Measuring arrangements and methods for the detection of trace gases in a measuring gas

Abstract

The invention discloses a measuring arrangement for detecting trace gases in a measurement gas, the measuring arrangement comprising at least one, in particular narrowband, excitation light source for emitting excitation light; a first chamber with an inlet for the measurement gas and an outlet, wherein the excitation light is emitted into the first chamber; a second chamber with an inlet, which is gas-tightly separated from the first chamber, wherein the excitation light is emitted into the second chamber; a scrubber, wherein the outlet of the first chamber leads to the scrubber and the measurement gas is passed through the scrubber, wherein the scrubber removes the trace gases from the measurement gas and background gas remains, wherein the background gas is passed to the inlet of the second chamber;at least one narrowband measuring light source for emitting measuring light, wherein the measuring light source is arranged such that the measuring light strikes the control light in the first chamber and the control light in the second chamber; a detector unit which is in optical contact with the first and the second chamber and which detects the measuring light from the first chamber and the measuring light from the second chamber and converts them into a first and a second electrical signal; and a data processing unit which calculates the proportion of trace gases in the measuring gas from the first and second electrical signals. The invention discloses an alternative measuring arrangement and method.

Description

The invention relates to measuring arrangements and methods for detecting trace gases in a measurement gas.The problem underlying the invention is to be explained without limiting generality on the basis of the measurement of H 2 S in natural gas.Spectroscopic measurement of trace gases such as H 2 S in a measurement gas such as natural gas or biogas is difficult, since the absorption of other gases in the gas mixture is at least 2 orders of magnitude higher. There are wavelength ranges in which the absorption lines of H 2 S normalized to the concentration are greater than those of methane. However, since the H 2 S concentration in natural gas and processed biogas must be less than 4 ppm, but the methane content is 50%-99%, the absorption of methane in the wave number range of 1000-6500 cm -1 is almost everywhere greater by a factor of 100 and more than the absorption of H 2 S. This results in variations in the methane concentration causing a large error in an H 2 S measurement.In order to avoid this, a differential TDLAS measurement is carried out, for example, in the commercially available product "Gas analyser SS2100" from the Endress+Hauser group (tunable diode laser absorption spectroscopy). For this purpose, a reference measurement is carried out at regular intervals. For this purpose, the gas mixture is passed through a scrubber (for example a scrubber in German), which removes H 2 S from the gas mixture. In this way, the background spectrum can be recorded without the H 2 S component. This background spectrum is subtracted from the later measurement with H 2 S.The drawback with this differential measurement method is the dead time of several minutes, which occurs while the background spectrum is being recorded. In addition, in TDLAS, a long-path cell is required to detect the low absorptions of H 2 S. Such long-path cells have a large volume of about 3 L. The large volume has two disadvantages: firstly, it leads to a high response time and, secondly, the large volume means a large loss of gas in an extractive measurement.Another method for measuring trace gases is photo-thermal spectroscopy. It is known that the absorption of photons leads to excitation of molecular energy levels, which in turn can lead to a change in the temperature, pressure and density of the sample. The temperature change occurs when the molecule relaxes from the excited energy level back to the original energy level through a nonradiative junction. The changes in these properties can be used for the detection of trace gases in photo-thermal spectroscopy (PTS). This method uses laser radiation to produce transient changes in sample properties. The temperature rise caused by photoabsorption produces a pressure change within the sample which propagates in an acoustic wave. As soon as the pressure has expanded to the equilibrium pressure, a density change proportional to the temperature remains. In combination, temperature and density changes affect other properties of the sample, e.g., the refractive index. PTS methods are based on changes in the sample temperature, which are generally monitored via the refractive index of the sample. In contrast to classic transmission spectroscopy according to Lambert-Beer law, in which the sensitivity increases with the path length, PTS is an indirect method for optical absorption analysis, in which a photoinduced change in the thermal state of the sample is measured. The heat output is proportional to the sample absorption coefficient and the incident light intensity.PTS arrangements for detecting changes in refractive index generally use an excitation laser for sample heating and a further laser for monitoring the changes resulting from the heating. The change in refractive index causes a phase shift of the light passing through the heated sample, and can be measured with high sensitivity with an interferometer.The use of Fabry-Perot interferometers (FPI) for the photo-thermal detection of trace gases is known, for example, from WO 2018 / 009953.Differential measurements are also known from photothermography. The measuring principle described in WO 2018 / 009953, however, does not serve for compensating background absorption, but for compensating background noises and laser noises.The invention is based on the object of compensating the background spectrum in photo-thermal spectroscopy.The object is achieved by a measuring arrangement for detecting trace gases in a measurement gas, the measuring arrangement comprising at least one, in particular narrow-band, excitation light source for emitting control light; a first chamber with an inlet for the measurement gas and an outlet, wherein the control light is sent into the first chamber; a second chamber with an inlet, wherein this is separated from the first chamber in a gas-tight manner, wherein the control light is sent into the second chamber; a scrubber, wherein the outlet of the first chamber leads to the scrubber and the measurement gas is passed through the scrubber, wherein the scrubber removes the trace gases from the measurement gas and background gas remains, wherein the background gas is passed to the inlet of the second chamber; at least one narrow-band measurement light source for emitting measurement light, wherein the measurement light source is arranged such that the measurement light impinges on the control light in the first chamber and the control light in the second chamber; a detector unit which is in optical contact with the first and the second chamber and respectively detects the measurement light from the first chamber and the measurement light from the second chamber and converts them into a first and a second electrical signal; and a data processing unit which calculates the proportion of the trace gases in the measurement gas from the first and the second electrical signal.The object is further achieved by a measuring arrangement for detecting trace gases in a measurement gas, the measuring arrangement comprising at least one narrow-band excitation light source for emitting control light; a first chamber with an inlet for the measurement gas and an outlet, wherein the control light is sent into the first chamber; a second chamber with an inlet, wherein the inlet is separated from the first chamber in a gas-tight manner, wherein the control light is sent into the second chamber; a scrubber, wherein the measurement gas is divided after a gas inlet, wherein a part of the measurement gas is conducted to the inlet of the first chamber and the other part of the measurement gas is conducted to the scrubber, wherein the scrubber removes the trace gases from the measurement gas and background gas remains, wherein the background gas is conducted to the inlet of the second chamber; at least one narrow-band measurement light source for emitting measurement light, the measurement light source being arranged such that the measurement light impinges on the control light in the first chamber and the control light in the second chamber; a detector unit which is in optical contact with the first and the second chamber and which detects the measurement light in the first chamber and the measurement light in the second chamber and converts them into a first and a second electrical signal, respectively; and a data processing unit which calculates the proportion of the trace gases in the measurement gas from the first and the second electrical signal.The claimed measuring arrangements modify the prior art in each case, so that compensation for background noises is possible and that compensation for a background spectrum can be realized. For this purpose, the two optical resonators are positioned in two separate chambers. The claimed arrangement produces a photo-thermal signal, wherein in the one chamber, in addition to the absorption of the background gas, the absorption of the target gas contributes to the photo-thermal signal. The optical resonators in the two chambers undergo a resonance shift in proportion to the photo-thermal signal detected by an interrogation laser (measurement light source). The difference of the signals of the interrogation laser proportional to the respective resonant wavelengths gives information about the absorption spectrum of the target gas and this ideally without influence of the background spectrum.The advantages of this measuring method are:A smaller volume (a few mL) compared to the prior art is possible since the optical resonators can be selected to be smaller; in one embodiment, these are 10×5×5 mm 3 large, optionally even smaller).No dead time results, since the background spectrum is recorded virtually simultaneously.It is possible to eliminate the noise of the measurement light source on the background spectrum.One embodiment provides that the measuring arrangement comprises exactly one excitation light source, wherein the control light is guided into the first and second chamber via optical components, in particular partially transmissive mirrors, lenses, beam splitters, dichroic beam splitters, fiber-based beam splitters or couplers, prisms or the like.In one embodiment, the detector unit is arranged such that it measures the measurement light in transmission through the first or second chamber.In one embodiment, the detector unit is arranged such that it measures the measurement light in reflection from the first or second chamber and an optical component, for example a circulator, separates the incident and reflected measurement light.One configuration provides that the excitation light source is configured as a laser, in particular as a diode laser, preferably a quantum cascade laser with continuous wave, in particular a quantum cascade laser with continuous wave and distributed feedback, or a quantum cascade laser with external resonator or an interband cascade laser, and / or wherein the measurement light source is configured as a laser, in particular as a diode laser, preferably a monomode diode laser, for example a diode laser with continuous wave and distributed feedback.In one embodiment, the measuring arrangement comprises a modulator which is connected to the excitation light source in order to modulate the wavelength of the control light and / or to switch the excitation light source on / off.One configuration provides that the detector unit is configured as a photodetector, in particular the detector unit comprises two photodetectors, a first photodetector for measurement light in the first chamber and a second photodetector for measurement light in the second chamber.In one embodiment, the measuring arrangement comprises a lock-in amplifier, in particular as part of the data processing unit, which demodulates the measurement light in the first and / or second chamber.In one embodiment, the excitation light source and the measurement light source are arranged such that the control light in the first chamber and the measurement light in the first chamber run parallel and that the control light in the second chamber and the measurement light in the second chamber run parallel.One configuration provides that the volume of the first and second chamber is between 1 ml and 125 ml, preferably between 1 ml and 25 ml, more preferably between 1 ml and 5 ml.In one embodiment, the trace gases are H 2 S, HF, SO 2, H 2 O, NH 3, or HCl.One configuration provides that the scrubber contains metal oxide particles having an aerodynamic diameter of less than about 250 nm, in particular of less than about 100 nm, wherein the metal oxide particles undergo one or more surface reactions with the measurement gas in order to form one or more metal-anion complexes on the surfaces of the metal oxide particles, wherein the metal oxide particles are selected from a group consisting of CuO, ZnO, potassium permanganate and alkaline unprep carbonate [CuCO 3·2] in particular the metal oxide particles are impregnated into or onto carrier particles which are inert with respect to the background gas, wherein the carrier particles have an aerodynamic diameter in a range of about 10 to 250 μm, in particular in a range of 40 to 250 μm.One configuration provides that the control light is absorbed in the first chamber by the background gas and the trace gases and in the second chamber by the background gas, and in each case leads to a temperature and / or refractive index change proportional to the absorption, wherein the refractive index change leads to a phase shift of the measurement light. The temperature change also leads to a phase shift, but only at the modulation frequency of the excitation light source.One embodiment provides that the measuring arrangement is designed as a Fabry-Perot interferometer with corresponding mirrors as optical resonators in the chambers, so that the phase shift of the measurement light is converted sensitively into an intensity change of the transmitted or reflected measurement light.One configuration provides that the resonances of the resonators are balanced by temperature control of the resonators via at least one Peltier element, as a result of which a length change of the resonators and a change of the refractive index take place; rotation of the resonators; a change of the distance of the mirrors from one another, in particular via a piezoelectric element; and / or a change of the wavelength of the measurement light radiated onto the first chamber and / or the second chamber via an ElectroOptischen modulator.The object is further achieved by a method for detecting trace gases in a measurement gas, comprising the steps of emitting control light into a first and second chamber, wherein the second chamber is separated from the first chamber in a gas-tight manner; conducting measurement gas into the first chamber; conducting the measurement gas from the first chamber to a scrubber, wherein the scrubber removes the trace gases from the measurement gas and background gas remains; conducting the background gas into the second chamber; emitting measurement light into the first and second chambers; detecting measurement light from the first and second chambers; and determining the proportion of trace gases by means of the detected measurement light.The object is further achieved by a method for detecting trace gases in a measurement gas, comprising the steps of emitting control light into a first and second chamber, wherein the second chamber is separated from the first chamber in a gas-tight manner; conducting measurement gas into the first chamber; conducting the measurement gas to a scrubber, wherein the scrubber removes the trace gases from the measurement gas and background gas remains; conducting the background gas into the second chamber; emitting measurement light into the first and second chamber; detecting measurement light from the first and second chamber; and determining the proportion of trace gases by means of the detected measurement light.This is explained in more detail with reference to the following figure. FIG. 1 shows the claimed measuring arrangement. FIG. 2 shows the claimed measuring arrangement in one embodiment. FIG. 3 shows the claimed measuring arrangement in one embodiment. FIG. 4 shows the claimed measuring arrangement in one embodiment.In the figures, identical features are identified by identical reference numerals.The claimed measuring arrangement in its entirety is denoted by the reference numeral 1 and is illustrated in FIG. 1.The measuring arrangement 1 is a photo-thermal spectroscopy arrangement. Generally, such arrangements use an excitation light source 8 for sample heating and a measurement light source 9 for monitoring the changes resulting from the heating, usually for detecting changes in refractive index. The change in refractive index causes a phase shift of the light passing through the heated sample, and can be measured with high sensitivity with an interferometer.An excitation light source 8, for example, a narrow-band excitation light source, emits control light into a first chamber 5 and a second chamber 6. the excitation light source 1 is designed as a laser, in particular as a diode laser, preferably a quantum cascade laser with a continuous wave, in particular a quantum cascade laser with a continuous wave and distributed feedback, or a quantum cascade laser with an external resonator or an interband cascade laser. A modulator 14 is connected to the excitation light source 8, which modulates the wavelength of the control light and / or switches the excitation light source 8 on / off, for example as a modulated voltage source. The modulator 14 can also be the laser 8 itself or the laser current and thus the laser driver.In the embodiment in FIG. 1, quantum cascade lasers or interband cascade lasers are used, for example, on account of the high absorptions in the MIR. In the embodiment in FIG. 2 (see below), wavelengths between 980 nm and 1650 nm are expedient, since a joint fiber guidance is then possible. The difference between the two configurations is explained further below, and above all, the beam guidance of the control light from the excitation light source 8 is concerned.The measurement gas 2 is therefore preferably heated periodically with modulated control light. Preferably, the modulation is achieved by wavelength modulation in which the emission frequency of the excitation light source 8 is modulated. Wavelength modulation spectroscopy (WMS) is capable of increasing the signal-to-noise ratio (SNR) by reducing the noise content of a measurement used for track detection. In wavelength modulation, the absorption of the excitation light source is converted into a periodic signal which is preferably isolated at its harmonics by a lock-in amplifier 13. This type of detection results in a marked improvement in signal-to-noise ratio (SNR) by limiting the detection passband to a narrow frequency interval as well as by shifting the detection to higher frequencies where the 1 / f laser noise is significantly reduced. Generally, a lock-in amplifier is an amplifier for measuring a weak alternating electrical signal modulated with a reference signal known in frequency and phase. The apparatus is an extremely narrow band pass filter and thereby improves the signal to noise ratio (SNR).One or more excitation light sources 8 can be used, exactly one excitation light source 8 is shown, wherein the control light is directed into the first and second chamber 5, 6 via optical components 12, in particular via partially transmissive mirrors, lenses, beam splitters, dichroic beam splitters, fiber-based beam splitters or couplers, prisms or the like.Via the inlet 5 a, measurement gas 2 is conducted into the first chamber 5. Via its outlet 5 b, the measurement gas 2 is conducted to a scrubber 7. The scrubber 7 removes the trace gases 3 from the measurement gas 2 and leaves behind background gas 4. Trace gases 3 are, for example, H 2 S, HF, SO 2, H 2 O, NH 3 or HCl.The volume of the first chamber 5 is, for example, between 1 ml and 125 ml, preferably between 1 ml and 25 ml, more preferably between 1 ml and 5 ml.The scrubber 7 comprises, for example, metal oxide particles having an aerodynamic diameter of less than about 250 nm, in particular of less than about 100 nm, wherein the metal oxide particles undergo one or more surface reactions with the measurement gas in order to form one or more metal-anion complexes on the surfaces of the metal oxide particles. The metal oxide particles may be selected from a group consisting of CuO, ZnO, potassium permanganate, and alkaline rnppe carbonate [CuCO 3 •C(OH) 2]. The metal oxide particles can be impregnated in or on carrier particles which are inert with respect to the background gas 4, wherein the carrier particles have an aerodynamic diameter in a range of approximately 10 to 250 μm, in particular in a range of 40 to 250 μm.The background gas 4 is conducted to the inlet 6 aof the second chamber 6 after the scrubber 7. The second chamber 6 comprises an outlet 6 b, whereby the gas can be discharged again and discarded.The volume of the second chamber 6 is, for example, between 1 ml and 125 ml, preferably between 1 ml and 25 ml, more preferably between 1 ml and 5 ml.The measuring arrangement 1 generally comprises exactly one narrowband measurement light source 9 for emitting measurement light. This is arranged such that the measurement light impinges on the control light in the first chamber 5 and the control light in the second chamber 6. In FIG. 1, call weight is sent "from left to right.". The measurement light appears "into the plane of the paper", i.e. from top to bottom. This is shown symbolically with the light source 9 or detector units 10 framed in dotted lines and the arrow shown in dotted lines in FIG. 1 which indicates where approximately the two components are arranged.The measurement light source 9 is designed as a laser, in particular as a diode laser, preferably a monomode diode laser, for example a diode laser with continuous wave and distributed feedback. For example, the laser 9 has a wavelength of 1550 nm or 1310 nm.The measuring arrangement 1 is configured as a Fabry-Perot interferometer. The Fabry-Perot interferometer consists of two partially reflecting mirrors 15 of high reflectivity at the "lower" and "upper" ends of the chambers 5, 6, which together form an optical resonator. The mirrors of the resonators are typically quartz glass. Each chamber 5, 6 thus has two mirrors 15, shown in FIG. 1, only the "lower" mirror in each case. The transmission spectrum of this arrangement shows narrow transmission maxima for wavelengths which meet the resonance condition, while other spectral ranges in the transmission are almost completely cancelled out. This is effected by constructive or destructive interference of the partial beams. In one embodiment, the dimensions of the chambers 5, 6 are adapted to the wavelength of the laser 9.There are further configurations for matching the resonances of the two resonators: in one configuration, the temperature of the resonators is controlled via a Peltier element, as a result of which, on the one hand, a length change of the resonators and, above all, a change of the refractive index takes place. In one embodiment, the resonators are rotated (angular tuning). In one embodiment, the distance between the mirrors is changed (for example via a piezoelectric element). In one configuration, the wavelength of one of the two beams is changed via an EOM (ElectroSwitch Modulator).The arrangement 1 further comprises a detector unit 10 each, which detects the measurement light in the first chamber 5 and the measurement light in the second chamber 6 and converts them into a first and a second electrical signal. The detector unit 10 is shown similarly to the light source 9.The detector unit 10 is designed as a photodetector. Shown here are two photodetectors 10, a first photodetector 10 for measurement light in the first chamber 5 and a second photodetector 10 for measurement light in the second chamber 6.Changes in the refractive index can be determined by measuring the intensity of the measurement light, which depends on the phase shift of the measurement light. As mentioned above, the wavelength of the control light is modulated by modulator 14. The photodetector 10 is arranged to detect modulation of the measurement light as it passes through the chambers 5, 6.The measuring arrangement 1 comprises, for example as part of the data processing unit 11, a lock-in amplifier 13 which receives an alternating current component of the electrical signal of the received measurement light generated by the photodetector 10. The current of the detector 10 is converted into a voltage signal which is then conducted to the lock-in amplifier 13. Thus, a harmonic of the modulation of the measurement light guided through the chambers 5, 6 can be determined. The detector 10 likewise receives a direct current component. The DC signal can be used to maintain the emission frequency of the measurement light source 9 at a predetermined value, preferably substantially at the inflection point, of the transfer function of the Fabry-Perot interferometer.As an alternative to the regulation via the direct current component, the measurement light can be modulated (typically with a frequency above the modulation frequency of the control oscillator 8), so that an error signal is generated via the Nth harmonic by the lock-in amplifier 14, which error signal resembles the Nth derivative of the transmission or reflection of the resonator. This allows control to the inflection point of the transmission function (or reflection function, see below). Without such modulation, it is controlled to a fixed transmission value. This has the disadvantage that, when the laser intensity is changed, regulation is no longer effected to the inflection point, but slightly above or below it.As a result of the above-described gas flow from the first chamber 5 via the scrubber 7 to the second chamber, the measurement gas (comprising the background gas 4 and the trace gases 3) is located in the first chamber 5. In the second chamber 6 there is only the background gas 4; the control layer is directed into both chambers 5, 6. This produces a photo-thermal signal, wherein in the first chamber 5, in addition to the absorption of the background gas 4, the absorption of the target gas 3 contributes to the photo-thermal signal.The measuring arrangement 1 comprises a data processing unit 11 which calculates the proportion of trace gases 3 in the measurement gas 2 from the first and second electrical signals. For this purpose, for example, one signal is subtracted from the other, for example in the data processing unit 11.FIG. 2 shows an embodiment.In FIG. 2, the excitation light source 8 and the measurement light source 9 are arranged such that the light in the first chamber 5 and the measurement light in the first chamber 5 are parallel, and that the light in the second chamber 6 and the measurement light in the second chamber 6 are parallel. In contrast to FIG. 1, control light and measurement light thus run here "from left to right.". For reasons of space, only one light source is shown in FIG. 2 (reference sign "8, 9"), but two are used. The beam path of excitation light and measurement light is the same. The mirrors 15 in one embodiment produce the resonators for the Fabry-Perot interferometer. The resonators can be adapted as described above.FIG. 3 shows the claimed measuring arrangement 1 in one embodiment and is similar to FIG. 2, and in FIG. 3 the resonators (two mirrors 15 with spacers) are placed in the cells 5, 6. The spacer can be made of quartz glass, which may possibly extend to a relatively large extent with the temperature, or the spacer alternatively consists of a temperature-stable material, for example Zerodur. The cell must then be so large that a resonator with holder fits in each case. The resonator is then adjusted to the corresponding wavelengths as above. In one configuration, this is effected thermally. The tuning range of the laser must be greater, in particular over the temperature, than the distance between two resonances. Thus, the wavelength of the laser can be adapted to the resonator.The embodiments described in FIGS. 1 to 3 take place in transmission. With few changes, a measurement in reflection is possible. The measurement light reflected by the resonator is then used for regulation (DC) and measurement (AC). For this purpose, a circulator is used, for example, in order to separate the incident and reflected light.The measuring arrangement 1 is designed in terms of acoustic design such that acoustic interference acts from the outside in the same way on the chambers 5, 6, so that acoustic interference is eliminated by differential measurement. This can be done, for example, by using a common base plate of the chambers 5, 6 or by rigidly connecting them, by acoustic filters, for example in the gas feed lines, or by placing the two chambers 5, 6 close to one another.FIGS. 1 to 3 show a "series connection" of the two chambers 5, 6 for the gas flow; FIG. 4 shows a "parallel circuit". The gas enters the measuring arrangement 1 at reference numeral 16 and the gas flow is then divided into two lines. One of the conduits is connected directly to inlet 5a. The other line is led via a scrubber 3 to inlet 6 a. The excitation light source 8 is not shown here. The above-mentioned embodiments apply analogously, for example with respect to the resonators. Data processing is also performed in a similar manner.List of reference characters1 Measuring arrangement 2 Measurement gas 3 Trace gas 4 Background gas 5 First chamber 5 a Einlass to 5 5 b Auslass to 5 6 Second chamber 6 a Einlass to 6 6 b Auslass to 6 7 Scrubber 8 Excitation light source 9 Measurement light source 10 Detector unit 11 Data processing unit 12 Optical component 13 Lock-in amplifier 14 Modulator 15 Mirror 16 Gas inletReferences included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedWO 2018 / 009953 [0008, 0009]

Claims

Measuring arrangement (1) for detecting trace gases (3) in a measurement gas (2), the measuring arrangement (1) comprising - at least one, in particular narrow-band, excitation light source (8) for emitting control light; - a first chamber (5) with an inlet for the measurement gas (2) and an outlet, wherein the control light is sent into the first chamber (6); - a second chamber (6) with an inlet, wherein this is separated from the first chamber (5) in a gas-tight manner, wherein the control light is sent into the second chamber (6); a scrubber (7), wherein the outlet of the first chamber (5) leads to the scrubber (7) and the measurement gas (2) is conducted through the scrubber (7), wherein the scrubber (7) removes the trace gases (3) from the measurement gas (2) and background gas (4) remains, wherein the background gas (4) is conducted to the inlet of the second chamber (6); at least one narrow-band measurement light source (9) for emitting measurement light, wherein the measurement light source (9) is arranged such that the measurement light impinges on the control light in the first chamber (5) and the control light in the second chamber (6); a detector unit (10) which is in optical contact with the first and the second chamber (5, 6) and which detects the measurement light from the first chamber (5) and the measurement light from the second chamber (6) and converts them into a first and a second electrical signal, respectively; and a data processing unit (11) which calculates the proportion of the trace gases (3) in the measurement gas (2) from the first and the second electrical signal.Measuring arrangement (1) for detecting trace gases (3) in a measurement gas (2), the measuring arrangement (1) comprising - at least one narrow-band excitation light source (8) for emitting control light; - a first chamber (5) with an inlet for the measurement gas (2) and an outlet, wherein the control light is sent into the first chamber (6); - a second chamber (6) with an inlet, wherein this is separated from the first chamber (5) in a gas-tight manner, wherein the control light is sent into the second chamber (6); a scrubber (7), wherein the measurement gas (2) is divided after a gas inlet (16), wherein a part of the measurement gas (2) is conducted to the inlet (5a) of the first chamber (5) and the other part of the measurement gas (2) is conducted to the scrubber (7), wherein the scrubber (7) removes the trace gases (3) from the measurement gas (2) and background gas (4) remains, wherein the background gas (4) is conducted to the inlet of the second chamber (6); - at least one narrow-band measurement light source (9) for emitting measurement light, wherein the measurement light source (9) is arranged such that the measurement light impinges on the control light in the first chamber (5) and the control light in the second chamber (6); a detector unit (10) which is in optical contact with the first and the second chamber (5, 6) and which detects the measurement light in the first chamber (5) and the measurement light in the second chamber (6) and converts them into a first and a second electrical signal, respectively; and a data processing unit (11) which calculates the proportion of the trace gases (3) in the measurement gas (2) from the first and the second electrical signal.Measuring arrangement (1) according to claim 1 or 2, wherein the detector unit (10) is arranged to measure the measurement light in transmission through the first or second chamber (5, 6).Measuring arrangement (1) according to one of the preceding claims, wherein the detector unit (10) is arranged such that it measures the measurement light in reflection from the first or second chamber (5, 6) and an optical component, such as a circulator, separates the incident and reflected measurement light.Measuring arrangement (1) according to one of the preceding claims, comprising a modulator (14) which is connected to the excitation light source (8) in order to modulate the wavelength of the control light and / or to switch the excitation light source (8) on / off.Measuring arrangement (1) according to one of the preceding claims, comprising a lock-in amplifier (13), in particular as part of the data processing unit (11), which demodulates the measurement light in the first and / or second chamber (6).Measuring arrangement (1) according to one of the preceding claims, wherein the excitation light source (8) and the measuring light source (9) are arranged such that the control light in the first chamber (5) and the measuring light in the first chamber (5) run parallel and that the control light in the second chamber (6) and the measuring light in the second chamber (6) run parallel.Measuring arrangement (1) according to one of the preceding claims, wherein the volume of the first and second chamber (5, 6) is between 1 mL and 125 mL, preferably between 1 mL and 25 mL, more preferably between 1 mL and 5 mL.The measuring arrangement (1) according to any one of the preceding claims, wherein the scrubber (7) comprises metal oxide particles having an aerodynamic diameter of less than about 250 nm, in particular of less than about 100 nm, wherein the metal oxide particles undergo one or more surface reactions with the measurement gas (2) to form one or more metal-anion complexes on the surfaces of the metal oxide particles, wherein the metal oxide particles are selected from a group consisting of CuO, ZnO, potassium permanganate and alkaline unppe carbonate [CuCO 3 •C(OH) 2] in particular the metal oxide particles are impregnated in or on carrier particles inert to the background gas (4), wherein the carrier particles have an aerodynamic diameter in a range of about 10 to 250 μm, in particular in a range from 40 to 250 μm.Measuring arrangement (1) according to one of the preceding claims, wherein the control light in the first chamber (5) is absorbed by the background gas (4) and the trace gases (3) and in the second chamber (6) by the background gas (4), and in each case leads to a temperature and / or refractive index change proportional to the absorption, wherein the refractive index change leads to a phase shift of the measurement light.Measuring arrangement (1) according to one of the preceding claims, in particular according to claim 10, wherein the measuring arrangement (1) is configured as a Fabry-Perot interferometer with corresponding mirrors (15) as optical resonators in the chambers (5, 6), such that the phase shift of the measurement light is sensitively converted into an intensity change of the transmitted or reflected measurement light.Measuring arrangement (1) according to the preceding claim, wherein the resonances of the resonators are balanced by - a temperature control of the resonators via at least one Peltier element, whereby a length change of the resonators and a change of the refractive index take place; - a rotation of the resonators; - a change of the distance of the mirrors (15) from one another, in particular via a piezoelectric element; and / or - a change of the wavelength of the measurement light radiated onto the first chamber (5) and / or the second chamber (6) via an electric modulator.Method for detecting trace gases (3) in a measurement gas (2), comprising the steps of - emitting control light into a first and second chamber (5, 6), wherein the second chamber (6) is separated from the first chamber (5) in a gas-tight manner; - guiding measurement gas (2) into the first chamber (5); - guiding the measurement gas (2) from the first chamber (5) to a scrubber (7), wherein the scrubber (7) removes the trace gases (3) from the measurement gas (2) and background gas (4) remains; - guiding the background gas (4) into the second chamber (5, 6); - emitting measurement light into the first and second chamber (5, 6); - detecting measurement light from the first and second chamber (5, 6); and - determining the proportion of trace gases (3) by means of the detected measurement light.Method for detecting trace gases (3) in a measurement gas (2), comprising the steps of - emitting control light into a first and second chamber (5, 6), wherein the second chamber (6) is separated from the first chamber (5) in a gas-tight manner; - guiding measurement gas (2) into the first chamber (5); - guiding the measurement gas (2) onto a scrubber (7), wherein the scrubber (7) removes the trace gases (3) from the measurement gas (2) and background gas (4) remains; - guiding the background gas (4) into the second chamber (5, 6); - emitting measurement light into the first and second chamber (5, 6); - detecting measurement light from the first and second chamber (5, 6); and - determining the proportion of trace gases (3) by means of the detected measurement light.

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