressure so that the absorption peak of .sup.14CO.sub.2 will be as high as possible, as shown in FIG. 11A. On the other hand, when the pressure of the measurement-target gas is increased to an appropriate level, the absorption peak of .sup.13CO.sub.2 disappears almost completely along with that of .sup.14CO.sub.2, as shown in FIG. 12A. However, this does not mean that the absorption by .sup.13CO.sub.2 has disappeared; it merely means that the absorption peak has disappeared, and the absorption itself is still present, as shown in FIGS. 12A and 12B. There is also the absorption by .sup.12CO.sub.2 having a higher concentration. That is to say, when the pressure of the measurement-target gas is increased as shown in FIG. 12A, the absorption at the position of the absorption peak of .sup.14CO.sub.2 can be considered to be due to the components other than the target component (.sup.14CO.sub.2), i.e., due to the background only. It should be noted that increasing the pressure increases the amount of absorption, so that the absorption coefficient itself has also significantly increased.

[0091] The CRDS measurement with the pressure of the measurement-target gas increased in the previously described manner yields a result (ring-down rate or ring-down time) which reflects the absorption by the non-targeted components. Based on the absorption coefficient calculated from this result, the absolute concentration of the non-targeted components can be calculated. From this absolute concentration, the spectrum of the baseline at the position of the absorption peak of .sup.14CO.sub.2 under the condition that the pressure of the measurement-target gas is relatively low as shown in FIG. 11A can be estimated. By subtracting this baseline from the absorption coefficient determined from a measurement result obtained by performing the CRDS measurement with a relatively low pressure of the measurement-target gas, the absorption coefficient of pure .sup.14CO.sub.2 can be determined. From this absorption coefficient, the absolute concentration of only the target component, .sup.14CO.sub.2, can be calculated.

[0092] Depending on the pressure of the measurement-target gas, the degree of absorption of .sup.14CO.sub.2 may be non-negligible. Even in that case, the absolute concentration of only the target component .sup.14CO.sub.2 can be calculated by solving a system of equations, as will be described later. As another possibility, the wavelength of the laser light used in the measurement may be varied in addition to the pressure of the measurement-target gas, to acquire a measurement result which reflects the baseline spectrum that is free from the influence of the absorption by the target component, and use that measurement result to determine the absorption coefficient of pure .sup.14CO.sub.2. This will also be described later.

[0093] One example of the CRDS device employing the previously described measurement principle is hereinafter described. FIG. 1 is a configuration diagram of the main components of the CRDS device according to the present embodiment.

[0094] The CRDS device according to the present embodiment has, as its measurement system, a laser source unit 1, laser driver 2, optical switch 3, optical resonator 4 and photodetector 5. The optical resonator 4 includes a substantially cylindrical measurement cell 40 configured to contain a sample gas as a measurement-target gas, as well as a pair of high-reflection mirrors 47 and 48 arranged at both ends of the measurement cell 40 and facing each other. A gas introduction tube 41 and gas discharge tube 43 are connected to the measurement cell 40. The gas introduction tube 41 has an introduction valve 42, while the gas discharge tube 43 has a discharge valve 44 and a vacuum pump 45. The measurement cell 40 is equipped with a pressure sensor 46 for detecting the pressure of the gas contained in the cell 40.

[0095] A control unit 6 is responsible for controlling the laser driver 2 and other related components in order to perform measurements and data processing as will be described later. This unit includes, as its functional blocks, a measurement controller 61, laser controller 62, pressure controller 63, measurement parameter storage section 64 and other related sections. In the measurement parameter storage section 64, measurement parameters, including the wavenumber (or wavelength) of the laser light and the pressure, are previously stored, being related to the kind of component to be subjected to the measurement or other types of information. A data processing unit 7, which receives detection signals from the photodetector 5, includes a measured data storage section 71, ring-down time calculator 72, concentration-computing operator 73, known information storage section for calculation 74, and other related sections as its functional blocks. The measured data storage section 71 includes an analogue-to-digital converter configured to digitize analogue detection signals. An output unit 8 connected to the data processing unit 7 is, for example, a display monitor.

[0096] A specific example of the operation in the CRDS device according to the present embodiment is hereinafter described on the assumption that the measurement-target gas is CO.sub.2, and the target component is .sup.14CO.sub.2, which is one of the isotopes of CO.sub.2. Concentration measurements of .sup.14CO.sub.2, which contains radioactive carbon .sup.14C, have been widely used in various areas.

[0097] FIG. 2A is a graph showing the result of a calculation of the absorption spectrum by CRDS for CO.sub.2 isotope gases at a pressure of 1013.25 Pa (=0.01 atm). The horizontal axis represents the wavenumber of light, and the vertical axis represents absorption coefficient. FIG. 2B is a pie chart showing a breakdown of the cause of the absorption at the wavenumber of light labelled “Condition 1” (the wavenumber of the absorption peak of .sup.14CO.sub.2) in FIG. 2A. For the calculation, it was assumed that the temperature was 200 K, the .sup.14CO.sub.2 concentration was 2×10.sup.−12, and the CO.sub.2 concentration exclusive of .sup.14CO.sub.2 was 0.2. It was also assumed that the CO.sub.2 isotopes, other than .sup.14CO.sub.2, contained in the measurement-target gas were introduced into the measurement cell 40 with their natural isotope abundance ratio.

[0098] As written in FIG. 2A, the calculated absorption coefficient at the wavenumber of the absorption peak of .sup.14CO.sub.2 at the aforementioned pressure was 3.49×10.sup.−10. As can be seen in FIG. 2B, approximately 82% of the absorption of light in the present case is due to .sup.14CO.sub.2, while the remaining portion, approximately 18%, is due to the other kinds of CO.sub.2 isotopes (.sup.12CO.sub.2 and .sup.13CO.sub.2). Accordingly, in order to calculate an accurate concentration of .sup.14CO.sub.2, it is necessary to subtract the baseline due to the absorption by CO.sub.2 exclusive of .sup.14CO.sub.2. The spectrum of this baseline can be determined if the concentration of CO.sub.2 exclusive of .sup.14CO.sub.2 is known. This information can be easily obtained if the absorption coefficient due to CO.sub.2 exclusive of .sup.14CO.sub.2 can be determined from the measured data. However, a problem exists in that accurate data which reflects the amount of absorption by CO.sub.2 exclusive of .sup.14CO.sub.2 cannot be obtained in the present situation since the amount of absorption in question is considerably low under measurement conditions that are optimized for .sup.14CO.sub.2 so that a sharp absorption peak of .sup.14CO.sub.2 can be observed as shown in FIG. 2A. In order to overcome this problem, in the present invention, measurements are performed at different pressures of the measurement-target gas, and the results of those measurements are used to determine the spectrum of the baseline due to the absorption by CO.sub.2 exclusive of .sup.14CO.sub.2.

[0099] In a common type of CRDS, the measurement for a measurement-target gas is performed with the pressure constantly maintained throughout the measurement. As is commonly known, the absorption coefficient of a component in a gas depends on temperature, pressure, wavelength of light and other related factors. Accordingly, in common cases, the pressure for a measurement of the absorption by .sup.14CO.sub.2 is set at a level that satisfies, for example, the condition that the difference between the absorption coefficient of the target component .sup.14CO.sub.2 and that of CO.sub.2 exclusive of .sup.14CO.sub.2 becomes as large as possible at the wavenumber of an absorption peak due to .sup.14CO.sub.2, because such a pressure condition is likely to yield the best signal-to-noise ratio for the observation of the absorption peak due to .sup.14CO.sub.2. Increasing the pressure of the measurement-target gas to be higher than the aforementioned level actually causes a dramatic increase in the heights of the absorption peaks of .sup.12CO.sub.2 and .sup.13CO.sub.2 at the wavenumbers of their respective absorption peaks, along with an increase in their peak widths. Consequently, the background level becomes considerably high, which lowers the signal-to-noise ratio of the absorption peak due to .sup.14CO.sub.2.

[0100] FIG. 3A is a graph showing the result of a calculation of the absorption spectrum by CRDS for CO.sub.2 isotope gases at a pressure of 10132.5 Pa (=0.1 atm), which is 10 times the pressure used in the case of FIG. 2A. FIG. 3B is a pie chart showing a breakdown of the cause of the absorption at a wavenumber labelled “Condition 2” in FIG. 3A (which is the same as the wavenumber under the aforementioned “Condition 1”). FIG. 3C is a pie chart showing a breakdown of the cause of the absorption at a wavenumber labelled “Condition 3” in FIG. 3A (which is an appropriate wavenumber smaller than “Condition 1”). Calculation conditions other than the pressure are identical to those in the case of FIGS. 2A and 2B.

[0101] The wavenumber of the absorption peak due to .sup.12CO.sub.2 is located beyond the left end of the graph shown in FIG. 3A. The height and peak width of that absorption peak rapidly increases with an increase in pressure. Although the height of the absorption peak due to .sup.14CO.sub.2 in the present situation is larger than ten times the height of the same absorption peak in the graph shown in FIG. 2A, the absorption peak is almost entirely buried in the tailing portion of the absorption peak due to .sup.12CO.sub.2. This demonstrates that increasing the pressure of the measurement-target gas considerably increases the overall level of the background, making it easier to detect the absorption by CO.sub.2 exclusive of .sup.14CO.sub.2, or improving the detection accuracy for that absorption. Accordingly, in the present invention, in addition to a CRDS measurement under a pressure condition in which the ratio of the absorption by the target component (.sup.14CO.sub.2) is relatively large, another CRDS measurement is performed for the same measurement-target gas under a pressure condition under which the level of the background becomes higher as just described.

[0102] As shown in FIGS. 3B and 3C, the absorption by .sup.14CO.sub.2 still forms approximately 7% of the entire absorption at the wavenumber of Condition 2, whereas the percentage of the absorption by .sup.14CO.sub.2 is zero at the wavenumber of Condition 3. The measurement which must be performed at a relatively high pressure for determining the background only needs to be performed under any one of the Conditions 2 and 3, although the method for processing the measurement result changes depending on which condition is used for the measurement. It should be noted that the wavenumber of Condition 3 is not limited to the position show in FIG. 3A. Any wavenumber that falls within a controllable range of the wavenumber of the laser light may be appropriately selected as long as the absorption peak of .sup.14CO.sub.2 is buried in the background, and the level of the background is high (i.e., as long as the wavenumber is located on the left side of the position of the absorption peak of .sup.14CO.sub.2 in FIG. 3A).

[0103] [Condition 3: When Absorption by .sup.14CO.sub.2 is Negligible]

[0104] At the wavenumber of Condition 3, the background due to the absorption by .sup.12CO.sub.2 and .sup.13CO.sub.2 is at such a high level that the absorption by .sup.14CO.sub.2 is negligible. Therefore, there is practically no influence of the absorption by .sup.14CO.sub.2 appearing in the result of the CRDS measurement. Accordingly, it is possible to calculate the absorption coefficient of the CO.sub.2 isotopes other than .sup.14CO.sub.2 based on the ring-down time determined from the thereby obtained measurement data, and to calculate, from that absorption coefficient, the concentration of the CO.sub.2 isotopes other than .sup.14CO.sub.2 at the relatively high pressure. By using the calculated concentration of the CO.sub.2 isotopes other than .sup.14CO.sub.2, the absorption coefficient corresponding to the background at the wavenumber of Condition 1 at the relatively low pressure with a strong absorption by .sup.14CO.sub.2 can be calculated. Accordingly, by subtracting the absorption coefficient corresponding to the background from the absorption coefficient determined from the measured result obtained under Condition 1, the absorption coefficient of only .sup.14CO.sub.2 can be determined, and from this absorption coefficient, the concentration of only the target component .sup.14CO.sub.2 can be calculated.

[0105] [Condition 2: When Absorption by .sup.14CO.sub.2 is Non-Negligible]

[0106] At the wavenumber of Condition 2, the absorption by .sup.14CO.sub.2 forms approximately 7% of the entire absorption, and is therefore non-negligible. In this case, a system of equations in which the concentration of .sup.14CO.sub.2 and that of the CO.sub.2 isotopes other than .sup.14CO.sub.2 are included as unknowns is prepared based on the measured result at the wavenumber of the absorption peak of .sup.14CO.sub.2 obtained at a relatively low pressure and the measured result at the wavenumber of Condition 2 obtained at a relatively high pressure. By solving the system of equations, the concentration of .sup.14CO.sub.2 and that of CO.sub.2 isotopes other than .sup.14CO.sub.2 can be calculated. Alternatively, a system of equations in which the absorption coefficient of .sup.14CO.sub.2 and that of the CO.sub.2 isotopes other than .sup.14CO.sub.2 are included as unknowns may be prepared and solved.

[0107] In any of these cases, the concentration of .sup.14CO.sub.2 calculated in the previously described manner is free from the influence of the absorption by the CO.sub.2 isotopes other than .sup.14CO.sub.2, or the influence is practically negligible. Therefore, the concentration of .sup.14CO.sub.2 in the measurement-target gas can be determined with a high level of accuracy. The pressure condition and the wavenumber of the used laser light for each of the measurements can be appropriately determined beforehand according to the kind of component to be subjected to the measurement or other related factors.

[0108] In the previously described methods, the concentration of the CO.sub.2 isotopes other than .sup.14CO.sub.2 is determined without discriminating between .sup.12CO.sub.2 and .sup.13CO.sub.2. When it is necessary to individually determine the concentrations of .sup.12CO.sub.2, .sup.13CO.sub.2 and .sup.14CO.sub.2, a measurement can be performed for each isotope gas under a different pressure condition which significantly increases the percentage of the absorption of the isotope gas in question, and the concentrations of the individual isotope gases can be calculated from the results of the three measurements.

[0109] Flowcharts of the operations for measuring the concentration of the target component (.sup.14CO.sub.2) in the measurement-target gas in the CRDS device according to the present embodiment are shown in FIGS. 4 and 5.

[0110] FIG. 4 is a flowchart showing one example of the procedure of the measurement and processing in the case of removing the background using a measured result obtained under Condition 3. It is assumed that the ring-down time at each pressure under the condition that the measurement-target gas containing CO.sub.2 is not present in the measurement cell 40 has been previously measured and is stored in the known information storage section for calculation 74. It is also assumed that prior information to be used for computing concentrations, such as the absorption cross section of the target component, is also stored in the known information storage section for calculation 74.

[0111] Initially, the pressure controller 63 in the control unit 6 opens the introduction valve 42, with the discharge valve 44 closed, to introduce a measurement-target gas into the measurement cell 40. When the pressure detected with the pressure sensor 46 has reached a predetermined level, the pressure controller 63 closes the introduction valve 42 to fill the measurement cell 40 with the measurement-target gas (Step S1). Subsequently, the pressure controller 63 opens the discharge valve 44 and energizes the vacuum pump 45, whereby the measurement-target gas in the measurement cell 40 begins to be discharged through the gas discharge tube 43. When the pressure detected with the pressure sensor 46 has decreased to a predetermined background (BG) measurement pressure P3 stored in the measurement parameter storage section 64, the pressure controller 63 closes the discharge valve 44 (Step S2). Consequently, the measurement cell 40 contains the measurement-target gas at pressure P3.

[0112] The laser controller 62 operates the laser source unit 1 through the laser driver 2 so that the wavenumber of the laser light will be a predetermined value v3 for the background (BG) measurement (Step S3). The measurement controller 61 performs the measurement under the condition of laser-light wavenumber v3 and pressure P3. Specifically, the measurement-target gas in the measurement cell 40 is irradiated with laser light, and the laser light is blocked at a predetermined timing with the optical switch 3. The data acquired with the photodetector 5 is collected from immediately before the blocking of the laser light until a predetermined period of time elapses (Step S4). The measurement data acquired at high pressure with the photodetector 5 in this step is temporarily stored in the measured data storage section 71. The measurement data acquired in this step is a set of data which contains, as a piece of information, a ring-down time t3 under Condition 3.

[0113] Subsequently, the pressure controller 63 opens the discharge valve 44 once more and energizes the vacuum pump 45, whereby the measurement-target gas in the measurement cell 40 begins to be discharged through the gas discharge tube 43 to the outside. When the pressure detected with the pressure sensor 46 has decreased to a target-component measurement pressure P1 stored in the known information storage section for calculation 74, the pressure controller 63 closes the discharge valve 44 (Step S5). Consequently, the measurement cell 40 contains the measurement-target gas at pressure P1, which is lower than P3.

[0114] Meanwhile, the laser controller 62 operates the laser source unit 1 through the laser driver 2 so that the wavenumber of the laser light will be a predetermined value v1 for the target-component measurement (Step S6). The measurement controller 61 performs the measurement under the condition of laser-light wavenumber v1 and pressure P1 to acquire measurement data over a predetermined period of time, as in Step S4 (Step S7). The measurement data sequentially acquired with the photodetector 5 in this step is also temporarily stored in the measured data storage section 71. This measurement data acquired at low pressure is a set of data which contains, as a piece of information, a ring-down time t1 under Condition 1 shown in FIG. 2A.

[0115] Strictly speaking, the measurement-target gas subjected to the measurement in Step S7 is not perfectly identical to the measurement-target gas in Step S4 since a portion of the measurement-target gas in the measurement cell 40 is discharged to the outside in Step S5 to decrease the pressure. However, since the distribution of the components in the measurement-target gas within the measurement cell 40 can be considered as uniform, the measurement-target gases subjected to the measurements in Steps S4 and S7 can be considered to be identical and merely different from each other in pressure.

[0116] In the data processing unit 7, the ring-down time calculator 72 calculates the ring-down time t3 based on the measurement data which has been acquired at high pressure and stored in the measured data storage section 71 (Step S8). Based on this calculated result as well as the ring-down time determined under Condition 3 with no measurement-target gas and stored in the known information storage section for calculation 74, the concentration-computing operator 73 calculates the absorption coefficient and determines the concentration from that absorption coefficient (Step S9). Those calculations are performed in the same manner as in a conventional method, in which the aforementioned equations (1) and (2) can be used, for example. In the present case, since the influence of the absorption by .sup.14CO.sub.2 is negligible, the value obtained in Step S9 is the concentration of the CO.sub.2 isotope gas exclusive of .sup.14CO.sub.2 in the measurement-target gas.

[0117] Subsequently, the ring-down time calculator 72 calculates the ring-down time t1 based on the measurement data which has been acquired at low pressure and stored in the measured data storage section 71 (Step S10). Based on this calculated result as well as the ring-down time determined under Condition 1 with no measurement-target gas and stored in the known information storage section for calculation 74, the concentration-computing operator 73 calculates the absorption coefficient (Step S11). The value obtained by this calculation is the absorption coefficient of the CO.sub.2 isotope gases inclusive of .sup.14CO.sub.2 in the measurement-target gas.

[0118] The concentration of the CO.sub.2 isotope gas exclusive of .sup.14CO.sub.2 has already been obtained in Step S9. From this concentration, the concentration-computing operator 73 calculates the absorption coefficient due to the CO.sub.2 isotope gas exclusive of .sup.14CO.sub.2 under the pressure and laser wavenumber of Condition 1. This absorption coefficient corresponds to the background. The concentration-computing operator 73 subtracts the absorption coefficient due to the CO.sub.2 isotope gas exclusive of .sup.14CO.sub.2 from the absorption coefficient due to the CO.sub.2 isotope gas inclusive of .sup.14CO.sub.2 to calculate the absorption coefficient due to .sup.14CO.sub.2 under Condition 1, and calculates the concentration of only .sup.14CO.sub.2 from this absorption coefficient (Step S12). The result is provided through the output unit 8.

[0119] As described to this point, the CRDS device according to the present embodiment can provide users with information of an accurate concentration of only .sup.14CO.sub.2 with the background removed.

[0120] Next, one example of the procedure of the measurement and processing in the case of removing the background using the measured result obtained under Condition 2 is described according to the flowchart shown in FIG. 5. Once again, it is assumed that a set of necessary information, including the ring-down time at each pressure under the condition that the measurement-target gas containing CO.sub.2 is not present in the measurement cell 40 and the absorption cross section of the target component, is already stored in the known information storage section for calculation 74.

[0121] The processing in Steps S21 and S22 is identical to the processing in Steps S1 and S2. By those steps, the measurement cell 40 is filled with the measurement-target gas at pressure P3. The laser controller 62 operates the laser source unit 1 through the laser driver 2 so that the wavenumber of the laser light will be a predetermined value v1 for the target-component measurement (Step S23). The measurement controller 61 performs a measurement under the condition of laser-light wavenumber v1 and pressure P3 to acquire measurement data (Step S24). This measurement data acquired at high pressure is a set of data which contains, as a piece of information, a ring-down time t2 under Condition 2.

[0122] Next, the same processing as in Step S is performed in Step S25 to decrease the pressure of the measurement-target gas contained in the measurement cell 40 to the target-component measurement pressure P1. While maintaining the laser wavenumber at the value v1 for the target-component measurement, the measurement controller 61 performs a measurement under the condition of laser-light wavenumber v1 and pressure P1 to acquire measurement data (Step S26). This measurement data acquired at low pressure is a set of data which contains, as a piece of information, a ring-down time t1 under Condition 1, as in the example of FIG. 4.

[0123] In the data processing unit 7, the ring-down time calculator 72 calculates the ring-down time t2 based on the measured data acquired at high pressure and stored in the measured data storage section 71 (Step S27). Based on the calculated result and the ring-down time determined under Condition 2 with no measurement-target gas, the concentration-computing operator 73 calculates an absorption coefficient α2 (Step S28). Since the influence of the absorption by .sup.14CO.sub.2 is non-negligible in the present case, the value obtained in this step is the sum of the absorption coefficient due to .sup.14CO.sub.2 and the absorption coefficient due to the CO.sub.2 isotope gas exclusive of .sup.14CO.sub.2 in the measurement-target gas under Condition 2.

[0124] Subsequently, the ring-down time calculator 72 calculates the ring-down time t1 based on the measurement data which has been acquired at low pressure and stored in the measured data storage section 71 (Step S29). This is identical to Step S10. Based on this calculated result as well as the ring-down time determined under Condition 1 with no measurement-target gas, the concentration-computing operator 73 calculates an absorption coefficient α1 (Step S30). The value obtained in this step is the sum of the absorption coefficient due to .sup.14CO.sub.2 and the absorption coefficient due to the CO.sub.2 isotope gas exclusive of .sup.14CO.sub.2 in the measurement-target gas under Condition 1.

[0125] Both the concentration x of .sup.14CO.sub.2 and the concentration y of the CO.sub.2 isotopes other than .sup.14CO.sub.2 are unknowns. Accordingly, a system of equations is prepared in which one equation expresses the relationship of the absorption coefficient α1 obtained by the measurement under Condition 1 and the concentrations x and y, while the other equation expresses the relationship of the absorption coefficient α2 obtained by the measurement under Condition 2 and the concentrations x and y. The concentration-computing operator 73 solves this system of equations to calculate the concentration of only .sup.14CO.sub.2 (Step S31).

[0126] As described to this point, an accurate concentration of only .sup.14CO.sub.2, with the background removed, can be calculated and presented to users.

[0127] In the previously described example, the absorption coefficient is determined from the ring-down time, and the concentration of .sup.14CO.sub.2 is subsequently calculated according to predetermined calculation formulae. Alternatively, the device may be configured so that the concentration of .sup.14CO.sub.2 can be derived by referring to a database instead of using calculation formulae. Specifically, for each of the measurement conditions (i.e., Conditions 1, 2 and 3), the values of the absorption coefficient observed for various combinations of the concentrations of .sup.14CO.sub.2 and other CO.sub.2 isotopes are previously determined and compiled into a database. After an absorption coefficient has been determined from measured data, that absorption coefficient can be used as an input to conduct a database search and derive the corresponding concentration of .sup.14CO.sub.2 and that of the other CO.sub.2 isotopes.

[0128] The previous description of the embodiment was concerned with the determination of the concentration of .sup.14CO.sub.2, which is one of the CO.sub.2 isotopes. Understandably, the CRDS device according to the present embodiment is also available for an analysis of other kinds of components in a measurement-target gas. Hereinafter briefly described is a measurement of H.sub.2O isotopes in a measurement-target gas as another application example. This type of measurement is comparable to the measurement of CO.sub.2 isotopes in terms of utility value.

[0129] FIG. 6A is a graph showing the result of a calculation of the absorption spectrum by CRDS acquired in the case of an absorption-peak measurement for DHO, which is an H.sub.2O isotope, at a gas pressure of 1013.25 Pa (=0.01 atm). FIG. 6B is a pie chart showing a breakdown of the cause of the absorption at a wavenumber indicated by the downward arrow in FIG. 6A (which is the position of the absorption peak of DHO). On the other hand, FIG. 7A is a graph showing the result of a calculation of the absorption spectrum by CRDS acquired in the case of an absorption-peak measurement for DHO, which is an H.sub.2O isotope, at a gas pressure of 101325 Pa (=1 atm), which is 100 times the value used in the case of FIGS. 6A and 6B. FIG. 7B is a pie chart showing a breakdown of the cause of the absorption at a wavenumber indicated by the downward arrow in FIG. 7A (which is the same wavenumber as indicated by the downward arrow in FIG. 6A).

[0130] For the calculation, it was assumed that the temperature was 353 K, and the H.sub.2O concentration was 0.03. It was also assumed that the H.sub.2O isotopes contained in the measurement-target gas were introduced into the measurement cell 40 with their natural isotope abundance ratio.

[0131] As shown in FIG. 6B, when the gas pressure is 1013.25 Pa, the absorption by DHO, which is one of the H.sub.2O isotopes, forms 93% of the entire absorption at the wavenumber of the absorption peak of DHO. On the other hand, as shown in FIG. 7B, increasing the gas pressure to a higher level which equals 100 times the aforementioned pressure considerably increases the degree of overall absorption, while dramatically decreasing the percentage of the absorption by DHO to 2%, with the absorption almost entirely caused by the H.sub.2O isotopes other than DHO. In this case, since the absorption by DHO cannot be entirely ignored, the background should be removed as in Condition 2 in the previously described example to determine an accurate absolute concentration of the target component, DHO.

[0132] Evidently, the device and method according to the present invention are effective for other various kinds of isotope gases in determining the concentration of an isotope having a particularly low concentration.

[0133] It should be noted that any of the previous embodiments is a mere example of the present invention, and any change, modification, addition or the like appropriately made within the spirit of the present invention will evidently fall within the scope of claims of the present application.

REFERENCE SIGNS LIST

[0134] 1 . . . Laser Source Unit [0135] 2 . . . Laser Driver [0136] 3 . . . Optical Switch [0137] 4 . . . Optical Resonator [0138] 40 . . . Measurement Cell [0139] 41 . . . Gas Introduction Tube [0140] 42 . . . Introduction Valve [0141] 43 . . . Gas Discharge Tube [0142] 44 . . . Discharge Valve [0143] 45 . . . Vacuum Pump [0144] 46 . . . Pressure Sensor [0145] 47, 48 . . . Mirror [0146] 5 . . . Photodetector [0147] 6 . . . Control Unit [0148] 61 . . . Measurement Controller [0149] 62 . . . Laser Controller [0150] 63 . . . Pressure Controller [0151] 64 . . . Measurement Parameter Storage Section [0152] 7 . . . Data Processing Unit [0153] 71 . . . Measured Data Storage Section [0154] 72 . . . Ring-Down Time Calculator [0155] 73 . . . Concentration-Computing Operator [0156] 74 . . . Known Information Storage Section for Calculation [0157] 8 . . . Output Unit