JP3505585B2 - Photoacoustic absorption meter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoacoustic absorptiometer for performing an absorption spectrum or a micro-spectroscopic analysis of a sample using a photoacoustic effect. 2. Description of the Related Art The photoacoustic effect is a phenomenon in which when a sample is irradiated with light whose intensity is modulated at an acoustic frequency, a photoacoustic signal is generated from the sample in accordance with the modulation frequency of the light. When a sample is irradiated with periodically modulated monochromatic light, the light absorbed by the sample is converted into heat through non-radiative relaxation, and the heat diffuses into the surrounding gas, etc., causing a periodic pressure change on the gas. Cause The photoacoustic absorptiometer detects and measures this periodic pressure change with an acoustic detector such as a microphone or a piezoelectric element. FIG. 6 is a schematic diagram of an apparatus of a photoacoustic spectroscopy system. Light from a high-intensity light source such as a Xe lamp or a light source 2 such as a laser is modulated by a chopper 4 and irradiated onto a sample in the photoacoustic cell 1. In the photoacoustic cell, a photoacoustic signal detected by a sound detector such as a microphone is converted into a lock-in amplifier 5 based on a reference signal synchronized with the modulated light source.
It is measured by signal processing 6. Note that the spectroscope 3 can be used to obtain monochromatic light. Conventionally, there has been known a photoacoustic absorption spectrometer that monitors the components and the like of airborne fine particles using the photoacoustic spectroscopy system. FIG. 7 is a block diagram of a conventional photoacoustic absorption meter. In FIG. 7, a double tube resonance type photoacoustic cell 10 is used as a photoacoustic cell. The sample air is sucked into the inner tube in the cell by a suction pump, and the semiconductor laser 2
Laser light from 0 is applied. The pressure change in the cell is detected by an acoustic detector such as a microphone, and is input to the lock-in amplifier 50 via the preamplifier 52. The lock-in amplifier 50 incorporates an oscillator 51, drives the semiconductor laser power supply 21, modulates the semiconductor laser 20, and measures a photoacoustic signal from the preamplifier 52 in synchronization with the cycle of the semiconductor laser. Then, the measured signal intensity and phase of the photoacoustic signal are recorded in the recorder 61 or the like. The photoacoustic signal from the preamplifier 52 is transmitted to the power meter 53 and the A / D
Based on the intensity of the laser beam detected by the converter 54, the light source intensity is compensated. [0004] However, the conventional photoacoustic absorptiometer has a problem that a measurement error occurs due to mixing of acoustic noise. By the conventional photoacoustic absorption meter, for example, when measuring the flue gas concentration of a diesel vehicle, simultaneously with the introduction of the sample atmosphere into the photoacoustic cell,
The running noise of a vehicle or noise in the measurement environment is mixed into the photoacoustic absorption meter as acoustic noise, and is measured together with the photoacoustic signal by the acoustic detector, which may cause a measurement error. FIG. 8 is a diagram for explaining the operation of noise in the conventional photoacoustic absorption spectrometer. In FIG. 8, the effective tube length (≒ L + D /
When noise having a wavelength twice as long as 2) (hereinafter referred to as a resonance wavelength L) enters, a sound signal of the wavelength causes a resonance phenomenon in the resonance tube, and its amplitude increases. Since the acoustic detector detects a noise component together with the photoacoustic signal from the measurement sample, if the amplitude of the noise component is increased, the noise component is increased and the detection sensitivity is reduced. [0005] As a method for preventing such noise from entering, a method of lengthening a suction pipe for introducing a measurement sample such as an air sample into the photoacoustic absorption spectrometer can be considered. In this case, the sample flows from the suction port. The measurement delays due to the delay time required to reach the photoacoustic cell, and the change is flattened. In addition, there is a problem that the measurement target is adsorbed in the suction pipe. Therefore, an object of the present invention is to solve the above-mentioned problems of the conventional photoacoustic absorption spectrometer and to provide a photoacoustic absorption spectrometer capable of reducing the influence of acoustic noise. SUMMARY OF THE INVENTION The present invention irradiates an intermittent light beam into a resonance tube of a double-tube resonance cell, and detects an intermittent sound generated by absorption of the light beam by a measurement sample in the resonance tube. And a photoacoustic absorption spectrometer for analyzing the measurement sample,
In the suction path for introducing a measurement sample into the resonance tube, sound absorption type acoustic resonance filter means for absorbing the resonance wavelength of the resonance tube is provided, and the sound absorption type acoustic resonance filter means is incorporated in the double tube type resonance cell. This achieves the above object. The acoustic absorptiometer of the present invention uses a resonance tube, intermittently irradiates a measurement sample in the resonance tube with light, and performs measurement by the photoacoustic effect.The measurement sample is supplied from outside via an intake path. It is introduced into the resonance tube. Further, the acoustic resonance filter means of the present invention has a function of reducing acoustic noise by absorbing sound waves of a specific wavelength from noise entering the resonance tube, and introducing a measurement sample from outside air into the resonance tube. It can be provided in the intake path of the vehicle, and absorbs an acoustic signal of a resonance wavelength of the resonance tube. A first embodiment of the present invention is a sound absorbing tube in which an acoustic resonance filter means is closed at one end, wherein the length of the sound absorbing filter is an odd multiple of half the length of the resonance tube. ,
The length may be half of the length of the resonance tube. Thus, the acoustic signal of the resonance wavelength of the resonance tube is resonated in the sound absorbing tube to absorb sound, and the introduction of the acoustic signal of the resonance wavelength to the resonance tube can be reduced. According to a second embodiment of the present invention, the frequency of the acoustic signal absorbed by the acoustic resonance filter means is the same as the frequency of the intermittent light beam, whereby
A frequency component corresponding to the frequency of the intermittent light beam can be removed from the acoustic signal entering from the intake pipe. In a third embodiment of the present invention, the length of the sound absorbing tube constituting the acoustic resonance filter means is made variable, whereby the wavelength of the sound signal to be absorbed is made variable or the temperature is changed. Temperature compensation can be performed by corresponding to the wavelength change of the acoustic signal. As a means for changing the length of the acoustic resonance filter means, a structure in which one end of the sound absorbing tube is closed by a movable stopper, or a structure in which the length of the sound absorbing tube itself is variable can be used. In a fourth embodiment of the present invention, the open end of the sound absorbing tube constituting the acoustic resonance filter means is closed by a flexible film to prevent the introduction of the measurement sample into the inside of the tube. Accelerates the introduction of the measurement sample into the resonance tube,
Responsiveness can be improved. According to a fifth embodiment of the present invention, the end of the intake pipe is arranged near the end of the resonance pipe and at a position where the light beam is not blocked, thereby introducing the measurement sample into the resonance pipe. And the responsiveness can be improved. The photoacoustic absorptiometer of the present invention has the above-described configuration, and emits an intermittent light beam into the resonance tube, introduces a measurement sample into the resonance tube, and generates light by absorption of the light beam by the measurement sample. The intermittent sound is detected and the measurement sample is analyzed. In this photoacoustic absorption spectrometer, in the intake path through which the measurement sample is introduced into the resonance tube, the noise having entered the intake tube together with the measurement sample has a frequency of 1/4 wavelength of the resonance wavelength of the resonance tube. The acoustic signal is absorbed by the acoustic resonance filter means, and the acoustic signal of the wavelength component causing resonance in the resonance tube is removed. Thus, the acoustic signal introduced into the resonance tube does not include an acoustic signal causing resonance in the resonance tube, and does not cause resonance. Therefore, the acoustic signal detected by the acoustic detector of the resonance tube is substantially an acoustic signal obtained by the photoacoustic effect based on the measurement sample, and the influence of acoustic noise can be reduced. In the case where the acoustic resonance filter means is a tube whose one end is closed and whose length corresponds to half the length of the resonance tube as described in the first embodiment, the wavelength at which resonance occurs in the resonance tube is performed. The acoustic signal of the tube causes resonance in this tube and is absorbed,
It does not reach the resonance tube. Therefore, the acoustic signal that reaches the resonance tube after passing through the acoustic resonance filter means is an acoustic signal of various wavelengths excluding the resonance signal. When this acoustic signal is introduced into the resonance tube, the acoustic signals do not cause an acoustic resonance phenomenon, but are mutually interfered and averaged in the resonance tube, and the peak value decreases. Therefore, the acoustic signal detected by the acoustic detector is mainly an acoustic signal obtained by the photoacoustic effect based on the measurement sample, and the measurement sample with little acoustic noise can be analyzed. An embodiment of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a block diagram illustrating the configuration of an embodiment of the photoacoustic absorption meter of the present invention. FIG. 2 is a block diagram illustrating the configuration of the photoacoustic absorption meter of the present invention. FIG. 3 is a perspective view including a partial cross section of the double-tube acoustic cell of FIG. FIG. 1 shows the configuration of a double-tube acoustic cell in a photoacoustic absorptiometer, an intake pipe for introducing a measurement sample into the double-tube acoustic cell, and an acoustic resonance filter. The double-tube acoustic cell 11 is an ordinary double-tube resonance photoacoustic cell, and includes a resonance tube 12 installed inside and an outer sound insulation tube 13 surrounding the entire resonance tube 12, and a measurement sample at one end. Is connected, and the other end is exhausted by an exhaust pump. Resonance tube 1
The length of the tube 2 is formed to be half the wavelength of the acoustic signal to be detected, and an acoustic detector 15 for detecting the detected acoustic signal is provided substantially at the center of the tube. As the acoustic detector 15, for example, a microphone or a piezoelectric element can be used. The detected sound signal detected by the sound detector 15 is detected by the lock-in amplifier in synchronization with the intermittent frequency of the excitation laser light. On the side of the intake pipe 31 opposite to the double-tube acoustic cell 11 side, an intake port 32 for taking in a measurement sample is formed open, and the measurement sample sucked from the intake port 32 is The sound is sent to the resonance tube 12 in the double-tube acoustic cell 11 through the intake pipe 31. Then, in the photoacoustic absorption meter of the present invention, an acoustic resonance filter 33 is provided in a part of the intake pipe 31. The installation position of this acoustic resonance filter can be an arbitrary position of the intake pipe 31. The acoustic resonance filter 33 is a tube whose one end is open and communicates with the intake pipe 31 and the other end of which is closed by a plug 34 or the like. Is formed to half the length of the tube. This tube length corresponds to a length of 1/4 of the wavelength of the acoustic signal to be detected. Therefore, the noise of the measurement environment as well as the measurement sample also intrudes from the suction port 32 of the suction pipe 31 at the same time. The acoustic resonance filter 33 absorbs a sound signal having a wavelength corresponding to the tube length of the acoustic resonance filter from the intruded noise. Proceeds toward the double-tube acoustic cell 11. FIG. 2 shows an embodiment of the above-described double-tube acoustic cell 11, in which the resonance tube 12 and the acoustic resonance filter 33 are incorporated in the sound insulating tube 13 of the double-tube acoustic cell 11. Is shown. A laser excited from a semiconductor laser or the like passes through the resonance tube 12 in the axial direction, and an acoustic detector tube 16 is attached to the outside of the sound insulation tube 13 at a substantially central position of the resonance tube 12. . The acoustic resonance filter 33 includes a sound absorbing tube 35, a plug 36 movable in the axial direction inside the sound absorbing tube 35, and a rod 37 for moving the plug 36.
By operating the rod 37 from outside the sound insulating tube 13, the plug 36 is moved within the sound absorbing tube 35, and the length of the acoustic tube in the sound absorbing tube 35 is adjusted. A branch pipe 39 is provided in the vicinity of an end of the acoustic resonance filter 33 opposite to the plug 30 of the sound absorbing pipe 35 through a communication port 38, and the other end of the branch pipe 39 is connected to one end of the resonance pipe 12. Are disposed in the vicinity of the open end. Note that the acoustic resonance filter 33 can be arranged almost in parallel with the resonance tube 12, although the positional relationship with the resonance tube 12 is arbitrary. In the case of this parallel arrangement, the acoustic resonance filter 33 can be compactly stored in the sound insulation tube 13. An intake pipe 31 is attached to the acoustic resonance filter 33 near the end opposite to the plug of the sound absorption pipe 35 toward the outside of the sound insulation pipe 13. The passed measurement sample is guided to the sound absorbing tube 35 and passes through the communication port 38 from the branch tube 39 to the resonance tube 1.
2 is introduced. The open end of the branch tube 39 is disposed at a position in the resonance tube 12 where laser light is not blocked. Further, the end of the intake pipe and the resonance pipe can be arranged so as to be close to each other so that the laser light is not blocked by the intake pipe. With this configuration, the measurement sample is introduced from the intake pipe to the resonance pipe. Can be accelerated, and responsiveness can be improved. Further, as a means for adjusting the length of the acoustic pipe in the sound absorbing tube, a configuration in which the length of the tube itself is variable may be used instead of moving the plug of the sound absorbing tube. (Operation of the embodiment of the present invention) Next, the operation of the embodiment of the present invention will be described with reference to the operation diagram of the embodiment of the present invention in FIG. 3 and the measurement result example in FIG. In the operation diagram of FIG. 3, noise in the measurement environment enters the double-tube acoustic cell 11 together with the measurement sample from the air inlet 32. When the intermittent excitation laser light acts on the measurement sample that has entered the resonance tube 12, an acoustic signal corresponding to the frequency of the excitation laser light is generated. By setting the length of the resonance tube 12 to be a half of the wavelength corresponding to the frequency of the excitation laser light, the acoustic signal generated in the resonance tube 12 resonates, and efficient detection of the acoustic signal is performed. On the other hand, the noise that enters the resonance tube 12 together with the measurement sample has various wavelengths, and includes wavelengths that are twice as long as the length of the resonance tube 12. When noise having a wavelength of this length enters the resonance tube 12, a resonance phenomenon occurs in the resonance tube 12, and
The sound detector 15 detects the noise as noise. In the embodiment of the present invention, when noise enters through the intake port 32, the noise having a wavelength twice as long as the length of the resonance tube 12 (hereinafter referred to as resonance wavelength L) is an acoustic resonance filter. The sound is absorbed by 33. Since the length of the acoustic resonance filter 33 is half the length of the resonance tube 12, it is １ ／ of the resonance wavelength L. In this wavelength relationship, the plug portion of the tube of the acoustic resonance filter 33 becomes a node,
The open end portion becomes an antinode, and the acoustic resonance filter 33 captures and absorbs the noise of the wavelength. Therefore, the intake pipe 3 after the acoustic resonance filter 33
The sound signal of the resonance wavelength L is removed during the noise in 1 and the sound signal of the wavelength excluding the resonance wavelength L is included in the noise introduced into the resonance pipe 12 through the intake pipe 31. When a sound signal having a wavelength other than the resonance wavelength L enters the resonance tube 12, the length of the resonance tube 12 is half of the resonance wavelength, so that no resonance phenomenon occurs in the resonance tube 12, and the resonance tube 12 12
There will be no loud noise inside. Therefore, most of the sound signal detected by the sound detector 15 is a photoacoustic signal generated by a laser beam, and the effect of acoustic noise is reduced to obtain a photoacoustic signal having a large S / N ratio. be able to. FIG. 4 shows an example of a measurement result according to the embodiment of the present invention, in which the horizontal axis indicates the relative value of the supplied sound volume that enters from the intake port, and the vertical axis indicates the noise sensitivity in μV. ing. Note that this noise sensitive amount is a value (root mean sq) obtained by calculating a root-mean-square square of the swing amount of the detection signal.
ure value). Further, in the figure, the symbol ■ indicates a value when the acoustic resonance filter is not used, and the symbol ● indicates a value when the acoustic resonance filter is used. From the results of FIG. 4, it can be seen that when the acoustic resonance filter is used, the fluctuation of the detection signal is suppressed low, and when the acoustic resonance filter is not used, large fluctuations are caused due to the mixing of sound near the resonance frequency. . (Other Embodiment) Next, another embodiment of the present invention will be described with reference to FIG. This embodiment is different from the above embodiment in the configuration of the acoustic resonance filter.
Since other configurations are the same, only different configurations will be described below. In another embodiment, in FIG. 5, a flexible film 41 is provided at an opening connected to an intake pipe of an acoustic resonance filter. The flexible film 41 is a material having a function of transmitting a pressure change such as a sound signal, but not transmitting gas or fine particles.
It can be formed by a thin rubber film. When the flexible film 41 is provided in the opening connected to the intake pipe of the acoustic resonance filter, the measurement sample such as gas and fine particles that have entered through the intake port is prevented from entering by the flexible film 41, Instead of entering the tube of the resonance filter, it is sent to the photoacoustic cell through the intake pipe as it is. On the other hand, the pressure change due to the noise that has entered from the intake port moves the flexible membrane 41, transmits the pressure change to the inside of the tube of the acoustic resonance filter, and absorbs the sound signal of the resonance wavelength. Therefore,
The acoustic resonance filter reduces the effect of acoustic noise, which is the object of the present invention, and prevents the measurement sample from being introduced into the tube of the acoustic resonance filter, thereby preventing a reduction in response speed. (Effects of the Embodiment) The length of the tube constituting the acoustic resonance filter means includes half the length of the resonance tube.
By setting the length to an odd multiple of the length of / 2, the acoustic signal of the resonance wavelength of the resonance tube is resonated in the tube to absorb sound, and the introduction of the acoustic signal of the resonance wavelength into the resonance tube can be reduced.
Further, by setting the frequency of the acoustic signal absorbed by the acoustic resonance filter means to be the same as the frequency of the intermittent light beam, a frequency component corresponding to the frequency of the intermittent light beam can be removed from the acoustic signal entering from the intake pipe. Also, by making the tube length of the tube part constituting the acoustic resonance filter means variable, it is possible to make the wavelength of the sound signal to be absorbed variable, or to perform temperature compensation in response to the wavelength change of the acoustic signal due to a temperature change or the like. it can. Further, by closing one end of the tube of the acoustic resonance filter means with a movable stopper, the length of the tube can be changed by an operation from outside the apparatus. Further, by adopting a configuration in which the open end of the tube part constituting the acoustic resonance filter means is closed by a flexible film and the introduction of the measurement sample into the tube part is prevented, the introduction of the measurement sample into the resonance tube is expedited,
Responsiveness can be improved. In addition, by arranging the end of the intake pipe near the end of the resonance pipe and not blocking the light beam that excites the photoacoustic effect, introduction of the measurement sample into the resonance pipe is expedited, and responsiveness is improved. Can be improved. As described above, according to the present invention,
A photoacoustic absorptiometer capable of reducing the influence of acoustic noise can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram illustrating a configuration of an embodiment of a photoacoustic absorption meter according to the present invention. FIG. 2 is a perspective view including a partial cross section of a double-tube acoustic cell of the photoacoustic absorption meter of the present invention. FIG. 3 is an operation diagram of the embodiment of the present invention. FIG. 4 is an example of a measurement result according to an example of the present invention. FIG. 5 is a block diagram illustrating a configuration of another embodiment of the present invention. FIG. 6 is a schematic diagram of a photoacoustic spectroscopy system. FIG. 7 is a block diagram of a conventional photoacoustic absorption meter. FIG. 8 is a diagram illustrating the operation of noise in a conventional photoacoustic absorption meter. [Description of Signs] 1 ... Photoacoustic cell, 2 ... Light source, 5 ... Loyne amplifier, 11
... double tube type resonance cell, 12 ... resonance tube, 13 ... sound insulation tube, 1
5 ... Acoustic detector, 31 ... Intake pipe, 32 ... Intake port, 33 ...
Acoustic resonance filter, 34 ... stopper.

──────────────────────────────────────────────────の Continuation of the front page (72) Inventor Kirisa Motosada 1-3-3 Kandanishikicho, Chiyoda-ku, Tokyo Shimadzu Corporation Tokyo branch office (56) References JP-A-55-50142 (JP, A) Kaihei 6-22558 (JP, U) Kenji Kato, "Development of real-time analysis method of elemental carbon suspended particles in the atmosphere by semiconductor laser photoacoustic method", Analytical Chemistry, Japan, Japan Society for Analytical Chemistry, 1994 April 5, 2003, Vol. 43, No. 4, pp. 317-323 (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 29/00-29/28 G01N 21/00 JICST file (JOIS )

Claims (1)

(57) [Claims 1] An intermittent light beam is irradiated into a resonance tube of a double tube type resonance cell, and an intermittent sound generated by absorption of the light beam by a measurement sample in the resonance tube is detected. A photoacoustic absorptiometer for analyzing a measurement sample, comprising: a sound absorption type acoustic resonance filter means for absorbing a resonance wavelength of the resonance tube in an intake path for introducing the measurement sample into the resonance tube; A photoacoustic absorptiometer characterized in that a filter means is built in the double tube type resonance cell.