Method and device for measuring concentration of gas species

Abstract

A method of determining a concentration of a first gas species in an interior space (3 ') of a container (3), where the method comprises the steps of:-transmitting a first electromagnetic radiation of a first wavelength along a first radiation path (1) across the container, where the first wavelength corresponds to a wavelength of an absorption line of the first gas species; transmitting a second electromagnetic radiation of a second wavelength along a second radiation path (2) across the container, the second wavelength corresponding to a 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 wherein the section extends at least across the interior space of the container; -receiving the transmitted first electromagnetic radiation and determining a first characteristic of the received first electromagnetic radiation; -receiving the transmitted second electromagnetic radiation and determining a second characteristic of the received second electromagnetic radiation; -determining the concentration of the first gas species in a function of the first characteristic and the second characteristic. The invention further relates to a device for determining the concentration of a gas species and to the use of the method.

Description

Technical Field

[0001] The invention described herein relates to a method of determining the concentration of a gas species in an interior space of a container. Under a further aspect, the invention relates to an apparatus for determining the concentration of a gas species and the use of the method. Background Art

[0002] In various applications, there are specific requirements for the composition of the gas present in containers with sensitive contents, such as pharmaceuticals or foods. Such contents, for example, need to be packaged at very low oxygen concentrations in order to prevent oxidation of the contents, which would result in degradation of the contents or shorten the life of the contents. There is a need to verify that the oxygen concentration is low and remains low. For process control and quality control, there is a need to determine the concentration of gas species in the container, such as, for example, the concentration of oxygen.

[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. The method allows the concentration of a gas species in a container to be determined in a non-invasive and non-destructive manner (i.e., without requiring access to the container with parts of the measuring device), and the container, which can be filled with valuable contents, can be further stored and used after the measurement. That is, infrared absorption spectroscopy provides a non-destructive analysis method for the contents of a container. Only infrared radiation passes through the container wall and through the gas in the inner space of the container to be analyzed. The radiation intensity of the infrared radiation decreases in absorption bands specific to different gas species.

[0004] Interpreting the decrease in intensity in order to obtain accurate information about the concentration of the gas species is hampered by various side effects. The size and form of the absorption band may depend not only on the concentration of the gas species to which the absorption band belongs, but also on the total pressure in the container. Background effects may also originate from materials outside the container (such as a thin film of water) and are difficult to remove. Summary of the invention

[0005] The object of the present invention is to provide a method or an apparatus which at least overcomes the problems of the prior art. In particular, the object of the present invention is to improve the accuracy when determining the concentration of a gas species under conditions of varying total pressure.

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

[0007] The method according to the invention is a method for determining the concentration of a first gas species in the interior space of a container. The method comprises the following steps: - sending first electromagnetic radiation of a first wavelength along a first radiation path across the container, wherein the first wavelength corresponds to the wavelength of an absorption line of the first gas species; - sending second electromagnetic radiation of a second wavelength along a second radiation path across the container, wherein the second wavelength corresponds to the wavelength of an absorption line of a second gas species, wherein the second radiation path is colinear with the first radiation path in a section of the first radiation path, and wherein the section extends at least across the interior space of the container; - receiving the transmitted first electromagnetic radiation and determining a first characteristic of the received first electromagnetic radiation; - receiving the transmitted second electromagnetic radiation and determining a second characteristic of the received second electromagnetic radiation; - determining a concentration of the first gas species in a function of the first characteristic and the second characteristic.

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

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

[0010] Variants of the method result from the features as defined in dependent claims 2 to 10 .

[0011] In a variant of the method according to the invention, the sending of the first electromagnetic radiation and / or the sending of the second electromagnetic radiation is performed by an electromagnetic radiation source, in particular a laser, having a tunable emission frequency. The emission frequency is periodically scanned in a spectral range covering the absorption lines of the first gas species or in a spectral range covering the absorption lines of the second gas species, respectively. The radiation source has a spectral bandwidth narrower than the spectral range.

[0012] In this way, the form of the absorption band can be sampled over time. The form can be directly interpreted, or it can be subjected to Fourier analysis. In the former case, for example, the peak height or the width at a defined fraction of the peak height (e.g., half the peak height) can 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 can be determined as a characteristic of the received radiation, for example, the ratio of a first or second order sideband to the main peak.

[0013] More specifically, a method known under the name "tunable diode laser absorption spectroscopy (abbreviated as TDLAS)" can be applied. According to this method, the emission wavelength of the diode laser is tuned in a time-dependent manner so that it sweeps in the wavelength region 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 swept. Typically, this wavelength scan occurs at a repetition rate of the order of 1kHz and in the form of a sawtooth modulation. Superimposed on this wavelength scan, a higher-frequency sinusoidal modulation with a significantly smaller amplitude can be superimposed. In this case, it is not a linear signal, that is, the direct value of the transmission being evaluated. The reason for this is that small changes on a large signal are difficult to detect. When analyzing the frequency components of the signal after passing through the container, the superposition frequency f and its multiples (2f, 3f, ...) can be found. This can be implemented in the so-called lock-in technology, for example as a specifically adapted electronic circuit. The signal intensity found at the frequency f corresponds to the first-order derivative of the absorption line, the signal intensity found at 2f corresponds to the second-order derivative of the absorption line, and so on.

[0014] Further properties of the transmission line can be analyzed, such as, for example, peak height, peak width, area under the peak. Similar analysis can be applied to first derivatives, second derivatives, and so on.

[0015] In a variation of the method involving a transmission source with a tunable transmission frequency, both transmission sources have this property. In this variation, the first electromagnetic radiation source is an electromagnetic radiation source with a first tunable transmission frequency. The second electromagnetic radiation source is an electromagnetic radiation source with a second tunable transmission frequency. The first tunable transmission frequency is periodically scanned with a first repetition rate in a spectral range covering an absorption line of the first gas species. The second tunable transmission frequency is periodically scanned with a second repetition rate in a spectral range covering an absorption line 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] The inventors have realised that, surprisingly, although the first and second transmit frequencies differ from each other, undesired beat frequencies may appear in the measurement results if the two repetition rates are too close to each other.

[0018] As an example, numerical values ​​are given to illustrate acceptable solutions. Both transmission sources can be tunable diode lasers operating with a control board having an 8MHz internal clock. A repetition rate of the order of 1kHz can be used. A complete wavelength scan of the first tunable transmission frequency can then be performed in 8000 clock cycles. In order to achieve a second repetition rate that is sufficiently different from the first repetition rate, a complete wavelength scan of the second tunable transmission frequency can be performed in 7999 or 8001 clock cycles, resulting in a relative difference of 1 / 8000=0.0125% with the first repetition rate. However, as the inventors have recognized, due to the subtle differences in the two oscillators that define the clocks of the two control boards involved, selecting the same number of 8000 clock cycles may result in a beat effect.

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

[0020] These gas species provide important information about the gas content of the container and about the condition of the product packaged in the container. All gas species of the list have absorption bands in the infrared wavelength range (in particular, in the near infrared range between 700 nanometers and 1.4 micrometer wavelengths), which are very suitable for carrying out the method according to the invention. The choice of gas species according to this variant has the following advantages. Oxygen is always present in the air. Even in an ambient environment that is deliberately made oxygen-free, humidity (i.e., residues of water resulting in water vapor in the head space of the container) is often present. The consumption of oxygen and the production of carbon dioxide and water are associated with most biological activities.

[0021] In a variant of the method according to the invention, the first characteristic is the peak height of the absorption line of the first gas species, and the second characteristic is the width of the absorption line of the second gas species. The total pressure inside the container is determined based on the width. The concentration is ultimately determined as a function of the peak height and the total pressure.

[0022] This variation of the method is useful if the form of the absorption band of the first gas species is strongly dependent on the total pressure of the gas. The line width of some gas species (such as water vapor) is a good indicator of the total pressure of the gas. Knowing the total pressure, the characteristics of the received first electromagnetic radiation can then be correctly interpreted to derive the concentration of the first gas species with high accuracy.

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

[0024] According to this variant, the total pressure in the container is derived from the width of the absorption line of water vapor.Knowing the total pressure, the peak height of the oxygen absorption line can be interpreted, so that an accurate value of the oxygen concentration is produced.

[0025] The absorption line of water vapor (for example, at a wavelength of 1380 nm) has the following properties: It changes its peak height almost independently of the water concentration. Its line width increases roughly in proportion to the total pressure.

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

[0027] This variant of the method can be used in the context of leak testing for containers, which need to be sealed in an airtight manner. The container is sealed in an environment under pressure. If the oxygen concentration inside the container increases over time, this indicates that ambient air is entering the container. A comparison of the measurement results of the oxygen concentration directly after closing the container and after, for example, two or three weeks gives a sensitive indicator of the presence of even small leaks.

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

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

[0030] In a 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 all combinations of both concentrations, the presence of biological activity can be detected or excluded. Almost any biological activity affects the ratio of oxygen concentration relative to carbon dioxide concentration. Bacteria and fungi consume oxygen and produce carbon dioxide, which results in measurable changes in the concentrations of the respective gases over time.

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

[0033] The inventors have realised that measurements of the concentration of water vapour benefit from knowledge of the concentration of carbon dioxide, which is a molecule with three atoms, as water, but has different properties with respect to adhesion on surfaces or penetration into media.

[0034] In a variant of the method according to the invention, the method comprises a calibration process and a measurement process. The calibration process performs all steps of the method according to the invention using a first container containing a known concentration of a second gas species. The measurement process performs all steps of the method according to the invention using a second container. Based on the calibration process, a background contribution of the first gas species is determined. Based on the measurement process, a provisional concentration of the first gas species in the second container is determined. The background contribution is subtracted from the provisional concentration of the first gas species in order 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 variation is discussed, in which the first gas species is water vapor, and wherein the second gas species is carbon dioxide, the method comprising a calibration process and a measurement process. The calibration process performs all steps of the method of the present invention using a first container containing a known concentration of carbon dioxide. The measurement process performs all steps of the method of the present invention using a second container. A background contribution of water is determined based on the calibration process. A provisional concentration of water vapor in the second container is determined based on the measurement process. 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 serves as a standard or calibration container, while the second container serves as the unit under test.

[0037] Measuring humidity in containers is generally difficult, apart from other factors defined by the concentration of water vapor, since, unlike other gases, it is difficult to completely replace it with a flushing gas such as nitrogen. Even if a defined gas composition is present inside the measuring device, preferably with a very low water vapor concentration, there may still be residual humidity on the windows of the device or on the surface of the container to be tested. If drastic measures, such as heating to high temperatures, must be avoided due to the fragile contents of the container. The requirement may be that at least all humidity is in the gas phase, which can only be achieved by applying drastic measures. Containers with unknown water content and unknown total pressure are difficult to measure. On the other hand, it is difficult to produce a defined humidity standard, since water is easily absorbed, it can penetrate deep into various media and the humidity is temperature dependent.

[0038] The inventors have realised that both problems can be avoided by proceeding according to the above-discussed variant of the method.For this purpose, only a reference container with an accurately defined carbon dioxide concentration needs to be provided, which is much easier to implement than a humidity standard of corresponding accuracy.

[0039] The discussed variation of the method can be further extended to a recurring method by using the values ​​determined for a first gas species to improve the results for a second gas species, then using the improved results from the second gas species to again improve the values ​​for the first gas species, and so on.

[0040] The method variations may be combined with one or more of the other variations unless incompatible.

[0041] Further within the scope of the present invention is a device according to claim 11.

[0042] The device of the present invention is a device for determining the concentration of a first gas species. The device comprises: - a first transmitter adapted to transmit first electromagnetic radiation of a first wavelength, wherein said first wavelength corresponds to the wavelength of an absorption line of said first gas species; - a second transmitter of second electromagnetic radiation of a second wavelength, wherein said second wavelength corresponds to the wavelength of an absorption line of a second gas species; - a first receiver for electromagnetic radiation; - a second receiver for electromagnetic radiation; wherein the means defines a space adapted to receive a container; wherein the apparatus is configured to transmit the first electromagnetic radiation along a first radiation path across the space and towards the first receiver, wherein the apparatus is configured to transmit the second electromagnetic radiation along a second radiation path across the space and towards the second receiver, Wherein the first radiation path and the second radiation path are collinear in a section of the first radiation path, and wherein the section extends at least across the space.

[0043] The first and second receivers may have filters in front of the radiation sensitive portion which pass only wavelengths close to the first wavelength or the second wavelength, respectively. At the receiver side of the segment where the radiation paths are collinear, the segment may be bounded by a prism or an optical grid, which imposes different directions on radiation of different wavelengths. Alternatively, a dichroic beam splitter may be configured to reflect electromagnetic radiation of a first wavelength and pass electromagnetic radiation of a second wavelength. In this alternative, the segment where the radiation paths are collinear ends at the point where the radiation paths hit the beam splitter.

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

[0045] In one embodiment of the device according to the invention, the segment is bounded by at least one dichroic beam splitter. The segment may be bounded by a dichroic beam splitter on both ends of the segment or on one end of the segment. In particular, on the transmitter side of the segment, the dichroic beam splitter may be arranged to bring together two radiation paths with originally different directions on a common co-linear segment.

[0046] In one embodiment of the device according to the invention, the first transmitter, the second transmitter, the first receiver and the second receiver are enclosed inside a housing having a window, wherein the space is located outside the housing, and wherein the first and second radiation paths pass through the window.

[0047] Embodiments can be implemented using a single window and a reflector element (such as a mirror) for electromagnetic radiation outside the housing. The first and second radiation paths can extend twice across the space for receiving the container, the first time after leaving through the window and before being reflected, and the second time after being reflected and before re-entering the housing through the window. In an alternative, there is a second window in the housing. The second window is arranged relative to the first window with respect to the space for receiving the container. In this alternative, the first and second radiation paths leave the housing through a window and enter the housing through another window. The housing including one or more windows can be airtight, and the interior of the housing can be filled with a gas component, which is adapted to the gas concentration to be measured. In particular, the concentration of the first and second gas species can be at least an order of magnitude lower than the concentration to be determined. The housing can contain a desiccant material to keep the humidity low inside the housing. If the first or second gas species is water vapor, this is particularly useful. The gas component can be free of oxygen molecules. The gas component inside the housing can be, for example, nitrogen. This embodiment can be combined with an embodiment including a dichroic beam splitter. In this combination, the dichroic beam splitter may be arranged inside the housing.

[0048] In an embodiment of the device according to the invention, the device is adapted and configured to perform the method according to the invention.

[0049] Embodiments of the apparatus may be combined with one or more of the other embodiments, or may be specifically adapted and configured to perform variations of the methods, unless incompatible.

[0050] In particular, the device may include means for determining the first and second characteristics and for determining the concentration of the first gas species. Such means may, for example, be implemented as a microprocessor. As an alternative, the device may be operatively connected to such means, for example to a desktop computer or a server. The operative connection may be established via a wired or wireless network. The device may include means for displaying the concentration of the gas species or means for storing the determined concentration (e.g. together with an identifier identifying an individual container).

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

[0052] It is used for the detection of microbial growth, especially in the context of media filling tests. It utilizes a variation of the method in which, in addition to the concentration of oxygen, the concentration of carbon dioxide is also determined.

[0053] In the field of aseptic production and packaging of pharmaceutical products, product quality cannot often be ensured by applying a final test. The safety of the product and the packaging process is ensured by a validation process in which the nutrient medium is packaged, the so-called medium filling. The inventors have recognized that the method according to the invention allows the oxygen and carbon dioxide concentrations in individual containers of a large collection of containers to be monitored over time and with high accuracy. The presence of microbial activity can be detected or excluded for each individual container. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The invention will now be further illustrated with the aid of the accompanying drawings. The accompanying drawings show: Figure 1 is a schematic and diagrammatic view of the situation surrounding a container which undergoes the steps of the method according to the invention; Figure 2 is a cross section through an embodiment of a device according to the invention. DETAILED DESCRIPTION

[0055] Figure 1A container 3 is schematically and simplifiedly shown undergoing determination of a gas species in an interior space 3'. A first electromagnetic radiation follows a first radiation path 1 shown as a dashed line. A second electromagnetic radiation follows a second radiation path shown as a dashed dotted line. The direction of radiation is indicated by arrows. The first and second electromagnetic radiations are transmitted from a first (11) and a second (12) transmitter, traverse the interior space 3' of the container 3, and are received by a first (21) or a second (22) receiver, respectively. The first radiation path 1 and the second radiation path 2 are collinear in a section 4 of said first radiation path. The section 4 extends across the interior space 3' of the container. In the case shown here, a common section of both radiation paths is bounded by dichroic beam splitters 5', 5". The beam splitters cause a first wavelength of a first electromagnetic radiation to be transmitted and a second wavelength of a second electromagnetic radiation to be reflected. In the embodiment shown here, the first radiation path follows a straight line from the first transmitter to the first receiver. Due to the dichroic beam splitter oriented at an angle of 45 degrees relative to the first radiation path, the second radiation path hits the dichroic beam splitter 5' at an angle of 90 degrees relative to the first radiation path and then extends colinearly with the first radiation path. After passing through the inner space of the container (in this case, the top space above some material packaged in the container), the two radiation paths are separated again by the beam splitter 5" before they are received by the corresponding first 21 and second 22 receivers.

[0056] As indicated by the slightly tilted container, the relative orientation of the container may vary - either by deliberately tilting the container in order to avoid disturbing effects from partial reflections on the container wall, or due to mechanical tolerances. With the method according to the invention, the path lengths of both radiation paths in the interior space of the container are the same. This facilitates the comparison and interpretation of characteristics derived from the received first and second radiations. If, for example, the path length of the first electromagnetic radiation inside the container becomes longer than the diameter of the container due to the tilt, the path length of the second electromagnetic radiation inside the container increases by the same amount.

[0057] Figure 2 A cross section through an embodiment of the device 10 is shown. It has a first 11 and a second 12 transmitter, a first 21 and a second 22 receiver and a beam splitter 5' and 5", which define a first radiation path 1 and a second radiation path 2, such as Figure 1. Also here, 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 the container. The discussed section of the first radiation path extends across the space 3", which in this embodiment is bounded, among other things, by windows 7' and 7". The section is bounded by dichroic beam splitters 5' and 5". Possible positions of the cylindrical container 3 shown in the top view are indicated by dashed circles. The housing 6 including the windows 7', 7" encloses the interior 8 of the device in an airtight manner. As represented by the dotted lines, the receivers 21, 22 are operatively connected to a component 9 for determining the concentration of at least a first gas species in the interior of the container. In this case, the component 9 is further adapted to display the concentration of the first gas species.

[0058] The windows 7', 7" are preferably highly transmissive at the first and second wavelengths of electromagnetic radiation. The dichroic beam splitters 5', 5" are transmissive at the first wavelength and mostly reflective at the second wavelength.

[0059] The zone 3" around the container can be flushed with a gas of defined composition, in particular, with a gas without the gas species to be measured. For example, if the gas species of interest are in the group comprising oxygen, water vapor and carbon dioxide, the surroundings can be flushed with nitrogen.

[0060] As a variant not shown in the figures, the walls of the device can be arranged so that they surround the zone in which the measurement takes place. In this variant, the container to be measured can enter the device through a lock chamber.

[0061] List of reference numerals 1 First radiation path 2 Second Radiation Path 3 Container 3' (Container's) interior space 3” space for receiving container 4 Segment (of the first radiation path) 5' Dichroic Beamsplitter 5” Dichroic Beamsplitter 6. Housing 7' (first) window 7” (second) window 8 Internal space (of the housing) 9 Components for determining / displaying concentration 10. Installation 11 First Transmitter 12 Second Transmitter 21 First Receiver 22 Second Receiver

Claims

1. A method for determining the concentration of a first gas species in an interior space (3') of a container (3), wherein the method comprises the following steps: - sending first electromagnetic radiation of a first wavelength along a first radiation path (1) across the container, wherein the first wavelength corresponds to the wavelength of an absorption line of the first gas species; - sending second electromagnetic radiation of a second wavelength along a second radiation path (2) across the container, wherein the second wavelength corresponds to the wavelength of an absorption line of a second gas species, wherein the second radiation path is colinear with the first radiation path in a section (4) of the first radiation path, and wherein the section extends at least across the interior space of the container; - receiving the transmitted first electromagnetic radiation and determining a first characteristic of the received first electromagnetic radiation; - receiving the transmitted second electromagnetic radiation and determining a second characteristic of the received second electromagnetic radiation; - determining said concentration of said first gas species in a function of said first characteristic and said second characteristic.

2. The method according to claim 1, wherein the sending of the first electromagnetic radiation and / or the sending of the second electromagnetic radiation is performed by an electromagnetic radiation source having a tunable emission frequency, in particular a laser, and wherein the emission frequency is periodically scanned within a spectral range covering the absorption lines of the first gas type or within a spectral range covering the absorption lines of the second gas type, respectively, wherein the radiation source has a spectral bandwidth narrower than the spectral range.

3. The method of claim 2, wherein the first electromagnetic radiation source is an electromagnetic radiation source having a first tunable emission frequency, and the second electromagnetic radiation source is an electromagnetic radiation source having a second tunable emission frequency, wherein the first tunable emission frequency is periodically scanned using a first repetition rate within a spectral range covering the absorption line of the first gas type, wherein the second tunable emission frequency is periodically scanned using a second repetition rate within a spectral range covering the absorption line of the second gas type, and wherein the second repetition rate differs from the first repetition rate by at least 0.01% of the first repetition rate.

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

5. The method according to any one of claims 1 to 4, wherein the first characteristic is a peak height of an absorption line of the first gas species, wherein the second characteristic is a width of an absorption line of the second gas species, wherein the total pressure inside the container is determined based on the width, and wherein 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 wherein the second gas species is water vapor.

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

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

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

10. The method according to claim 9, wherein the method comprises: - a calibration process, performing all the steps of the method according to claim 1 using a first container containing a known concentration of the second gas species, and - a measurement process, performing all the steps of the method according to claim 1 using a second container, wherein a background contribution of a first gas species is determined based on the calibration process, wherein a temporary concentration of the first gas species in the second container is determined based on the measurement process, and wherein the background contribution is subtracted from the temporary concentration of the first gas species in order to determine the concentration of the first gas species in the second container.

11. An apparatus (10) for determining a concentration of a first gas species, wherein the apparatus comprises: - a first transmitter (11) adapted to transmit first electromagnetic radiation of a first wavelength, wherein said first wavelength corresponds to the wavelength of an absorption line of said first gas species; - a second transmitter (12) of second electromagnetic radiation of a second wavelength, wherein said second wavelength corresponds to the wavelength of an absorption line of a second gas species; - a first receiver (21) for electromagnetic radiation; - a second receiver (22) for electromagnetic radiation; wherein the device defines a space (3") adapted to receive a container (3); wherein the apparatus is configured to transmit the first electromagnetic radiation along a first radiation path across the space and towards the first receiver, wherein the apparatus is configured to transmit the second electromagnetic radiation along a second radiation path across the space and towards the second receiver, The first radiation path and the second radiation path are colinear in a section (4) of the first radiation path, and the section (4) extends at least across the space (3").

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

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

14. The device (10) according to any one of claims 11 to 13, wherein the device is adapted and configured to perform the method according to any one of claims 1 to 10.

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

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