System for hydrogen detection at low volume mixing ratios

Abstract

The invention relates to a system (1) for detecting molecular hydrogen in a gas, wherein the system (1) comprises the following components: - A gas inlet (10) for sampling gas, - A gas dryer (2), configured to remove water from the gas - A vacuum section (3) configured to maintain gas at a gas pressure at or below 100 mPa, - A mass spectrometer (4) configured to determine a volume mixing ratio of molecular hydrogen in the gas comprised in the vacuum section (3), - A first vacuum pump (5-1) configured to generate the gas pressure in the vacuum section (3), characterized in that the gas dryer (2) is arranged between the gas inlet (10) and the vacuum section (3), wherein the gas dryer (2) is configured to reduce a water mixing ratio in the gas below 3000 ppm.

Description

[0001] ams101wo

[0002] 1

[0003] System for Hydrogen Detection at Low Volume Mixing Ratios

[0004] The invention relates to a system for detecting molecular hydrogen in a gas.

[0005] In the art, hydrogen detection in a gas stream, e.g., for leak detection at low volume mixing ratios, is performed by mass spectrometry.

[0006] In the so-called “sniffing mode” operation, an object to be leak-tested is filled with a hydrogen-containing gas, and a stream of air is sampled into the mass spectrometer from the outside of the test object.

[0007] The mass spectrometric measurement of hydrogen in a gas stream is based on the following basic principle: (1) a gas stream is sampled and brought to high vacuum, (2) energetic electrons ionize a fraction of the gas molecules (electron ionization), (3) a mass spectrometer separates hydrogen cations, H2+, with a mass-to-charge ratio (m / z- ratio) of two from ions with other m / z-ratios, (4) an ion detector collects H2+ions with the m / z- ratio of 2, and the thereby generated electric signal is converted into a quantitative measurement of the hydrogen volume mixing ratio in the gas stream.

[0008] State-of-the-art mass spectrometers for measuring hydrogen in a gas stream exclusively use electrons as the ionizing agents.

[0009] State-of-the-art mass spectrometers are capable of measuring hydrogen volume mixing ratios down to single-digit-ppm (ppm: parts per million) levels in a gas stream. However, mass spectrometers suffer from a significant positive bias (typically >1 ppm) in the measured hydrogen volume mixing ratio when the gas stream contains water at typical ambient humidity levels. This bias makes it impossible to accurately measure low volume mixing ratios of hydrogen in a humid gas and, in particular, in ambient air, where typical hydrogen volume mixing ratios are in the range between 0.5 and 1.5 ppm.

[0010] It is an object of the invention to overcome this problem and to provide a system for accurately measuring a hydrogen volume mixing ratio in the low-ppm and sub-ppm region. The object is achieved by the system having the features of claim 1 . ams101wo

[0011] 2

[0012] Advantageous embodiments are described in the dependent claims.

[0013] According to claim 1 , a system for detecting molecular hydrogen in a gas comprises the following components:

[0014] - A gas inlet for sampling gas,

[0015] - A gas dryer, configured to efficiently remove water from the gas,

[0016] - A vacuum section configured to maintain gas at a gas pressure at or below 100 mPa,

[0017] - A mass spectrometer configured to detect molecular hydrogen in the gas comprised in the vacuum section,

[0018] - A first vacuum pump configured to generate the reduced gas pressure in the vacuum section, e.g. by pumping gas from the gas inlet through the vacuum section to a gas outlet, wherein the gas dryer is arranged between the gas inlet and the vacuum section, wherein the gas dryer is configured to reduce a water mixing ratio in the gas below 3,000 ppm.

[0019] Throughout the specification, the abbreviation “ppm” (parts per million) refers to 10'6volume by volume.

[0020] The inventors have found that a positive measurement bias of hydrogen mixing ratios at low volume mixing ratios in a gas is caused by the presence of water (H2O) comprised by the gas. Under typical electron-ionization (El) conditions in mass spectrometry, water undergoes partial dissociation in both its neutral and ionic forms to generate hydrogen cations, F , which are detected by the mass spectrometer, thereby causing a positive bias in the hydrogen measurement.

[0021] The inventors have further found that in state-of-the-art mass spectrometers used for hydrogen measurements the positive bias in the hydrogen volume mixing ratios amounts to approximately 0.01% but can be as much as 0.1% of the water vapor mixing ratio in the gas. As the Earth’s atmosphere typically contains 1 % to 2% (10,000- 20,000 ppm) of water, any state-of-the-art measurement in atmospheric conditions results in a 10 ppm to 20 ppm positive bias in atmospheric hydrogen measurements. ams101wo

[0022] 3

[0023] The invention solves the problem by employing a gas dryer that is arranged upstream of the vacuum section and that is configured to reduce the amount of water in the gas, such that a remaining bias of residual water molecules in the gas that reach the mass spectrometer is below a desired threshold.

[0024] In the context of the current specification, the expression “upstream” particularly refers to a net flow direction of the gas once it enters the system via the gas inlet. The expression “upstream” may relate to a direction oriented against said net flow direction.

[0025] Similarly, the expression “downstream” may be understood as a direction oriented along the net flow direction.

[0026] It is understood that the notion of water comprised by the gas particularly refers to molecular water vapor in the gas, which the invention aims to reduce.

[0027] In particular, the volume mixing ratio of water in the gas after the gas dryer is set to be below 3,000 ppm, which translates to a bias of not more than 3 ppm in hydrogen detection, therefore allowing determination of comparably unbiased hydrogen volume mixing ratios of gases.

[0028] Peltier cooling elements, which, for example, are used in the prior art for gas drying in mass spectrometry, are incapable of reducing the water mixing ratio to below 3,000 ppm. Such a gas cooler typically cools the gas to 2°C to 5°C, which corresponds to a saturation water vapor mixing ratio of 7,000 ppm to 9,000 ppm at standard atmospheric pressure (101 ,325 Pa). Therefore, Peltier cooler-based systems in the art are incapable of achieving the goal of the invention.

[0029] Other systems in the art arrange for cryogenic cooling of the gas, e.g. by liquid nitrogen traps, in the vacuum section of such a system, which, however, renders the system prone to frequent maintenance, such as frequent defreezing.

[0030] The system according to the invention not only allows for an accurate hydrogen volume mixing ratio measurement in atmospheric conditions but also provides a solution that has a greatly reduced need for maintenance.

[0031] According to the invention, the gas dryer is arranged upstream of the vacuum section such that the water comprised by the gas is removed from the gas before the gas enters the vacuum section. ams101wo

[0032] 4

[0033] The pressure in the gas dryer may still be below atmospheric pressure, but only to a moderate degree. For example, it is conceivable that a pressure in the gas dryer is in the range of atmospheric pressure down to 100 Pa, or even lower. It is also conceivable that the pressure in the gas dryer is above atmospheric pressure.

[0034] The components of the system are fluidically connected, such that gas that entered the inlet of the system may be transported from one component to the next component that may be arranged downstream.

[0035] That is, from the inlet that gas may be guided, e.g. via a corresponding tubing, to the gas dryer, where the water is removed before it is directed further to the vacuum section, and from the vacuum section the gas is provided to the mass spectrometer.

[0036] A gas flow may be obtained by the vacuum section that induces a gas stream from the inlet to the vacuum section.

[0037] Further, regarding pressure values provided in the context of the current specification, it is noted that pressure values may be understood to refer to pressure at temperatures at the location where the pressure is determined. The temperature may be a surrounding temperature of the system or a component of the system, or an ambient temperature or a temperature around or at 293K.

[0038] The vacuum section is preferably configured to maintain a so-called high vacuum. That is a pressure in the vacuum section, when the system is operating, that may be in the ranges and conditions as specified in ISO 3529-1 :2019.

[0039] The first vacuum pump is configured to generate the vacuum in the vacuum section. It is conceivable that a second vacuum pump is comprised by the system, wherein said second pump is arranged downstream of the first vacuum pump to reduce a pressure gradient between the vacuum section and the gas outlet.

[0040] According to another embodiment of the invention, the gas dryer comprises a moisture exchanger with an exchange membrane.

[0041] Such membrane-based gas dryers have several advantages. Membrane gas dryers require comparably low maintenance may be operated at various temperatures - in particular, it may not be required to cool the gas to very low temperatures to force condensation of the water. Further, the exchange membrane may act as an additional ams101wo

[0042] 5 filter for other compounds comprised by the gas, such as ammonia and / or hydrocarbons.

[0043] According to another embodiment of the invention, the moisture exchanger comprises an inlet and an outlet, wherein the moisture exchanger comprises a first and a second channel that form two flow channels. The first and the second channel may be arranged concentrically with respect to each other and have a tube-like shape so as to form, for example, an inner and an outer channel.

[0044] According to another embodiment of the invention, the moisture exchanger comprises a plurality of first and second channels that are arranged fluidically parallel to each other. This allows for parallel and thus high-volume gas drying and for a more compact build space of the gas dryer.

[0045] The exchange membrane forms at least a part of a wall of the first channel.

[0046] The system is further configured to flow a drying or purge gas through the second channel, in particular against a net flow direction of the gas (to be dried) flowing in the first channel. Via the exchange membrane, water may be exchanged from the gas to be dried to the drying or purge gas. That is, the exchange membrane is permeable to water.

[0047] This allows for an efficient and low-maintenance gas dryer that may be operated in various conditions and temperatures at ambient pressure.

[0048] The exchange membrane is further configured to be impermeable to hydrogen in the gas (to be dried).

[0049] According to another embodiment of the invention the exchange membrane comprises an ionomer, such as a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (NAFION). Particularly, the exchange membrane may consist of an ionomer, such as sulfonated tetrafluoroethylene-based fluoropolymer-copolymer.

[0050] Ionomers are particularly well-suited for exchange membranes for gas drying.

[0051] According to another embodiment of the invention, the gas dryer comprises a desiccant granulate.

[0052] This embodiment may be understood as an alternative or additional embodiment to the moisture exchanger. ams101wo

[0053] 6

[0054] Desiccant granulate allows for cost-efficient gas drying, while regeneration of the desiccant granulate may be facilitated by simply heating the granulate.

[0055] Alternatively, the desiccant granulate may be repeatedly regenerated by adjusting a pressure surrounding the granulate. This process is referred to as pressure swing absorption.

[0056] According to another embodiment of the invention, the desiccant granulate consists of one or more selected from the group:

[0057] -anhydrous calcium sulfate,

[0058] - silica gel

[0059] - activated alumina,

[0060] -calcium chloride.

[0061] This embodiment allows for highly efficient gas drying.

[0062] According to another embodiment of the invention, the gas dryer comprises one or more cryogenic traps, such as liquid nitrogen traps.

[0063] According to another embodiment of the invention, the gas dryer comprises two or more cryogenic traps, wherein the traps are arranged fluidically parallel.

[0064] According to another embodiment of the invention, the system comprises a switch configured to adopt at least a first and a second state, wherein in the first state, the gas is guided from the inlet to a first of the two or more cryogenic traps and particularly not to a second cryogenic trap of the two or more cryogenic traps, and wherein in the second state, the gas is guided from the inlet to the second of the two or more cryogenic traps, and particularly not to the first cryogenic trap of the two or more cryogenic traps.

[0065] For example, in case the gas comprises more than 1% of water, a single cryogenic trap would quickly freeze over by the amount of water condensing in the trap, rendering the trap inoperable or inefficient. This embodiment allows alternating operation of the first or the second cryogenic trap, such that the respective other trap that is not exposed to the gas may be de-iced, e.g. by heating and purging the trap with a dry gas, such as dry air or nitrogen.

[0066] The system may comprise respective components for facilitating the de-icing. ams101wo

[0067] 7

[0068] According to another embodiment of the invention, the gas dryer is configured to reduce the volume mixing ratio of water in the gas below 1 ,000 ppm, particularly below 500 ppm, more particularly below 100 ppm, even more particularly below 50 ppm, or even below 10 ppm. Typically, the water mixing ratio of the dried gas may not necessarily be below 1 ppm or 0.1 ppm, as other biasing effects may take place at such low mixing ratios.

[0069] The lower the volume mixing ratio of water, the lower the positive bias in the measured hydrogen volume mixing ratio of the system.

[0070] Particularly, with the gas dryer according to the preceding embodiments, such a dryer comprising a moisture exchanger and / or a desiccant granulate, it is possible to achieve water volume mixing ratios below 100 ppm in the dried gas, which translates to a bias of the estimated hydrogen volume mixing ratio of approximately 0.1 ppm, a value that has not been reported in the art.

[0071] Accordingly, the bias of the hydrogen volume mixing ratio is still only 1 ppm, in case the dried gas comprises 1 ,000 ppm of water, a value that is not attained by instruments in the art.

[0072] According to another embodiment of the invention, the dried gas at the gas dryer has a temperature higher than 100K, particularly higher than 150K.

[0073] According to another embodiment of the invention, the gas dryer is configured to adjust, particularly to reduce the dew point temperature of the gas to a range below -30°C, particularly to a range below -40°C, more particularly below -50°C.

[0074] At standard atmospheric pressure (101 ,325 Pa), a dew point temperature below -30°C is equivalent to a water volume mixing ratio below 416 ppm, wherein for a temperature below -40°C the water volume mixing ratio is below 141 ppm, wherein at -50°C the water volume mixing ratio is below 43 ppm.

[0075] According to another embodiment of the invention, the system comprises a hydrocarbon filter that is configured to filter hydrocarbons from the gas, particularly gaseous hydrocarbons, particularly methane and ethane, particularly wherein the hydrocarbon filter is comprised by the gas dryer and / or formed as a filter unit separate from the gas dryer. ams101wo

[0076] 8

[0077] This embodiment allows for removing residual sources of hydrogen bias, as hydrocarbons may falsely contribute to the count of molecular hydrogen in the gas when analyzed by the mass spectrometer due to ionization products of the hydrocarbons.

[0078] The filter may be comprised in the gas dryer, e.g. for example, by selecting an appropriate exchange membrane, or the filter may be embodied as a separate component that is not comprised by the gas dryer. The hydrocarbon filter is preferably arranged upstream the vacuum section.

[0079] In case the hydrocarbon filter is a separate filter unit from the gas dryer, that hydrocarbon filter may be arranged upstream of the gas dryer.

[0080] Alternatively, the filter unit, i.e. the hydrocarbon filter, may be arranged downstream of the gas dryer.

[0081] This embodiment benefits from the filter unit, as at least some filter types may exhibit a water-sensitive performance that decreases as the water mixing ratio rises. Therefore, arranging the filter unit downstream provides, at least for some types of filters, an improved performance.

[0082] According to another embodiment of the invention, the system comprises an ammonia filter that is configured to filter ammonia from the gas, particularly wherein the ammonia filter is comprised by the gas dryer and / or formed as a filter unit separate from the gas dryer.

[0083] This embodiment allows for removing residual sources of hydrogen bias, as ammonia may falsely contribute to the count of molecular hydrogen in the gas when analyzed by the mass spectrometer due to ionization products of ammonia.

[0084] The filter may be comprised in the gas dryer, e.g. for example, by selecting an appropriate exchange membrane, or the filter may be embodied as a separate component that is not comprised by the gas dryer. The ammonia filter is preferably arranged upstream of the vacuum section.

[0085] In case the ammonia filter is a separate filter unit from the gas dryer, that ammonia filter may be arranged upstream of the gas dryer. ams101wo

[0086] 9

[0087] Alternatively, the filter unit, i.e. the ammonia filter, may be arranged downstream of the gas dryer.

[0088] This embodiment benefits the filter unit, as at least some filter types may exhibit a water-sensitive performance that decreases as the water mixing ratio rises. Therefore, arranging the filter unit downstream provides, at least for some types of filters, an improved performance.

[0089] Ammonia particularly refers to NH3.

[0090] According to another embodiment of the invention, the system comprises a temperature-stabilizing device that is configured to maintain a preset temperature for one or more components of the system.

[0091] The components may be selected from one or more of the group consisting of: a detector of the mass spectrometer (4); detector electronics of the mass spectrometer (4); the mass-spectrometer (4), the gas dryer.

[0092] In particular, with respect to the gas dryer, there are significant advantages to keeping the gas dryer at a preset temperature, particularly wherein the preset temperature is lower than a temperature of the surroundings. Adjusting the temperature of the gas dryer to the preset temperature serves to control and stabilize the drying performance, which varies with temperature. Operating the gas dryer at a temperature below the temperature of the surroundings enables additional reduction of the dried-gas dew point temperature, wherein heating increases the dew point temperature of the gas to be dried. To achieve the claimed low water mixing ratio in the dried gas, heating would work against said goal.

[0093] The temperature-stabilizing device may comprise one or more temperature sensors configured to determine a temperature, wherein the temperature-stabilizing device may further comprise a cooling component configured to adjust a temperature in response to the temperature determined by the one or more temperature sensors. ams101wo

[0094] 10

[0095] According to another embodiment of the invention, the system comprises one or more temperature sensors to measure the temperature of the surrounding and the preset temperature.

[0096] In addition, or alternatively, the temperature-stabilizing device may comprise temperature-insulating walls forming a housing for the one or more components of the system.

[0097] The temperature-stabilizing device particularly ensures that temperature-driven sensitivity or detection efficiency drifts of electronics and sensors of the components of the system are avoided.

[0098] The preset temperature may be in the range of 15°C to 25°C.

[0099] Additionally or alternatively, the preset temperature may be 1°C, 2°C, 3°C, 4°C, 5°C or more degrees Celsius lower than the temperature of the surroundings. According to another embodiment of the invention, the temperature-stabilizing device maintains the preset temperature within a range of ± 0.5°C, particularly ± 0.3°C, more particularly in a range of ± 0.1°C, around the preset temperature.

[0100] This embodiment allows for a more precise determination of the hydrogen volume mixing ratio, as a temperature-induced drift in an output signal of the system in particular the mass spectrometer, is excluded as a source of error. The output signal is indicative of and / or comprises information on the hydrogen volume mixing ratio.

[0101] According to another embodiment of the invention, the temperature-stabilizing device comprises, particularly houses or encloses, one or more components from a list consisting of: the detector of the mass spectrometer; the detector electronics of the mass spectrometer; the mass spectrometer the gas dryer.

[0102] These components may therefore be kept within the range around the preset temperature.

[0103] It is noted that the temperature-stabilizing device may house all components from said list.

[0104] According to another embodiment of the invention, the temperature-stabilizing device is configured to adjust the preset temperature of the one or more comprised ams101wo

[0105] 11 components, in particular the gas dryer, to be lower than a temperature of the surroundings of the system.

[0106] The term “surroundings” particularly refers to the temperature of the ambient air. Changes in the temperature of the ambient air are slow in comparison to the measurement time of the system and can be assumed quasi-static for the duration of the measurement.

[0107] According to another embodiment of the invention, the system comprises a temperature controller, wherein the temperature controller is configured to determine the temperature of the surroundings and to control the temperature-stabilizing device to adjust the temperature of the component to be lower than the temperature of the surroundings by a predefined amount, and / or wherein the temperature controller is configured to control the temperature-stabilizing device to adjust the preset temperature to a preset value, wherein the preset temperature is lower than the temperature of the surroundings.

[0108] The temperature controller renders the system an active and controlled cooling system that is configured to control the temperature actively for the gas dryer and / or other components of the system. The control may be facilitated relative to the temperature of the surroundings, e.g. 5°C colder than the surrounding temperature, or in terms of an absolute temperature value to be adjusted, e.g. in the range of -10°C to 15°, or in terms of the dew point temperature of the dried gas to be kept stable within a range of ± 1.0°C, particularly ± 0.5°C, more particularly in a range of ± 0.1 °C.

[0109] According to another embodiment of the invention, the temperature-stabilizing device is configured to cool, particularly solely to cool the one or more components, particularly wherein the temperature-stabilizing device is not configured to heat the component.

[0110] The temperature-stabilizing device may comprise a Peltier element to cool the component, e.g. the gas dryer.

[0111] According to another embodiment of the invention, the system comprises one or more temperature sensors to measure the temperature of the surrounding and the preset temperature, wherein the temperature controller is configured to obtain measured temperature values from the one or more temperature sensors.

[0112] This embodiment allows controlling the temperature-stabilizing device in a relative or an absolute fashion in terms of the preset temperature.

[0113] It is important to note that to reduce the water mixing ratio of the gas below the claimed value, the gas dryer has to be adjusted to a temperature that is not greater than the ams101wo

[0114] 12 temperature of the surroundings. As the gas is typically sampled from the surroundings, the gas to be dried, i.e. the gas flown through the gas dryer, initially has the temperature of the surroundings and is cooled down in the gas dryer, in case the gas dryer is temperature-controlled by the temperature-stabilizing device.

[0115] According to another embodiment of the invention, the system is configured to detect molecular hydrogen in the gas in volume mixing ratios from 0.01 ppm, particularly 0.05 ppm upwards.

[0116] According to another embodiment of the invention, the mass spectrometer is configured to ionize all or a fraction of the gas, to subsequently separate ionized molecular hydrogen by means of a mass-to-charge ratio separation, and to detect the corresponding mass-to-charge ratio of the ionized molecular hydrogen on a detector of the mass spectrometer configured to generate a output signal indicative, particularly quantitative, of the hydrogen mixing ratio of molecular hydrogen in the gas.

[0117] Ionization of the gas may be effected, e.g. by means of electron ionization, photoionization, chemical ionization, electrical field ionization, or Penning ionization.

[0118] That is, the invention described herein may be applied to other embodiments in which hydrogen ionization is effected via interactions with photons (photoionization), with gaseous ions (chemical ionization), with a strong electric field (field ionization), or with energetic neutrals (Penning ionization).

[0119] In particular, the mass spectrometer comprises a magnetic sector mass spectrometer, which allows for a compact and low-cost design.

[0120] According to another embodiment of the invention, the mass spectrometer comprises a quadrupole mass spectrometer, an ion trap mass spectrometer, a time-of-flight mass spectrometer, or another type of m / z-separating device.

[0121] The term “m / z-separating” particularly refers to a device that separates ions by a mass- to-charge ratio (m / z).

[0122] According to another embodiment of the invention, the moisture exchanger comprises a first and a second channel that form two flow channels, wherein the exchange membrane forms at least a part of a wall shared by the first channel and the second channel, wherein the system is configured to flow the gas through the first channel and to flow a purge gas through the second channel.

[0123] According to another embodiment of the invention, the system is arranged and configured to flow the purge gas against a net flow direction of the gas to be dried, i.e. to have the gas and purge gas flow in countercurrent. ams101wo

[0124] 13

[0125] The counterflow architecture of the gas dryer is particularly efficient, as this architecture allows maintaining a gradient throughout the length of the exchange membrane, whereas in an architecture in which purge gas and gas to be dried flow along in the same direction, the purge gas may assume the same water concentration, and the moisture gradient is reduced or even nulled.

[0126] According to another embodiment of the invention, the system comprises a split sample section, wherein the split sample section is arranged and configured to feed a portion of the gas coming out of the gas dryer as the purge gas into the gas dryer.

[0127] Said portion may be greater than 50%, 60%, 70%, 80%, or even greater than 90% of the volume flow of gas exiting the gas dryer.

[0128] The split sample section provides two advantages: a first advantage is that the purge gas is obtained from the gas to be dried and does not need to be provided by a separate dried gas storage with a limited amount of purge gas. This in turn renders the system easier to transport and even portable - while requiring no consumables.

[0129] Further, the system produces its own purge gas and increases its performance continuously once the system starts, as the drier the gas becomes, the drier the purge gas becomes, which allows achieving even drier gas exiting the gas dryer, and so on. That is, the system adaptively adjusts the purge gas to be dryer than the gas to be dried.

[0130] The amount of available and suitable purge gas is therefore essentially unlimited.

[0131] According to another embodiment of the invention, a purge gas dryer is arranged in the split sample section to dry the gas before it flows as the purge gas through the gas dryer, wherein the purge gas dryer is selected from the group consisting of: a desiccant, a molecular sieve, a cryogenic trap.

[0132] The purge gas dryer is therefore arranged in series with the gas dryer for the portion of the gas that is used as purge gas. The remaining portion of gas exiting the gas dryer and not entering the split section is not flowing through the purge gas dryer.

[0133] As the amount of moisture in the gas after the gas dryer is already comparably low, the purge gas dryer may be a gas dryer that is prone to recovery or to reactivation procedures once performance decreases due to saturation or similar effects. This drawback of frequent replacement or recovery / reactivation procedures, however, is ams101wo

[0134] 14 negligible, as the frequency of this maintenance is very low due to the low moisture content of the dried gas exiting the gas dryer before entering the purge gas dryer.

[0135] The advantage is that the purge gas may be dried to an extent that is vastly different from the gas to be dried flowing through the gas dryer so that the gas to be dried is purged very efficiently.

[0136] This embodiment allows for very low water mixing ratios of the dried gas that may otherwise be unobtainable.

[0137] According to another embodiment of the invention, the split sample section comprises a mass flow controller to control a flow of gas that is fed into the gas dryer as the purge gas.

[0138] The split sample section may comprise tubing connected to an outlet of the gas dryer that is also referred to as the exit of the gas dryer in the context of the current specification that allows flowing the gas exiting the gas dryer as purge gas through the gas dryer, namely through the second channel.

[0139] The split sample section may further comprise a pump to control a flow rate of the purge gas, in particular in response to control signals from the mass flow controller.

[0140] The purge gas may exit into the surrounding air.

[0141] According to another embodiment of the invention, the purge gas is entirely obtained from the surrounding air via the gas dryer and the split sample section.

[0142] This embodiment allows for a compact system that allows for an essentially unlimited volume of purge gas.

[0143] According to another embodiment of the invention, the system is configured to adjust a dew point of the gas after it exits the gas dryer to a preset dew point temperature, particularly wherein said preset dew point temperature is lower than -40°C, particularly lower than -50°C, more particularly lower than -60°C.

[0144] This embodiment provides precise control of the water mixing ratio in the dried gas exiting the gas dryer toward the mass spectrometer.

[0145] As the humidity of the ambient air that is taken in the gas dryer might vary, the water mixing ratio at the exit of the gas dryer might vary as well. For this reason, it is advantageous to adjust the dew point temperature to allow for the intended water mixing ratio to be obtained.

[0146] According to another embodiment of the invention, the system comprises a dew point sensor, wherein the dew point sensor is configured to determine a dew point temperature of the gas exiting the gas dryer. ams101wo

[0147] 15

[0148] The dew point sensor may be arranged at the exit of the gas dryer, in particular in the split sample section. The dew point sensor may advantageously be arranged in the split sample section upstream of the purge gas dryer. The dew point sensor may also be arranged upstream of the mass flow controller in the split sample section. However it is advantageous to place the dew point sensor at or in a tubing that is comprised by the split sample section and not a tubing leading to the vacuum section.

[0149] Arranging the dew point sensor upstream the purge gas dryer and / or the mass flow controller in the split sample section allows for a higher gas flow on the dew point sensor, as in front of the flow restrictor that connects the gas dryer with the vacuum section. There the gas flow rate may be comparably low, so that measurements may take longer and may be unreliable due to the low gas flow.

[0150] This embodiment allows determining the dew point temperature of the gas exiting the gas dryer for the mass spectrometer and thus obtaining a water mixing ratio connected to said dew point temperature. This allows determining any bias of the hydrogen volume mixing ratio determined by the mass spectrometer due to the water mixing ratio of the dried gas.

[0151] The dew point sensor may be selected from one or more of the group consisting of: a chilled-mirror hygrometer, a capacitive or a polymer humidity sensor, a metal-oxide, resistive or thin-film sensor, another dew-point sensing device known in the art.

[0152] According to another embodiment of the invention, the system is configured to adjust the temperature of the gas dryer in response to the dew point temperature determined by the dew point sensor.

[0153] This embodiment allows for adjusting the dew point temperature of the dried gas and therefore adjusting the water mixing ratio of the dried gas for the measurement at the mass spectrometer.

[0154] This embodiment allows further active control and adjustment of the water mixing ratio even if the humidity of the ambient air varies, which - if left unaccounted for - would alter the water mixing ratio of the dried gas and thus introduce a varying bias in the hydrogen volume mixing ratio determination.

[0155] According to another embodiment of the invention, the system is arranged to adjust a temperature of the gas dryer such that a dew point temperature of the gas at the exit of the gas dryer is within a range of a preset dew point temperature, particularly ams101wo

[0156] 16 wherein said range extends ± 1.0°C, particularly ± 0.5°C, more particularly in a range of ± 0.1 °C around the preset dew point temperature.

[0157] This embodiment specifies a range of tolerance that may be acceptable in terms of a resulting measurement bias at the mass spectrometer. It is noted that the more precise the hydrogen measurement has to be, the lower the dew point temperature has to be, and advantageously also the range within which the preset dew point temperature should remain .

[0158] In the following, a table is presented that elaborates on the dew point temperature, the water mixing ratio at said dew point temperature, and the resulting interference of the water in the determination of molecular hydrogen in the gas.

[0159] The magnitude of the interference of water with the determination of the hydrogen concentration has not been known nor observed previously. The inventor found that even minute levels of water in the gas surprisingly lead to comparably large biases in the determination of hydrogen using mass spectrometry.

[0160] This motivation in the first place led the inventor to design a system according to the invention that would allow for reducing the water mixing ratios to the levels claimed.

[0161] As can be seen from this table, in order to obtain a result that is affected by less than 0.141 ppm of water-caused hydrogen bias, a dew point temperature of at least -40°C has to be achieved for the dried gas, which corresponds to a water mixing ratio in the dried gas of less than 141 ppm.

[0162] These particular low water mixing ratios are only achievable with the system according to the invention. The combination of the cooling of the gas dryer and the addition of a ams101wo

[0163] 17 purge gas dryer in the split sample section effectuates the goal of achieving water mixing ratios below 100 ppm, if necessary.

[0164] Particularly, exemplary embodiments are described below in conjunction with the Figures. The Figures are appended to the claims and are accompanied by text explaining individual features of the shown embodiments and aspects of the present invention. Each individual feature shown in the Figures and / or mentioned in said text of the Figures may be incorporated (also in an isolated fashion) into a claim relating to the device according to the present invention.

[0165] Fig. 1 shows an exemplary embodiment of a system according to the invention;

[0166] Fig. 2 shows another exemplary embodiment of a system according to the invention;

[0167] Fig. 3 shows exemplary embodiments of gas dryers; and

[0168] Fig. 4 shows an exemplary embodiment of a system according to the invention.

[0169] In Fig. 1 , an exemplary schematic drawing of a system 1 according to the invention is depicted.

[0170] The system 1 comprises a gas inlet 10 that may be formed as an opening through which gas 100 may enter a channel system 9, e.g. a tubing, connecting the components of the system 1. Said channel system 9 is depicted as solid lines connecting the components. The system 1 is configured to determine a volume mixing ratio of molecular hydrogen in the gas entering the gas inlet 10.

[0171] The system 1 comprises a gas dryer 2 arranged downstream of the gas inlet 10, wherein the gas dryer 2 is connected via the channel system 9 to the gas inlet 10 and configured to operate at atmospheric or near-atmospheric pressure (indicated by box A).

[0172] The gas dryer 2 is a high-efficiency gas dryer 2 and is configured to reduce a volume mixing ratio of water vapor in the gas below 3,000 ppm, particularly below 1 ,000 ppm or even lower than 500 ppm. ams101wo

[0173] 18

[0174] It is noted that conventional gas coolers, e.g. Peltier-driven gas coolers, known in the art are not suitable nor configured to achieve water volume mixing ratios below 3,000 ppm.

[0175] After passing the gas dryer 2, the gas comprises less than 3,000 ppm water vapor.

[0176] Following the gas dryer 2 in a downstream direction of the channel system 9, there is arranged a vacuum section 3, in which a pressure of the dried gas is reduced to high- vacuum conditions. The high vacuum may be lower than 100 Pa.

[0177] For this, the system 1 may comprise a flow-restricting device 6, such as a choke, a throttle, or a valve, configured to maintain a pressure gradient between the gas dryer

[0178] 2 and the vacuum section 3. The pressure gradient over the flow-restricting device 6 may be generated by pumping gas from the gas inlet 10 through the vacuum section

[0179] 3 to a gas outlet 20 downstream of the vacuum section 3, e.g. by means of a first vacuum pump 5-1.

[0180] For this, the channel system 9 is connected to said first vacuum pump 5-1 downstream of the vacuum section 3. The first vacuum pump, 5-1 allows pumping gas from the gas inlet 10 through the vacuum section 3 to a gas outlet 20. The first vacuum pump 5-1 is configured to generate and maintain the high vacuum condition in the vacuum section

[0181] 3. The first vacuum pump 5-1 , may be a turbomolecular pump. Downstream of the first pump 5-1 there is the gas outlet 20, through which the dried gas may exit the system 1. To reduce a pressure gradient against which the first pump 5-1 must operate, a second vacuum pump 5-2, in form of a backing pump may be arranged between the gas outlet 20 and the first vacuum pump 5-1.

[0182] For determining the hydrogen volume mixing ratio in the dried gas, a mass spectrometer 4 is comprised by the system 1 , wherein the mass spectrometer 4 is connected to the high-vacuum section 3, where dried gas samples are obtained for the mass spectrometer 4.

[0183] The mass spectrometer 4 is configured to ionize the gas, e.g. by means of an electron ionization method and system. Other ionization methods may be suitable, as elaborated in previous paragraphs.

[0184] To exclude systematic errors due to varying temperatures at the mass spectrometer

[0185] 4, the system 1 may comprise a temperature-stabilizing device 7 that comprises the ams101wo

[0186] 19 mass spectrometer 4 or parts or components thereof, such that temperature-sensitive electronics and detectors may not suffer from a temperature-induced drift.

[0187] Further, the gas dryer 2 may be configured to filter hydrocarbons and / or ammonia from the gas that, due to ionization at the mass spectrometer 4 could otherwise contribute to a false hydrogen signal at the detector of the mass spectrometer 4.

[0188] In Fig. 2 a variant of the system of Fig. 1 is shown. While the system 1 of Fig. 2 is in large parts identical to the system of Fig. 1 , a difference is that upstream of the gas dryer 2, a separate filter unit 8 is arranged that can operate at the same pressure conditions as the gas dryer 2 and that is configured to filter hydrocarbons and / or ammonia (alternatively, the separate filter unit 8 may be arranged downstream of the gas dryer 2 (not shown)). This configuration allows for a reduced bias in hydrogen mixing ratio determination, particularly in case the gas to be tested comprises comparably large amounts of hydrocarbons or ammonia. In addition to the separate filter unit 8, the gas dryer 2 may nonetheless be configured to filter for hydrocarbons and / or ammonia as well.

[0189] In Fig. 3, various advantageous embodiments of a gas dryer 2 are depicted. In Fig. 3A, a gas dryer 2 comprising a moisture filter 21 with an exchange membrane 210 is schematically depicted. In Fig. 3B a gas dryer 2 comprising a desiccating granulate 220 is depicted.

[0190] In Fig. 3A the moisture filter 21 comprises two channels 211 , 212, wherein a first channel 211 of the two channels extends concentrically inside a second channel 212 of the two channels. Other moisture filter / channel geometries are conceivable. The first channel 211 is configured to carry the gas 100 to be dried. The gas to be dried, comprises water 400 and molecular hydrogen 401 and flows along a gas flow direction as depicted by arrow 100. A drying or purge gas flows in the second channel 212 that encloses the first channel 211. A channel wall of the first channel 211 comprises or consists of an exchange membrane 210 permeable for water vapor 400, such that the water vapor 400 may pass from the first channel 211 to the second channel 212. At the same time, the exchange membrane is impermeable and inert with respect to molecular hydrogen 401 , such that molecular hydrogen 401 is retained in the first channel 211 . The drying gas, e.g. dried air, flows in the opposite direction 300 than the gas to be dried, and thus carries any water molecule 400 away, depriving the gas to be dried from water. ams101wo

[0191] 20

[0192] Advantageously, the exchange membrane 210 may be configured to further allow hydrocarbons and / or ammonia to pass to the second channel 212. The degree of water vapor reduction depends on the flow speed of the gas to be dried as well as a length of the first channel and second channel 211 , 212, wherein it is possible to recirculate the gas to be dried several times through the first channel 211 , if necessary. The drying gas 300 may be replenished from a dry gas source that provides drying gas with a mixing ratio of water that is lower than the mixing ratio of the gas to be dried. Said replenishing may be even facilitated with a fraction of the dried gas that may be recirculated back in the second channel 212 as the purge / drying gas (not shown).

[0193] In Fig. 3B, an embodiment of the gas dryer 2 comprising a granulate dryer 22 that comprises a desiccating granulate 220 is shown. The gas 100 to be dried is flown through the granulate 220, or flown by the granulate, wherein the granulate 210 is hydrophilic and deprives the gas to be dried from the water vapor 400. The granulate does not deprive the gas form of molecular hydrogen 401 , such that the gas, once dried, comprises the same hydrogen volume mixing ratio as the gas before it was dried.

[0194] When the granulate 220 is saturated, it may be replenished, e.g. by heating the granulate 220 to evaporate the water. Additionally, or alternatively, replenishing the granulate’s desiccant capacity may be achieved by adjusting the pressure.

[0195] Clearly, a gas dryer 2 comprising both the moisture filter 21 as well as the desiccating granulate filter 22 may yield excellent results.

[0196] It is noted that also other gas dryers 2 may be suitable for reducing the volume mixing ratio of water vapor below the desired threshold of at least 3,000 ppm. For example, one or more cryogenic traps (e.g. liquid nitrogen trap) may be suitable, as long as they are arranged upstream and outside the vacuum section 3.

[0197] In Fig. 4 another exemplary embodiment of a system 1 according to the invention is shown. The system 1 depicted is similar to the system depicted in Fig. 1. In addition to the components elaborated and depicted in Fig. 1 , which bear the same numerals in Fig. 4, the system 1 comprises a membrane-based gas dryer 2, having a water-vapor- permeable exchange membrane 210 that forms a wall separating a first 211 and second channel 212 of the gas dryer 2. At an inlet 10 of the first channel 211 , ambient air enters the gas dryer 2 and flows along the first channel 210. At an outlet 20 of the first channel 210 a split sample section 215 is connected such that a portion of the gas exiting the first channel 210 enters the split sample section 215. Typically, more than ams101wo

[0198] 21

[0199] 50%, e.g. 80%-90% or more, of the gas exiting the first flow channel 210 is sucked into the split sample section 215. There the gas flows toward a purge gas dryer 214, where the gas is dried further, particularly by means of a cryogenic trap or a desiccant dryer. The purge gas dryer 214 is particularly a gas dryer that requires periodic maintenance, e.g. in form of heating or other means to reactivate the drying capacity of the purge gas dryer 214. As the amount of water in the purge gas supplied to the purge gas dryer 214 is already dramatically reduced, as it is supplied only by the gas dried by the gas dryer 2, the maintenance intervals are comparably long.

[0200] The purge gas dryer 214 then circulates the further dried gas back to the second channel 212 of the gas dryer 2 so as to remove moisture from the gas flowing in the first channel 211 via the exchange membrane 210. This way the system 1 generates its own purge gas in a very efficient way. The additional drying of the gas entering the split sample section 215 allows obtaining a very dry purge gas so that the system 1 achieves superior drying performance as compared to systems that do not possess the additional purge gas dryer. To control a flow of the purge gas, the split sample section 215 comprises a mass flow controller 213 and a pump (not shown) that pumps the purge gas through the split sample section 215 and the second channel 212.

[0201] The purge gas, after having passed through the second channel 212, exists in the ambient air.

[0202] This architecture allows achieving the claimed low water mixing ratios.

[0203] In addition to the use of the purge gas dryer 214 and in order to adjust the water mixing ratio of the gas exiting the gas dryer 2 to the intended low value of less than 3,000 ppm, particularly less than 1 ,000 ppm or even less, e.g. less than 200 ppm or less than 100 ppm (= -44°C dew point) as listed in the table associated the water mixing ratio with the resulting interference of hydrogen determined from the residual water in the gas, the gas dryer 2 may be temperature stabilized by means of a temperaturestabilizing device 7’ that controls the temperature of the gas dryer 2 to be at a temperature that is either a preset amount lower, e.g. 5°C, than the ambient temperature, i.e. a temperature of the gas that enters the gas dryer or to be at a preset temperature that is lower than the ambient temperature, e.g. 15°C, or to be at a temperature that is actively controlled to keep the dew point temperature of the dried gas to be stable, e.g. -50°C. ams101wo

[0204] 22

[0205] Both relative adjustment of the temperature and / or the absolute setting of the preset temperature are advantageous to achieve the low water mixing ratios.

[0206] Having the preset temperature adjusted relative to the ambient temperature allows for an automatic setting of the temperature, wherein setting the preset temperature to a preset value may require user input. In both cases, the temperature-stabilizing device is configured to cool the gas dryer and is not heating the gas dryer, which would be counterproductive when trying to reduce the dew point of a gas.

[0207] The moisture content, i.e. the humidity in the ambient air or atmospheric air and thus the gas to be measured, can vary; for example, the moisture content in the atmosphere changes slowly. A membrane-based gas dryer 2, e.g. the moisture exchanger, particularly the moisture exchanger comprising a Nation membrane, fails to compensate for these fluctuations in moisture content of the gas, and therefore the humidity of the dried gas varies slightly at the exit 20 of the gas dryer 2 depending on the humidity at the inlet 10 of the gas dryer 2. In case the system 1 is used for atmospheric hydrogen measurements over several hours / days, for example, this fluctuation may lead to a fluctuation in the interference signal caused by water in the dried gas. However, as the drying efficiency of the gas dryer 2 is temperaturedependent, it is possible to compensate for these fluctuations by controlling the temperature of the gas dryer and thus the dew point temperature of the dried gas at the exit 20 of the gas dryer 2. To achieve this, the system may be arranged and configured to measure the dew point temperature of the dried gas downstream of the dryer, preferably at the exit 20 of the gas dryer 2; if the dew point temperature of the dried gas rises due to increased gas humidity at the inlet, the gas dryer is set to a colder temperature, particularly by way of the temperature-stabilizing device 7’, such that the dew point temperature of the dried gas drops and the dried gas arrives at a preset dew point temperature or at least within a range of the preset dew point temperature. The range may be within ±1°C around the preset dew point temperature. If the temperature-stabilizing device 7’ cools the gas dryer 2 to a lesser degree, the temperature of the gas dryer 2 may rise and thus the dew point temperature of the dried gas at the exit of the gas dryer 2 rises as well. Therefore, this embodiment allows for an actively controlled water mixing ratio of the dried gas.

[0208] For this, a dew point sensor 11 may be arranged in the split sample section 215 upstream of the purge gas dryer 214 and particularly upstream of the mass flow controller 213. In comparison to the portion of the gas flowing toward and through the ams101wo

[0209] 23 flow restrictor 6, the gas flow flowing through the split sample section 215 is generally higher and thus more suited for a dew point measurement, as long as it is upstream the purge gas dryer 214. The dew point sensor 11 may be connected to an evaluation unit (not shown) or the temperature-stabilizing device controller (not shown), to enable the system 1 to actively control and adjust the water mixing ratio of the dried gas.

[0210] The example in Fig. 4 shows two temperature-stabilizing devices, 7, 7’ that adjust the temperature of either the gas dryer or the mass spectrometer.

[0211] With the system according to the invention, minute concentrations of molecular hydrogen in a gas may be determined accurately down to the sub-ppm range. The system 1 may be applied to measure hydrogen mixing ratios in the atmosphere or in leak detection application, where such systems operate in “sniffing mode”, to detect any hydrogen escaping the system under test.

[0212] ams101wo

[0213] 24

[0214] List of Reference Signs

[0215] 1 system

[0216] 2 gas dryer

[0217] 21 moisture filter

[0218] 210 exchange membrane

[0219] 211 first channel

[0220] 212 second channel

[0221] 213 mass flow controller

[0222] 214 purge gas dryer

[0223] 215 split sample section

[0224] 22 granulate filter

[0225] 220 desiccant granulate

[0226] 3 high vacuum section

[0227] 4 mass-spectrometer

[0228] 5-1 first pump

[0229] 5-2 second pump

[0230] 6 flow restrictor

[0231] 7 temperature-stabilizing device

[0232] 8 filter unit for hydrocarbons or ammonia

[0233] 9 channel system / tubing

[0234] 10 gas inlet

[0235] 11 dew point sensor

[0236] 20 gas outlet

[0237] 100 gas flow

[0238] 300 drying or purge gas flow

[0239] 400 water

[0240] 401 molecular hydrogen

[0241] *****

Claims

ams101wo25Claims1. A system (1) for detecting molecular hydrogen (401) in a gas, wherein the system (1) comprises the following components:- A gas inlet (10) for sampling gas,- A gas dryer (2), configured to remove water (400) from the gas- A vacuum section (3) configured to maintain gas at a gas pressure at or below 100 mPa,- A mass spectrometer (4) configured to determine a volume mixing ratio of molecular hydrogen (401) in the gas comprised in the vacuum section (3),- A first vacuum pump (5-1) configured to generate the gas pressure in the vacuum section (3), characterized in that the gas dryer (2) is arranged between the gas inlet (10) and the vacuum section (3), wherein the gas dryer (2) is configured to reduce a water mixing ratio in the gas below 3,000 ppm.

2. The system (1) according to claim 1, the gas dryer (2) comprises a moisture exchanger (21) with an exchange membrane (210).

3. The system (1) according to claim 2, wherein the exchange membrane (210) comprises a ionomer.

4. The system (1) according to one of the preceding claims, wherein the gas dryer (2) comprises a desiccant granulate (220).

5. The system (1) according to one of the preceding claims, wherein the desiccant granulate consists of one or more selected from the group: anhydrous calcium sulfate, silica gel activated alumina-calcium chloride.

6. The system (1) according to one of the preceding claims, wherein the gas dryer (2) is configured to reduce the volume mixing ratio of water in the gas below 1,000 ppm.ams101wo267. The system (1) according to one of the preceding claims, wherein the gas dryer (2) is configured to adjust a dew temperature of the gas to a range below- 30°C.

8. The system (1) according to one of the preceding claims, wherein the system (1) comprises a hydrocarbon filter (8) that is configured to filter hydrocarbons from the gas.

9. The system (1) according to one of the preceding claims, wherein the system (1) comprises an ammonia filter (8) that is configured to filter ammonia from the gas.

10. The system (1) according to one of the preceding claims, wherein the system (1) comprises a temperature-stabilizing device (7), that is configured to maintain a preset temperature for one or more components of the system (1).

11. The system (1) according to claim 10, wherein the temperature-stabilizing device (7) is configured to maintain the preset temperature within a range of ± 0.5°C around the preset temperature.

12. The system (1) according to claim 10 or 11, wherein the temperature-stabilizing device (7) comprises one or more components from a list consisting of: a detector of the mass spectrometer (4); detector electronics of the mass spectrometer (4); the mass-spectrometer (4), the gas dryer.

13. The system according to one of the claims 10 to 12, wherein the temperaturestabilizing device is configured to adjust the preset temperature of the one or more comprised components to be lower than a temperature of the surroundings of the system.

14. The system according to one of the claims 10 to 13, wherein the system comprises a temperature controller, wherein the temperature controller is configured to determine the temperature of the surroundings and to control theams101wo27 temperature-stabilizing device to adjust the temperature of the component to be lower than the temperature of the surroundings by a predefined amount, and / or wherein the temperature controller is configured to control the temperaturestabilizing device to adjust the preset temperature to a preset value, wherein the preset temperature is lower than the temperature of the surroundings, and / or wherein the temperature controller is configured to control the temperature-stabilizing device to keep the dew point temperature of the dried gas to be stable.

15. The system (1) according to one of the claims 10 to 14, wherein the temperature-stabilizing device is configured to cool, particularly solely to cool, particularly wherein the temperature-stabilizing device is not configured to heat.

16. The system (1) according to one of the claims 10 to 15, wherein the system comprises one or more temperature sensors to measure the temperature of the surrounding and the preset temperature, wherein the temperature controller is configured to obtain measured temperature values from the one or more temperature sensors.

17. The system (1) according to one of the preceding claims, wherein the system (1) is configured to detect molecular hydrogen (401) in the gas in volume mixing ratios from 0.01 ppm.

18. The system (1) according to one of the preceding claims, wherein the mass spectrometer (4) is configured to ionize the gas, to subsequently separate ionized molecular hydrogen by means of a mass-to-charge ratio separation, and to detect the corresponding mass-to-charge ratio of the ionized molecular hydrogen on a detector of the mass-spectrometer (4) configured to generate a output signal indicative for the hydrogen volume mixing ratio of molecular hydrogen in the gas.

19. The system (1) according to one of the preceding claims, when referring to claim 2, wherein the moisture exchanger comprises a first and a second channel that form two flow channels, wherein the exchange membrane forms at least a part of a wall shared by the first channel and the second channel,ams101wo28 wherein the system (1) is configured to flow the gas through the first channel and to flow a purge gas through the second channel.

20. The system according to claim 19, wherein the system is arranged and configured to flow the purge gas against a net flow direction of the gas to be dried.

21. The system according to one of the claims 19 or 20, wherein the system comprises a split sample section, wherein the split sample section is arranged and configured to feed a portion, particularly wherein said portion is greater than 50% to 90%, of the gas coming out of the gas dryer as the purge gas into the gas dryer.

22. The system according to claim 21, wherein a purge gas dryer is arranged in the split sample section to dry the gas before it flows as the purge gas through the gas dryer, wherein the purge gas dryer is selected from the group consisting of: a desiccant, a molecular sieve, a cryogenic trap.

23. The system according to claim 21 or 22, wherein the split sample section comprises a mass flow controller to control a flow of gas that is fed into the gas dryer as the purge gas.

24. The system according to one of the claims 21 to 23, wherein the purge gas is entirely obtained from the surrounding air via the split sample section.

25. The system according to one of the preceding claims, wherein the system is configured to adjust a dew point of the gas after it exits the gas dryer to a preset dew point temperature, particularly wherein said preset dew point temperature is lower than -40°C, particularly lower than -50°C, more particularly lower than -60°C.

26. The system (1) according to one of the preceding claims, wherein the system comprises a dew point sensor (11), wherein the dew point sensor (11) isams101wo29 configured to determine a dew point temperature of the gas exiting the gas dryer (2).

27. The system according to claim 26, wherein the dew point sensor (11) is arranged in split sample section (215) upstream of the purge gas dryer (214).

28. The system according to claim 26 or 27, wherein the system (1) is configured to adjust the temperature of the gas dryer (2) in response to the dew point temperature determined by the dew point sensor (11).

29. The system (1) according to one of the preceding claims, wherein the system (1) is arranged to adjust a temperature of the gas dryer (2) such that a dew point temperature of the gas at the exit (20) of the gas dryer (2) is within a range of a preset dew point temperature, particularly wherein said range extends ± 1.0°C, particularly ± 0.5°C, more particularly in a range of ± 0.1°C around the preset dew point temperature.

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