Method and apparatus for measuring the concentration of a gas species

Abstract

1. A method for determining the concentration of a first gas species in an interior space 3′ of a container 3, said method comprising: - transmitting a first electromagnetic wave of a first wavelength along a first radiation path (1) traversing the vessel, said first wavelength corresponding to the wavelength of an absorption line of said first gas species; - transmitting a second electromagnetic wave of a second wavelength along a second radiation path (2) across the container, the second wavelength corresponding to the wavelength of an absorption line of a second gas species, the second radiation path being collinear with the first radiation path in a section (4) of the first radiation path, and the section extending across at least the interior space of the container; - receiving a transmitted first electromagnetic wave and determining a first characteristic of the received first electromagnetic wave; - receiving the transmitted second electromagnetic wave and determining a second characteristic of the received second electromagnetic wave; - determining the concentration of the first gas species in function of the first characteristic and of the second characteristic; Furthermore, the present invention relates to an apparatus for determining the concentration of a gas species and to the use of said method.

Description

[Technical Field]

[0001] The invention addressed here relates to a method for determining the concentration of a gas species in the interior space of a container. Under further aspects, the invention relates to an apparatus for determining the concentration of a gas species and to the use of said method.

[0002] In various applications, there are specific requirements for the composition of gases present in containers with sensitive contents, such as pharmaceuticals or food. Such contents need to be packaged under very low oxygen concentrations to prevent oxidation of the contents, which may cause degradation or a reduced shelf life of the contents. It is necessary to ensure that the oxygen concentration is low and remains low. For process control and quality control, it is necessary to determine the concentration of gas species, such as oxygen concentration, within the container.

[0003] As an example, infrared absorption spectroscopy is a known method suitable for determining the concentration of a specific monitored gas species in a container. This method allows for the determination of the concentration of a gas species in a container in a non-invasive and non-destructive manner, i.e., no part of the measurement device needs to enter the container, and the container, which may be filled with valuable contents, can still be stored and used after the measurement. Thus, infrared absorption spectroscopy provides a non-destructive method for analyzing the contents of a container. Only infrared radiation passes through the container wall and through the gas in the container's internal space to be analyzed. The radiation intensity of the infrared radiation is reduced in absorption bands specific to different gas species. Summary of the Invention [Problem to be solved by the invention]

[0004] Interpreting the decrease in intensity to obtain accurate information about the concentration of the gas species is hindered by various side effects: the dimension and the shape of an absorption band may depend not only on the concentration of the gas species to which it belongs, but also on the total pressure inside the container; background effects may also come from substances outside the container, such as a thin water film, and are difficult to remove.

[0005] It is an object of the present invention to provide a method or apparatus that overcomes at least the problems of the prior art, and in particular to improve the accuracy of determining the concentration of a gas species under varying total pressure conditions. [Means for solving the problem]

[0006] This object is achieved by the method according to claim 1.

[0007] The method according to the present invention is a method for determining the concentration of a first gas species in the interior space of a container, said method comprising: - transmitting a first electromagnetic wave at a first wavelength along a first radiation path across the vessel, the first wavelength corresponding to the wavelength of an absorption line of the first gas species; - transmitting a second electromagnetic wave at a second wavelength along a second radiation path across the vessel, the second wavelength corresponding to the wavelength of an absorption line of a second gas species, the second radiation path being collinear with the first radiation path over a section of the first radiation path, the section extending across at least the interior space of the vessel; - receiving a transmitted first electromagnetic wave and determining a first characteristic of the received first electromagnetic wave; - receiving a transmitted second electromagnetic wave and determining a second characteristic of the received second electromagnetic wave; - determining the concentration of the first gas species in function of the first characteristic and of the second characteristic; Includes:

[0008] According to the present invention, the role of the second gas species is to provide information, which is then used to determine the concentration of the first gas species. In addition, the concentration of the second gas species may also be determined. Furthermore, it is envisaged that in a further step the roles of the first and second gas species may be reversed, so that both concentrations can be measured with high accuracy.

[0009] The container may contain a liquid or solid material and the gas concentration may be measured in the headspace above the liquid or solid material. The walls of the container, or at least a section of the walls, are of course made from a material that is at least partially transparent to electromagnetic radiation of the first and second wavelengths.

[0010] Variations of the above method result from the features as defined in the dependent claims 2-10.

[0011] In one variant of the method according to the invention, the emission of the first electromagnetic wave and / or the emission of the second electromagnetic wave is carried out by an electromagnetic wave source, in particular a laser, with a variable emission frequency, which is periodically swept over a spectral region containing the absorption lines of the first gas species or over a spectral region containing the absorption lines of the second gas species, respectively, and the radiation source has a spectral bandwidth narrower than the spectral regions.

[0012] In this way, the shape of the absorption band is sampled over time. This shape may be interpreted directly or it may be subjected to Fourier analysis. In the former case, for example, the peak height or the width at a defined portion of the peak height, for example at half the peak height, may be determined as a first or second characteristic of the received radiation. In the latter case, the peak height of the main peak and the relative peak heights of the sidebands may be determined as a characteristic of the received radiation, for example, the ratio of the first or second sidebands to the main peak.

[0013] More specifically, a method known as Tunable Diode Laser Absorption Spectroscopy (abbreviated as TDLAS) may be applied. According to this method, the emission wavelength of a diode laser is tuned in a time-dependent manner, thus scanning it over the wavelength range of the absorption line to be detected. The bandwidth of the laser line is much narrower than the absorption line, so that the wavelength dependence of the absorption line can be scanned. Typically, such wavelength sweeps occur at a repetition rate of the order of 1 kHz and in the form of a sawtooth modulation. By superimposing this wavelength sweep, a sine-shaped modulation with a higher frequency and much smaller amplitude can be superimposed. In this case, it is not the linear signal, i.e., the direct value of the emission, that is evaluated. The reason for this is that small changes in a large signal are difficult to detect. When analyzing the frequency composition of the signal after its passage through the container, the superimposed frequency f and their overlaps (2f, 3f, ...) can be found. This can be realized, for example, in the so-called lock-in technique, in particular as applied to electronic circuits: the signal intensity found at frequency f corresponds to the first derivative of the absorption line, the signal intensity found at 2f corresponds to the second derivative of the absorption line, etc.

[0014] Further properties of the emitted line may be analyzed, such as, for example, peak height, peak width, area under the peak, etc. Similar analyses may be applied to the first derivative, the second derivative, etc.

[0015] In one variation of the above method including a source with a variable emission frequency, both sources have this property. In this variation, the first electromagnetic wave source is an electromagnetic wave source with a first variable emission frequency. The second electromagnetic wave source is an electromagnetic wave source with a second variable emission frequency. The first variable emission frequency is periodically swept at a first repetition rate over a spectral region including the absorption lines of the first gas species. The second variable emission frequency is periodically swept at a second repetition rate over a spectral region including the absorption lines of the second gas species.

[0016] Thereby, the second repetition rate differs from the first repetition rate by at least 0.01% of the first repetition rate.

[0017] Surprisingly, the inventors have realised that despite the first and second emission frequencies being different from one another, unwanted beat frequencies can occur in the measurement, even if the two repetition rates are very close to one another.

[0018] By way of example, numerical values are given to illustrate an acceptable solution. Both sources may be tunable diode lasers operated by a control board with an 8 MHz internal clock. A repetition rate of the order of 1 kHz may then be used. A complete wavelength sweep of the first variable emission frequency may be performed in 8000 clock periods. To achieve a second repetition rate that is sufficiently different from the first repetition rate, a complete wavelength sweep of the second variable emission frequency may be performed in 7999 or 8001 clock periods, thus introducing a relative difference of 1 / 8000 = 0.0125% from the first repetition rate. However, as recognized by the inventors, due to the slight differences in the two oscillators defining the clocks of the two control boards involved, choosing the same number of 8000 clock periods may introduce beat effects.

[0019] In one variant of the method according to the invention, the first and second gas species are - oxygen, - water vapor, - carbon dioxide is selected from the above list.

[0020] These gas species provide important information about the gas contents of the container and about the state of the product packed in the container. All gas species in the list above have absorption bands in the infrared wavelength range, especially in the near-infrared range between wavelengths of 700 nanometers and 1.4 micrometers, which is well suited to carrying out the method according to the invention. This variant of the selection of gas species has the following advantages: oxygen is always present in the atmosphere; humidity, i.e., residual water that leads to water vapor in the headspace of a container, is often present even in environments intentionally made to be oxygen-free; the consumption of oxygen and the production of carbon dioxide and water are associated with most biological activities.

[0021] In one variation of the method according to the invention, the first characteristic is a peak height of an absorption line of the first gas species and the second characteristic is a width of an absorption line of the second gas species, the total pressure inside the vessel is determined based on the width, and the concentration is finally determined as a function of the peak height and the total pressure.

[0022] This variation of the method is effective even if the shape of the absorption band of the first gas species is strongly dependent on the total pressure of the gas. The linewidth of some gas species, such as water vapor, is an excellent indicator of the total pressure of the gas. With knowledge of the total pressure, the characteristics of the received first electromagnetic wave can then be correctly interpreted to obtain the concentration of the first gas species with high accuracy.

[0023] In one variant of the method according to the invention, the first gas species is oxygen and the second gas species is water vapor.

[0024] According to this variant, the total pressure in the vessel is obtained from the width of the water vapor absorption line. With knowledge of the total pressure, the peak height of the oxygen absorption line can be interpreted, thus yielding an accurate value for the oxygen concentration.

[0025] The absorption line of water vapor (e.g., at a wavelength of 1380 nm) has the property that its peak height varies little depending on the water concentration, and its linewidth increases approximately proportional to the total pressure.

[0026] The oxygen absorption line (e.g., at a wavelength of 760 nm) has a peak height that increases approximately proportionally to the oxygen concentration when the total pressure is below approximately 700 mbar. Above approximately 700 mbar, the peak height of the oxygen absorption line no longer changes significantly, either in peak height or peak width. However, above 700 mbar, the peak width of the HO line continues to change with increasing total pressure. From this combination of both properties, it is possible to obtain oxygen partial pressures over a larger range of total pressures, including total pressures above atmospheric pressure.

[0027] This variation of the method may be used in the context of leak testing of containers that need to be sealed in a gas-tight manner. The container is sealed in a pressurized environment. If the oxygen concentration inside the container increases over time, this indicates that ambient atmosphere has entered the container. A comparison of direct oxygen concentration measurements after closing the container and after, for example, two or three weeks, gives a sensitive indication of the presence of even small leaks.

[0028] In one variant of the method according to the invention, the first gas species is oxygen and the second gas species is carbon dioxide.

[0029] This combination of first and second gas species is particularly useful for investigating the sterility of a sample.

[0030] In one variant of the method according to the invention, in addition to the concentration of oxygen as the first gas species, the concentration of carbon dioxide is also determined.

[0031] From the combination of both concentrations, the presence of biological activity can be detected or ruled out. Almost all biological activity affects the ratio of oxygen to carbon dioxide concentrations. Bacteria and fungi consume oxygen and produce carbon dioxide, causing measurable changes in the concentrations of each gas over time.

[0032] In one variant of the method according to the invention, the first gas species is water vapor and the second gas species is carbon dioxide.

[0033] The inventors have recognized that the above measurements of water vapor concentration would benefit from knowledge of the concentration of carbon dioxide, which, like water, is a three-atom molecule but has different properties with respect to adhering to surfaces or penetrating into media.

[0034] In one variant of the method according to the invention, the method includes a calibration procedure and a measurement procedure. The calibration procedure involves performing all of the steps of the method according to the invention with a first container containing a known concentration of a second gas species. The measurement procedure involves performing all of the steps of the method according to the invention with a second container. A background contribution of a first gas species is determined based on the calibration procedure. A provisional concentration of the first gas species in the second container is determined based on the measurement procedure. The background contribution is subtracted from the provisional concentration of the first gas species to determine the concentration of the first gas species in the second container. The first and second gas species are different gas species.

[0035] In the following, a specific variant will be discussed in which the first gas species is water vapor and the second gas species is carbon dioxide, and the method includes a calibration procedure and a measurement procedure. The calibration procedure involves performing all of the steps of the inventive method with a first container containing a known concentration of carbon dioxide. The measurement procedure involves performing all of the steps of the inventive method with a second container. A background contribution of water is determined based on the calibration procedure. A provisional concentration of water vapor in the second container is determined based on the measurement procedure. The background contribution is subtracted from the provisional concentration of water vapor to determine the concentration of water vapor in the second container.

[0036] The first container has the role of a standard or calibration container, while the second container has the role of the unit under test.

[0037] Measuring humidity inside a container, among other factors defined by the water vapor concentration, is generally difficult because, unlike other gases, water is difficult to completely replace with a flushing gas such as nitrogen. Even if a defined gas composition, preferably with a very low water vapor concentration, is present inside the measurement device, residual humidity may still exist on the device window or on the surface of the container being tested. Harsh measurements, such as heating to high temperatures, must be avoided due to the delicate contents of the container. A requirement may be that at least all of the humidity be in the gas phase, which can only be achieved by applying harsh measurements. A container with unknown water content and unknown total pressure is difficult to measure. On the other hand, water is easily adsorbed, can penetrate deeply into various media, and humidity is temperature-dependent, making it difficult to create a defined humidity standard.

[0038] The inventors have realised that by continuing to follow the method variants discussed above, both problems can be avoided. To this end, only a reference vessel with a precisely defined carbon dioxide concentration needs to be provided, which is much easier to realise than a humidity standard of corresponding accuracy.

[0039] Variations of the method discussed above can be further extended into an iterative method by using the value determined for a first gas species to improve the results of a second gas species, then again using the improved results from the second gas species to improve the value of the first gas species, etc.

[0040] Any of the above method variations may be combined with one or more of the other variations described above, unless inconsistent.

[0041] A further scope of the invention relates to an apparatus as defined in claim 11.

[0042] The device according to the invention is a device for determining the concentration of a first gas species. a first transmitting device adapted to transmit a first electromagnetic wave of a first wavelength, said first wavelength corresponding to said wavelength of an absorption line of said first gas species; - a second transmitter of a second electromagnetic wave of a second wavelength, said second wavelength corresponding to said wavelength of an absorption line of a second gas species; - a first receiving device for electromagnetic waves; - a second receiving device for electromagnetic waves; Including, the apparatus defining a space adapted to receive a container; the device is configured to transmit the first electromagnetic wave along a first radiation path across the space and to the first receiving device; the device is configured to transmit the second electromagnetic wave along a second radiation path across the space and to the second receiving device; the first radiation path and the second radiation path are collinear in a section of the first radiation path; and The section extends across at least the space.

[0043] The first and second receivers may have filters in front of their radiation-sensitive portions, which filters pass only wavelengths close to the first wavelength or the second wavelength, respectively. On the receiver side of the section where the radiation paths are collinear, this section may be separated by a prism or an optical grating that gives different directions to radiation of different wavelengths. Alternatively, a dichroic beam splitter may be configured to reflect electromagnetic waves of the first wavelength and pass electromagnetic waves of the second wavelength. In this alternative, the section where the radiation paths are collinear ends at the point where the radiation paths strike the beam splitter.

[0044] Embodiments of the device according to the invention result from the features as defined in claims 11-14.

[0045] In one embodiment of the device according to the invention, the sections are delimited by at least one dichroic beam splitter. The sections may be delimited by a dichroic beam splitter on both ends of the section or on one end of the section. In particular, on the emitter side of the section, a dichroic beam splitter may be arranged to bring two radiation paths with originally different directions into a common collinear section.

[0046] In one embodiment of the device according to the present invention, the first emitting device, the second emitting device, the first receiving device, and the second receiving device are enclosed inside a housing having a window, the space is located outside the housing, and the first and second radiation paths traverse the window.

[0047] The above embodiment may be realized with a single window on the outside of the housing and a reflective element for the electromagnetic waves, such as a mirror. The first and second radiation paths may traverse the space for receiving a container twice: once after exiting through the window and before being reflected, and a second time after being reflected and before re-entering the housing through the window. In one alternative, there is a second window within the housing. The second window is positioned opposite the first window with respect to the space for receiving a container. In this alternative, the first and second radiation paths exit the housing through one window and enter the housing through the other window. The housing containing the window or windows may be gas-tight, and the interior of the housing may be filled with a gas component adapted to the gas concentration to be measured. In particular, the concentrations of the first and second gas species may be at least one order of magnitude lower than the determined concentrations. The housing may include a desiccator material to maintain low humidity within the interior of the housing. This is particularly useful when the first and second gas species are water vapor. The gas components may be free of molecular oxygen. The gas component inside the housing may be, for example, nitrogen. This embodiment may be combined with the embodiment including a dichroic beam splitter. In this combination, the dichroic beam splitter may be located inside the housing.

[0048] In one embodiment of the device according to the invention, said device is adapted and configured to carry out the method according to the invention.

[0049] Any of the above apparatus embodiments may be combined with one or more of the other embodiments, unless inconsistent, or may be specifically adapted and configured to perform any of the above method variations.

[0050] In particular, the apparatus may include means for determining the first and second properties and for determining the concentration of the first gas species. Such means may be realized, for example, as a microprocessor. Alternatively, the apparatus may be operatively coupled to such means, for example to a desktop computer or a server. The operative coupling may be established via a wired or wireless network. The apparatus may also comprise means for displaying the concentration of the gas species or means for storing the determined concentration, for example together with an identifier for identifying the individual container.

[0051] The invention is further directed to the use according to claim 15.

[0052] It is of use particularly for the detection of microbial cultures in the context of the medium fill test method, using the variant of that method in which the carbon dioxide concentration is determined in addition to the oxygen concentration.

[0053] In the field of packaging of sterile and pharmaceutical products, product quality often cannot be guaranteed by applying final tests. In so-called media fills, where nutrient media are packaged, the safety of the product and the packaging process is guaranteed by a validation process. The inventors have recognized that the method described in the present invention makes it possible to monitor the oxygen and carbon dioxide concentrations in individual containers of a large set of containers over time and with high accuracy. The presence of biological activity can be detected or excluded in each individual container.

[0054] The invention will now be further illustrated with the aid of the drawings. [Brief explanation of the drawings]

[0055] [Figure 1] 1 is a schematic and illustrative view of the situation around a container undergoing the steps of the method according to the invention; [Figure 2] 1 is a cross section through an embodiment of the device according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0056] FIG. 1 shows, in a schematic and simplified manner, a vessel 3 undergoing gas species determination in its interior space 3'. A first electromagnetic wave follows a first radiation path 1, shown as a dashed line. A second electromagnetic wave follows a second radiation path, shown as a dot-dash line. The direction of the radiation is indicated by arrows. The first and second electromagnetic waves are emitted from first (11) and second (12) emitting devices, pass across the interior space 3' of the vessel 3, and are received by first (21) or second (22) receiving devices, respectively. The first radiation path 1 and the second radiation path 2 are collinear in section 4 of the first radiation path, which section 4 extends across the interior space 3' of the vessel. In the situation disclosed here, the common section of both radiation paths is separated by a dichroic beam splitter 5', 5", which transmits the first wavelength of the first electromagnetic wave and reflects the second wavelength of the second electromagnetic wave. In the embodiment disclosed here, the first radiation path follows a straight line from the first emitting device to the first receiving device. With the dichroic beam splitter aligned at an angle of 45 degrees with respect to the first radiation path, the second radiation path strikes the dichroic beam splitter 5' at an angle of 90 degrees with respect to the first radiation path and then runs collinear with the first radiation path. After passing through the interior space of the container, in this case the headspace above some material packed in the container, the two radiation paths are separated again by a beam splitter 5", before they are received by the corresponding first (21) and second (22) receiving devices.

[0057] As indicated by the slightly tilted container, the relative orientation of the container may vary—either by intentionally tilting the container to avoid interference effects from partial reflections at the container walls, or for mechanical durability. With the method described in the present invention, the path lengths of both radiation paths in the interior space of the container are identical. This aids in the comparison and interpretation of the characteristics from reception of the first and second radiation. If, for example, tilting makes the path length of the first electromagnetic wave inside the container longer than the diameter of the container, the path length of the second electromagnetic wave inside the container increases by the same amount.

[0058] 2 shows a cross section through one embodiment of the device 10. It has first (11) and second (12) emitting devices, first (21) and second (22) receiving devices, and beam splitters 5' and 5" that define a first radiation path 1 and a second radiation path 2, as discussed above in the context of FIG. 1. Here, too, the first radiation path 1 and the second radiation path 2 are collinear in a common section. The device defines a space 3" for receiving a vessel. The above-discussed section of the first radiation path runs across the space 3", which in this embodiment is delimited along the other by the windows 7' and 7". The sections are delimited by dichroic beam splitters 5' and 5". A possible position of the cylindrical vessel 3 shown in plan view is indicated by a dashed circle. A housing 6 containing the windows 7', 7" surrounds the interior 8 of the device in a gas-tight manner. As symbolized by the dotted lines, the receiving devices 21, 22 are operatively coupled to means 9 for determining the concentration of the first gas species in the interior of at least the vessel, whereby this means 9 is further adapted to display the concentration of the first gas species.

[0059] The windows 7', 7" are preferably highly transmissive at the first and second wavelengths of the electromagnetic radiation. The dichroic beam splitters 5', 5" are transmissive at the first wavelength and largely reflective at the second wavelength.

[0060] The area 3" around the container may be flushed with a gas of defined composition, in particular a gas that does not contain the gas species to be measured. For example, the environment may be flushed with nitrogen if the gas species of interest are in the group including oxygen, water vapor, and carbon dioxide.

[0061] In a variant not shown in the present drawing, the walls of the device may be arranged so that they enclose the area in which the measurement takes place. In such a variant, the container to be measured may enter the device through a lock chamber. [Explanation of symbols]

[0062] 1. First Radial Path 2 Second Radial Path 3 containers 3' Internal space (of the above container) 3" space to receive container 4 Section (of the first radiation path) 5' Dichroic Beam Splitter 5" Dichroic Beam Splitter 6. Housing 7' (1st) window 7” (2nd) window 8. Internal space (of the above housing) 9. Means for determining / indicating the above concentrations 10 equipment 11 First transmitter 12 Second transmitter 21 First receiving device 22 Second receiving device

Claims

1. 1. A method for determining the concentration of a first gas species in an interior space (3') of a container (3), said method comprising: - transmitting a first electromagnetic wave of a first wavelength along a first radiation path (1) traversing said container, said first wavelength corresponding to the wavelength of an absorption line of said first gas species; - transmitting second electromagnetic waves of a second wavelength along a second radiation path (2) across the container, said second wavelength corresponding to the wavelength of an absorption line of a second gas species, said second radiation path being collinear with said first radiation path in a section (4) of said first radiation path, said section extending at least across said interior space of the container; receiving the transmitted first electromagnetic wave and determining a first characteristic of the received first electromagnetic wave; receiving the transmitted second electromagnetic wave and determining a second characteristic of the received second electromagnetic wave; - determining the concentration of the first gas species in function of the first characteristic and of the second characteristic; A method comprising:

2. 2. The method of claim 1, wherein the step of emitting the first electromagnetic wave and / or the step of emitting the second electromagnetic wave are performed by an electromagnetic wave source, in particular a laser, with a variable emission frequency, the emission frequency being periodically swept over a spectral region containing the absorption lines of the first gas species or over a spectral region containing the absorption lines of the second gas species, respectively, and the radiation source having a spectral bandwidth narrower than said spectral regions.

3. 3. The method of claim 2, wherein the first electromagnetic wave source is an electromagnetic wave source with a first variable emission frequency and the second electromagnetic wave source is an electromagnetic wave source with a second variable emission frequency, wherein the first variable emission frequency is periodically swept across a spectral region containing the absorption lines of the first gas species at a first repetition rate and the second variable emission frequency is periodically swept across a spectral region containing the absorption lines of the second gas species at a second repetition rate, the second repetition rate differing from the first repetition rate by at least 0.01% of the first repetition rate.

4. The first and second gas species are - oxygen, - water vapor, carbon dioxide, The method according to any one of claims 1 to 3, wherein the compound is selected from the list

5. 5. The method of claim 1, wherein the first characteristic is a peak height of an absorption line of the first gas species and the second characteristic is a width of an absorption line of the second gas species, a total pressure inside the container is determined based on the width, and the concentration is determined as a function of the peak height and the total pressure.

6. The method of claim 5 , wherein the first gas species is oxygen and the second gas species is water vapor.

7. 4. The method of claim 1, 2, or 3, wherein the first gas species is oxygen and the second gas species is carbon dioxide.

8. 8. The method of claim 7, wherein in addition to the oxygen concentration, the carbon dioxide concentration is also determined.

9. 4. The method of claim 1, 2, or 3, wherein the first gas species is water vapor and the second gas species is carbon dioxide.

10. 10. The method of claim 9, comprising: a calibration procedure, in which all the steps of the method of claim 1 are carried out with a first container containing a known concentration of a second gas species; a measurement procedure, carrying out all the steps of the method of claim 1 with a second container; Including, a background contribution of a first gas species is determined based on the calibration procedure, an interim concentration of the first gas species in the second container is determined based on the measurement procedure, and the background contribution is subtracted from the interim concentration of the first gas species to determine the concentration of the first gas species in the second container.

11. An apparatus (10) for determining the concentration of a first gas species, said apparatus comprising: a first transmitting device (11) adapted to transmit a first electromagnetic wave of a first wavelength, said first wavelength corresponding to the wavelength of an absorption line of said first gas species; a second emitter (12) of a second electromagnetic wave of a second wavelength, said second wavelength corresponding to said wavelength of an absorption line of a second gas species; a first receiving device (21) for electromagnetic waves, a second receiver (22) for electromagnetic waves, Equipped with the device defines a space (3") adapted to receive a container (3); the device is configured to transmit the first electromagnetic wave along a first radiation path across the space to the first receiving device; the device is configured to transmit the second electromagnetic wave along a second radiation path across the space to the second receiving device; the first radiation path and the second radiation path are collinear in a section (4) of the first radiation path; The device wherein said section (4) extends across at least said space (3'').

12. 12. The device (10) according to claim 11, wherein the section (4) is delimited by at least one dichroic beam splitter (5', 5").

13. 13. The device (10) according to claim 11 or 12, wherein the first emitting device, the second emitting device, the first receiving device and the second receiving device are enclosed in an interior (8) of a housing (6) having a window (7', 7"), the space (3") being located outside the housing, and the first (1) and second (2) radiation paths traverse the window.

14. Apparatus (10) according to any one of claims 11 to 13, wherein said apparatus is adapted and configured to carry out the method according to any one of claims 1 to 10.

15. 9. Use of the method according to claim 8 for the detection of microbial cultures.

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