CH719928A2 - Photo-acoustic analyzer. - Google Patents

Abstract

The present invention relates to a photo-acoustic measuring device comprising a photo-acoustic chamber, a source of modulated electromagnetic waves (51) and at least one acoustic transducer (533). The energy of modulated electromagnetic waves excites several acoustic modes, at least one of which has regions having amplitudes of opposite signs. Said mode is excited so as to prevent complete compensation of its positive and negative components, thus generating a photo-acoustic signal. Using ratios between the intensity of this mode and that of other modes leads to robust measurement of chemical species. The invention also relates to a use of such a device for the analysis of chemical species.

Description

Technical area

[0001] The present invention relates to a photo-acoustic device for detecting and measuring chemical species.

State of the art

[0002] It is known from patent applications US3820901, US3938365 and EP0855592, photo-acoustic systems allowing the measurement of gaseous species. These use a source of electromagnetic waves modulated in amplitude and whose emitted wavelength is adapted to the species to be measured; said species being within a gas located in a chamber equipped with an acoustic transducer. The species to be measured present in the gas and having an absorption band in the range of wavelengths emitted by the electromagnetic wave source, are thus excited and will transfer the energy acquired to the gaseous medium, thus generating an acoustic signal . This acoustic signal has a frequency corresponding to the modulation frequency of the source and an amplitude which depends on the concentration of the species having absorbed the energy of the electromagnetic waves and on the intensity of the source of electromagnetic waves. Measuring this acoustic signal via a suitable transducer, for example a microphone, thus allows the detection and quantification of a given chemical species. The selectivity of the analysis comes from the fact that the wavelength position of the absorption bands varies depending on the chemical species, which makes it possible to target a particular species by choosing the range of wavelengths emitted by the source. electromagnetic waves. In order to increase the selectivity of such analyzers (also called photo-acoustic analyzers), they often use laser sources or optical filters in order to limit the bandwidth, in wavelength, of the source of the electromagnetic waves. Other photoacoustic analyzers use optical spectral correlation to select the chemical species to analyze, such analyzers are described, for example, in the following articles: Luft KF, Z. Tech. Phys. 5 pages 97 to 104 (1943), Fastie WG and Pfund AH, J. Opt. Soc. Am. 37 pages 762 to 768 (1947) as well as Rey JM and Sigrist MW, Rev Sci. Instrument. 78 Article No. 063104 (2007). Different types of photo-acoustic detectors have been described in the article by Miklôs A, Hess P and Bozôki Z, Rev. of Sci. Instrument. 74 pages 1937 to 1955 (2001) and in that of Schmid T, Anal. Bioanal. Chem. 384 pages 1071 to 1086 (2006). The use of resonant acoustic cavities as photoacoustic chambers containing the gas to be measured is also described. Such chambers have the advantage of amplifying the acoustic signal and thereby increasing the sensitivity of the photo-acoustic device when the source of electromagnetic waves is modulated at a frequency close to or equal to the resonance frequency of said chambers. When the acoustic wave of a room or its contents oscillates at a given resonance frequency, the oscillation takes place in its own acoustic mode, also called normal mode or simply mode in the rest of the text. For a given mode, the spatial and temporal components of the acoustic oscillation can be represented separately. Each mode can therefore be characterized or represented by the spatial distribution of the amplitude of its acoustic pressure, the acoustic pressure representing the pressure deviation relative to the average pressure in the room. The normal modes are determined mainly by the physical properties of the medium, by the boundary conditions and the geometry of the chambers or resonant systems. In the remainder of the presentation of the invention, a mode must be understood as the spatial distribution of the acoustic pressure amplitude for a system oscillating at a frequency close to or equal to one of its resonance frequencies. For photo-acoustic analyzers, different types of acoustic resonances have been proposed: longitudinal, axial, radial, azimuthal and Helmholtz resonance.

[0003] In general, a photo-acoustic signal being proportional to the intensity of the electromagnetic wave source used, a device for measuring this intensity is necessary in order to allow the normalization of said photo-acoustic signal and to obtain thus a standardized signal allowing the quantification of the chemical species to be measured. Measuring the intensity of the electromagnetic wave source is generally based on the use of semiconductor photodiodes, pyroelectric or thermoelectric detectors.

Presentation and advantage of the invention

[0004] The aim of the present invention is to propose a photo-acoustic measuring device for chemical species in the gas phase and/or in contact with a gas phase. Said device is simple, compact, easily adaptable to the detection of various chemical species and one of its advantages comes from its increased selectivity and obtained without requiring the use of sources of electromagnetic waves with low bandwidth or optical filters with low bandwidth. passing. Another advantage of said device is that it can do without a separate device for measuring the intensity emitted by the source of electromagnetic waves used. Another advantage of said device comes from its low sensitivity to variations and drifts both in intensity and in wavelength of the electromagnetic wave source used. Another advantage of said device comes from its low sensitivity to drift from the acoustic transducer(s) used. Other characteristics and advantages of the invention compared to the current state of the art will appear in the preferred embodiments of the invention.

[0005] This aim is obtained by a photo-acoustic device according to any one of claims 1 to 9. The photo-acoustic chamber of said device is provided with orifices equipped with closures allowing the introduction of the gaseous, liquid or solid species required according to the different embodiments of the invention. At least one source of electromagnetic energy, for example coming from a laser, a light-emitting diode (LED), a black body or the radiative de-excitation of previously excited atoms or molecules, emitting at least one length wave corresponding to at least one absorption band of the chemical species(s) to be measured is used to generate an acoustic signal in said photo-acoustic chamber. One or more beam(s) of electromagnetic waves coming from the source of the electromagnetic waves is/are modulated in amplitude, or in frequency or spatially so as to contain, simultaneously or successively, at least one or more frequency(s) corresponding to one or more mode(s) of the photo-acoustic chamber. This modulation can be generated, for example, by direct modulation of the current exciting the electromagnetic wave source or by the use of a chopper or by the use of one or more oscillating mirror(s). s).

Said beam(s) of electromagnetic energy is/are conducted into the photo-acoustic chamber via one or more transparent windows for the useful wavelengths. Inside the photo-acoustic chamber, part of the electromagnetic energy is absorbed by the chemical species(s) to be measured which, by de-energizing, will transmit its/their energy ( s) to the gas which surrounds it/them thus generating an acoustic wave. The acoustic wave thus generated is then detected using at least one acoustic transducer such as, for example, an optical, electret, condenser, piezoelectric or piezoresistive element microphone. The signal coming from the acoustic transducer(s) is then processed according to methods known to those skilled in the art, these treatments including for example: phase detection (Lockin), the various transforms (for example: Fourier or Laplace), analog or digital filtration, averaging and digitalization. When the photo-acoustic chamber is equipped with several acoustic transducers, one or more linear combination(s) of the amplitudes of the signals coming from said acoustic transducers can be formed and used, instead of places the amplitudes themselves, for the measurement of the chemical species(s) to be measured. In the simplest cases, such linear combinations can be obtained by directly adding or subtracting, before or after demodulation, the signals from the acoustic transducers.

[0007] In a photo-acoustic device and for a given mode, the amplitude of the acoustic wave generated depends on the intensity of the incident electromagnetic waves and depends on the location where the electromagnetic excitation energy is absorbed. Each mode having its own resonant frequency, it is possible to excite one or more mode(s) using modulated or pulsed electromagnetic waves so as to contain one or more frequency(s) close to or equal to it. to those of the mode(s) considered.

[0008] Thus, by way of example, for a cylindrical tube closed at both ends the fundamental longitudinal mode presents an acoustic pressure node at the center of the cylinder. This node (or more precisely the plane passing through this node and perpendicular to the axis of the cylinder) separates the cylinder into two regions (called left half-cylinder and right half-cylinder in the following), each having an amplitude of the fundamental mode longitudinal of opposite sign from each other. This leads to the fact that, if an electromagnetic wave absorbed in the left half-cylinder generates a positive acoustic signal, an electromagnetic wave (of identical phase) absorbed in the right half-cylinder generates a negative acoustic signal. If an electromagnetic wave propagates along the axis of the cylinder and homogeneously deposits energy along its path, the fundamental longitudinal mode will be excited both positively (in the left half-cylinder) and negatively (in the right half-cylinder). In this example, the positive and negative components compensate for each other and the fundamental longitudinal mode is therefore almost not excited and the acoustic signal is almost zero. On the other hand, if the energy of the electromagnetic wave is deposited mainly in the left half-cylinder, the compensation of the positive and negative components is incomplete and an acoustic signal is obtained. Still as an example, if the cylinder contains a chemical species which strongly absorbs electromagnetic waves at a wavelength λ1 and very little at a wavelength λ2, the energy absorption at λ1 will not be uniform along the axis of the cylinder and an acoustic signal will be obtained for the fundamental longitudinal mode. For electromagnetic waves of wavelength λ2. the absorption will be almost uniform along the axis of the cylinder and the acoustic signal for the fundamental longitudinal mode almost zero. Thus, only wavelengths where the absorption is high lead to a significant acoustic signal for this mode, this leads to a detector having a very selective optical spectral response. Furthermore, and again by way of example, the cylinder described above can be excited at the frequency of its first radial mode. This radial mode has an amplitude of constant sign along the axis of the cylinder, compensation between positive and negative components is therefore not present for this mode along the axis of the cylinder. The acoustic signal obtained for this radial mode with a beam propagating along the axis of the cylinder is therefore proportional to the intensity of the incident electromagnetic waves and can therefore be used to compensate for possible variations in intensity or drifts from the source. electromagnetic waves.

The sample to be analyzed can either be present in the photo-acoustic chamber, or be introduced into a sample chamber traversed by at least part of the electromagnetic waves which then enter the photo-acoustic chamber. In the latter case, the photo-acoustic chamber contains a fixed quantity of the chemical species(s) to be measured and serves as a specific detector for wavelengths strongly absorbed in the photo-acoustic chamber. Changing the chemical species(s) present in the photo-acoustic chamber makes it possible to adjust the chemical species(s) that this device can measure, detect or quantify.

The device of this present invention is based on inhomogeneous absorption of electromagnetic waves in the photo-acoustic chamber, which is possible when the optical absorption coefficient (alpha) in the photo-acoustic chamber is sufficiently high. If this is low, inhomogeneity of the absorption can be obtained by circulating the electromagnetic waves between partially reflecting surfaces (for said electromagnetic waves) placed in the photo-acoustic chamber or on its walls. This also makes it possible to adjust, via the arrangement of the partially reflective surfaces, the region(s) where the electromagnetic waves are absorbed and thus makes it possible to adjust the excitation of the acoustic mode(s). ). Absorption inhomogeneity can also be introduced when one or more partially reflecting surface(s) is/are replaced by one or more diffusing or diffracting surface(s).

[0011] The present invention also applies to the case where electromagnetic energy is absorbed by one or more non-gaseous sample(s) which then transmit(s) part of this energy to the gas which surrounds it/them and thus excite one or more modes of the photo-acoustic chamber. In this case, the non-gaseous sample(s) is/are placed either on the walls of the photoacoustic chamber or in the volume of the photoacoustic chamber. The present invention also applies to the case where the excitation of the mode(s) comes from absorption of electromagnetic energy both by the gas present in the photo-acoustic chamber and by one or more samples. non-gaseous present either on the walls or in the volume of the photo-acoustic chamber.

The device of this invention is not limited either to cavities of cylindrical shape, nor to cavities having sections of constant shapes and/or surfaces perpendicular to the direction of propagation of the electromagnetic waves. The existence of discontinuities in these sections makes it possible to adjust the shape of the acoustic resonance modes and/or to introduce walls that can accommodate non-gaseous samples and/or reflective, semi-reflective, diffusing or diffracting surfaces.

[0013] It should be clear that the above description is given by way of example and that the invention is not limited only to fundamental or longitudinal or radial modes, nor to the simultaneous use of two wavelengths. , nor the simultaneous or successive use of two modulation frequencies. The device of the invention can also include the use of a wide range of modulated or pulsed wavelengths so as to contain, simultaneously or successively, one or more frequency(s) corresponding to one or more mode(s). acoustics of the photo-acoustic chamber. The device may also include simultaneous or successive excitation over a wide frequency band in order to collect an acoustic spectral response over a frequency band containing one or more acoustic resonance frequencies.

List of designs

[0014] Some embodiments of the present invention are illustrated by the accompanying drawings, among which: – Figure 1 is a schematic view of a first embodiment of the device for measuring chemical species according to the invention. – Figure 2 shows the spectral response of the acoustic signal obtained with a device corresponding to Figure 1. The bottom curve is obtained with 100%vv of nitrogen N2 in the sample chamber, the top curve with 2%vv of methane CH4 in nitrogen N2. – Figure 3 presents the quotient of the amplitude of the fundamental longitudinal mode with that of the first radial mode for a device corresponding to Figure 1, for different concentrations of methane in the sample chamber. – Figure 4 is a schematic view of a second embodiment of the invention. – Figure 5 is a schematic view of a third embodiment of the invention. – Figure 6 is a schematic view of a fourth embodiment of the invention where the sample(s) to be measured is/are introduced into the photo-acoustic chamber.

Description of embodiments of the invention

[0015] The present invention is described below using some embodiments designed for purposes of illustration.

[0016] Figure 1 shows a first embodiment of the invention. A light-emitting diode (LED)11generates a beam of infrared electromagnetic waves, one of the directions of propagation of which is schematized by112(maximum of emission at 3.4 micro-m, bandwidth at half-maximum of 0.5 micro-m, average power approximately 100 micro-W) which is amplitude modulated by direct current modulation. This beam passes through a sample chamber 12 containing the gas to be measured before reaching a photo-acoustic chamber 13. The 50 mm long sample chamber12 is equipped with transparent windows121 for the wavelengths used as well as orifices122 equipped with a valve allowing the exchange of the gas to be measured. The intensity of the beam at the entrance to the photo-acoustic chamber 13 thus depends on the absorption in the sample chamber 12, this absorption being a measure of the concentration of the chemical species present in the sample chamber 12. The photo-acoustic chamber13consists of a cylinder 40 mm long and 10 mm in diameter equipped with transparent windows131 for the useful wavelengths and an orifice132 equipped with a valve allowing, if necessary, the exchange of the gas present in the photo-acoustic chamber13. The cylinder of the photo-acoustic chamber is equipped with one or more electret microphones133, the signal of which is transmitted to an electronic device14 allowing the detection of the amplitude and/or phase of the photo-acoustic signal generated in the photo-acoustic chamber13 . The gas present in the photo-acoustic chamber13 contains a fixed concentration of the chemical species to be measured (in this example approximately 6%vv of methane CH4 in nitrogen N2), the photo-acoustic cell is thus a selective detector for lengths wave absorbed by the chemical species to be analyzed (in this example, methane). Figure 2 shows the signal from the microphone for different modulation frequencies of the electromagnetic wave source11. Line 21 in Figure 2 is obtained with sample chamber 12 containing pure nitrogen while line 22 in Figure 2 represents the signal obtained when sample chamber 12 contains 2% methane in nitrogen. The signal around 4.7 kHz corresponds to the fundamental longitudinal mode of the photo-acoustic chamber13 and the signal around 21 kHz to the first radial mode of the photo-acoustic chamber13. Figure 3 presents the quotient (R_Long_Rad) of the maximum amplitude of the acoustic signal for the fundamental longitudinal mode with the maximum amplitude of the acoustic signal for the first radial mode, for different methane concentrations in the sample chamber. Figure 3 shows that the R_Long_Rad quotient depends on the methane concentration in the sample chamber. Measuring this quotient therefore makes it possible to determine, after calibration, the methane concentration of a sample introduced into the sample chamber12. Experience shows that the R_Long_Rad quotient is not significantly affected when the average intensity of the electromagnetic wave source is divided by two. This confirms that this embodiment of the invention is robust with respect to variations in intensity of the source of electromagnetic waves and this without the use of an additional sensor of intensity of the source of electromagnetic waves. The addition of 4% vv of butane C4H10 in the sample reduces the maximum amplitude of the resonances by approximately 30% but does not significantly affect the R_Long_Rad quotient. This shows that this embodiment of the invention is very specific for methane measurement.

[0017] Figure 4 shows a second embodiment of the invention. A source of electromagnetic waves41generates a beam of electromagnetic waves, some directions of propagation of which are represented by412. The source41 is modulated so as to contain, simultaneously or successively, one or more frequency(s) corresponding to one or more mode(s) of the photo-acoustic chamber43. This beam passes through a sample chamber42 containing the gas to be measured before reaching a photo-acoustic chamber43. The sample chamber42 is equipped with orifices422 equipped with a valve allowing the exchange of the gas to be measured. The photo-acoustic chamber43 is equipped with transparent windows431 for the useful wavelengths and an orifice432 equipped with a valve allowing, if necessary, the exchange of the gas present in the photo-acoustic chamber43. The photo-acoustic chamber 43 is equipped with several acoustic transducers 433 whose signals are transmitted to an electronic device 44 allowing the measurement of the amplitude and phase of the acoustic signal detected by each of the transducers 433. The gas present in the photo-acoustic chamber43 contains, among other things, the chemical species(s) to be measured. One or more ratios between the signals from the acoustic transducers, or between linear combinations of the signals from the acoustic transducers, is/are used to quantify, after calibration, the species(s) to be analyzed.

[0018] Figure 5 shows a third embodiment of the invention. A source of electromagnetic waves51generates a beam of electromagnetic waves, some directions of propagation of which are represented by512. The source51 is modulated so as to contain, simultaneously or successively, one or more frequency(s) corresponding to one or more mode(s) of the photo-acoustic chamber53. This beam passes through a sample chamber52 containing the gas to be measured before reaching a photo-acoustic chamber53. The sample chamber52 is equipped with orifices522 equipped with a valve allowing the exchange of the gas to be measured. The photo-acoustic chamber53 is equipped with transparent windows531 for the useful wavelengths and an orifice532 equipped with a valve allowing, if necessary, the exchange of the gas present in the photo-acoustic chamber53. The photo-acoustic chamber53 has a discontinuity in a section perpendicular to the direction of propagation of the electromagnetic waves, this discontinuity influences the shape of the acoustic resonance modes. This embodiment also makes it possible to place one or more non-gaseous species(s) or reflective or diffusing surfaces on the surface introduced by the discontinuity, thus allowing the latter(s) to be irradiated by electromagnetic waves. The photo-acoustic chamber53 is equipped with several acoustic transducers533, the signal of which is transmitted to an electronic device54 allowing the measurement of the amplitude and/or phase of the signal detected by each of the acoustic transducers533. Following a principle similar to that described for the previous embodiment, one or more ratio(s) between the signals of the acoustic transducers, or between linear combinations of signals from the acoustic transducers, is/are used to quantify, after calibration, the species(s) to be analyzed.

[0019] Figure 6 shows a fourth embodiment of the invention. A source of electromagnetic waves61generates a beam of electromagnetic waves, some directions of propagation of which are represented by612. The source61 is modulated so as to contain, simultaneously or successively, one or more frequency(s) corresponding to one or more mode(s) of the photo-acoustic chamber62. This beam joins the photo-acoustic chamber62 containing the sample(s) to be measured. The photo-acoustic chamber62 is equipped with orifices622 equipped with valves allowing the exchange of the sample contained therein and with transparent windows621 for the useful wavelengths. The photo-acoustic chamber62 has a discontinuity in a section perpendicular to the direction of propagation of the electromagnetic waves. This embodiment of the invention makes it possible to place one or more non-gaseous species(s) to be analyzed on the surface introduced by the discontinuity, thus allowing it to be irradiated. by electromagnetic waves. The excitation of the mode(s) comes from absorption of electromagnetic energy either by the gaseous sample, or by the non-gaseous sample, or by both. The photo-acoustic chamber62 is equipped with several acoustic transducers623, the signal of which is transmitted to an electronic device63 allowing the measurement of the amplitude and/or phase of the acoustic signal detected by each of the transducers623. Following a principle similar to that described for the previous embodiment, one or more ratio(s) between the signals of the acoustic transducers, or between linear combinations of signals from the acoustic transducers, is/are used to quantify, after calibration, the species(s) to be analyzed.

[0020] It should be obvious that, to those skilled in the art, the present invention allows embodiments in many other specific forms without departing from the invention as claimed. Therefore, the embodiments should be considered merely illustrative and in no way limiting and may be modified within the scope defined by the scope of the appended claims.

Claims (10)

1. Photo-acoustic measuring device comprising at least one photo-acoustic chamber and at least one source of electromagnetic waves and at least one acoustic transducer characterized in that at least a part of the electromagnetic waves which reach at least one photo chamber -acoustic is absorbed by at least one chemical species present or to be measured, and in that at least two acoustic resonance modes of the photo-acoustic chamber are simultaneously or successively excited by the absorption of said electromagnetic waves, and in that at least one of said modes is excited by the absorption of electromagnetic waves of a beam of said electromagnetic waves passing successively through at least two regions of space where the amplitudes of said acoustic mode are of opposite sign or out of phase at least 179 degrees apart, and in that the absorption of said electromagnetic waves in at least one of said regions of space of said mode is at least three percent greater than the absorption in all of the regions of space having an amplitude of opposite sign or a phase shift of at least 179 degrees for said mode, and characterized in that the acoustic signal from said mode is used, with that or those coming from at least one other mode, to measure at least one chemical species. 2. Measuring device according to claim 1, characterized in that the amplitude or phase of the photo-acoustic signal provided by at least one acoustic transducer for several modulation frequencies is used in order to establish one or more signal/signals. used to measure one or more chemical species(s). 3. Measuring device according to claim 1 or 2, characterized in that one or more linear combination(s) of the amplitudes or phases of the signals coming from several acoustic transducers equipping the photo-acoustic chamber is/are used ( s) to establish a signal used for measuring one or more chemical species(s). 4. Measuring device according to any one of claims 1 to 3, characterized in that the sample to be analyzed is in a sample chamber (12, 42 or 52) through which at least a part of the electromagnetic waves which then enter into a photo-acoustic chamber (13, 43 or 53) containing, among other things, the chemical species(s) to be measured. 5. Measuring device according to any one of claims 1 to 4, characterized in that the electromagnetic waves circulate at least once between reflecting or partially reflecting or diffusing or diffracting surfaces placed in the photo-acoustic chamber of said device or on its walls . 6. Measuring device according to any one of claims 1 to 5, characterized in that the photo-acoustic chamber has at least two sections of varied shapes and/or surfaces perpendicular to the direction of propagation of the electromagnetic waves. 7. Measuring device according to any one of claims 1 to 6, characterized in that at least one of the modes is excited by the absorption of at least a part of the electromagnetic waves by one or more non-chemical species(s). gaseous gas placed(s) on one or more wall(s), and/or on at least one of the windows of the photo-acoustic chamber and/or in the volume of the photo-acoustic chamber. 8. Device according to any one of claims 1 to 7, characterized in that it comprises means of adapting the wavelength of the electromagnetic wave source as a function of the chemical species(s) to be measured. 9. Device according to any one of claims 1 to 8, characterized in that several photo-acoustic chambers are placed in the path of at least one beam of electromagnetic waves coming from the source of electromagnetic waves. 10. Use of a device according to any one of claims 1 to 9 in order to quantify or identify or characterize one or more chemical species present in one or more samples.