WO2004008113A1

WO2004008113A1 - Absorption spectrometer and corresponding measuring method

Info

Publication number: WO2004008113A1
Application number: PCT/CH2002/000381
Authority: WO
Inventors: Markus Naegele; Klaus Bohnert; Hubert Brändle
Original assignee: Abb Research Ltd
Priority date: 2002-07-12
Filing date: 2002-07-12
Publication date: 2004-01-22
Also published as: AU2002344911A1

Abstract

The invention relates to an absorption spectrometer for determining a concentration CM of a molecule species M in a sample volume (2), comprising a continuously tunable light source (1) for emitting pulsed light, a detection unit (3), and an evaluation unit (4) for evaluating signals SM of the detection unit (3) which depend on the concentration CM. The invention is characterized in that the absorption spectrometer comprises a photoacoustic reference cell (6) disposed along the optical path (1a) in which cell molecules of at least one predetermined concentration CR,Ref are contained and in that photoacoustic signals SR,Ref which stem from the molecules of the reference molecule species R contained in the reference cell (6) are evaluated for the purpose of calibration. The invention thereby allows for a simple and continuous calibration of the absorption spectrometer with high detection sensitivity and excellent accuracy of measurement. The absorption spectrometer is highly useful for trace gas analysis for defined molecules species and for monitoring chemical processes.

Description

ABSORPTION SPECTROMETER AND CORRESPONDING MEASURING METHOD

DESCRIPTION

10

Technical field

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.

State of the art

20

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. If 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. By collisions with other gas molecules in the absorption cell, 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. After excitation, 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. If 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. At a pressure of about 10 ² Pa, the energetic uniform distribution and thus the pressure increase takes place within about 10 ^{~ 5} s. If 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.

If the light wavelength is tuned over a characteristic absorption line of the molecules to be detected in the cell, 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 α and the absorption length L. For a given absorption peak, 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. Due to this high selectivity, it is possible by means of photoacoustic absorption spectroscopy to detect a to be detected trace gas and to determine its concentration, even if the Kon ^¬ concentration of the corresponding molecular species only some ppm or even a few 100 ppb.

In order to increase the sensitivity and reduce the detection limit, the publication by D. Hofstetter et al. on the one hand, 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. On the other hand, 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. In terms of manufacturing technology, the wavelength of the QC-DFB can typically be selected between 3 μm and 25 μm. 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. In addition, it is generally necessary to also calibrate the wavelength of the light emitted by the light source, so that measured absorption peaks can be assigned to the corresponding molecular species.

In M. Nagele, MW Sigrist, "Mobile Laser Spectrometer with Novel Resonant multipass Photoacoustic Cell for trace gas analysis", Applied Physics B, Vol. 70, p. 895-901 (2000) describes an absorption spectrometer with photoakus ^¬ genetic detection , in which for the purpose of standardizing the light intensity emitted by the light source and for controlling the emitted th light wavelength is coupled out of the light path using a beam splitter. The light coupled out in this way is fed on the one hand to a means for measuring light output. This is typically a photoelectric infrared detector, such as an InSb photodiode. On the other hand, the light is fed via a further beam splitter to a spectrum analyzer or a spectrometer, in which the wavelength of the light is determined.

In T. Keltz et al., "Detection of CO in Air Using Diode Laser Pumped Difference Frequency Generation in a Modular Setup", J. Quant. Spectrosc. Radiat. Transfer, Vol. 61, p. 591-601 (1 999) two absorption spectrometer with photoelectric detection means InSb photodiodes are illustrated. In the first absorption spectrometer, a relative wavelength calibration is realized in that is coupled out of the light path by means of a beam splitter light Siert then after passing through an etalon analy ^¬. the When the laser wavelength is tuned, 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. In the second absorption spectrometer, 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. After passing through the reference cell, the remaining light intensity is detected by a InSb photodiode, while the non ausgekop 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.

The methods mentioned for intensity or wavelength calibration have the disadvantage that either the light path has to be changed and no measurement can take place while a calibration w is being carried out, or that in favor of the possibility of calibration, losses of light intensity and thus of detection sensitivity and / or measuring speed must be accepted.

75 Presentation of the invention

It is 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. In particular, 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.

This problem is solved by an absorption spectrometer with the features of claim 1 and a method with the features of claim 16.

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,

(b) a detection unit, and

(c) an evaluation unit operatively connected to the detection unit for evaluating signals SM of the detection unit, which signals SM are dependent on the concentration CM, wherein

(d) the sample volume and the detection unit are arranged along the light path, and

(e) light from the light source is absorbable by the molecular species M. w It is characterized by

(f) that the absorption spectrometer contains a photoacoustic reference cell arranged along the light path,

(g) that the reference cell contains molecules of at least one reference molecule species R in a predeterminable concentration CR.Ref,

15 (h) that light of the light source is absorbable by the molecular species R, and

(i) that 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.

20

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

Light from the light source is passed, whereby

(c) Light of the light source is detected by means of a detection unit, where ^¬ are generated by signals SM, which are dependent on the concentration CM, and wherein (d) the signals SM are evaluated by means of an evaluation unit which is operatively connected to the detection unit.

It is characterized

(e) 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,

(f) light of the light source is absorbed by the reference molecule species R in the reference cell, and

(g) that, by means of the evaluation unit, photoacoustic signals S ^ Ref, which w of the molecules of the at least one contained in the reference cell

Reference molecule species R originate, are evaluated for calibration purposes.

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.

With a suitable choice of the reference molecule species R occur through the

Calibration practically no loss of light intensity when the wave

30 length of light from the light source to a characteristic absorption world length XM of the molecular species R to be measured is matched. This makes it possible to achieve high sensitivity. Because the reference cell can remain in the light path, no calibration of the light path is required for such a calibration using a photoacoustic reference cell. This enables improved measurement accuracy and time resolution. And no moving parts are needed. Intensity and / or wavelength calibration of the absorption spectrometer can be achieved in a simple manner.

w In particular, 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

15 rows feasible. 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.

20 volume can be obtained, and based on a temporal course of the measured signals, predictions can be made about the need for maintenance work. If a characteristic absorption wavelength of the reference XR molecular species R is known, a abso lute can ^¬ wavelength calibration are made, otherwise at least

25 a relative wavelength calibration. k

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.

30 le, which are generated by the fact that light is one for the molecular species M characteristic wavelength XM is at least partially absorbed by the M molecules contained in the detection cell. Photoacoustic detection makes it possible to measure with a high level of measurement accuracy and sensitivity. The reference cell can be identical to or different from the detection cell.

In a further advantageous embodiment with photoacoustic detection, 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. In addition, 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.

In a particularly advantageous embodiment of the invention, 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 ^¬ toakustischen 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-

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. In particular with continuous gas or liquid measurements in the flow, 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.

Furthermore, 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.

In addition, greater measuring accuracy can be achieved, since the linear relationship between the photoacoustic signal and the absorbed radiation power for “L << 1 is relatively simple, provided that the

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

There are 20 molecular species present in the photoacoustic cell.

In a particularly preferred embodiment of 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.

In the case of a detection cell that is not identical to the reference cell, 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.

In a further advantageous embodiment of the subject matter of the invention, 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. If 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. In this way, the same methods mentioned above and with the same method have been used described absorption spectrometer, several different molecular species and several sample volumes can be examined. 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.

Further preferred embodiments and advantages emerge from the dependent patent claims and the figures.

Brief description of the drawings

The subject matter of the invention is explained in more detail below on the basis of preferred exemplary embodiments which are illustrated in the accompanying drawings. Show it:

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

Multiple reflection cell, the reference cell being identical to the photoacoustic detection cell, schematic; 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.

5 The reference symbols used in the drawings and their meaning are summarized in the list of reference symbols. Basically, the same or at least equivalent parts are provided with the same reference numerals in the figures.

w

Ways of Carrying Out the Invention

1 schematically shows a simply constructed absorption spectrometer according to the invention with photoacoustic detection for trace gas

15 Analysis. 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. In the case of a cw laser, for example, this can be achieved by a mechanical interrupter (chopper). Typically 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

25 concentration C .Ref. In general, in addition to the molecules, the reference k

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. ef dependent and are transmitted to an evaluation unit 4 operatively connected to the reference cell 6 via a signal line (active connection shown as a dashed line). These and the operative connections mentioned below can also be referred to as connections and are typically (electrical) signal lines. In the evaluation unit 4, the signals SR. ef evaluated.

In FIG. 1, 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

Absorbs light. In general, only some of the light is absorbed. Due to the periodic modulation of the light then in the De ^¬ 3 tektionszelle periodic pressure waves are generated which are detected by means of the ^micro-¬ phon Anodnung 31st 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. If the product of the absorption coefficient α and the absorption length L (in the sample volume) is much smaller than one (OCL << 1) and interference factors such as the Kinetic Cooling Effect are negligible, 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). Such a 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. From a thus obtained absorption spectrum can by means of the evaluation unit 4 with great accuracy the absorbed optical power and hence the absorption coefficient o "of the molecular species M, and it constant with knowledge of the appropriate molecular determined concentration to be determined CM to investigate ^¬. In front- In this evaluation, an adaptation of a curve to the absorption spectrum is carried out in part by the evaluation unit 4 (curve fitting). By measuring a (quasi-continuous) spectrum, the cross-sensitivity of the measurement method to molecular species that are present in the sample volume or otherwise along the light path la and have an absorption peak near the wavelength XM is minimized. In such a case, the curve shape of the spectrum is generally changed characteristically, which is noticeable in curve fitting and enables corresponding corrections.

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. In addition, 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. In this case, 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. However, separate wavelength ranges and thus separate spectra can also be measured for the molecular species M and the reference molecule species R.

Advantageous is the characteristic of the reference molecule species R Wel ^¬ lenlänge XR known, so that the signals S _R .REF to an absolute Wellenlän ^¬ gen-calibration are used. For example, with QC-DFB lasers, aging processes or changing operating conditions can cause changes such that the relationship between between the light wavelength emitted by the QC-DFB laser and a control parameter (typically temperature or injection current) used to select the light wavelength. By means of the signals SR. Such changes can be compensated for. The S .Ref signals serve as a wavelength standard.

Alternatively or additionally, 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. If the light source 1 comprises a laser, the laser beam geometry can change, for example. And 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. Furthermore, interference can also occur due to light intensity losses on the light path l a. For example, by soiling or adsorption-related variable reflection, absorption and / or scattering on optical elements along the light path la, such as on the mirrors or on entrance or exit windows of photoacoustic cells 3, 6, or by scattering or absorption on the light path la, such as due to moisture or soot particles. In order to compensate for such fluctuations in intensity or loss of light intensity, 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.

In the exemplary embodiment in FIG. 1, there is an optional active connection between the light source 1 and the evaluation unit 4 (active connection shown as a dashed line). This active connection is used to transmit control signals from the light source 1 to the evaluation unit 4. In the case of a measurement, 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. For example, 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. However, 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). In this way, optionally after a wavelength calibration, a photoacoustic signal can be assigned to the associated light wavelength at any time during the measurement.

2 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. However, here the reference cell 6 is identical to the detection cell 3. This means that 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. Is the wavelength of the light from the light source 1 at a characteristic wavelength λ _M or .lambda.r tuned so light is absorbed by the jeweili ^¬ gen molecules M and R, respectively, and corresponding photo- acoustic signals SM, SR. ef are generated and sent to the evaluation unit 4.

Wavelength and / or intensity calibrations can be carried out as described above. In particular, simple and accurate compensation of all light intensity losses or fluctuations that occur in front of the detection cell 3 (= reference cell 6) is possible.

An optional operative connection between the light source 1 and the evaluation unit 4 is not provided here, but can advantageously be provided.

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. However, here 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.

Due to the concave mirrors 7, 7 ', 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-

25 toacoustic cells 3,6, larger photoacoustic signals S .S .Ref k are generated. An enlarged absorption length L and thus a stronger From ^¬ sorption in sample volume 2 is achieved.

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, for example, 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.

15 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

20 can be increased. As shown in FIG. 4, the multiple reflection cell 5 can be suitable for measurements in the gas flow. For this purpose, the multiple reflection cell 5 in FIG. 4 has a gas inlet 51 and a gas outlet 52. In addition, 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

25 be volume 2 can be determined. It is advantageously possible to use the k

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). In the sample volume there are molecules of a molecular species M in the concentration C to be determined in a carrier gas. When 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.

After leaving the multiple reflection cell 5 and the sample volume 2, the light penetrates into 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

75 creates and forwarded to the evaluation unit 4, which are dependent on the concentration CM and the concentration CM.O. If the light wavelength is matched to a wavelength XR of a characteristic absorption peak of the reference molecule species R, photoacoustic signals S .Ref are generated and passed on to the evaluation unit 4 which is generated by the

20 concentration C. ef are dependent.

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

25 Light of the wavelength XM characteristic of the molecular species M. If k is the concentration CM of molecules of the molecular species M in the sample volume zero (CM = 0), the signals SM are maximal at the absorption wavelength XM. For the light of the light source 1 reaches a maxima ^¬ len light intensity in the detection cell 3 and generates a maximum 30 photoacoustic signal. The greater the concentration CM in the sample volume is, the less light of the characteristic absorption wavelength XM reaches the detection cell 3, so that the corresponding photoacoustic signals SM become smaller. If the product of absorption coefficient a and absorption length L (in the sample volume) is much smaller than one (od _ << 1), the decrease in photoacoustic signals SM is proportional to the concentration CM, since SM is proportional to 1 -CM.

Using the predeterminable concentration CM.O and / or the predefinable absorption length L, it is possible to select a desired measuring range of concentrations CM ZU. As a result, 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. In particular, it is possible to optimize the detection sensitivity of the absorption spectrometer by choosing CM.O and / or L.

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.

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. In contrast to FIG. 4, in the absorption spectrometer in FIG. 5 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. This means in particular that 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 , As a result, photoacoustic signals of the two cells 3, 6 can be compared more easily and directly, so that calibrations are easier and more accurate.

In FIG. 5, 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. This contains molecules of the molecular species M in one predeterminable concentration CM.O. If the wavelength of the light source 1 is tuned to XM, photoacoustic signals SM are generated. If the concentration CM disappears, the signals SM are at a maximum. The larger the CM, the smaller the signals SM, since SM is proportional to 1 -CM.

5

The signals S .Ref can be used in the evaluation unit 4 for wavelength calibration and / or for intensity calibration, as described above. In particular, in the structure of FIG. 5, it is possible to monitor the light output emitted by the light source and to make appropriate corrections so that a changing light output of the light source 1 does not lead to falsified values for CM.

6 shows a particularly advantageous embodiment of the subject matter of the invention schematically. The structure of the absorption spectrum

75 ters largely corresponds to the structure described in connection with FIG. 5. In FIG. 6, 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 -

20 on C .De before. 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. As a result, 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

25 if the detection cell 3 and the reference cell 6 are of the same design.

In addition to the signals SM, which originate from the molecules of the molecular species M in the detection cell 3 and depend on the concentration CM,

30 are 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. By comparing the signals S .Ref and S .Det, those between the reference cell 6 and the detection cell 3, that is to say those in the sample Volume 2 occurring light intensity losses can be determined in the evaluation unit 4. This is particularly advantageous if, for example, soiling occurs on optical elements arranged between the reference cell 6 and the detection cell 3, such as the mirrors of the multiple reflection cell 5. Or if, for example, soot particles contained in the gas to be examined lead to light scattering and a corresponding loss of light intensity. For an exact compensation of the light losses, it must be assumed that the light

75 would have no strong wavelength dependence in the range of the characteristic absorption wavelengths XM and XR. 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

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

25 maintenance work that becomes necessary, 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. There is an optional operative connection between the light source 1 and the evaluation unit 4 for the transmission of control signals from the light source 1 to the evaluation unit 4. As already described in connection with FIG. 1, 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. By means of the transmission of such control signals, 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

15 Change using the signals SR. ef and / or SR.Det are recognized by the evaluation unit 4. A new, corrected relationship between the control signal and the emitted light wavelength can then be determined by the evaluation unit 4 by means of the wavelength calibration. In this way, high accuracy and operational reliability of the absorptive

20 on spectrometers reached. 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

25 of the intensity calibration are transmitted from the evaluation unit 4 to the light source 1 k. In the case of such a control of the light source 1 ^¬ can be obtained in that wavelength and / or time-dependent no or only small variations in intensity occur. 7 schematically shows a further exemplary embodiment of the subject matter of the invention. The absorption spectrometer largely corresponds to that shown in FIG. 5. However, a photoelectric detector 3, in particular a photodiode 3, is used as the detection unit 3 here.

5 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. And the SR.Ref signals can be used as a wavelength standard.

75 If, as in the embodiment of Fig. 7, for the detection unit 3 is not necessary to use pulsed light for the measurement to work son ^¬ countries in cw mode, it can, of course, during the measurements with continuous light to work and the pulsed light only ge ^¬ be used occasionally for calibrations.

20

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. The wavenumber k is given by the equation k = 2π / λ with the world

25 length X and is typically given in cm ^-1 . k

- Light source: a quantum cascade laser with a tuning range of 1045 cm- ¹ - 1048 cm- ¹

- Detuning the quantum cascade laser by varying the quantum cascade laser between -1 5 ° C and 25 ° C

30 - Line width of the light emitted by the quantum cascade laser: 0.05 cm ^-1 . - Measured characteristic absorption peak of NH3 at the wave number 1046.4 cm- ¹

- Measured width of the measured characteristic absorption peak of NH ₃ : 0.25 cm- ¹ (HWHM) at 1 bar total pressure in the sample volume - absorption length L = 2.4 m

- CM.O = 100 ppm (vol.) NH3 in synthetic air

- Measuring range for CM: 0.1 ppm - 100 ppm

- Reference molecule species: OCS with CR.Ref = 1 l OOppm.

The alternative or additional features listed above and below are optional and can be combined as desired with one another and with the exemplary embodiments illustrated in the description.

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. In this case, 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. In order to achieve the emission of pulsed light, 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.

Instead of a quantum cascade laser, another light ^generator can also be used. For example, 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.

w 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. However, light of other wavelength ranges can be used, such as. for example visible light or ultraviolet light.

75

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

25, which can emit light of a different wavelength at which there is no characteristic absorption peak of a molecular species arranged along the light path la. By means of this third Halbleiterla ^¬ sers, a background signal may then be determined which rection to Kor ^¬ measured at the wavelengths of the first two laser signaled may be used len 30th In this way, a few individual measurement points recorded at different wavelengths, while quasi-continuous absorption spectra can be measured in a simple manner by means of a continuously detunable light generator.

The 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. In the infrared range, infrared-transmitting glass fibers or “hollow tube” infrared fibers are particularly advantageous.

10

The term 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.

15

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.

It is also possible to use a light source 1 (parallel) for simultaneously Operator Op ^¬ ben several absorption spectrometer to use, for example by means of beam splitters. In this way, at least one molecular species M in several sample volumes can be measured simultaneously. If necessary, a common evaluation unit 4 can be used.

It is also possible to use 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.

w If, as in the exemplary embodiments of FIGS. 4, 5 and 6, 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

15 sample) or be flowed through by a sample to be examined (flow measurement). However, incomplete sample volumes 2 can also be examined (open-path measurement). For example, the light path la can be guided through a gas stream to be examined that is freely spreading. The detection cell 3 can be largely independent

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

25 can lead. The separate arrangement of 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.

30 If gases are investigated using the method according to the invention, 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. When examining solids or adsorbates, the concentration CM essentially corresponds to a (surface) coverage. In this case, 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).

In the case of photoacoustic detection, 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. The lower the detection limit, the greater the detection sensitivity, the detection limit indicating the minimum 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.

w 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.

75 A possible reference molecule species R is carbonyl sulfide (OCS, empirical formula: COS). OCS has the advantage of having a large number of very narrow absorption peaks in the infrared range. 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-

If 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.

25 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. Furthermore, 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.

To determine CM from the signals SM corrected on the basis of an intensity calibration, for example theoretical calculations or calibration measurements can be made with gases with known compositions.

These features can be advantageous jointly or individually or in combination liebiger be ^¬.

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.

LIST OF REFERENCE NUMBERS

1 light source 1 a light path

2 sample volumes

3 detection unit; Photoacoustic detection cell 31 Microphone arrangement of the detection cell

4 Evaluation unit 5 Multiple reflection cell

51 gas inlet

52 gas outlet

53 pressure gauge

6 Photoacoustic reference cell 61 Microphone arrangement of the reference cell

7.7 'concave mirror

CM concentration to be determined of the molecular species M in the sample

Volume CM.O Predeterminable concentration of the molecular species N in the detection cell C Concentration of the molecular species N to be determined

CN.O predetermined concentration of the molecular species N in the detection cell C .Det concentration of the reference molecule species in the R photoakusti ^¬ rule detection cell (when the detection cell not ferenz cell with the re ^¬ identical) CR.Ref concentration of the reference Molecular species R in the reference ^cell L absorption length (in the sample volume) M molecular species to be detected

N further molecular species to be detected

R reference molecule species

SM signals of the detection unit, depending on CM, "photoacoustic signals of the detection cell; photoelectric signals of the photodiode

SN signals of the detection unit, depending on CM; photoacoustic

Signals from the detection cell; Photoelectric signals of the photodiode SR.Det photoacoustic signals of the detection cell, depending on CR.Det

SR.Ref photoacoustic signals of the reference cell, depending on CR.Ref

XM absorption wavelength, characteristic of the molecular

Species M XR absorption wavelength, characteristic of the reference

Molecular species R

Claims

PATENT CLAIMS

1 . Absorption spectrometer for determining a concentration CM of a molecular species M in a sample volume (2), comprising (a) a tunable light source (1) for the emission of pulsed light which can be propagated along a light path (la), w (b) a detection unit (3), and

(c) an evaluation unit (4) operatively connected to the detection unit (3) for evaluating signals SM of the detection unit (3), which signals SM are dependent on the concentration CM, wherein

(d) the sample volume (2) and the detection unit (3) are arranged along the 75 light path (l a), and

(e) light of the light source (1) can be absorbed by the molecular species M, characterized in that

(f) that the absorption spectrometer contains a photoacoustic reference cell (6) arranged along the light path (1 a), 0 (g) that molecules in the reference cell (6) have at least one reference

Molecular species R are contained in a predeterminable concentration CR.Ref,

(h) that light from the light source (1) can be absorbed by the molecular species R, and 5 (i) that by means of the evaluation unit (4) photoacoustic signals SR.Ref which are generated by the molecules contained in the reference cell (6) Refe rence ^¬ the molecule species R originate CKEN to Kalibrierungszwe ^'can be evaluated.

2. Absorption spectrometer according to claim 1, characterized in that the reference molecule species R has at least one known absorption peak, so that the signals S .Ref in the evaluation unit (4) for a wavelength calibration of the absorption spectrometer.

5 meters can be used.

3. absorption spectrometer according to claim 1 or 2, characterized in that the signals S. ef can be used in the evaluation unit (4) for an intensity calibration of the absorption spectrometer.

10

4. Absorption spectrometer according to one of the preceding claims, characterized in that the detection unit (3) is a photoacoustic detection cell (3).

15 5. absorption spectrometer according to claim 4, characterized in that

(a) the reference cell (6) is a photoacoustic cell different from the detection cell (3), and

(b) in particular the reference cell (6) is arranged in the light path (l a) between the 20 light source (1) and the sample volume (2).

6. absorption spectrometer according to claim 4 or 5, characterized ^¬ characterized in that

(a) the sample volume (2) is arranged separately from the detection cell (3), and k

(b) there is a predeterminable concentration CM.O of the molecular species M in the detection cell (3).

7. absorption spectrometer according to claim 6, characterized in that the sample volume (2) along the light path (l a) between the light source (1) and the detection cell (3) is arranged.

5 8. absorption spectrometer according to claim 7, characterized in

(a) that the reference cell (6) is a photoacoustic cell different from the detection cell (3),

(b) that the reference cell (6) is arranged in the light path (l a) between the light source (1) and the sample volume (2), w (c) that in the detection cell (3) molecules of the reference molecule

Species R are contained in a predeterminable concentration CR.Det, and (d) that photoacoustic signals S .Det, which originate from the molecules of the reference molecule species R contained in the detection cell (3), in the evaluation unit (4 ) to determine light intensity

15 loss of activity on the light path (l a) between the reference cell (6) and the detection cell (3) can be used.

9. absorption spectrometer according to claim 5 or 8, characterized labeled in ^¬ characterized in that the detection cell (3) and the reference cell (6) gleichar-

20 are trained,

10. absorption spectrometer according to one of the preceding Ansprü ^¬ che, characterized in that the sample volume (2) a in a multi ^¬ multiple reflection cell (5), particularly in a Herriott cell, or

25 white cell, is arranged.

1 1. absorption spectrometer according to one of the preceding Ansprü ^¬ che, characterized in that the evaluation unit (4) is operatively connected to the light source (1).

30

12. Absorption spectrometer according to one of the preceding claims, characterized in that the light source (1) comprises a continuously tunable light generator, in particular a quantum cascade laser.

5

1 3. absorption spectrometer according to one of claims 4 to 1 2, characterized in that

(a) the absorption spectrometer comprises at least a second sample volume which contains molecules of at least one further w molecular species N to be detected in a concentration CN to be determined,

(b) the second sample volume is arranged along the light path (l a),

(c) light from the light source (1) can be absorbed by the at least one further molecular species N,

75 (d) in the detection cell (3) molecules of at least one other

Molecular species N are included, and

(e) means of the evaluation unit (4) photoacoustic signals SN of De ^¬ tektionszelle be evaluated (3) which are dependent on the concentration CN. 0

14, absorption spectrometer according to claim 1 3, characterized in that the second sample volume by sample volume (2) is identical ^¬ table gekennzeich ^¬ net.

1 5. absorption spectrometer according to any one of claims 4 to 14, characterized in that

(a) the molecular species M is gaseous in the sample volume (2), 5 (b) the molecular species M is gaseous in the detection cell (3), and

(c) the concentration CM to be determined is a partial pressure of the molecular species M in the sample volume (2).

w

1 6. Method for determining the concentration CM of a molecular species M in a sample volume (2), wherein

(a) from a tunable light source (1) pulsed light is emitted along a light path (l a), wherein

(b) the sample volume (2) is arranged 15 along the light path (1 a) and through which the light from the light source (1) is passed, whereby

(c) light of the light source (1) is detected by means of a detection unit (3), whereby signals SM are generated which are dependent on the concentration C, and wherein

(d) the signals SM are evaluated by means of an evaluation unit (4) which is operatively connected to the detection unit (3), characterized in that

(e) that of a photoacoustic reference cell (6) arranged along the light path (l a) molecules of at least one reference molecule species R in a predeterminable concentration CR. ef are shown

25 (f) that light from the light source (1) is absorbed by the reference molecule species R in k of the reference cell (6), and

(g) that photoacoustic means of the evaluation unit (4) signals SR.Ref which from those contained in the reference cell (6) molecules of the min ^¬ least originate a reference molecular species R, are evaluated to calibration purposes 30th

17. The method according to claim 16, characterized in that a molecular species with at least one known absorption peak is used as the reference molecule species R, so that the signals S .Ref in the

5 evaluation unit (4) can be used for a wavelength calibration of the absorption spectrometer.

18. The method according to claim 16 or 1 7, characterized in that the signals SR.Ref in the evaluation unit (4) to an intensity

10 calibration of the absorption spectrometer can be used.

1 9. The method according to any one of claims 1 6 to 1 8, characterized in that

(a) a photoacoustic detection cell (3) 15 is used as the detection unit (3),

(b) the detection cell (3) contains molecules of the molecular species M and

(c) the detection cell (3) is traversed by the light from the light source (1), and wherein

(d) the signals SM are photoacoustic signals SM which are generated due to absorption of light from the light source (1) by the molecules of the molecular species M contained in the detection cell (3).

20. The method according to claim 19, characterized in that

(a) the light from the light source (1) passes through the reference cell (6) on the light path (l a) separately from the detection cell (3), and k

(b) that in particular the sample volume (2) is traversed by the light from the light source (1) at a time after the reference cell (6).

2 I. Method according to one of claims 19 or 20, characterized in that

(a) that the sample volume (2) on the light path (l a) is passed through separately from the detection cell (3) by the light from the light source (1),

5 (b) that a concentration CM.O of the molecular species M in the detection cell (3) is predetermined, and

(c) that the sample volume (2) is used as a light absorber with CM-dependent light absorption for light from the light source (1).

w

22. The method according to claim 21, characterized in that the detection cell (3) is passed through temporally after the sample volume (2) by the light of the light source (1).

23. The method according to claim 22, characterized in that (a) the reference cell (6) on the light path (1 a) is passed through separately from the detection cell (3) by the light from the light source (1),

(b) that the sample volume (2) is traversed by the light from the light source (1) temporally after the reference cell (6),

(c) that the detection cell (3) has molecules of the molecular species R in a predeterminable concentration CR.Det, and

(d) that photoacoustic signals SR.Det, which originate from the molecules of the reference molecule species R contained in the detection cell (3), in the evaluation unit (4) for determining light intensity losses on the light path (la) between the Reference cell (6) and 5 of the detection cell (3) are used.

24. The method according to claim 20 or 23, characterized in that photoacoustic cells of the same design are used as the detection cell (3) and as the reference cell (6).

5 25. The method according to any one of claims 16 to 24, characterized in that the sample volume (2) is traversed several times by the light from the light source (1).

26. The method according to any one of claims 16 to 25, characterized in that control signals of the light source (1) are used in the evaluation of the signals SM by the evaluation unit (4).

27. The method according to any one of claims 16 to 26, characterized in that a quasi-continuous measurement of spectra of Ab-

75 sorption peaks of the molecular species M is carried out.

28. The method according to any one of claims n to 27, characterized in that

(a) that are exhibited by the sample volume (2) molecules of at least a further 20 to be detected in a molecular species to be determined N Kon ^¬ concentration CN,

(b) that the detection cell (3) has molecules of the molecular species N,

(c) that the detection cell (3) generates N photoacoustic signals SN due to absorption of light 25 from the light source (1) by the molecules of the molecular species present in the detection cell (3) are dependent

(d) that the signals SN evaluated by the evaluation unit (4) ^¬ the.

30