IMR / PTR-ms instrument

Abstract

lon-Molecule-Reaction - Mass Spectrometry or Proton-Transfer- Reaction - Mass Spectrometry (IMR / PTR-MS) instrument comprising: a reagent ion source (1), an IMR / PTR drift tube (2) with a sample inlet line (5), a mass analyzer region (3), a controlling device (8), and an H2O source (9), wherein the reagent ion source (1) is in fluid connection with the IMR / PTR drift tube (2) and wherein the IMR / PTR drift tube (2) is in fluid connection with the mass analyzer region (3), characterized in that the IMR / PTR-MS instrument further comprises, (a) an adjustable flow restricting device (7), (b) an input device for entering a humidity target value (H_Target), and (c) means for determining humidity in the IMR / PTR drift tube (2), wherein the H2O source (9) is in fluid connection with the IMR / PTR drift tube (2), wherein the controlling device (8) controls the gaseous H2O flow from the H2O source (9) into the IMR / PTR drift tube (2) by adjusting the adjustable flow restricting device (7), wherein the controlling device (8) calculates a humidity value (H_Measure) correlating with the current humidity in the IMR / PTR drift tube (2) from data obtained from the means for determining humidity, wherein the controlling device compares the target value (H_Target) with the humidity value (H_Measure) and adjusts the gaseous H2O flow into the IMR / PTR drift tube (2) to adjust the humidity value (H_Measure) to the target value (H_Target).

Description

[0001] IMR / PTR-MS INSTRUMENT

[0002] The invention relates to an lon-Molecule-Reaction - Mass Spectrometry or Proton-Transfer- Reaction - Mass Spectrometry (IMR / PTR-MS) instrument comprising: a reagent ion source, an IMR / PTR drift tube with a sample inlet line, a mass analyzer region, a controlling device, and an H2O source, wherein the reagent ion source is in fluid connection with the IMR / PTR drift tube and wherein the IMR / PTR drift tube is in fluid connection with the mass analyzer region. The invention further relates to a method of operating an lon-Molecule-Reaction - Mass Spectrometry or Proton-Transfer- Reaction - Mass Spectrometry (IMR / PTR-MS) instrument, the IMR / PTR-MS instrument comprising a reagent ion source, an IMR / PTR drift tube with a sample inlet, and a mass analyzer region, wherein the reagent ion source is in fluid connection with the IMR / PTR drift tube and wherein the IMR / PTR drift tube is in fluid connection with the mass analyzer region.

[0003] BACKGROUND OF THE INVENTION lon-Molecule-Reaction I Proton-Transfer- Reaction Mass Spectrometry (IMR / PTR-MS) is a direct sample injection method for online detection and quantification of trace gases. Crucially, reagent ions are generated in a spatially separated reagent ion source. Subsequently, the reagent ions and gas containing traces of analytes are injected into an IMR / PTR drift tube, where the analytes undergo chemical ionization by interacting with the reagent ions under the influence of well controlled electric fields. Finally, neutral gas is removed and the ions are introduced into a mass spectrometer I analyzer.

[0004] It is well known that in IMR / PTR-MS the humidity of the sample air has an influence on the measured concentrations of trace analytes. That is, if the concentration of an analyte remains constant but the humidity of the air matrix changes, the IMR / PTR-MS instrument will measure a putative change in analyte concentration.

[0005] The extent of this effect is strongly dependent on the analyte and can range from negligible differences between dry and humid sample air to more than an order of magnitude. For PTR ionization via H3CF particularly compounds with Proton Affinities (PAs) only slightly higher than the PA of H2O (691 kJ / mol) experience a strong humidity effect because of backward reactions between the protonated molecules and water. The most prominent examples are: formaldehyde (CH2O; PA = 713 kJ / mol), hydrogen cyanide (HCN; PA = 713 kJ / mol), hydrogen sulfide (H2S; PA = 705 kJ / mol). However, also many compounds with PAs considerably higher than the PA of water show humidity dependences, as it was e.g. demonstrated in a publication by P. Trefz et al. [4],

[0006] In general, the humidity in the IMR / PTR drift tube affects the ion chemistry, e.g. by suppressing or increasing analyte fragmentation. Therefore, it is not surprising that during the last decades countermeasures against effects caused by varying humidity have been introduced.

[0007] Humidity dependent calibrations

[0008] The most widespread countermeasure is to calibrate the IMR / PTR-MS instrument for the compounds of interest at various humidity levels and subsequently correct the measurement data. Obviously, this is an extremely time-consuming and labor-intensive process.

[0009] For several analytes the relation between detection efficiency and sample air humidity is strictly non-linear (e.g. [5]) so that the range between 0 and 100% Relative Humidity (RH) must be calibrated with an excessive amount of data points and complicated interpolation functions between these points need to be derived.

[0010] One common device for performing humidity dependent calibrations is the "Liquid Calibration Unit" (LCU; [9],

[0010] ). In an LCU liquid water is actively pumped into a specially designed nozzle were the water gets nebulized into a heated chamber with the aid of compressed air. Advanced LCU models have two supply ports for water and two for pressurized gas, so that pure water and aqueous standards, as well as pure air I N2 and gas standards can be fed to the device simultaneously. With this setup, a wide variety of calibration gas concentrations (of liquid and gaseous origin) can be provided at selected humidity levels. Wo Caihong, et al. (CN 117 936 353 A;

[0011] ) introduced an advanced LCU setup in combination with a PTR-MS instrument for more comfortable humidity adjustment via measuring humidity dependent internal standards and a feedback loop to the mass flow controllers for gases and liquids feeding the nebulizer nozzle. The proposed workflow for analyzing formaldehyde, for example, is that one gas cylinder with clean air and one cylinder with a formaldehyde standard are connected to the device. A vessel containing pure water is connected to the suction port of one of the liquid pumps. By adjusting the mixing ratio between clean air and the formaldehyde standard and measuring the instrument's response, a calibration curve can be acquired. In the example given in

[0011] this process is performed ten times, while the flow of liquid water that is nebulized into the diluted standard gas stream is changed. Obviously, the gas flow through the nebulizing chamber has to be rather high so that complete evaporation of the water is guaranteed. As PTR-MS instruments have rather low sampling flows, the setup is equipped with an overflow port to discard the excess gas. Finally, after an extensive series of humidity dependent calibration curves for formaldehyde have been recorded, they can be subsequently used for correcting I calibrating formaldehyde data measured from real-life samples. Notably, this process has to be repeated for each compound of interest. That is, although humidity dependent calibrations can be performed with high accuracy, the whole process is still very time-consuming.

[0011] Drying the sample air

[0012] Several attempts to remove humidity from sample air have been published. One common example is membrane filters installed in the IMR / PTR-MS sample inlet line ([6]). However, also cold traps to condense and freeze water vapor have been utilized.

[0013] All of these measures have in common that not only humidity but also analytes of interest can and will get trapped, which again adulterates the measured concentrations. Furthermore, particularly cold traps need considerable amounts of time to equilibrate, which restrains realtime quantification, one of the major advantages of IMR / PTR-MS.

[0014] Adding excessive amounts of humidity

[0015] Krechmer et al. [7] introduced a method to presumably reduce the effect of varying sample air humidity by constantly injecting excessive amounts of water vapor into the IMR / PTR reaction region via the reagent ion source. That is, even when dry air is sampled, the concentration of gaseous water in the reaction region is about 20%. This comes with two major drawbacks: i) Humidity variations in the reaction region caused by changing humidity of the sample air are not eliminated but still present. Just the relative variation of the total humidity is somewhat reduced because of the very high "baseline" humidity, which has a beneficial effect only for a limited number of compounds. ii) The perpetual excessively high water concentration in the reaction region would lead to profuse clustering of reagent ions HsO+to form higher order water clusters H3O+.(H2O)n. This is not acceptable in IMR / PTR-MS and must be suppressed by applying high E / N to the reaction region via DC and RF fields. However, high E / N induces fragmentation of analytes and thus even more limits direct quantification capabilities.

[0016] Breitenlechner et al.

[0012] utilized the same method but injected the water vapor directly into the reaction region via a "humidification inlet". This was necessary because they used different reagent ion sources, which rendered the original route of injection via the source non-feasible. However, the basic principle remains the same, about 20 seem pure water vapor are constantly injected into about 100 seem gas, which results in a water concentration of about 20% plus the varying humidity introduced by the sample air into the reaction region. BRIEF DESCRIPTION OF THE INVENTION

[0017] The object of the present invention is to provide an IMR / PTR-MS instrument and a method to operate the instrument, wherein the effects of changing humidity in the sample air on the quantification results of IMR / PTR-MS instruments are eliminated without introducing any of the drawbacks of existing solutions.

[0018] The object is solved by an IMR / PTR-MS instrument comprising:

[0019] • a reagent ion source,

[0020] • an IMR / PTR drift tube with a sample inlet line,

[0021] • a mass analyzer region,

[0022] • a controlling device, and

[0023] • an H2O source, wherein the reagent ion source is in fluid connection with the IMR / PTR drift tube and wherein the IMR / PTR drift tube is in fluid connection with the mass analyzer region, characterized in that the IMR / PTR-MS instrument further comprises,

[0024] (a) an adjustable flow restricting device,

[0025] (b) an input device for entering a humidity target value H_Target, and

[0026] (c) means for determining humidity in the IMR / PTR drift tube, wherein the H2O source is in fluid connection with the IMR / PTR drift tube, wherein the controlling device controls the gaseous H2O flow from the H2O source into the IMR / PTR drift tube by adjusting the adjustable flow restricting device, wherein the controlling device calculates a humidity value H_Measure correlating with the current humidity in the IMR / PTR drift tube from data obtained from the means for determining humidity, wherein the controlling device compares the target value H_Target with the humidity value H_Measure and adjusts the gaseous H2O flow into the IMR / PTR drift tube to adjust the humidity value H_Measure to the target value H_Target.

[0027] The object is also solved by a method of operating an IMR / PTR-MS instrument, the IMR / PTR- MS instrument comprising

[0028] • a reagent ion source,

[0029] • an IMR / PTR drift tube with a sample inlet, and

[0030] • a mass analyzer region, wherein the reagent ion source is in fluid connection with the IMR / PTR drift tube and wherein the IMR / PTR drift tube is in fluid connection with the mass analyzer region, characterized in that a humidity value H_Measure correlating with the current humidity in the IMR / PTR drift tube is determined, wherein a controlled flow of gaseous H2O is introduced into the IMR / PTR drift tube and the controlled flow of gaseous H2O is adjusted so that the humidity value H_Measure adjusts to a set humidity target value H_Target.

[0031] Definition IMR / PTR-MS

[0032] Herein, the abbreviation IMR / PTR-MS is used as the more accurate term for what in literature is commonly depicted as Proton-Transfer-Reaction - Mass Spectrometry (PTR-MS). When PTR-MS was invented in the 1990s, H3O+was the only relevant reagent ion available. Chemical ionization with H3O+reagent ions proceeds mainly via proton transfer to the analyte, hence the name PTR. However, throughout the following decades methods were developed to produce alternative reagent ions (NO+, C , NH4+, Kr+, etc.) with only minor modifications to the PTR-MS setup. Some of these devices were labeled SRI-MS (Switchable Reagent Ions or Selective Reagent Ionization) instruments but many publications still use PTR-MS despite of the minor incorrectness.

[0033] "IMR / PTR-MS" in this document should be understood as the technology utilizing all major components of a PTR-MS instrument, but without the limitation to chemical ionization via proton transfer. "IMR / PTR-MS" in this document should not be understood in its broadest sense, i.e. including all types of instrumentation where ion molecule reactions can occur (e.g. methods and devices for increasing the internal energy of ions with electric fields in Electro Spray Ionization (ESI)).

[0034] IMR / PTR-MS instruments comprise the following elements:

[0035] Reagent ion source

[0036] Most IMR / PTR-MS instruments utilize hollow cathode glow discharge (HCD) reagent ion sources with source drift (SD) regions. The principal setup can be found in the first publications about PTR-MS in the 1990s [1] as well as in recent ones from 2020 [2], In short, a source gas is introduced into the HCD region where it is partly ionized. In the SD region ion-molecule reactions form the reagent ions at very high purity for subsequent injection into the drift tube. Other types of reagent ion sources are known in the art.

[0037] Various reagent ions have been successfully produced and utilized in IMR / PTR-MS instruments, which include but are not limited to: NO+, O2+, NH4+, OH; etc. Drift tube

[0038] Directly adjacent to and in fluid connection with the reagent ion source there is the IMR / PTR reaction chamber. In the very comprehensive PTR-MS literature this reaction chamber is predominantly referred to as the PTR drift tube. Unfortunately, this expression can lead to confusion with the drift tube in Ion Mobility Spectrometry (IMS), which is operated in a pulsed mode and used for separating the ionized analytes according to their mobility in a matrix. Furthermore, in IMS the drift tubes are predominantly operated at or close to atmospheric pressure. Thus, it should be noted that an IMS drift tube is a fundamentally different setup.

[0039] In the IMR / PTR drift tube chemical ionization of the analytes via interactions with the reagent ions takes place. While a certain flow of gas containing the analytes is continuously injected, an electric field draws ions along the drift tube. Commonly, air containing traces of impurities (e.g. traces of volatile organic compounds) is analyzed by IMR / PTR-MS, but many other matrices containing compounds of interest (e.g. remaining impurities in purified gases, gas standards, etc.) have been successfully investigated with various reagent ions.

[0040] Some of the common reactions between the reagent ion and the analyte taking place in the IMR / PTR drift tube are:

[0041] Proton transfer reactions, either non-dissociative or dissociative, with A.H+being the reagent ion (in most cases H2O.I ) and BC being the analyte

[0042] A.H++ BC A A + BC.H+

[0043] A.H++ BC A A + B + C.H+

[0044] Charge transfer reactions, either non-dissociative or dissociative, with A+being the reagent ion (e.g. O2+, NO+, Kr+, etc.) and BC being the analyte:

[0045] A++ BC A A + BC+

[0046] A++ BC T A + B + C+

[0047] Clustering reactions, with A+being the reagent ion (e.g. HsO+, NO+, etc.) and BC being the analyte:

[0048] A++ BC A BC.A+

[0049] In addition, other types of reactions can occur (e.g. ligand switching).

[0050] H+extraction can be observed in case of the negatively charged reagent ion OH'

[0051] OH' + MH A H2O + M-. Most common IMR / PTR drift tubes consist of a series of ring electrodes electrically connected via resistors with equal resistance (other reported embodiments are e.g. tubes with resistive coating), so that a DC voltage U can be applied along an IMR / PTR drift tube of the length d, resulting in the electric field strength E = U / d (in V I cm) (1). Another important IMR / PTR drift tube parameter is the gas number density N, which is defined by equation (2):

[0052] NA273.15 Pd

[0053] N = — - - — (2)

[0054] VMTd1013.251 J

[0055] Here, NA is the Avogadro constant (6.022 x 1023mol-1), VM (22.414 X 103cm3mol-1) is the molar volume at 1013.25 hPa and at 273.15 K, Td is the temperature in K and Pd is the pressure in hPa in the IMR / PTR drift tube.

[0056] Dividing E by N leads to the reduced electric field strength, which is related to the collision energies of ion-molecule reactions in the IMR / PTR drift tube and most commonly simply denoted as E / N with the unit Townsend (1 Td = 10'17V cm2).

[0057] Recently, novel IMR / PTR drift tubes, which provide improved sensitivity and / or selectivity, have been introduced. Most of these include one or more RF (Radio Frequency) devices, such as ion funnels or ion guides for focusing the ions and thus, avoiding losses on the walls and on the orifices to the mass analyzer.

[0058] However, the crucial prerequisite for any IMR / PTR drift tube is, that the chemical ionization conditions can be well controlled, either via direct adjustment of the parameters in equations (1) and (2) or via the method introduced in [3], This is one of the main differences between IMR / PTR-MS as being used herein and IMR-MS in general, which comprises chemical ionization technologies utilizing generic reaction chambers that offer less to no control of the ion chemistry during the ionization process.

[0059] IMR / PTR drift tubes operate between 1 and 100 hPa (commonly 1 - 10 hPa, most commonly 2 - 4 hPa). At pressures <1 hPa ionization of the analytes becomes inefficient and thus the overall sensitivity of the device is insufficient for real-time trace gas analysis. For pressures >100 hPa the ion chemistry is not well-controlled anymore (e.g. strong formation of H3O+.(H2O)nreagent ion clusters, extremely high voltages needed), which considerably complicates quantification. Mass analyzer and detector

[0060] Various types of mass analyzers / spectrometers have been employed in IMR / PTR-MS instruments. The most prominent example for a low mass resolution mass spectrometer is the quadrupole mass filter, whereas for high mass resolution measurements Time-Of-Flight (TOF) analyzers are commonly used. However, the use of other types of mass analyzers / spectrometers, such as e.g. ion trap analyzers, has also been reported and even MSncould be realized. The mass analyzer / spectrometer separates the ions injected from the IMR / PTR drift tube according to their m / z and quantifies the ion yields of the separated m / z with a suitable detector (e.g. secondary electron multiplier, microchannel plate, etc.). It has to be noted that each mass analyzer / spectrometer has a mass dependent ion transmission, which is further influenced by the transfer system between the IMR / PTR drift tube and the analyzer and other devices. Therefore, in order to get comparable measurement results and, even more importantly, comparable branching ratios, the obtained ion yields should be corrected for the mass dependent transmission. This can be done rather easily by analyzing a gas standard containing well-defined amounts of compounds distributed over a (preferably) broad mass range and approximating the correction factors with an appropriate fitting function. With this fitting function the correction factors for all relevant m / z can be calculated with high accuracy.

[0061] Means for determining humidity and humidity measurement

[0062] According to the invention the humidity in the IMR / PTR drift tube is measured. It is not essential to determine the absolute humidity but rather a humidity value (H_Measure) correlating with the current humidity in the IMR / PTR drift tube.

[0063] It is well-known in IMR / PTR-MS that the intensity of certain ions and / or ratios of certain ion intensities correlate with the humidity in the IMR / PTR drift tube. Thus, in one embodiment the humidity can be determined by measuring the intensities of certain ions with the mass analyzer I detector arrangement of the IMR / PTR-MS instrument. In this case no additional hardware is needed. In this embodiment the means for determining humidity includes the mass analyzer region. The mass analyzer region can then be configured to measure ion intensity profiles which correlate with the current humidity in the IMR / PTR drift tube. With these data the humidity value H_Measure can be calculated.

[0064] Alternatively, humidity sensors can be installed and used for the determination of the humidity in the IMR / PTR drift tube. In this embodiment the means for determining humidity comprises one or more humidity sensors and the humidity value H_Measure can be calculated from the sensor data. Preferably, a humidity sensor is installed in the gas line that is used for evacuating the IMR / PTR drift tube. The air in this pump line contains the same level of humidity as the air in the IMR / PTR drift tube, but a sensor at this position does not interfere with the ionization process, nor introduces impurities or contaminations into the IMR / PTR drift tube. In some embodiments, a humidity sensor can be installed in the sample inlet line.

[0065] Introduction of a controlled flow of gaseous H2O

[0066] Via an adjustable flow restricting device, such as a valve (preferably electronically controllable) or a mass flow controller, gaseous H2O is continuously introduced at a well-defined flow rate into the IMR / PTR drift tube. Preferably, the source of the gaseous H2O is a container with liquid H2O from which the H2O evaporates either because of the low pressure in the IMR / PTR drift tube or because of heating.

[0067] The vapor pressure of water at room temperature of 20 - 25 °C is 23 - 32 hPa. This is already more than sufficient for common IMR / PTR drift tube pressures of 1 - 10 hPa and even more so for more common IMR / PTR drift tubes, which are operating below 4 hPa (vapor pressure of ice at -5 °C is 4 hPa). In case of IMR / PTR drift tubes operating at the high end of the 1 - 100 hPa range and / or in case of very low temperatures (due to low ambient temperature) in the H2O source, some heating can be required. In embodiments where heating is required, the heating only needs to be very moderate, i.e. well below the boiling point of water at atmospheric pressure. At 46 °C the vapor pressure of water already exceeds 100 hPa. In summary, although the original aggregate state of H2O is liquid, the H2O leaves the H2O source in the gas phase, thus the H2O source is a source of gaseous H2O.

[0068] Crucially, the introduction of a controlled flow of gaseous H2O is adjusted according to the humidity measurement so that the humidity in the IMR / PTR drift tube remains essentially constant regardless of the sample air humidity. In order to achieve this, first a target humidity has to be set. This is done by setting a humidity value H_Target which corresponds to a humidity value H_Measure at the target humidity. Preferably, this target humidity is equal to or higher than the highest humidity that is expected to occur during the subsequent IMR / PTR-MS measurements.

[0069] That is, preferably, the target value H_Target is set to a value corresponding to a higher humidity than the expected maximum humidity to be introduced into the IMR / PTR drift tube by a sample via the sample inlet line. The expected maximum humidity is determined by the sample and sample collection, respectively. As long as the measured humidity in the IMR / PTR drift tube is below the target humidity the flow of gaseous H2O is increased. If the measured humidity in the IMR / PTR drift tube is higher than the target humidity the flow of gaseous H2O is decreased. As soon as the target humidity has been reached the flow of gaseous H2O is kept at the corresponding level. In other words, the measured humidity is adjusted to the target humidity I the humidity value H_Measure is adjusted to the set value H_Target by adjusting the gaseous H2O flow into the IMR / PTR drift tube.

[0070] In case the target humidity is reached by the humidity introduced via the sample air, no additional gaseous H2O is introduced into the IMR / PTR drift tube. Obviously, with the invention no humidity can be removed from the IMR / PTR drift tube, i.e. the gas cannot be "dried". Therefore, it is preferable to set the target humidity sufficiently high so that it is not exceeded by the humidity introduced with the sample air. However, even if the sample air humidity is slightly higher than the set target humidity, the invention is highly beneficial as the effects caused by this small humidity variation are miniscule to negligible compared to an IMR / PTR instrument without the invention.

[0071] In one embodiment, the adjustable flow restricting device directly connects the H2O source with the IMR / PTR drift tube, preferably via a gaseous H2O introduction line. This allows easy control of adjusting the calculated value H_Measure to the target value H_Target.

[0072] In another embodiment, the adjustable flow restricting device connects the H2O source with the IMR / PTR drift tube via the sample inlet line. In this embodiment sample gas and H2O are introduced simultaneously, providing a pre-mixture of the sample gas with H2O before entering the IMR / PTR drift tube. Obviously, this embodiment is only possible if the connection to the sample inlet line is at a point where the pressure in the sample inlet line is low. As already mentioned, IMR / PTR drift tubes operate below 100 hPa, preferably below 10 hPa, more preferably below 4 hPa. In proximity to the IMR / PTR drift tube the pressure in the sample inlet line is similar to the pressure in the IMR / PTR drift tube and thus low enough for the water in the H2O source to evaporate, when the H2O source is evacuated via the adjustable flow restricting device. The pressure in the sample inlet line downstream of a T-piece, e.g. for connecting a sample inlet line with a larger inner diameter or a bypass line, or an (inlet) flow restricting device, e.g. mass flow controller or needle valve, will be too high so that insufficient gaseous H2O could be provided by the H2O source.

[0073] Some readily-available adjustable flow- restricting devices, particularly mass flow controllers, require a minimum gas flow in order to function correctly. This could lead to problems if very low gaseous H2O flows (into the IMR / PTR drift tube) are needed. Therefore, in one embodiment there is an additional T-piece between the adjustable flow restricting device and the IMR / PTR drift tube or the sample inlet line. One port of this T-piece is connected to a vacuum pump via another gas flow restricting device. Preferably, the vacuum pump is one that is already installed in the IMR / PTR-MS device. This additional pumping enables a(n) (adjustable) minimum gas flow through the adjustable flow restricting device which adjusts the gaseous H2O flow from the H2O source. That is, even if H_Target equals H_Measure and no additional gaseous H2O has to be introduced into the IMR / PTR drift tube, a gas flow at or above the lowest flow of the dynamic range of the adjustable flow restricting device connected to the H2O source is enabled. Particularly in case a non-adjustable gas flow restricting device between the additional T-piece and the vacuum pump (e.g. gas line with small inner diameter) is utilized, the gas flow via this route will change when the adjustable flow restricting device connected to the H2O source is increased from zero. However, it is not necessary to know or measure this “bypass flow”, because the adjustable flow restricting device connected to the H2O source is controlled by adjusting H_Measure (dependent on the added gaseous H2O) and H_Target, i.e. any offset or non-linearity will be compensated for.

[0074] In one embodiment of the method, the method is further characterized in that

[0075] (a) the controlled flow of gaseous H2O introduced into the IMR / PTR drift tube is reduced or stopped as long as the humidity value H_Measure in the IMR / PTR drift tube is larger than the target value H_Target,

[0076] (b) the controlled flow of gaseous H2O introduced into the IMR / PTR drift tube is increased as long as the humidity value H_Measure in the IMR / PTR drift tube is smaller than the target value H_Target, and

[0077] (c) the controlled flow of gaseous H2O introduced into the IMR / PTR drift tube is kept constant as long as the humidity value H_Measure is essentially equal to the target value H_Target.

[0078] While it is possible to have different H2O sources for the reagent ion source and for controlling the humidity in the IMR / PTR drift tube, there can be a shared H2O source. In this case it is possible that the H2O source is connected with the reagent ion source, preferably via a source gas line.

[0079] The IMR / PTR-MS instrument can also comprise an alarm unit, wherein the controlling device is configured to issue an alarm with the alarm unit if adjusting the humidity value H_Measure to the target value H_Target requires a reduction of gaseous H2O flow from the H2O source into the IMR / PTR drift tube and the adjustable flow restricting device is in a closed state. In the method, it is possible that an alarm is issued if adjusting the humidity value H_Measure to the target value H_Target requires reducing gaseous H2O flow, while no gaseous H2O is introduced into the IMR / PTR drift tube.

[0080] DETAILED DESCRIPTION OF THE INVENTION

[0081] Further details and advantages of the invention are shown in the attached figures and in the following sections.

[0082] Fig. 1 shows schematics of one embodiment of an IMR / PTR-MS instrument according to the invention.

[0083] Fig. 2 shows schematics of one embodiment of an IMR / PTR-MS instrument according to the invention.

[0084] Fig. 3 shows schematics of one embodiment of an IMR / PTR-MS instrument according to the invention.

[0085] Fig. 4 shows IMR / PTR-MS measured ion yield for protonated formaldehyde (m / z 31.018) at different sample air humidities, without the invention applied.

[0086] Fig. 5 shows IMR / PTR-MS measured ion yield for protonated formaldehyde (m / z 31.018) at different sample air humidities, with the invention applied.

[0087] Fig. 6 shows schematics of one embodiment of an IMR / PTR-MS instrument according to the invention.

[0088] The following reference signs will be used throughout the description of the figures:

[0089] 1 Reagent ion source

[0090] 2 IMR / PTR drift tube

[0091] 3 Mass analyzer region

[0092] 4 Source gas line

[0093] 5 Sample inlet line

[0094] 6 Gaseous H2O introduction line

[0095] 7 Adjustable flow restricting device

[0096] 8 Controlling device

[0097] 9 H2O source

[0098] 10 Humidity sensor(s)

[0099] 11 T-piece

[0100] 12 flow restricting device

[0101] 13 vacuum pump Figs. 1 to 3 show schematics of exemplary embodiments of IMR / PTR-MS instruments according to the invention. In a reagent ion source 1 reagent ions are produced and subsequently injected into an IMR / PTR drift tube 2 for chemical ionization of analytes. In mass analyzer region 3 the ions transferred from IMR / PTR drift tube 2 are separated according to their m / z and detected with a detector. One or more vacuum pumps evacuate the IMR / PTR- MS instrument. In many embodiments a transfer region between IMR / PTR drift tube 2 and mass analyzer region 3 focuses the ions and acts as a differential pumping stage. Various electronic devices, such as power supplies, control units, computers, etc. are necessary for operating the IMR / PTR-MS instrument and well known in the art.

[0102] The reagent ion source 1 can be any IMR / PTR reagent ion source. In many embodiments the reagent ion source 1 is a hollow cathode discharge ion source with an integrated source drift region. Source gas line 4 can be split to various source gas supplies. Preferably, the source gas flow(s) introduced via source gas line 4 is / are controlled with mass flow controllers. In case of an IMR / PTR-MS instrument being operated with HsO+reagent ions at least a vessel containing essentially pure liquid H2O is connected to source gas line 4 via a flow regulating device (e.g. valve, mass flow controller, etc.). Due to the low pressure in the reagent ion source 1 , which typically ranges between 0.1 and 100 hPa, preferably between 0.5 and 10 hPa, liquid H2O will vaporize upon evacuation of the vessel. Additional heating of the vessel can be necessary if this vaporization process produces insufficient amounts of gaseous H2O, e.g. because of low ambient temperatures. Preferably, there is at least one pumping port connected to a vacuum pump at the reagent ion source 1. In some embodiments reagent ion source 1 is evacuated solely through the fluid connection to IMR / PTR drift tube 2.

[0103] IMR / PTR drift tube 2 can be of any kind known in the art. This includes, but is not limited to a series of ring electrodes with constant and / or varying orifice diameters, tubular structures with conductive I resistive coating, etc. The reduced electric field strength E / N necessary for controlling the ion chemistry is created by applied DC voltages. Additionally, RF voltages can be applied. The sample is introduced into IMR / PTR drift tube 2 via sample inlet line 5 and pumped out either through the transfer region between IMR / PTR drift tube 2 and mass analyzer region 3 or a pump port connected to a vacuum pump preferably close to the downstream end of IMR / PTR drift tube 2 or a combination of both. Typical sample gas flows through the IMR / PTR drift tube 2 are between 1 and 1000 seem (cubic centimeter per min at standard conditions), preferably between 10 and 100 seem. Pressures inside IMR / PTR drift tube 2 are typically between 1 and 100 hPa, preferably between 1 and 10 hPa. Through gaseous H2O introduction line 6 gaseous H2O is introduced from an H2O source 9 into IMR / PTR drift tube 2 (Fig. 1). Alternatively, gaseous H2O introduction line 6 is connected to sample inlet line 5, so that a mixture of sample gas and added gaseous H2O is introduced into IMR / PTR drift tube 2 (Fig. 2). The flow of gaseous H2O through gaseous H2O introduction line 6 is controllable via an adjustable flow restricting device 7. Preferably, this device is an electronically adjustable valve or a mass flow controller. Preferably, the H2O source 9 is the same that is used for supplying H2O source gas to reagent ion source 1 via source gas line 4. This has the additional advantage that trace impurities in the gaseous H2O used for reagent ion production can easily be analyzed and quantified. In some embodiments an additional H2O source 9 is installed, e.g. again in form of a vessel containing liquid H2O that can optionally be heated.

[0104] In one embodiment as depicted in Fig. 6 there is a T-piece 11 between the adjustable flow restricting device 7 and the IMR / PTR drift tube 2 (optionally via the sample inlet line 5). One port of this T-piece 11 is connected to a vacuum pump 13 via a flow restricting device 12. Preferably, the vacuum pump 13 is one that is already installed in the IMR / PTR-MS device. This additional pumping enables a(n) (adjustable) minimum gas flow through the adjustable flow restricting device 7.

[0105] In some embodiments one or more humidity sensor(s) 10 is / are installed. Preferably, at least one humidity sensor is installed in the IMR / PTR drift tube 2, in the transfer region between IMR / PTR drift tube 2 and mass analyzer region 3 or in a pump line connected to these elements. This has the advantage that the measured humidity already includes the gaseous H2O added via gaseous H2O introduction line 6. In case gaseous H2O introduction line 6 is connected to sample inlet line 5 a humidity sensor 10 can be installed in sample inlet line 5 between the connection point (to 6) and IMR / PTR drift tube 2.

[0106] If there is only a humidity sensor installed in sample inlet line 5 prior to the addition of gaseous H2O the resulting humidity including the added gaseous H2O via H2O introduction line 6 has to be calculated. The utilized humidity sensors can either measure the absolute or relative humidity or give any value correlating to the humidity in the IMR / PTR drift tube 2.

[0107] Mass analyzer region 3 can comprise any mass analyzer I detector combination known in the art (e.g., as mentioned above in the corresponding section). Preferably, a TOF mass analyzer in combination with a microchannel plate detector is used. In a preferred embodiment, specific ions correlating with the humidity in the IMR / PTR drift tube 2 are detected in mass analyzer region 3. One example well-known in the art [8] is the ratio of the measured H2O.H3O+signal divided by the measured H3O+signal. At a fixed reduced electric field strength E / N in the IMR / PTR drift tube 2 this ratio increases with increasing humidity and decreases with decreasing humidity. Other examples are the intensities of higher order water cluster ions H3O+.(H2O)nand / or their relative proportions to each other. In case of reagent ions other than HsO+also the HsO+signal intensity itself can be used. Further correlations between ion signals and IMR / PTR drift tube 2 humidity exist and can be, for example, easily experimentally determined, regardless of the utilized reagent ions (positively or negatively charged).

[0108] As a next step a target humidity value H_Target is chosen. Preferably, H_Target is given in the same unit as the humidity in the IMR / PTR drift tube 2 is measured (H_Measure). For example, if H_Measure is measured with an absolute humidity sensor, H_Target is in g / m3, if H_Measure is measured via the intensity of a certain detected ion, H_Target is in cps (counts-per-second), if H_Measure is measured via the ratio of certain ion intensities, H_Target is in percent, etc.

[0109] Preferably, H_Target corresponds to the maximum humidity that is to be expected during the following measurement. For example, if the expected maximum humidity of the sample air is 45 g / m3AH (Absolute Humidity), H_Target should be at least the corresponding H_Measure in the IMR / PTR drift tube 2 when 45 g / m3AH sample air is introduced via sample inlet line 5, plus any additional humidity introduced, e.g. from reagent ion source 1 (source gas).

[0110] A controlling device 8 can take control over the gaseous H2O flow from the H2O source 9 into the IMR / PTR drift tube 2 by adjusting the adjustable flow restricting device. The controlling device 8 can also calculate the humidity value H_Measure, compare it with the target value H_Target and adjust the gaseous H2O flow into the IMR / PTR drift tube 2 to adjust the humidity value H_Measure to the target value H_Target.

[0111] In the special case when human breath should be analyzed, H_Target is preferably set to a value corresponding to H_Measure at 100% RH at about 37°C, i.e. the typical humidity in human breath.

[0112] In some embodiments H_Target is automatically determined via a temperature sensor in vicinity of the sampling point and by assuming 100% RH at the measured temperature. For example, if a metabolomic process in a vial should be analyzed, the temperature sensor is connected to the vial and sample inlet line 5 is fed into the vial. H_Target is then automatically calculated from 100% RH at the vial temperature. In some embodiments H_Target is set to a value well above the maximum humidity that is to be expected during the following measurement. In this case there is some safety margin in case unforeseeable humidity spikes in the sample air occur (e.g. in outdoor measurements, periods of extremely high temperature and humidity that are abnormal to a certain region).

[0113] In case H_Measure is inversely proportional to the humidity in the IMR / PTR drift tube 2 (e.g. specific ion intensity ratios, which increase with decreasing humidity), the opposite actions must be taken in the following.

[0114] Procedure according to the invention a) If H_Measure is lower than H_Target, the flow of gaseous H2O into the IMR / PTR drift tube 2 via the adjustable flow restricting device 7 is increased. b) If H_Measure is higher than H_Target, the flow of gaseous H2O into the IMR / PTR drift tube 2 via the adjustable flow restricting device 7 is decreased. c) If H_Measure is essentially equal to H_Target, the flow of gaseous H2O into the IMR / PTR drift tube 2 via the adjustable flow restricting device 7 is kept constant at the current level.

[0115] Preferably, H_Measure is measured, compared to H_Target and one of steps a) - c) is performed in frequent intervals. Preferably, the procedure is performed after each measurement cycle, while in case of a TOF based IMR / PTR-MS instrument one measurement cycle is the integration time for one mass spectrum, which is typically between 100 ms and 10 min.

[0116] In a preferred embodiment steps a) - c) are performed in an automated way, i.e. via a control unit. The control unit can be the instrument control software running on the computer controlling the IMR / PTR-MS instrument. A smoothing algorithm can be implemented into the control unit in order not to transfer noise of H_Measure to the controlling of the gaseous H2O injection flow.

[0117] In a preferred embodiment a warning is triggered in case H_Measure is higher than H_Target and the gaseous H2O injection flow is already 0.

[0118] Exemplary embodiment A

[0119] H_Measure is measured with an AH sensor in the pumping line for evacuating the sample gas from IMR / PTR drift tube 2. The IMR / PTR instrument is used for quantifying VOCs in outside air with an expected maximum temperature of 30°C. 100% RH (Relative Humidity) at 30°C equals about 30 g / m3AH. Therefore, H_Target is set to 30 g / m3. The sample gas flow through the IMR / PTR drift tube 2 for this specific instrument is 20 seem.

[0120] When dry air is connected to the sample inlet line 5 (e.g. in order to determine the chemical background of the instrument) H_Measure will be about 0 g / m3. The gaseous H2O flow into IMR / PTR drift tube 2 via the adjustable flow restricting device 7 (in this case a mass flow controller) is increased until H_Measure essentially equals 30 g / m3. In this example this corresponds to a gaseous H2O flow of about 0.75 seem that is controlled with the mass flow controller between gaseous H2O introduction line 6 and the H2O source 9. In other words, 0.75 seem is the maximum gaseous H2O flow that is only needed while sampling dry air and, in order to adjust H_Measure with H_Target the gaseous H2O flow is varied between 0 - 0.75 seem.

[0121] For this exemplary embodiment a mass flow controller is used as the adjustable flow restricting device 7. If this mass flow controller is of the type that can only either be fully closed (0 seem) or set to a distinct value of at least 1 seem, the use of the "bypass flow setup" in Fig. 6 may be beneficial. I.e. via a T-piece 11 and a flow restricting device 12 1 seem of gaseous water is always pumped into vacuum pump 13. The minimum setting of the mass flow controller 7 in this case is 1 seem, which corresponds to 0 seem gaseous water injection into IMR / PTR drift tube 2. By varying the set value between 1 - 1.75 seem, the required 0 - 0.75 seem gaseous water are injected into the IMR / PTR drift tube 2 with extremely high precision and accuracy.

[0122] H_Measure is displayed on the screen of the instrument's control computer. During the measurement the operator continuously compares H_Measure with H_Target and adjusts the gaseous H2O flow via the mass flow controller, which is again controlled by the instrument's control computer, in order to match the two values. Alternatively, the IMR / PTR-MS instrument's control software continuously compares H_Measure with H_Target and adjusts the gaseous H2O flow via the mass flow controller, i.e. the process is performed in an automated way.

[0123] It is very important to note that if, for example, a gas standard, which is typically a mixture of well-defined concentrations of trace compounds in dry air, is analyzed for subsequent calibration of the IMR / PTR-MS instrument, the humidity in the IMR / PTR drift tube 2 is the same as the humidity during an outdoor air measurement at, e.g., 40% RH at 25°C, 80% RH at 20°C, 100% RH at 30°C, etc. That is, neither complex humidity-dependent corrections, nor any manipulations of the sample air (e.g. drying) have to be performed and the results from the gas standard measurements can be directly applied to the outdoor air measurement results. Furthermore, any VOC concentration variations measured in outside air are "real concentration variations and not artefacts caused by the effects of varying humidity.

[0124] Exemplary embodiment B

[0125] H_Measure is measured via the ratio of l(H2O.H3O+) divided by l(sum of all reagent ions) in percent, with I standing for intensity of the respective ion yields. That is:

[0126] H_Measure = 100 x l(H2O.H3O+) I [l(H3O+) + l(NO+) + l(O2+) + l(H2O.H3O+)].

[0127] As the source for sample gas a "Liquid Calibration Unit" [9] is used. This device enables mixing gas standards with dry zero air and adding well-defined amounts of humidity to the sample gas stream. The gas standard in the present example contains traces of formaldehyde. H3O+is used as reagent ions, which protonate formaldehyde to CH2O.H+at m / z 31.018.

[0128] In Fig. 4 the measurement results without the invention are shown. The formaldehyde concentration in the sample gas was kept constant throughout the whole measurement, only the sample gas humidity was changed. During measurement phase A the sample gas was humidified to an AH of 6.7 g / m3, during phase B to 13.3 g / m3, during phase C to 20 g / m3(about 100% RH at 23°C), and during phase D dry sample gas was used. It can be seen that the detected signal intensity for protonated formaldehyde substantially changes with changing sample gas humidity. This means that, for example, in outdoor air measurements formaldehyde cannot be quantified because intensity changes can either result from actual concentration changes in the sample or from humidity variations.

[0129] The whole measurement was repeated, but this time with the invention. Fig. 5 shows the resulting data. First, H_Target was set to 8.9%, which corresponds to 27 g / m3AH or about 100% RH at 28°C, i.e. well above the maximum humidity that is expected during the measurement. A software program on the instrument's control computer calculated H_Measure from the IMR / PTR-MS data according to the above-mentioned formula for each measurement cycle (1 s). H_Measure was then compared to H_Target and the mass flow controller controlling the gaseous H2O flow into the IMR / PTR drift tube 2 via gaseous H2O introduction line 6 was adjusted until H_Measure essentially matched H_Target, using a simple PID (Proportional-lntegral-Derivative controller) logic. The "outliers" between the measurement phases result from this PID processes.

[0130] However, although again in phase A the sample gas was humidified to an AH of 6.7 g / m3, during phase B to 13.3 g / m3, during phase C to 20 g / m3(about 100% RH at 23°C), and during phase D dry sample gas was used, the humidity in the IMR / PTR drift tube 2 remained essentially constant throughout the whole measurement. Most importantly, the detected signal intensity for protonated formaldehyde remains constant from phase A through phase D, which perfectly represents the actual formaldehyde concentration.

[0131] Advantages of the invention

[0132] The invention solves the problems caused by changing sample gas humidity in IMR / PTR-MS, without introducing any of the drawbacks of existing solutions.

[0133] The humidity in the IMR / PTR drift tube 2 stays constant regardless of the humidity introduced by the sample air. This makes any corrections of measured compound intensities with humidity-dependent calibrations and / or complex models, which is susceptible to errors, obsolete. Changes in measured signal intensities directly reflect changes in the concentrations of the respective compounds. That is, artefacts from changes in the ion-chemistry caused by changes in humidity, are eliminated. Furthermore, no manipulation of the sample is necessary, i.e. no drying, diluting, filtering, etc. This considerably reduces maintenance, improves response times, and lowers detection limits. In contrast to solutions where constantly excessive amounts of gaseous H2O are introduced into the IMR / PTR drift tube in order to lower (but by far not eliminate) the relative humidity variations introduced by the sample air, the present innovation does not require very high reduced electric field strengths E / N, which again cause problems, e.g. because of analyte fragmentation.

[0134] REFERENCES

[0135] [1] W. Lindinger, A. Hansel, A. Jordan, On-line monitoring of volatile organic compounds at pptv levels by means of Proton-Transfer-Reaction Mass Spectrometry (PTR-MS) Medical applications, food control and environmental research. Int. J. of Mass Spectrom. and Ion Proc. 173 (1998) 191-241.

[0136] [2] M. Muller, F. Piel, R. Gutmann, P. Sulzer, E. Hartungen, A. Wisthaler, A novel method for producing NH4+reagent ions in the hollow cathode flow discharge ion source of PTR-MS instruments. Int. J. of Mass Spectrom. 447 (2020) 116254.

[0137] [3] P. Sulzer, S. Jurschik, J. Herbig, A. Jordan, L. Mark, IMR-MS Device. EP 3 309 817 B1.

[0138] [4] P. Trefz, J.K. Schubert, W. Miekisch, Effects of humidity, CO2 and O2 on real time quantitation of breath biomarkers by means of PTR-ToF-MS. J. Breath Res. 12 (2018) 026016. [5] W.B. Knighton, E.C. Fortner, A. J. Midey, A.A. Viggiano, S.C. Herndon, E.C. Wood, C.E. Kolb, HCN detection with a proton transfer reaction mass spectrometer. In. J. of Mass Spec. 283 (2009) 112-121.

[0139] [6] C.A. Mayhew, Clinical Applications of Exhaled Volatiles. In: A. Hansel, J. Dunkl (Eds.), 9th International Conference on Proton Transfer Reaction Mass Spectrometry and its Applications. 2024, 100-104.

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Claims

CLAIMS1. lon-Molecule-Reaction - Mass Spectrometry or Proton-Transfer- Reaction - Mass Spectrometry (IMR / PTR-MS) instrument comprising:• a reagent ion source (1),• an IMR / PTR drift tube (2) with a sample inlet line (5),• a mass analyzer region (3),• a controlling device (8), and• an H2O source (9), wherein the reagent ion source (1) is in fluid connection with the IMR / PTR drift tube (2) and wherein the IMR / PTR drift tube (2) is in fluid connection with the mass analyzer region (3), characterized in that the IMR / PTR-MS instrument further comprises,(a) an adjustable flow restricting device (7),(b) an input device for entering a humidity target value (H_Target), and(c) means for determining humidity in the IMR / PTR drift tube (2), wherein the H2O source (9) is in fluid connection with the IMR / PTR drift tube (2), wherein the controlling device (8) controls the gaseous H2O flow from the H2O source (9) into the IMR / PTR drift tube (2) by adjusting the adjustable flow restricting device (7), wherein the controlling device (8) calculates a humidity value (H_Measure) correlating with the current humidity in the IMR / PTR drift tube (2) from data obtained from the means for determining humidity, wherein the controlling device compares the target value (H_Target) with the humidity value (H_Measure) and adjusts the gaseous H2O flow into the IMR / PTR drift tube (2) to adjust the humidity value (H_Measure) to the target value (H_Target).

2. IMR / PTR-MS instrument according to claim 1 , wherein the adjustable flow restricting device (7) directly connects the H2O source (9) with the IMR / PTR drift tube (2), preferably via a gaseous H2O introduction line (6).

3. IMR / PTR-MS instrument according to claim 1 , wherein the adjustable flow restricting device (7) connects the H2O source (9) with the IMR / PTR drift tube (2) via the sample inlet line (5).

4. IMR / PTR-MS instrument according to one of claims 1 to 3, wherein the target value (H_Target) is set to a value equal to the expected maximum humidity to be introduced into the IMR / PTR drift tube (2) by a sample via sample inlet line (5).

5. IMR / PTR-MS instrument according to one of claims 1 to 4, wherein the means for determining humidity includes the mass analyzer region (3).

6. IMR / PTR-MS instrument according to claim 5, wherein the mass analyzer region (3) is configured to measure ion intensity profiles which correlate with the current humidity in the IMR / PTR drift tube (2).

7. IMR / PTR-MS instrument according to one of claims 1 to 6, wherein the means for detecting humidity comprises one or more humidity sensors (10).

8. IMR / PTR-MS instrument according to one of claims 1 to 7, wherein the H2O source (9) is connected with the reagent ion source (1) via a source gas line (4).

9. IMR / PTR-MS instrument according to one of claims 1 to 8, further comprising an alarm unit, wherein the controlling device (8) is configured to issue an alarm with the alarm unit if adjusting the humidity value (H_Measure) to the target value (H_Target) requires a reduction of gaseous H2O flow from the H2O source (9) into the IMR / PTR drift tube (2) and the adjustable flow restricting device (7) is in a closed state.

10. Method of operating an lon-Molecule-Reaction - Mass Spectrometry or Proton- Transfer- Reaction - Mass Spectrometry (IMR / PTR-MS) instrument, the IMR / PTR-MS instrument comprising• a reagent ion source (1),• an IMR / PTR drift tube (2) with a sample inlet (5), and• a mass analyzer region (3), wherein the reagent ion source (1) is in fluid connection with the IMR / PTR drift tube (2) and wherein the IMR / PTR drift tube (2) is in fluid connection with the mass analyzer region (3), characterized in that a humidity value (H_Measure) correlating with the current humidity in the IMR / PTR drift tube (2) is determined, wherein a controlled flow of gaseous H2O is introduced into the IMR / PTR drift tube (2) and the controlled flow of gaseous H2O is adjusted so that the humidity value (H_Measure) adjusts to a set humidity target value (H_Target).

11. Method according to claim 10, wherein the controlled flow of gaseous H2O is adjusted by adjusting an adjustable gas flow restricting device (7) between a source for gaseous H2O and the IMR / PTR drift tube (2).

12. Method according to claim 10 or claim 11 , wherein the target value (H_Target) is set to a value equal to the expected maximum humidity introduced into the IMR / PTR drift tube (2) by a sample via sample inlet line (5).

13. Method according to one of claims 10 to 12, wherein the humidity value (H_Measure) in the IMR / PTR drift tube (2) is determined with the mass analyzer region (3).

14. Method according to one of claims 10 to 12, wherein the humidity value (H_Measure) in the IMR / PTR drift tube (2) is determined with one or more humidity sensors (10).

15. Method according to one of claims 10 to 14, wherein an alarm is issued if adjusting the humidity value (H_Measure) to the target value (H_Target) requires reducing the controlled flow of gaseous H2O, while no gaseous H2O is introduced into the IMR / PTR drift tube (2).

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