METHOD AND DEVICE FOR DETERMINING A PROPERTY OF A FLUID

Description

[0001] The present invention relates to a method for determining a property of a fluid comprising the steps: A) generating electromagnetic excitation radiation modulated with a modulation frequency, wherein a carrier frequency of the excitation radiation is selected such that a first fluid absorbs the excitation radiation, B) illuminating the first fluid with the excitation radiation, C) detecting a sound wave generated by absorption of the excitation radiation in the first fluid with a sound measuring device.

[0002] The present invention further relates to a device for determining a property of a fluid, comprising a first fluid, a source configured such that the source generates modulated electromagnetic excitation radiation during operation of the device at a modulation frequency, wherein a carrier frequency of the excitation radiation is selected such that a first fluid absorbs the excitation radiation, wherein the source is arranged such that the first fluid can be illuminated with the excitation radiation during operation of the device, and a sound measuring device, wherein the sound measuring device is configured and arranged such that a sound wave generated by absorption of the excitation radiation in the first fluid can be detected by the sound measuring device during operation of the device.

[0003] For a variety of reasons, the demand is growing for analytical methods and devices for determining the properties of a fluid, especially a target gas within a gas mixture. In addition to absorption spectrometric methods, photoacoustic methods and sensors can also be used to determine the properties of such a fluid.

[0004] In a photoacoustic method, the fluid, in particular a gas mixture containing the target gas, is irradiated with electromagnetic excitation radiation. The excitation radiation has a carrier frequency, i.e., a frequency of its alternating electromagnetic field, which is equal to an absorption frequency of the fluid. The fluid absorbs the excitation radiation. This absorption leads to heating and thus thermal expansion of the fluid.

[0005] To enable conclusions about a fluid's properties to be drawn from its irradiation with excitation radiation, the excitation radiation is modulated such that the fluid's thermal expansion occurs periodically, thus generating a sound wave within the fluid. By measuring at least the amplitude or phase of this sound wave with a suitable acoustic measuring device, the fluid's properties can be determined.

[0006] Photoacoustic methods and the associated devices have the potential to form particularly cost-effective alternatives to optical methods of spectroscopy.

[0007] The possibility of addressing fluids with such a photoacoustic method depends on the accessibility of sources for suitable excitation radiation. The frequency of the alternating electromagnetic field must be selected such that the fluid absorbs at this frequency. This sometimes proves to be either expensive or even impossible. Therefore, it is an object of the present invention to provide a method and a device for determining a property of a fluid, which makes it possible to address even a fluid for which electromagnetic excitation radiation at the absorption frequency cannot be generated with conventional sources, or only with great difficulty.

[0008] Furthermore, photoacoustic methods repeatedly produce ambiguous results in the sense that it is not possible to distinguish whether the detected sound wave is caused by absorption of the excitation radiation in the fluid under investigation or by another cause.

[0009] WO 01 / 57498 A1 describes the execution of a chemical or physical or chemical and physical reaction of a reactant, whereby an outflow comprising at least one reaction product is obtained. The analysis of the outflow obtained during the reaction is carried out by recording and evaluating at least one photoacoustic signal. A photoacoustic detection system usable for this purpose has the following essential features: 1. A light source that excites molecular vibrations, rotational movements, electronic excitation states, chemical reactions, or combinations of these processes of chemical compounds present in the reaction product and / or the reactant; 2. At least one measuring device for recording the photoacoustic signals, e.g., pressure gradient transducers such as microphones, pressure transducers such as barometers, or combinations thereof, such as ultrasonic transducers; 3. At least one data acquisition and evaluation unit for evaluating the generated photoacoustic signals; and 4. Optionally, a collection device, preferably a measuring cuvette. in which the outflow is contained in gaseous, liquid or solid form, although in the simplest case work can also be carried out in the free gas space.

[0010] The scientific publication by S.L. Firebaugh et al., "Miniaturization and integration of photoacoustic detection with a microfabricated chemical reactor system," Journal of Microelectromechanical Systems, June 2001, pages 232-237, New York, reveals the miniaturization of photoacoustic detection and, in particular, the use of 3.4-micrometer light to detect propane in a carbon dioxide background. A miniature photoacoustic cell was incorporated into the assembly block of a microfabricated chemical reactor, demonstrating the integration of a photoacoustic detector into a miniaturized system. The cell used a hearing aid microphone and an infrared diode, which was modulated at the cell's initial acoustic resonance. As the gas composition of the cell changed from carbon dioxide to propane, the resonance peak was observed to shift and increase in size, as predicted by theory.

[0011] The scientific publication by N. Murakami et al., "In situ observation of photocatalytic reaction by photoacoustic spectroscopy: Detection of heat of exothermic photocatalytic reaction," Chemical Physics Letters, Elsevier BV, NL, Vol. 451, No. 4-6, December 15, 2007, pages 316-320, discloses the application of modulation frequency-dependent photoacoustic spectroscopy to gold-coated titanium(IV) oxide powders in the presence of adsorbed methanol to detect the photoacoustic signal due to phenomena occurring during photocatalytic reactions, such as the release of heat of reaction. Analysis of the peak frequency of the Helmholtz resonance provided qualitative information about the changes in gas composition caused by photocatalytic reactions (oxidation or dehydrogenation of methanol), which depended on the presence or absence of oxygen.The photoacoustic signal, which can be attributed to the heat of the photocatalytic reaction, was observed by removing the effect of changes in gas composition using two types of modulated light as excitation and non-excitation for the photocatalytic reaction.

[0012] Therefore, it is also an object of the present invention to provide a method and a device for determining a property of a fluid that exhibit increased clarity in the ability to express the result when detecting the sound wave.

[0013] The present invention is defined in claims 1 and 6.

[0014] At least one of the aforementioned problems is solved by a method according to independent claim 1. The method of the type mentioned at the outset further comprises the following steps: D) at least before or during the illumination of the first fluid in step B) initiating a chemical reaction, E) converting the first fluid as a reaction reactant into a second fluid as a reaction product in the chemical reaction, and F) determining a measure for a property of the first fluid from at least one amplitude or one phase of the sound wave generated by the absorption of the excitation radiation by the first fluid.

[0015] Furthermore, the following steps are carried out before the initiation of the chemical reaction in step D). G) Illuminating the first fluid with the excitation radiation (3), H) Detecting a sound wave generated by the absorption of the excitation radiation (3) in the first fluid with the sound measuring device (5, 12), I) Determining a comparative measure for the property of the first fluid from at least the amplitude or the phase of the sound wave generated by the absorption of the excitation radiation (3) by the first fluid, and furthermore the method comprises step J) Determining a differential measure for the property of the first fluid from the measure for the property of the first fluid from step F) and the comparative measure for the property of the first fluid from step I).

[0016] The present invention is based on the idea of ​​using a chemical conversion reaction, in which the first fluid is involved as a reaction product, to determine the properties of the first fluid.

[0017] For the purposes of this application, the first fluid is the fluid that absorbs the excitation radiation and through whose absorption the sound wave is generated.

[0018] For the purposes of this application, the fluid, in particular the first fluid or the second fluid, is a liquid or a gas, in particular a gas in a gas mixture.

[0019] According to the invention, the first fluid participates as a reactant in the chemical conversion reaction. Simultaneously, the first fluid is the one whose property is to be determined. The chemical reaction allows the concentration of the first fluid to be selectively changed, making it possible to determine whether the detected sound wave is caused by the absorption of the excitation radiation in the first fluid or not.

[0020] According to the invention, the first fluid is the target or sample fluid whose property is determined.

[0021] In one embodiment of the invention, the property of the first fluid is the presence or concentration of the first fluid.

[0022] For the purposes of this application, the carrier frequency of the excitation radiation refers to the frequency of the alternating electromagnetic field of the excitation radiation.

[0023] In one embodiment of the invention, the intensity of the excitation radiation is modulated by the modulation frequency. In another embodiment, the carrier frequency of the excitation radiation is modulated by the modulation frequency. Crucially, the applied modulation modulates, or switches on and off, the excitation in the first fluid. The periodic modulation of the excitation radiation generates a sound wave whose sound frequency is equal to the modulation frequency of the excitation radiation.

[0024] According to the invention, the method comprises, in a further step D), initiating the chemical reaction. The targeted initiation of the chemical reaction, i.e., the conversion of the first fluid into the second fluid, makes it possible to precisely determine the measure of the property of the first fluid by controlling the chemical reaction. Initiating the chemical reaction allows for the differentiation of two measurements, one with and one without the chemical reaction. In this way, for example, background effects can be detected and factored out.

[0025] In other words, the chemical reaction is controllable in the sense that it can be switched on. In one embodiment of the invention, the chemical reaction can be modulated with a reaction modulation frequency, i.e., it can be switched on and off periodically.

[0026] In one embodiment of the invention, the initiation of the chemical reaction in step D) is carried out by at least one of the following steps: Adding thermal energy to the reaction product, irradiating the reaction product with electromagnetic activation radiation, introducing a catalyst into the reaction product, activating a catalyst in the reaction product, or chemically activating the reaction product.

[0027] One way to supply thermal energy is by heating with an electric heating element, an ignition device, or electromagnetic radiation, which generates heat through absorption.

[0028] Another example of the supply of thermal energy is a gas discharge between two electrodes.

[0029] In one embodiment, the conversion is carried out by electrolysis via a chemical reaction. Electrolysis can be initiated by applying a voltage to two electrodes in contact with the electrolyte. In such an embodiment, preferably at least the first or the second fluid is an aqueous solution or a plasma.

[0030] It is also possible to initiate the chemical reaction beyond thermal initiation by irradiating the reaction product with electromagnetic activation radiation, thereby bringing the reaction product into an excited state.

[0031] In the case of irradiation of the reaction product with electromagnetic activation radiation, this activation radiation can have the same or a different carrier frequency as the excitation radiation.

[0032] In one embodiment of the invention, if the chemical reaction in step D) is initiated by irradiating the reactant with electromagnetic excitation radiation, the electromagnetic excitation radiation has a modulation depth that varies between a first value prior to step D) and a second value during the execution of step D). In one embodiment, the second value of the modulation depth is smaller than the first value. In other words, during the execution of step D), the excitation radiation is provided with an unmodulated DC component. This DC component exclusively initiates the chemical reaction without influencing the sound wave generated by absorption.

[0033] In the context of this application, the modulation depth is understood to be the ratio between a maximum and a minimum value of the excitation radiation intensity. A modulation where the minimum intensity is 0 has a maximum modulation depth of 1. In one embodiment, the amplitude of the excitation radiation intensity modulation is increased as the modulation depth decreases. In this case, the absolute range between the minimum and maximum intensity values ​​remains constant, while the addition of the DC component reduces the modulation depth.

[0034] Chemical activation of the reaction product can be achieved, for example, by injecting a reactive fluid.

[0035] Furthermore, it is possible to initiate the chemical reaction catalytically. This can be achieved either by introducing a catalyst into the reaction mixture or by activating such a catalyst, for example thermally.

[0036] According to the invention, the following steps are carried out before the initiation of the chemical reaction in step D): G) Illuminating the first fluid with the excitation radiation, H) Detecting a sound wave generated by the absorption of the excitation radiation in the first fluid with the sound measuring device, I) Determining a comparative measure for the property of the first fluid from at least the amplitude or the phase of the sound wave generated by the absorption of the excitation radiation by the first fluid, and the method further comprises step J) Determining a differential measure for the property of the first fluid from the measure for the property of the first fluid from step G) and the comparative measure for the property of the first fluid from step K).

[0037] In this embodiment, the property of the first fluid is determined twice. A comparative measure of the property is determined before the initiation of the chemical reaction, and the measure is determined a second time after the initiation of the chemical reaction. From the comparative measure and the second measure, a differential measure can be determined, revealing what proportion of the detected sound wave is actually attributable to absorption by the first fluid.

[0038] An example of the method according to the invention is described below.

[0039] In one embodiment of the invention, the method is used to detect nitrogen dioxide (NO₂) or to determine the concentration of nitrogen dioxide. The nitrogen dioxide constitutes the first fluid according to the present application. In step F), the nitrogen dioxide is converted into nitric oxide by a chemical reaction. The nitric oxide constitutes the second fluid according to the present invention. The chemical reaction selectively reduces the concentration of the nitrogen dioxide. By comparing the results with and without the chemical reaction, the nitrogen dioxide can be unambiguously identified as the origin of the sound wave. In one embodiment of the invention, the chemical reaction is initiated by the supply of thermal energy. In another embodiment, the chemical reaction is initiated by irradiation with electromagnetic activation radiation.In one embodiment of the invention, the excitation radiation lies in the green spectral range, while the activation radiation lies in the ultraviolet spectral range, preferably in a range of 350 nm to 400 nm.

[0040] Furthermore, at least one of the aforementioned tasks is also solved by a device for determining a property of a fluid according to the attached independent device claim. For this purpose, the device of the type mentioned at the outset further comprises an activation device and a control and evaluation device, wherein the control and evaluation device is effectively connected to the sound measuring device in such a way that the control and evaluation device receives a measurement signal from the sound measuring device during operation of the device, and wherein the control and evaluation device is effectively connected to the activation device in such a way that the activation device is controllable by the control and evaluation device during operation of the device. The activation device is set up and arranged in such a way that, during operation of the device, the activation device initiates a chemical reaction to convert the first fluid as a reaction reactant into the second fluid as a reaction product, and the control and evaluation device is set up in such a way that, during operation of the device, the control and evaluation device determines a measure of a property of the first fluid from the measurement signal of the sound wave generated by the absorption of the excitation radiation by the first fluid.

[0041] Furthermore, the control and evaluation unit is arranged such that, during operation of the device, the control and evaluation unit determines a comparative measure for the property of the first fluid from the measurement signal of the sound wave generated by the absorption of the excitation radiation by the first fluid, prior to the initiation of the chemical reaction, and determines a differential measure for the property of the first fluid from the measure for the property of the first fluid and the comparative measure for the property of the first fluid.

[0042] Insofar as aspects of the invention are described below with regard to the apparatus, these also apply to the corresponding method for determining a property of a fluid and vice versa. Insofar as the method is carried out with an apparatus according to this invention, this apparatus includes the corresponding means for carrying out the method. In particular, embodiments of the apparatus according to the invention are suitable for carrying out the embodiments of the method described above.

[0043] In one embodiment of the invention, the activation device is selected from a heating device for supplying thermal energy to the reaction product, a source of electromagnetic activation radiation for irradiating the reaction product, an activatable catalyst, a device for injecting a chemical substance to activate the reaction product or a combination thereof.

[0044] In one embodiment of the invention, the activatable catalyst is a heatable element with a surface section comprising a metal, preferably an element of the platinum group (Ni, Pd, Pt or Ds), in particular platinum or palladium.

[0045] In one embodiment of the invention, the device comprises a gas measuring cell, the gas measuring cell being configured such that the reactant and the product can be received in the gas measuring cell and the chemical reaction can take place in the gas measuring cell, with the acoustic measuring device being arranged in the gas measuring cell. It is understood that in such an embodiment, the gas measuring cell must be configured such that the excitation radiation can either be directed into the gas measuring cell or generated directly within it by the source.

[0046] In one embodiment of the invention, the activation device, in particular a heating device or a catalyst, is arranged in the gas measuring cell.

[0047] In one embodiment of the invention, the gas measuring cell forms an acoustic resonator with a natural frequency, wherein the natural frequency of the resonator is equal to the modulation frequency of the excitation radiation. This leads to an increase in the amplitude of the sound wave to be detected by the sound measuring device.

[0048] In one embodiment of the invention, the sound measuring device is an alternating pressure transducer or an alternating fluid flow transducer.

[0049] An example of an alternating pressure transducer is a microphone, although the specific design of the microphone is irrelevant according to the invention. An alternating pressure transducer detects, preferably with time resolution, a pressure change in the fluid with which the transducer is in contact.

[0050] An alternating fluid flow transducer, on the other hand, detects, preferably with time resolution, a change in the flow of a fluid surrounding the transducer. An example of an alternating fluid flow transducer is a measuring device that relies on a temperature change in a wire, where the wire is located within the sound wave.

[0051] Further advantages, features, and applications of the present invention will become clear with reference to the following description of embodiments and the accompanying figures. In the figures, identical elements are designated by the same reference numerals. Figure 1 is a schematic cross-sectional view of a first embodiment of a device for determining a property of a fluid. Figure 2 is a schematic cross-sectional view of a second embodiment of a device for determining a property of a fluid. Figure 3 is a schematic cross-sectional view of a further embodiment of a device for determining a property of a fluid.

[0052] The following describes three embodiments of the device for determining a property of a fluid from the Figures 1 to 3 described in detail. First, the elements of the respective device are described with their individual function, and then the procedure is described based on a specific use of the device.

[0053] The photoacoustic sensor 1 from Figure 1The photoacoustic sensor 1 comprises a laser diode 2 as a source for the electromagnetic excitation radiation 3. It forms a device for determining a property of a fluid as defined in the present application. The sensor 1 further comprises a gas measuring cell 4, an alternating gas flow converter 5 as a sound measuring device as defined in the present application, and a catalytically active heating element 6 as an activation device as defined in the present application. The alternating gas flow converter 5 is based on a heated wire, the temperature change of which is detected due to the flowing gas.

[0054] The intensity of the electromagnetic excitation radiation 3 is modulated at a frequency of 5 kHz. In the illustrated embodiment, this intensity modulation of the excitation radiation 3 is imposed on the laser diode 2 by a current modulation of the current flowing through the diode. If a gas in the interior 7 of the cell absorbs the excitation radiation 3, this absorption is switched on and off by means of the modulation frequency of the excitation radiation 3. This periodicity ensures that the pressure change in the absorbing gas is also modulated by the modulation frequency. In this way, the frequency of the sound wave generated in the interior 7 is equal to the modulation frequency of the excitation radiation 3.

[0055] The gas measuring cell 4 provides a substantially cylindrical interior space 7 for receiving the fluid to be measured, i.e., in this embodiment, a gas. The interior space 7 is bounded by side walls 8 and two cell windows 9, 10.

[0056] The cell windows serve to couple the excitation radiation 3 generated by the light-emitting diode 2 into the interior 7 of the gas measuring cell 4 through the coupling window 9 and to couple it out of the interior 7 of the gas measuring cell through the exit window 10. The material of both windows 9 and 10 is selected to exhibit minimal absorption in the region of the carrier frequency of the excitation radiation 3. This minimizes interference with the measurement signal, since any absorption in the cell 4 can, in principle, also result in a sound wave.

[0057] A sound wave generated by the absorption of excitation radiation by a gas located in the interior 7 of cell 4 can be detected using the gas flow sensor 5. The gas flow sensor 5 consists of a heated wire located within the sound wave, so that a gas flow cools the wire. This cooling is measurable as an electric current through the wire. The flow sensor 5 is positioned at a flow maximum of this standing wave to optimize the measurable signal.

[0058] The interior space 7 of the gas measuring cell 4 is designed as a resonator for the resulting sound wave. Thus, the sound wave forms as a standing wave in the interior space 7 of the gas measuring cell 4.

[0059] The catalytic heating element 6 comprises an electrically heated surface section made of platinum, the platinum acting as a catalyst when it exceeds a certain critical temperature.

[0060] In the illustrated embodiment of the Figure 1 Source 2 and heating element 6 are connected to a control and evaluation unit 11. The flow sensor 5 is also connected to the control and evaluation unit 11. During operation, the control and evaluation unit 11 sets the modulation frequency for the diode and receives a measured value from the flow sensor 5 for the amplitude of the sound wave generated in the interior 7 of the gas measuring cell 4. Furthermore, the control and evaluation unit 11 controls the heating element, thus allowing control of the activity of the catalytic surface.

[0061] The in Figure 1The embodiment shown in the present invention serves to detect hydrogen in a gas mixture introduced into the gas measuring cell 4. The hydrogen constitutes the second fluid according to the present application. Since hydrogen does not exhibit any absorption bands achievable with commercial sources, the detection of hydrogen is carried out indirectly according to the invention.

[0062] The carrier frequency of the excitation radiation 3 is chosen such that it is absorbed by water in the gas measuring cell. The excitation radiation 3 therefore has a carrier wavelength of 1.4 µm. The water constitutes the first fluid within the meaning of the present application.

[0063] If the interior 7 of the gas measuring cell 4 is illuminated with the excitation radiation 3 while the heating element 6 is switched off, only the water already present in the gas mixture generates the sound wave with a low amplitude. If the control and evaluation unit 11 switches on the heating element 6, a chemical reaction of hydrogen with the oxygen contained in the gas mixture to form water takes place, initiated by the catalytic surface 12. This water produced from the hydrogen contributes to the absorption of the excitation radiation 3 and increases the amplitude of the sound wave generated in the interior 7. Subtracting the comparative measure for the concentration of hydrogen without the initiated conversion reaction from the measure for the concentration of hydrogen with the initiated conversion reaction yields a unique differential measure for the concentration of hydrogen in the gas measuring cell 4.

[0064] The structure of the photoacoustic sensor 1 from Figure 2 differs in particular from sensor 1 in that... Figure 1 that no heating element is provided in the interior 7 of cell 4. Furthermore, in this embodiment, a pressure transducer, namely a microphone 12, serves to detect the sound wave in the interior 7. In addition to the source 2 for the excitation radiation 3, the device 1 has a source 13 for electromagnetic activation radiation 14. The source 13 for the activation radiation 14 and the microphone 12 are connected to the control and evaluation unit 11. Both the excitation radiation 3 and the activation radiation 14 are focused into the interior 7 of the gas measuring cell 4 by means of a lens 15.

[0065] The in Figure 2The embodiment shown serves, in a first use, to detect the concentration of oxygen in a gas mixture in the interior 7 of the gas measuring cell 4. In this embodiment, the oxygen constitutes the second fluid within the meaning of the present application. The absorption frequencies of oxygen also lie outside a range easily addressed with commercial sources 2. Therefore, the detection of the oxygen concentration in the cell 4 is carried out indirectly by absorption of the excitation radiation 3 by ozone. The ozone constitutes the first fluid within the meaning of the present application.

[0066] For this purpose, the oxygen is split into oxygen radicals by a chemical reaction, which then react with the oxygen to form ozone. The splitting of the oxygen into oxygen radicals is initiated by irradiating the oxygen with the activation radiation 14. In this embodiment as well, a measurement is performed before initiating the chemical reaction, with the source 13 for the activation radiation 14 switched off. In this way, a background of ozone already present in the gas mixture within the gas measuring cell 4 can be determined as a reference measure. In a second measurement, after initiation, i.e., after switching on the source 13 for the activation radiation 14, the sound wave generated by the ozone, or rather its amplitude, is recorded, and a measure of the ozone concentration is derived from this.From the comparative measure and the measure, a comparative measure for the concentration of oxygen is then formed as before.

[0067] In an alternative embodiment, the sensor 1 is made of Figure 2 This device is used for the highly sensitive detection of nitrogen dioxide. Nitrogen dioxide forms the first fluid as defined in the present application. The nitrogen dioxide is converted into nitrogen monoxide, the second fluid as defined in the present application, through a chemical reaction. This chemical reaction selectively reduces the concentration of nitrogen dioxide in the interior 7 of the gas measuring cell 4. A comparison with and without the chemical reaction allows the nitrogen dioxide to be clearly identified as the origin of the sound wave. The excitation radiation lies in the green spectral range, while the activation radiation lies in the ultraviolet spectral range.

[0068] The embodiment of the Figure 3shows a variant of the embodiment Figure 1 , whereby in Figure 3 The gas measuring cell 4 is not fully shown. Instead of an inlet window 9, the source 2 for the excitation radiation 3 is arranged directly in the cell, so that the excitation radiation 3 does not first have to be injected into the cell. Furthermore, the embodiment features Figure 3 also a microphone 13 as a sound measuring device.

[0069] The embodiment from Figure 3 The device is used to detect the concentration of natural gas, represented by methane, in a gas mixture introduced into the interior 7 of the gas measuring cell 4. The methane constitutes the second fluid as defined in the present application. A chemical reaction is initiated by the heating element 6 to convert the methane, together with oxygen, into carbon dioxide. The carbon dioxide absorbs the excitation radiation 3 at an excitation wavelength of 4.2 µm.

[0070] For the purposes of the original disclosure, it is pointed out that all features as they can be deduced by a person skilled in the art from the present description, the drawings, and the claims, even if they are specifically described only in connection with certain other features, can be combined individually or in any combination with other features or groups of features disclosed herein, unless this has been expressly excluded or technical circumstances render such combinations impossible or pointless. A comprehensive, explicit description of all conceivable combinations of features is omitted here solely for the sake of brevity and readability.

[0071] While the invention has been illustrated and described in detail in the drawings and the preceding description, this illustration and description are merely exemplary and are not intended to limit the scope of protection as defined by the claims. The invention is not limited to the disclosed embodiments.

[0072] Variations of the disclosed embodiments are obvious to a person skilled in the art from the drawings, the description, and the accompanying claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. Reference numerals in the claims are not intended to limit the scope of protection. Reference symbol list

[0073] 1 Photoacoustic sensor 2 Source of excitation radiation 3 Excitation radiation 4 Gas measuring cell 5 Gas flow sensor 6 Heating element 7 Interior 8 Side walls 9 Inlet window 10 Outlet window 11 Control and evaluation unit 12 Microphone 13 Source of activation radiation 14 Activation radiation 15 Lens

Claims

1. A method for determining a property of a fluid comprising the steps: A) generating an electromagnetic excitation radiation (3) modulated with a modulation frequency, wherein a carrier frequency of the excitation radiation (3) is selected such that a first fluid absorbs the excitation radiation (3), B) illuminating the first fluid with the excitation radiation (3) and C) detecting an acoustic wave generated by an absorption of the excitation radiation (3) in the first fluid with a sound measuring device (5, 12), D) in time, at least before or during the illuminating of the first fluid in step B) initiating a chemical reaction, E) converting the first fluid as reactant into a second fluid as reaction product in the chemical reaction and F) determining a measure for a property of the first fluid from at least an amplitude or a phase of the acoustic wave generated by the absorption of the excitation radiation (3) by the first fluid, characterized in that, in time, the following steps G) to I) are carried out before the initiating of the chemical reaction in step D): G) illuminating the first fluid with the excitation radiation (3), H) detecting an acoustic wave generated by the absorption of the excitation radiation (3) in the first fluid with the sound measuring device (5, 12), I) determining a comparison measure for the property of the first fluid from at least the amplitude or the phase of the acoustic wave generated by the absorption of the excitation radiation (3) by the first fluid and that the method further comprises the step: J) determining a differential measure for the property of the first fluid from the measure for the property of the first fluid from step F) and the comparison measure for the property of the first fluid from step I).

2. The method according to the preceding claim, wherein the initiating of the chemical reaction in step D) is effected by at least one of the following steps: - supplying thermal energy into the reactant, - irradiating the reactant with electromagnetic activation radiation, - introducing a catalyst into the reactant, - activating a catalyst in the reactant and - chemical activating of the reactant.

3. The method according to one of the preceding claims, wherein the initiating of the chemical reaction in step D) is effected by irradiating the reactant with the electromagnetic excitation radiation (3), wherein the excitation radiation (3) has a modulation depth variable between a first value temporally before step D) and a second value temporally during an execution of step D).

4. The method according to one of the preceding claims, wherein the property of the first fluid is a presence or a concentration of the first fluid or wherein the property of the second fluid is a presence or a concentration of the second fluid.

5. Use of the method according to one of the preceding claims for detecting nitrogen dioxide or for detecting a concentration of nitrogen dioxide, wherein the first fluid is nitrogen dioxide and the second fluid is nitrogen monoxide, wherein the nitrogen dioxide in step E) is converted by a chemical reaction into the nitrogen monoxide, wherein the chemical reaction is preferably initiated by irradiation with electromagnetic activation radiation.

6. A device (1) for determining a property of a fluid with the first fluid, a source (2) which is configured such that the source (2) in an operation of the device (1) generates electromagnetic excitation radiation (3) modulated with a modulation frequency, wherein a carrier frequency of the excitation radiation (3) is selected such that the first fluid absorbs the excitation radiation (3), wherein the source (2) is arranged such that in the operation of the device (1) the first fluid is illuminated with the excitation radiation, and a sound measuring device (5, 12), wherein the sound measuring device is configured and arranged such that in the operation of the device (1) an acoustic wave generated by an absorption of the excitation radiation (3) in the first fluid is detected with the sound measuring device (5, 12), an activation device (6, 13) and a control and evaluation device (11), wherein the control and evaluation device (11) is operatively connected to the sound measuring device (5, 12) such that the control and evaluation device (11) in the operation of the device (1) receives a measurement signal from the sound measuring device (5, 12), wherein the control and evaluation device (11) is operatively connected to the activation device such that the activation device (5, 12) in the operation of the device (1) is controllable by the control and evaluation device (11), and wherein the activation device (6, 13) is configured and arranged such that the activation device (6, 13) in the operation of the device (1) initiates a chemical reaction for converting the first fluid as reactant into a second fluid as reaction product, and the control and evaluation device (11) is configured such that the control and evaluation device (11) in the operation of the device (1) determines from the measurement signal of the acoustic wave generated by the absorption of the excitation radiation (3) by the first fluid a measure for a property of the first fluid, characterized in that the control and evaluation device (11) is configured such that the control and evaluation device (11) in the operation of the device (1) temporally before the initiating of the chemical reaction determines a comparison measure for the property of the first fluid from the measurement signal of the acoustic wave generated by the absorption of the excitation radiation (3) by the first fluid and determines a differential measure for the property of the first fluid from the measure for the property of the first fluid and the comparison measure for the property of the first fluid.

7. The device (1) according to the preceding claim, wherein the activation device (6, 13) is selected from - a heating device (6) for supplying thermal energy into the reactant, - a source (13) for electromagnetic activation radiation (14) for irradiating the reactant, - an activatable catalyst (6), - a device for injecting a chemical substance for activating the reactant or a combination thereof.

8. The device according to claim 6 or 7, wherein the device comprises a gas measurement cell (4), wherein the gas measurement cell (4) is configured such that in the gas measurement cell (4) the reactant and the reaction product are receivable and the chemical reaction can take place in the gas measurement cell (4), and wherein the sound measuring device (5, 12) is arranged in the gas measurement cell.

9. The device (1) according to the preceding claim, wherein the gas measurement cell (4) forms an acoustic resonator with a natural frequency, wherein the natural frequency is equal to the modulation frequency.