WO2004008113A1 - Spectrometre d'absorption et procede de mesure correspondant - Google Patents

Spectrometre d'absorption et procede de mesure correspondant Download PDF

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Publication number
WO2004008113A1
WO2004008113A1 PCT/CH2002/000381 CH0200381W WO2004008113A1 WO 2004008113 A1 WO2004008113 A1 WO 2004008113A1 CH 0200381 W CH0200381 W CH 0200381W WO 2004008113 A1 WO2004008113 A1 WO 2004008113A1
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Prior art keywords
light
cell
light source
signals
sample volume
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PCT/CH2002/000381
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German (de)
English (en)
Inventor
Markus Naegele
Klaus Bohnert
Hubert Brändle
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Abb Research Ltd
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Publication date
Application filed by Abb Research Ltd filed Critical Abb Research Ltd
Priority to PCT/CH2002/000381 priority Critical patent/WO2004008113A1/fr
Priority to AU2002344911A priority patent/AU2002344911A1/en
Publication of WO2004008113A1 publication Critical patent/WO2004008113A1/fr

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids

Definitions

  • the invention relates to the field of absorption spectroscopy. It relates to an absorption spectrometer according to the preamble of claim 1 and a corresponding method according to the preamble of claim 16.
  • absorption spectrometer Such an absorption spectrometer and a corresponding method are described, for example, in the publication “Photoacoustic Spectroscopy with Quantum Cascade Distributed-Feedback Lasers”, Opt. Lett., Vol. 26, No. 12 (June 2001), pages 887-889 by D. Hofstetter et al ..
  • the absorption spectrometer disclosed there is used to measure a partial pressure of a trace gas to be detected in a carrier gas, making use of the photoacoustic effect.
  • the photoacoustic effect is used primarily for the sensitive measurement of low concentrations of a molecular species in the presence of other molecules.
  • the spectrally narrow-band light from a light source shines through the substance mixture to be examined.
  • the wavelength of the light source is matched to an absorption wavelength of the molecular species to be investigated, some of the molecules of this molecular species are brought into an energetically excited state by absorption of the light emitted by the light source.
  • Rotational vibration states of gaseous molecules are typically excited by means of infrared radiation, the energy levels of which are characteristic of the type of molecule to be detected.
  • the excited molecules can release all or part of their excitation energy and convert it into translation, rotation or vibration energy of the shock partners.
  • a thermal equilibrium is achieved: The absorbed energy is evenly distributed over all degrees of freedom of the molecules, and thus the translation energy is increased.
  • the increase in the translation energy is equivalent to an increase in the temperature of the gas mixture.
  • the gas density remains approximately constant, for example because a sample volume taken up by the gas mixture to be examined remains constant, there is therefore an increase in pressure.
  • a pressure of about 10 2 Pa the energetic uniform distribution and thus the pressure increase takes place within about 10 ⁇ 5 s.
  • the light beam from the light source is interrupted periodically, typically with frequencies in the Hz to kHz range, periodic pressure fluctuations in the absorption cell are obtained. These pressure fluctuations can be detected for example by means of a sensitive micro ⁇ PHONS.
  • a photoacoustic apparatus such as that described, for example, in the publication by D. Hofstetter et al. is described, has a tunable laser as a light source, a photoacoustic cell and an evaluation unit.
  • the photoacoustic cell consists of a microphone arrangement and a housing in which the gas mixture to be examined is located.
  • the evaluation unit is used to evaluate the photoacoustic signals generated by the microphone arrangement.
  • the photoacoustic signal is in good approximation proportional to the absorbed light output and thus in good approximation proportional to the absorption coefficient.
  • Small absorptions are assumed, i.e. 0 (L ⁇ 1 with the absorption coefficient ⁇ and the absorption length L.
  • a photoacoustic signal proportional to the concentration results in good approximation.
  • Numerous gas molecules show characteristic wavelength dependencies in the area of the vibration-rotation bands of the absorption coefficient, which can be detected with high resolution photoacoustic spectroscopy with great accuracy.
  • the photoacoustic cell is arranged between two spherical concave mirrors. Because of this arrangement, the light from the light source passes through the photoacoustic cell several times (for example 36 times), so that the photoacoustic signals are amplified by more than an order of magnitude.
  • D. Hofstetter et al. a quantum cascade laser with distributed feedback (QC-DFB laser) is used as the light source. This type of laser allows continuous tuning of the emitted light wavelength over a small wavelength range, typically around 1% of its wavelength.
  • the wavelength of the QC-DFB can typically be selected between 3 ⁇ m and 25 ⁇ m.
  • the small spectral width of the radiation emitted by a QC-DFB laser and the fact that the QC-DFB lasers can be easily tuned make it possible to record quasi-continuous absorption spectra, i.e. absorption spectra measured at many wavelength measurement points that are close to one another. This enables reliable data evaluation to be achieved because absorption peaks which originate from a component of the carrier gas and which may spectrally overlap with the peaks to be measured can be recognized as such and taken into account accordingly in the evaluation. In addition, great measuring accuracy can be achieved if absorption peaks can be measured precisely.
  • Light sources such as the quantum cascade lasers mentioned, can have fluctuations in the power of the emitted light, for example due to aging processes or temperature fluctuations.
  • the wavelength-dependent transmission behavior of the etalon leads to a periodically structured signal at the detector, whose period (peak) spacing can be used for a relative wavelength calibration.
  • an intensity calibration of the one sample volume is performed supplied light realized by means of a beam splitter ⁇ light from the Li right out of the sample volume, which is then passed through a reference cell.
  • This reference cell ent ⁇ maintains a defined CO concentration, so that there can be a defined absorption of the light in the reference cell.
  • the remaining light intensity is detected by a InSb photodiode, while the non latestkop through the beam splitter ⁇ pelte light after passing through the sample volume InSb photodiode is also detected by means of a.
  • Absorption spectrometers are also known, for example from Norsk Elektro Optikk A / S, Norway, which contain switchable mirrors (flip mirrors). Controlled by relays, mirrors of this type can be brought into the beam path in order to redirect the light to a calibration unit.
  • an object of the invention to provide an absorption spectrometer of the type mentioned at the outset and a corresponding method which does not have the disadvantages mentioned above.
  • an absorption should onsspektrometer 20 and a corresponding method are provided which allows an improved calibration with high detection sensitivity and gros ⁇ ser measurement accuracy.
  • the absorption spectrometer according to the invention for determining a concentration CM of a molecular species M in a sample volume comprises (a) a tunable light source for the emission of pulsed light which can be propagated along a light path,
  • photo-acoustic signals SR, Ref which originate from the molecules of the reference molecule species R contained in the reference cell, can be evaluated for calibration purposes by means of the evaluation unit.
  • the corresponding method for determining the concentration CM of a molecular species M in a sample volume comprises the steps of (a) emitting pulsed light from a tunable light source along a light path, 25 (b) the sample volume along the light path is arranged and by the k
  • Molecules of at least one reference molecule species R are shown in a predeterminable concentration CR.Ref by a photoacoustic reference cell arranged along the light path,
  • a photoacoustic cell is therefore arranged along the light path as a reference cell for calibration purposes.
  • This reference cell contains molecules of the reference molecule species R in a predeterminable concentration CR.Ref. If the wavelength of the light from the light source is matched to a characteristic absorption wavelength XR of the reference molecule species R, this light is absorbed in the reference cell and photoacoustic signals SR.Ref are generated. If the wavelength of the light from the light source is matched to a characteristic absorption wavelength u of the molecule species R to be measured, this light is absorbed in the sample volume and signals SM are generated in the evaluation unit, which are dependent on the concentration CM. When evaluating the signals SM in the evaluation unit, the photoacoustic signals SR. ef calibrations.
  • a self-calibrating absorption spectrometer can be implemented.
  • the calibrations can be carried out during the operation of the absorption spectrometer and can also be carried out automatically by means of the evaluation unit. That is why long absorption measurements or measurement
  • the absorption spectrometer has a low maintenance requirement.
  • Light intensity losses in the optical path before the Refe rence ⁇ cell can be determined and used to correct the signals SM.
  • Information about the intensity of the light emitted by the light source and / or about the state of contamination of the sample can be obtained.
  • the absorption spectrometer In an advantageous embodiment of the absorption spectrometer, the
  • Detection unit a photoacoustic detection cell, the molecules of the
  • Molecular species M contains.
  • the signals SM are then photoacoustic signals.
  • the reference cell can be identical to or different from the detection cell.
  • the reference cell and the detection cell are not identical. This has the advantage that the choice of a suitable reference molecule species R is not restricted by chemical or other incompatibility with the molecule species M to be detected.
  • the sequence of sample volume and reference cell can advantageously be selected such that the sample volume is only passed through by the light after the light has passed through the reference cell.
  • the sample volume is arranged separately from the detection cell, a predefinable concentration CM, O of the molecular species M being present in the detection cell.
  • the sample volume which molecules of the molecular species M contains in the measured concentration C, so is not in the pho ⁇ toakustica detection cell, but is arranged outside of this. If the wavelength of the light from the light source is matched to a characteristic absorption wavelength XM of the molecular species M, this light is absorbed in the sample volume, and the stronger the more molecules of the molecular species M along the light path in the sample-
  • the sample volume is available. At constant pressure and constant temperature, the greater the concentration C, the greater the absorption.
  • the sample volume thus serves as a CM-dependent absorber for light of a wavelength XM that can be absorbed by the molecular species M.
  • the detection cell also contains molecules of the molecular species M, but it is the one present there Concentration CM, O of the M molecules can be specified. With increasing concentration CM, less light of the wavelength XM absorbed by the molecular species M reaches the detection cell. The photoacoustic signals SM of the detection cell decrease accordingly. The signals SM are therefore dependent on the concentration CM. For weak absorption (cxL ⁇ l with absorption coefficient a and absorption length L) and in good approximation, the signals SM are proportional to 1 -CM.
  • the separation of sample volume and photoacoustic detection cell makes it possible to achieve high detection sensitivity.
  • there is no pressure fluctuation or flow-related interference signal in the photoacoustic detection cell which would generate a signal background (noise) and thus an increase in the detection limit.
  • the detection cell is also unaffected by further disturbances of the sample volume, such as temperature fluctuations or chemical properties of the sample to be examined.
  • such an absorption spectrometer can be easily and flexibly adapted to a measurement task, because the properties of the photoacoustic detection cell (including geometry and material) can be selected and optimized independently of requirements for the sample volume. In particular, it is possible to examine an incomplete sample volume (open path measurement). The measurement of aggressive molecular species M that are incompatible with materials in the photoacoustic cell is also made possible.
  • Such an absorption spectrometer and measuring method can be inexpensively adapted to a measuring task, since typically only minor modifications are necessary, such as changing the concentration CM.O. or exchange of the molecules of the molecular species M in the detector cell for molecules of another molecule species to be detected or the change in the absorption path length L.
  • Such an absorption spectrometer and measurement method can also be better adapted to a measurement task with regard to the measurement range (typical concentrations to be measured) as well as optimizable with regard to the dynamic measuring range (ratio of smallest to largest measurable concentration). This is achieved by the predeterminability of the M concentration CM, O present in the detection cell and also by the fact that an absorption length L in the sample volume w can be chosen to be large and independent of the detection cell.
  • 15 detection cell present molecular species can be specified or at least known. This relationship between the photoacoustic signal and the absorbed radiation power can be significantly more complex and difficult to interpret if different, determined by the composition of the sample to be examined and possibly varied
  • the subject invention with photoacoustic detection of the sequence is ges along the Lichtwe ⁇ as follows: light source, reference cell, sample volume, detection cell. 25
  • the detection cell contains not only molecules of the molecular species M, but also molecules of the molecular species R in a predeterminable concentration C .Det, which generate photoacoustic signals Sß.Det in the detection cell.
  • the signals S. ef to surveil ⁇ monitoring the stability of the light emitted from the light source 30 are used.
  • By comparing the signals SR.Ref and S .De regardless of the stability of the light source, light intensity losses on the light path between the reference cell and the detection cell are detected. Such light intensity losses can occur, for example, due to progressive contamination of a multiple reflection cell containing the sample volume, typically due to deposits or adsorption on mirrors or windows. In this way, high measuring accuracy and great long-term stability can be achieved.
  • the reference cell and the detection cell are advantageously of the same design. They therefore preferably have the same optical, acoustic, photoacoustic and / or geometric properties and preferably also the same materials and the same microphone arrangement. This optimally simplifies the intensity calibration and the compensation of light losses.
  • the sample volume is advantageously arranged in a multiple reflection cell. This ensures a high level of detection sensitivity.
  • the absorption spectrometer comprises at least a second sample volume which contains molecules of a molecular species N to be detected in a concentration CN to be determined.
  • the detection unit is a photoacoustic detection cell
  • the detection cell also contains molecules of the molecular species N, but in a predeterminable concentration c N) o.
  • the second sample volume can also be identical to the sample volume already mentioned, in which case it is also it is possible that the sample volume is arranged in the detection cell and the concentrations in the detection cell cannot be predetermined, but rather the concentrations to be determined are CM.C.
  • FIG. 1 shows a simple absorption spectrometer according to the invention with photoacoustic detection, schematically; 2 shows a simple absorption spectrometer according to the invention, the reference cell being identical to the photoacoustic detection cell, schematically; 3 shows an inventive absorption spectrometer with photoacoustic detection and concave mirrors, schematically; Fig. 4 is an absorption spectrometer according to the invention with a
  • FIG. 5 shows an inventive absorption spectrometer with photo- acoustic detection and a multiple-reflection cell, schematically ⁇ table
  • Fig. 6 is an inventive absorption spectrometer with photo ⁇ acoustic detection, a multiple reflection cell and refer- ence molecules in the detection cell, schematically
  • Fig. 7 shows an inventive absorption spectrometer with photoelectric detection and a multiple reflection cell, schematically.
  • FIG. 1 schematically shows a simply constructed absorption spectrometer according to the invention with photoacoustic detection for trace gas
  • a light source 1 emits light along a light path l a (shown in dotted lines).
  • the light is pulsed, so it exhibits periodic intensity modulation.
  • this can be achieved by a mechanical interrupter (chopper).
  • the breaker frequency is on the order of 1 kHz. That from the
  • Light source 1 emitted light is advantageously spectrally narrow-band and preferably of low divergence.
  • the light first passes through a photoacoustic reference cell 6.
  • This contains molecules of a reference molecule species R in a predefinable one
  • Molecule species R also contain a carrier gas in the reference cell 6, for example nitrogen or technical air.
  • the reference cell 6 has a specially designed housing and a microphone arrangement 61.
  • the geometry of the housing is advantageously designed such that the housing 30 has a strong acoustic resonance, typically in frequency range of the order of 1 kHz.
  • the housing is advantageously sealed gas-tight. But it could also be measured in flow. If the light wavelength is matched to an absorption peak of the reference molecule species R, periodic pressure waves are generated by the periodically modulated light, which can be detected by means of the microphone arrangement 61.
  • the photoacoustic signals SR generated in this way. ef are from C.
  • the light is reflected by two optional mirrors and then reaches a sample volume 2.
  • the sample volume 2 is formed by an amount of gas to be analyzed, which in this case is obtained by taking a sample.
  • This sample contains molecules of a molecular species M to be examined in a concentration CM to be determined.
  • the sample volume 2 is contained in a photoacoustic detection cell 3.
  • the detection cell 3 here is advantageously constructed in exactly the same way as the reference cell 6. It has a microphone arrangement 31. If the light wavelength of the light emitted by the light source 1 is matched to the wavelength XM of a characteristic absorption peak of the molecular species M, the molecules of the molecular species M
  • the photoacoustic signals SM generated in this way are sent to the detection cell 3 via a signal line functionally connected evaluation unit 4 transmitted (active connection shown as dashed line).
  • the signals SM are evaluated in the evaluation unit 4, so that information about the size of the concentration CM is obtained.
  • the increase in the photoacoustic signals SM is for concentration CM proportional since SM is proportional to CM.
  • the S .Ref signals are used to calibrate the absorption spectrometer and the SM signals, as will be described below.
  • the light source 1 is advantageously a quantum cascade laser with distributed feedback (QC-DFB laser).
  • QC-DFB laser can be continuously tuned over a certain wavelength range. For example, this allows a continuous absorption spectrum to be recorded by tuning the wavelength of the QC-DFB laser over a wavelength range which contains at least one absorption peak at the wavelength XM which is characteristic of the molecular species M.
  • a quasi-continuous absorption spectrum is preferably measured by continuously tuning the QC-DFB laser over a wavelength range which contains at least one absorption peak which is characteristic of the molecular species M, with photoacoustic signals from the detection cell 3 of the evaluation unit 4 are recorded.
  • the reference molecule species R and the light source 1 are selected such that light can be emitted from the light source 1, which light can be absorbed by molecules of the reference molecule species R.
  • the reference molecule species R is preferably also selected such that it has such a characteristic absorption wavelength XR that is only slightly different from a wavelength XM characteristic of the molecular species M, but without the absorption peaks at XM and XR would overlap or at least in such a way that the absorption peaks for XM and XR can be distinguished in the evaluation.
  • spectra can advantageously be recorded during the measurement, which extend over a wavelength range which contains at least these two characteristic wavelengths XM and XR.
  • separate wavelength ranges and thus separate spectra can also be measured for the molecular species M and the reference molecule species R.
  • the characteristic of the reference molecule species R Wel ⁇ lendorf XR known so that the signals S R .REF to an absolute Wellenlän ⁇ gen-calibration are used.
  • a control parameter typically temperature or injection current
  • the S .Ref signals serve as a wavelength standard.
  • the S .Ref signals can also be used for intensity calibration.
  • the intensity of the light generating the photoacoustic signals SM and SR.Ref can change even when the concentration CM remains the same. This can happen, for example, due to aging processes or changing operating conditions of the light source 1.
  • the light source 1 comprises a laser
  • the laser beam geometry can change, for example.
  • changes in the beam path, ie the course of the light path l a can also occur and have a disruptive influence on the signal intensities.
  • interference can also occur due to light intensity losses on the light path l a.
  • the signals SR. ef can be used as an intensity standard. For an exact compensation it is assumed that the intensity fluctuations do not have a strong wavelength dependency or are at least approximately the same size for the wavelengths XM and XR.
  • the control signals serve to simplify the evaluation of the photoacoustic signals, in that the wavelength emitted by the light source 1 is used to interpret and evaluate the photoacoustic signals SM .S .Ref in the evaluation unit 4 is related.
  • a control signal can be transmitted for a spectrum at the beginning and at the end of the wavelength tuning, so that it is clear for the evaluation which of the photoacoustic signals transmitted from the detection cell 3 to the evaluation unit 4 belong to a spectrum.
  • control signals are preferably used which are directly dependent on the control parameters used for the selection of the light wavelength (typically temperature or injection current).
  • control parameters typically temperature or injection current.
  • a photoacoustic signal can be assigned to the associated light wavelength at any time during the measurement.
  • the absorption spectrometer shows a further simple embodiment of the absorption spectrometer according to the invention, which can be implemented particularly cost-effectively and works with photoacoustic detection.
  • This absorption spectrometer largely corresponds to that from FIG. 1.
  • the reference cell 6 is identical to the detection cell 3.
  • the absorption spectrometer contains only a single photoacoustic cell. This contains both the sample volume 2 to be examined with molecules of the molecular species M in a concentration CM to be determined and also molecules of the reference molecule species R in a predetermined concentration C .Ref.
  • An optional operative connection between the light source 1 and the evaluation unit 4 is not provided here, but can advantageously be provided.
  • FIG. 3 schematically shows a further advantageous embodiment of an absorption spectrometer according to the invention.
  • This absorption spectrometer largely corresponds to that from FIG. 1.
  • the photo-75 acoustic reference cell 6 and the detection cell 3 are arranged between two spherical concave mirrors 7, 7 '.
  • the light from the light source 1 is coupled into the light path between the concave mirrors 7, 7 'by means of a mirror.
  • the light from the light source 1 is reflected several times in such a way that the detection cell 3 together with the sample volume 2 and the reference cell 6 are traversed several times by the light. Characterized accuracy can be achieved a high detection sensitivity, and an improved measurement ⁇ . Because by going through the pho-
  • the optional operative connection between the light source 1 and the evaluation unit 4 is not provided here, but can be provided.
  • 4 schematically shows a further advantageous exemplary embodiment of the subject matter of the invention. Photoacoustic detection is carried out in this absorption spectrometer for trace gas analysis, although the sample 5 volume 2 is arranged separately from the photoacoustic detection cell 3. As in the exemplary embodiment in FIG. 1, the reference cell 6 is identical to the detection cell 3.
  • a QC-DFB laser serves as light source 1, the light intensity of which is modulated by varying the injection current of the QC-DFB laser, so that pulsed light is emitted.
  • the light first arrives along the light path 1 a in a multiple reflection cell 5, which contains the sample volume 2.
  • the multiple reflection cell 5 is advantageously a White or a Herriott cell.
  • the absorption length L in the sample volume can be selected by means of the multiple reflection cell 5, and large absorption lengths L can be set. This allows the absorption spectrometer to be adapted to a measurement problem and the detection sensitivity
  • the multiple reflection cell 5 can be suitable for measurements in the gas flow.
  • the multiple reflection cell 5 in FIG. 4 has a gas inlet 51 and a gas outlet 52.
  • a pressure meter 53 can also be provided for pressure measurement in the multiple reflection cell 5, as a result of which pressure fluctuations in the
  • the evaluation unit 4 To transmit pressure variations in the sample volume 2 to the intensity and / or lines ⁇ width correction of the signals SM to the evaluation unit 4 (not shown).
  • the sample volume there are molecules of a molecular species M in the concentration C to be determined in a carrier gas.
  • the light wavelength is tuned to a wavelength XM of a characteristic absorption peak of the molecular species M, light is absorbed by the molecules of the molecular species 5.
  • a photoacoustic detection cell 3 which also serves as a reference cell 6. It therefore contains molecules of the molecular species M in a predeterminable, advantageously known concentration CM.O and molecules of the reference molecule species R in a predeterminable, advantageously known concentration C. ef. If the light wavelength is matched to a wavelength XM of a characteristic absorption peak of the molecular species M, photoacoustic signals SM are
  • the photoacoustic signals SM depend not only on CM.O but also on CM. This is because the sample volume 2 serves as a CM-dependent light absorber for light from the light source 1, in particular for light emitted by the light source 1
  • the decrease in photoacoustic signals SM is proportional to the concentration CM, since SM is proportional to 1 -CM.
  • CM.O and / or the predefinable absorption length L it is possible to select a desired measuring range of concentrations CM ZU.
  • the measurement accuracy can be optimally adapted to a measurement task, and the dynamic range available for the photoacoustic measurement, typically about three orders of magnitude, can be used optimally.
  • the signals SR.Ref can be used in the manner described above for wavelength and / or intensity calibration. Loss of light intensity, which is caused by fluctuations in intensity of the light source 1 or by losses in the multiple reflection cell 5, for example by absorbates on the mirror surfaces of the multiple reflection cell 5, can be used to correct the signals SM.
  • FIG. 5 schematically shows a further advantageous absorption spectrometer according to the invention with photoacoustic detection. It is very similar to the absorption spectrometer from FIG. 4 and is therefore described on the basis of this.
  • the detection cell 3 is not used as a reference cell 6.
  • a separate reference cell 6 is provided.
  • the detection cell 3 and the reference cell 6 are different photoacoustic cells. This is special advantageous if the reference molecule species R is not compatible with the molecules present in the detection cell 3, for example because R molecules can react chemically with M molecules.
  • the reference cell 6 is advantageously arranged on the light path la between the light source 1 and the sample volume 2.
  • the reference cell 6 can also be arranged on the light path la between the sample volume 2 and the detection cell 3.
  • the detection cell 3 and the reference cell 6 are advantageously of the same design.
  • the two photoacoustic cells have the same resonance frequencies or better still have essentially the same photoacoustic properties and in particular have essentially the same acoustic, optical and geometric properties and a similar microphone arrangement and are advantageously also made from the same materials ,
  • photoacoustic signals of the two cells 3, 6 can be compared more easily and directly, so that calibrations are easier and more accurate.
  • the pulsed light from the light source 1 first passes through the photoacoustic reference cell 6, which contains molecules of a reference molecule species R in a predeterminable concentration CR.Ref. If the light of the light source 1 is tuned to a wavelength XR which is characteristic of the reference molecule species R, photoacoustic signals SR. ef generated and transmitted to the evaluation unit 4 for evaluation. The light then passes through the multiple reflection cell 5, which contains the sample volume 2, which contains molecules of the molecular species M in the concentration C to be determined. The sample volume serves as a CM-dependent absorber for light of the wavelength XM characteristic of the molecular species M to be detected. Then the light passes through the photoacoustic detection cell 3.
  • the signals S .Ref can be used in the evaluation unit 4 for wavelength calibration and / or for intensity calibration, as described above.
  • the detection cell 3 in addition to the molecules of the molecular species M, the detection cell 3 also contains molecules of the same reference molecule species R as the reference cell 6. In the detection cell 3, the molecules of the reference molecule species R are in a predeterminable concentration -
  • This concentration CR.Det is advantageously chosen to be the same size as the concentration CR, ef of the molecules of the reference molecule species R in the reference cell 6.
  • photoacoustic signals from the two cells 3, 6 can be compared more easily and directly, so that intensity calibrations are easier and more precise. It is also advantageous
  • the evaluation unit 4 still photoacoustic signals S .Ref and SR.Det transmitted.
  • the signals SR.Ref originate from the molecules of the reference molecule species R contained in the reference cell 6 and can be used to monitor the light output emitted by the light source 1.
  • the signals SR.Det originate from the 5 molecules of the reference molecule species R contained in the detection cell 3.
  • the evaluation unit 4 can advantageously comprise a self-diagnosis unit which, on the basis of the signals S. ef obtained information about the intensity stability of the light source 1 and based on the signals SR.Ref and S ⁇ Det
  • the evaluation unit 20 differentiates when the light source 1 is to be checked or replaced and when the multiple reflection cell 5 is to be checked or cleaned. In the case of appropriate operating conditions (exceeding predefinable error tolerances), the evaluation unit can emit 4 warning signals. Based on a time course of the measured signals, predictions can be made about
  • the source of errors for an upcoming maintenance or repair (light source 1 or sample volume 2) can be specified.
  • Information about the intensity of the light emitted by the light source 1 and, in addition, information about the state of contamination of the sample volume 2 can be obtained.
  • the signals SR.Ref and / or signals S .Det can be used to calibrate the wavelength.
  • these control signals serve to simplify the evaluation of the measurement the photoacoustic signals in that the wavelength selection of the light source 1 is related to the interpretation and evaluation of the photoacoustic signals in the evaluation unit 4.
  • a photoacoustic signal can be uniquely assigned to the corresponding light wavelength at any time during a measurement. If the relationship between the control signal and the emitted light wavelength changes in the course of time or due to interference, one can
  • the new, corrected relationship between the control signal and the emitted light wavelength can be transmitted from the evaluation unit 4 to the light source 1 via the above-mentioned active connection, so that measurements can always be carried out in the desired wavelength range. Results can also be obtained via the active connection mentioned
  • sample cell 2 arranged in the flexion cell 5 serves as a CM-dependent light absorber, which produces a reduction in the light intensity at characteristic wavelengths XM of the M molecules, which light intensity can then be detected by means of the photodiode 3.
  • the corresponding photoelectric signals SM are transmitted to the evaluation unit 4 and can there by means of the signals SR. ef can be corrected in a manner mentioned above. Fluctuations in the light intensity of the light source 1 can thus be easily compensated.
  • the SR.Ref signals can be used as a wavelength standard.
  • Typical parameters and orders of magnitude for trace gas detection in the infrared range are given below by way of example for NH3 detection.
  • the structure of the absorption spectrometer corresponds to that shown in FIG. 6.
  • - Light source a quantum cascade laser with a tuning range of 1045 cm- 1 - 1048 cm- 1
  • An advantageous quantum cascade laser in particular a QC-DFB laser, can be selected as the light source 1.
  • Quantum cascade lasers are inexpensive and enable a high spectral resolution and thus a high measurement accuracy, since they emit light of a small spectral width.
  • the tuning of the wavelength of the emitted light can advantageously be achieved by temperature modulation of the laser and / or by modulation of the injection current of the laser.
  • the injection current of the laser can be modulated in a suitable manner. It is also possible to operate the laser in cw mode and to use an additional, preferably mechanical, chopper in order to obtain pulsed light.
  • a quantum cascade laser instead of a quantum cascade laser, another light generator can also be used.
  • a tunable laser diode, a color material ⁇ laser, a fiber laser or a CO or a CO 2 laser, the Lichterzeu ⁇ ger spectrally narrow-band light should emit so that for a molecule to be detected species characteristic absorption peak ascending is solvable. If necessary, this can be achieved with an additional color filter.
  • the light source 1 can comprise a single or also a plurality of light generators, for example a plurality of QC-DFB lasers. This is particularly advantageous if several absorption peaks are to be measured and the wavelength range which can be covered by a single light generator does not include all absorption peaks to be measured.
  • the wavelength of the light from the light source 1 is preferably in the infrared range, since there are typically very characteristic absorption peaks of the molecular species M to be measured.
  • light of other wavelength ranges can be used, such as. for example visible light or ultraviolet light.
  • the tunability of the light source 1 does not necessarily mean that the light source 1 comprises a continuously detunable light generator . It is also possible to use a light source 1 which emits light of several discrete wavelengths. For example, a light
  • 20 source 1 comprise two (semiconductor) lasers, of which a first light of a wavelength XM characteristic of the molecular species M can emit and a second light of a wavelength XR characteristic of the reference molecule species R. A calibration can then be carried out using the second laser. A third laser can also be used
  • optical coupling of the light source 1 to the next component of the absorption spectrometer along the light path 1 a can be implemented, for example, via an air gap or via a glass fiber.
  • infrared-transmitting glass fibers or “hollow tube” infrared fibers are particularly advantageous.
  • molecular species includes any type of particle with characteristic absorption properties and in particular also atomic species.
  • the thermodynamic state of the molecular species is preferably gaseous, but can also be liquid or solid.
  • molecular species M instead of a single molecular species M, several molecular species M, N, ... can also be examined. These can be present in the same sample volume 2 or can also be contained in a plurality of sample volumes which are advantageously arranged along the light path 1 a between the light source 1 and the detection unit 3 and which each contain molecules of one or more molecular species M to be detected. N, ... can contain. Light from the light source 1 must of course be examined in this case, and by the other to be ⁇ molecular species be absorbable. Individual measuring points or a single quasi-continuous absorption spectrum 25 or several quasi-continuous absorption spectra can then be measured.
  • a light source 1 for the sequential operation of a plurality of 5 absorption spectrometers, for example by means of movable mirrors. In this way, for example, different molecular species can be measured in different sample volumes.
  • a common evaluation unit 4 can advantageously be used.
  • the sample volume 2 and the detection cell 3 are arranged separately from one another, these two can be selected and optimized largely independently of one another.
  • the sample volume 2 can be contained in a container of almost any shape and filled with a sample to be examined (pro
  • the detection cell 3 can be largely independent
  • the sample 20 can be selected from the properties of the sample to be examined. This is particularly advantageous if in the sample to be examined chemically reac ⁇ tive substances are present, or if a light gas mixture to be examined moisture or other adsorbed substances containing the light intensity to losses or other distortions of measuring signals
  • sample volume 2 and detection cell 3 increases the detection sensitivity in flow measurements, since there are no disturbing flow noises in the detection cell 3. With the separate arrangement, the measurement is significantly less sensitive to temperature fluctuations of the sample to be examined.
  • the concentration CM corresponds to a partial pressure, which is typically specified in a volume fraction. Gases are then advantageously contained in all photoacoustic cells 3, 6 and sample volumes. However, liquids and solids or adsorbates can also be examined using an absorption spectrometer according to the invention.
  • the photoacoustic effect in liquids is known, so that the above-described embodiments can easily be transferred to the use of liquids in the reference cell 6 and, if appropriate, in the detection cell 3 for examining liquids in the sample volume 2. It is also conceivable for gaseous molecular species R, M to be present in the reference cell 6 (and possibly a detection cell 3) when examining a liquid in the sample volume 2.
  • the concentration CM essentially corresponds to a (surface) coverage.
  • reflection on the surface of a solid to be examined advantageously takes place along the light path la and has molecules of the molecular species to be analyzed on its surface ,
  • the detection unit 3 can, as in FIGS. 1, 2, 3, 4, 5 and 6, advantageously a light detector based on the photoacoustic effect or, as in FIG. 7, also advantageously a light detector based on the photoelectric effect. tiplier). However, it can also be any other light-sensitive detector, for example a photochemical or a photothermal (for example a thermopile or a pyroelectric detector).
  • the dynamic measuring range and also the detection sensitivity can be selected via the concentration CM.O of the molecular species M present in the detection cell 3 and via the absorption length L in the sample volume 2.
  • the dynamic measuring range gives the Size of the concentration range between the minimum and the maximum measurable concentration CM.
  • CM.O is advantageously chosen on the one hand 5 to be large enough to achieve a large signal-to-noise ratio of the microphone arrangement 31 for the photoacoustic signals transmitted to the evaluation unit 4 and on the other hand to be chosen so small that the relationship between CM and SM is as linear as possible.
  • the resonance frequency of the reference cell 6 (and possibly the detection cell 3) and the interruption frequency of the light source 1 are advantageously chosen to be the same size. This results in an improved detection sensitivity.
  • a possible reference molecule species R is carbonyl sulfide (OCS, empirical formula: COS).
  • OCS carbonyl sulfide
  • the reference molecule species R is advantageously selected depending on the molecule species present in the sample to be examined. If the reference cell 6 with the de-
  • the detection cell 3 is identical, as for example in FIGS. 2 and 4, a defined amount of the R molecules is advantageously added to the molecules present in the detection cell 3. This can also be done with flow measurements.
  • the signals S .Ref can, if the characteristic wavelength XR is known for the reference molecule species R, be used for an absolute wavelength calibration. If the characteristic wavelength XR is not absolutely known, the signals SR.Ref can be used for a relative wavelength calibration. A particularly high accuracy of the wavelength calibration can be achieved if at least two cha- characteristic wavelengths of the reference molecule species R are measured. Several different reference molecule species can also be used, and corresponding characteristic absorption peaks of these reference molecule species can be measured, in particular if several absorption peaks of one or more molecule species to be examined are to be measured.
  • a particular advantage of the options for wavelength and intensity calibration shown is that these calibrations can be carried out while the absorption spectrometer is in operation.
  • a self-calibrating absorption spectrometer can thus be implemented. This has the advantages that an inexpensive operation of the absorption spectrometer is made possible, that a low maintenance requirement is achieved, and that long, uninterrupted measurement intervals with great long-term stability can be achieved.
  • the use of a reference cell 6 described for calibration purposes is very advantageous in that there is no or only a small loss of light intensity in the case of measured characteristic absorption wavelengths of the molecular species to be examined.
  • CM CM corrected on the basis of an intensity calibration
  • theoretical calculations or calibration measurements can be made with gases with known compositions.
  • An absorption spectrometer according to the invention can be used for the precise measurement of a concentration CM. But it can also be simple the falling below or exceeding a limit concentration are detected, so that the absorption spectrometer acts as a safety or monitoring device which may give an alarm. If several measurements are made in a chronological sequence, the temporal development of CM can be measured.
  • An absorption spectrometer according to the invention is very easy to adapt to a specific measurement problem. It is very suitable for measuring the concentration CM of a certain molecular species M in the presence of other gases.
  • An absorption spectrometer according to the invention and the corresponding method can be used particularly advantageously in the control of chemical or physical processes and in the monitoring of compliance with maximum workplace concentrations or in the measurement and monitoring of environmentally relevant gases, such as for example in the case of immission and emission measurements in industrial plants , Road traffic or in agricultural businesses.
  • SM signals of the detection unit depending on CM, "photoacoustic signals of the detection cell; photoelectric signals of the photodiode

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Abstract

L'invention concerne un spectromètre d'absorption servant à déterminer une concentration CM d'une espèce moléculaire M dans un volume d'essai (2), ce spectromètre comprenant une source lumineuse ajustable (1) servant à émettre une lumière pulsée, une unité de détection (3), ainsi qu'une unité d'analyse (4) servant à analyser des signaux SM de l'unité de détection (3) qui sont fonction de la concentration CM. Ce spectromètre d'absorption se caractérise en ce qu'il comprend une cellule de référence photo-acoustique (6) disposée sur le trajet du faisceau lumineux (1a), cette cellule contenant des molécules d'au moins une espèce moléculaire de référence R dans une concentration prédéterminable CR,Ref, et en ce que l'unité d'analyse (4) permet d'analyser des signaux photo-acoustiques SR,Ref provenant des molécules de l'espèce moléculaire de référence R contenues dans la cellule de référence (6) à des fins d'étalonnage. Ce spectromètre d'absorption peut être étalonné de façon simple et continue tout en garantissant une haute sensibilité de détection et une grande précision de mesure. Il est particulièrement adapté à l'analyse de traces de gaz pour des espèces moléculaires définies, ainsi qu'au contrôle de processus chimiques.
PCT/CH2002/000381 2002-07-12 2002-07-12 Spectrometre d'absorption et procede de mesure correspondant WO2004008113A1 (fr)

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AU2002344911A AU2002344911A1 (en) 2002-07-12 2002-07-12 Absorption spectrometer and corresponding measuring method

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DE10345507A1 (de) * 2003-09-30 2005-05-04 Siemens Ag Diodenlaser-Spektrometer
WO2007000297A1 (fr) * 2005-06-28 2007-01-04 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e. V. Detecteur photoacoustique en champ libre
WO2008116659A1 (fr) * 2007-03-27 2008-10-02 Fraunhofer-Gesellschaft Zur Förderung Der Angewandten Forschung Détecteur photoacoustique cylindrique à excitation de la deuxième résonance azimutale
DE102007014518B3 (de) * 2007-03-27 2008-11-13 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Photoakustische Multipass-Zelle mit konzentrierenden Reflexionsmitteln
DE102015106373A1 (de) * 2015-04-24 2016-10-27 Infineon Technologies Ag Photoakustisches gassensormodul mit lichtemittereinheit undeiner detektoreinheit
FR3117590A1 (fr) * 2020-12-15 2022-06-17 Mirsense Dispositif de mesure d’un rayonnement laser par effet photoacoustique

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Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE10345507A1 (de) * 2003-09-30 2005-05-04 Siemens Ag Diodenlaser-Spektrometer
WO2007000297A1 (fr) * 2005-06-28 2007-01-04 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e. V. Detecteur photoacoustique en champ libre
WO2008116659A1 (fr) * 2007-03-27 2008-10-02 Fraunhofer-Gesellschaft Zur Förderung Der Angewandten Forschung Détecteur photoacoustique cylindrique à excitation de la deuxième résonance azimutale
DE102007014518B3 (de) * 2007-03-27 2008-11-13 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Photoakustische Multipass-Zelle mit konzentrierenden Reflexionsmitteln
US8479559B2 (en) 2007-03-27 2013-07-09 Fraunhofer-Gesellschaft Zur Foerderung Der Angewandten Forschung E.V. Cylindrical photoacoustic detector with excitation of the second azimuthal resonance
US8534129B2 (en) 2007-03-27 2013-09-17 Fraunhofer-Gesellschaft Zur Foerderung Der Angewandten Forschung E.V. Photoacoustic multipass cell with concentrating reflection means
DE102015106373A1 (de) * 2015-04-24 2016-10-27 Infineon Technologies Ag Photoakustisches gassensormodul mit lichtemittereinheit undeiner detektoreinheit
DE102015106373A8 (de) * 2015-04-24 2016-12-22 Infineon Technologies Ag Photoakustisches gassensormodul mit lichtemittereinheit und einer detektoreinheit
US10241088B2 (en) 2015-04-24 2019-03-26 Infineon Technologies Ag Photo-acoustic gas sensor module having light emitter and detector units
DE102015106373B4 (de) 2015-04-24 2023-03-02 Infineon Technologies Ag Photoakustisches gassensormodul mit lichtemittereinheit und einer detektoreinheit
FR3117590A1 (fr) * 2020-12-15 2022-06-17 Mirsense Dispositif de mesure d’un rayonnement laser par effet photoacoustique
WO2022128892A1 (fr) * 2020-12-15 2022-06-23 Mirsense Dispositif de mesure d'un rayonnement laser par effet photoacoustique

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