Ion mobility spectrometer and method for operating an ion mobility spectrometer

Abstract

The present invention relates to an ion mobility spectrometer (100) for determining the ion mobility of a sample, the ion mobility spectrometer comprising a drift chamber (10) and a cylindrical reaction chamber (20) adjacent to the drift chamber (10) along an axial direction (1). The drift chamber (10) is designed to transport ions from a switch grid (11) to an ion detector (12) against an axial drift gas flow (13). The reaction chamber (20) has a sample gas inlet (21) adjacent to the switch grid (11) for introducing a sample gas (22), a gas outlet (23) opposite to the switch grid (11) for discharging the drift gas (13) and the sample gas (22), and a local ionization source (24) arranged at the gas outlet (23). The sample gas inlet (21) has opposite gas inlets (25) on the inner circumference of the reaction chamber (20). The present application also relates to a method for operating the ion mobility spectrometer (100), the method being used to determine the ion mobility spectrum of a sample.

Description

Technical Field

[0001] The present invention relates to an ion mobility spectrometer (or ion mobility mass spectrometer), in particular a mobile ion mobility spectrometer with an improved sample gas supply device, and a method for operating an ion mobility spectrometer, in particular for calibrating the ion mobility spectrometer so as to measure the ion mobility of a sample based on a reference. Background Art

[0002] An ion mobility spectrometer is an instrument that characterizes ions based on their mobility in a drift medium under the influence of electromagnetic forces. An ion mobility spectrometer usually has a reaction chamber and an adjacent drift chamber. The reaction chamber is used here to ionize the sample material and form product ions, and the drift chamber is used to determine the flight time or drift time of the ions formed.

[0003] The sample enters the reaction chamber through the inlet and the carrier gas, and is ionized there by energy supply. The formed ions are guided into the drift chamber by the action of the electric field, and the drift chamber is defined by a grid as an ion modulator on the inlet side, for example, and by a detector unit for ion flow measurement on the opposite side. In the drift chamber, the ions move toward the detector by further field action. The drift medium with defined physicochemical properties contained in the drift chamber interacts with the ions and counteracts the ion motion mediated by the field effect. Therefore, the transmission of ions in the drift chamber depends on the electric field strength and the interaction with the drift gas, and can be characterized by the resulting drift velocity.

[0004] Therefore, the ion mobility K is a measure of the molecular friction of ions in a given drift medium under the action of electromagnetic forces, and the greater the ion mobility, the smaller the friction. The ion mobility K is determined as a proportionality factor between the drift velocity v and the electric field component E effective in the drift direction. In a first approximation, these variables have the following linear relationship

[0005] v=K*E (1)

[0006] Assuming that the properties of the drift medium in the drift chamber are uniform and that the electric field in the ion drift region is constant, the ion mobility on a given path L is determined by determining the drift time t and the voltage difference U between the end and the start of the drift path based on the following equation derived from (1):

[0007] K=L 2 / (U*t) (2)

[0008] The ion mobility K determined in this way always depends on the set drift medium and is not a specific constant for a particular ion. Instead, the value of the ion mobility depends on the interaction of the ion with the drift medium. If this interaction is assumed to be a collision process, then according to the kinetic gas theory its strength depends on the masses of the collision partners involved, the particle number density, the pressure and temperature of the drift medium and the cross section of the interaction of the ion with the molecules of the drift gas.

[0009] For ion mobility spectrometers used under atmospheric conditions, the range of values ​​of the particle number density of the drift gas, which in turn depends on its temperature T and pressure p, extends over a wide range. For example, a range from -30°C to +60°C for (absolute) temperature T and a range from 50 kPa to 110 kPa for pressure are possible. Thus, by converting to a normalized ion mobility, the dependence of the ion mobility on the particle number density is eliminated. However, even the normalized ion mobility K0(T) is temperature-dependent, since it depends on the temperature-dependent action cross section of the specific interaction.

[0010] Therefore, when determining the ion mobility, it is usually assumed that the drift medium is homogeneous and has constant physicochemical properties. However, if air is used as drift medium, for example, the proportion of water present therein can already significantly influence the value of the ion mobility. Therefore, dry air, preferably with an absolute water concentration of less than 100 ppm, should be used as drift medium. Filter systems are usually used for this purpose.

[0011] In order for the drift medium to maintain its defined physicochemical properties, it is also continuously renewed in the drift chamber by the gas flow. In other words, a continuous drift gas flow flows through the drift chamber and the gas flow is determined by integrating the gas flow density over the cross-sectional area through which the gas flow passes. In this case, the gas flow density is the product of the gas concentration and the gas velocity. The gas flow in the drift chamber is superimposed on the ion flow and can in principle influence the effective drift time. However, a drift gas velocity of, for example, 4 cm / s is significantly smaller than an ion drift velocity of, for example, 4 m / s and can therefore usually be ignored for determining the ion mobility.

[0012] The ionization of the sample to be analyzed is first carried out in the ion mobility spectrometer by providing energy through the ionization source. A large number of ionization sources are known to those skilled in the art, such as plasma sources. The primary ions formed by the source can form further product ions through complex molecular ion reactions and charge transfer processes with sample substances. Due to the multi-stage nature of ionization, this occurs in the reaction chamber formed by the area between the ionization source and the ion flow modulator.

[0013] The effective ionization at the ionization source is a prerequisite for subsequent ionization and effective measurement operation. Although the amount of the primary ion produced increases with higher ionization source energy usually, the amount of NOx compound and the ozone formed is also disadvantageous. In this case, ozone, nitrogen oxides and nitrogen oxide compounds (NOx compounds) are unfavorable for use in ion mobility spectrometers, because they can suppress or prevent the ionization of some sample molecules. For example, NOx compounds are due to their high electron affinity and in conjunction with negative charge, therefore can not be used for the further ionization of primary ions, and the increase formed by ozone can cause unnecessary side reactions and the components in the gas path, such as pumps and sensors, which are aggravated by wear. Therefore, this limits the energy that can be used for sample ionization.

[0014] Therefore, structural measures must be taken to ensure that the sample gas consisting of sample material and carrier gas effectively reaches the ionization area near the ionization source. However, it should be noted that if the inlet velocity of the sample gas is too low, only a small part of the sample gas may reach the ionization source. At the same time, too high a sample gas inlet velocity may lead to turbulence and complex flow paths. However, it is also disadvantageous to supply the sample material to the ionization source in a targeted manner, because some product ions, such as dimers, require a larger formation area, in which the primary monomer ions meet other neutral sample gas molecules. In addition, this "targeting" of the ionization source is also prone to errors due to tolerances.

[0015] As already described at the beginning, the ion mobility K in a material well-defined drift medium can be determined by measuring the drift time and based on equation (2). In addition, the temperature-dependent normalized ion mobility K0(T) can be determined from the ion mobility K determined in this way. However, it is disadvantageous that this requires measuring a large number of values, such as pressure, temperature, drift voltage and drift time, and the measurement errors of the individual measurements may add up. Therefore, it is advantageous to use a reference-based method to determine the ion mobility.

[0016] In this case, a reference substance is used whose normalized, temperature-dependent ion mobility K0 R (T) is known. If the physicochemical conditions in the drift chamber are the same for the reference and sample measurements, K can be calculated by comparing the drift times of the reference and sample substances. R (T) = t R / t P *K0 R (T) from the drift ratio t R / t P Calculate the sample material K0 P (T) with drift time t PThe ion mobility of the sample is determined by the ion flow rate. Advantageously, only the measurement error of the drift time measurement (if the measurement sampling is high enough, it is within the noise range) and the measurement error of the drift chamber temperature T (only becomes significant when the ion mobility is strongly correlated with T) are important in this method. The reference material is usually supplied from the outside, either through the sample supply line or through an additional gas inlet that can be opened by a valve. Disadvantageously, this requires a more complex ion mobility spectrometer design.

[0017] The technical problem to be solved by the present invention is to overcome or minimize the disadvantages of the prior art and to provide an improved ion mobility spectrometer and an improved method for operating an ion mobility spectrometer. Summary of the invention

[0018] A first aspect of the present invention relates to an ion mobility spectrometer (IMS), which is used to determine the ion mobility of a sample, preferably the ion mobility spectrum of a sample, wherein the sample is particularly present in a sample gas (composed of a sample and a carrier gas). Here, the ion mobility spectrometer according to the present application has a cylindrical drift chamber, which is designed to transport ions from a switch grid preferably composed of at least one gate electrode to an ion detector. The drift chamber here is further designed to make the ion flow therein against the axial drift gas flow. The drift gas flow therefore flows from the direction of the ion detector toward the direction of the switch grid. The drift velocity depends on the electric field effect between the switch grid and the ion detector and the interaction between the ions and the drift gas flow. Preferably, for a given electric field and a given drift gas flow, a constant drift velocity is generated for a given ion type, so that macroscopic acceleration of the ions does not occur in the drift chamber. Within the scope of the present application, axial refers to the height of the cylindrical drift chamber.

[0019] The ion migration spectrometer of the present application also has a cylindrical reaction chamber adjacent to the drift chamber in the axial direction. Axial refers to the height of the cylindrical reaction chamber here and is preferably the same as the axial direction of the cylindrical drift chamber. According to the present invention, the reaction chamber has a sample gas inlet adjacent to the switch grid for introducing a sample gas. The sample gas inlet is designed so that the sample gas is mixed with the drift gas, as explained in detail below. The sample gas inlet is located on a side away from the drift chamber, adjacent to the switch grid. There is a gas outlet at the other end of the reaction chamber for discharging drift gas and sample gas, i.e., a mixture of these gases. In addition, a local ionization source is arranged at the gas outlet of the ion migration spectrometer according to the present invention. Preferably, the range in which sample ionization occurs around the local ionization source is less than 5mm, and most of the primary ions thus produced are produced in the immediate vicinity of the source.

[0020] According to the present invention, the sample gas inlet has (multiple) gas inlets arranged relatively to each other on the inner circumference of the reaction chamber. Such a sample gas inlet allows the sample gas flow and the drift gas flow to be combined in an advantageous manner and guided together to the gas outlet in a unidirectional flow mode, wherein an optimal flow occurs around the local ionization source. Preferably, the sample gas inlet has gas inlets arranged relatively to each other in pairs on the inner circumference of the reaction chamber. However, in principle, it is also conceivable to assign two relatively arranged gas inlets to one gas inlet on one side. In a preferred paired arrangement, the sample gas inlet has at least one pair of relatively arranged gas inlets, but preferably has multiple pairs. What is important is not the number of relatively arranged gas inlets, but the flow pattern achieved by the specific arrangement of the gas inlets, which will be explained below.

[0021] About the merging of gas flow in the ion mobility spectrometer with drift chamber and adjacent reaction chamber, it is usually distinguished that there is a gas guide variant of bidirectional or unidirectional gas flow.For bidirectional flow, drift gas flow and sample gas flow point to opposite directions and are discharged through the common outlet in the transition region.If this outlet is close to the drift chamber, some neutral sample gas flows and moisture therein may enter the drift chamber unfavorably.Sample gas flow and ion flow point to the same direction.If ionization is carried out using a plasma source, the NOx compounds formed may enter the reaction chamber due to directional and non-directional motion, where they can suppress the formation of negative product ions of the sample to be analyzed due to their high electron affinity.

[0022] In the case of unidirectional flow guidance, the drift gas flow and sample gas flow in the reaction chamber combine to form a synthetic gas flow in the opposite direction of the ion flow. In the absence of an electric field, the ions generated in the reaction chamber will also directly reach the gas outlet.

[0023] Using the unidirectional flow around the local ionization source, the advantage is that the neutral NOx compound formed by ozone and free radicals, such as nitric oxide, directly reaches the gas outlet and therefore cannot be further ionized. However, the problem is that the NOx compound usually has a higher ion mobility than the product ions to be analyzed, and if the flow rate is too high and the field strength is too low, the product particles to be analyzed will preferentially rush to the outlet. Therefore, the optimal flow around the local ionization source is of decisive importance for generating the maximum product ion signal, and because the electron affinity of many substances is also lower than that of the NOx compound, it is therefore impossible to detect it at low concentrations. Advantageously, the sample gas inlet according to the present invention can also detect ions with very low ion mobility. Therefore, using the ion mobility spectrometer according to the present invention, it is advantageous to set the minimum gas flow so that the formation of the NOx compound and its ionization in the reaction chamber are suppressed.

[0024] In a similarly preferred embodiment of the ion mobility spectrometer according to the present invention, the drift chamber and the reaction chamber have a cylindrical shape. In other words, the drift chamber and the reaction chamber have a circular cross-sectional area, and the radial direction of these circular areas is perpendicular to the axial direction of the specific chamber. Particularly preferably, the circular cross-sections of the drift chamber and the reaction chamber are concentric with each other, so that these axial directions coincide. Also preferably, the gas outlet of the reaction chamber is arranged concentrically with the reaction chamber and the drift chamber relative to the axial direction. According to this preferred embodiment, the sample gas is advantageously supplied with a radial flow component through the gas inlet. This enables the sample gas flow and the drift gas flow to be particularly effectively combined into an axial unidirectional gas flow and an optimal gas flow around the local ionization source.

[0025] In a particularly preferred embodiment, the sample gas inlet, particularly preferably the gas inlet arranged oppositely in pairs, is designed so that the radial components of the flow velocity of the partial flows introduced by the preferably oppositely arranged gas inlets of the sample gas compensate each other (or balance). Preferably, the gas inlet is therefore arranged and designed so that the radial inflow of the carrier gas (with or without a sample) is compensated in a manner that no gas flow reaches the opposite outer wall of the reaction chamber. This is particularly preferably achieved by the oppositely arranged paired flows from the oppositely arranged paired gas inlets. But it is important that the synthetic radial velocity component of the introduced sample gas flow or the radial total flow of the gas flow must be zero. Therefore, it is important for the present invention that the gas inlet is arranged at the edge so that the sum of the partial flows prevents radial flow to the outer wall of the reaction chamber. In principle, the inflow on one side can also be compensated by two or more inflows on the other side. However, a paired symmetrical arrangement of the gas inlet (symmetric about the axial point) is preferred.

[0026] When calculating ion mobilities, it is usually assumed that the drift gas always has the same physicochemical properties, so that the impact cross section of the ion under investigation remains constant. However, if air is used as drift gas in particular, the proportion of water in the air can significantly change the values ​​of the ion mobility of some (especially negatively charged) ions due to ion-dipole interactions, which impairs the distinguishability of many molecular ions.

[0027] The effect of ion-dipole interactions decreases with increasing temperature, but the generation of high temperatures and thermal insulation in mobile IMS devices is difficult to provide economically. The adverse effect of humidity on ion mobility can be greatly reduced by using defined dry air with an absolute water concentration below 100 ppm. In principle, this dryness requirement in the drift chamber can be maintained for a long time in a sealed IMS device with suitable filters.

[0028] In order to keep the drift chamber pollution-free and dry as far as possible, a continuous drift gas flow (used synonymously with drift gas flow in this article) passes through the chamber. Gas flow is the gas flow density integrated on the cross section (flow cross section). Gas flow density is the product of gas concentration and gas velocity. The gas flow in the drift chamber is superimposed on the ion flow and affects the effective drift time. In order to keep this influence low or negligible, compared with the drift velocity of ions (such as 4m / s), the drift gas velocity has a relatively small value (such as 4cm / s).

[0029] As mentioned above, the sample is supplied to the reaction chamber through one or more inlets. Through the sample gas flow (here used synonymously with the sample gas flow), not only the sample to be detected but also a certain amount of water is guided into the reaction chamber by the carrier gas flow (here used synonymously with the carrier gas flow). If this water enters the drift chamber, it will have an uncertain effect on the drift characteristics. Using the sample gas inlet according to the present invention, especially the radial flow distributor, the sample gas flow is advantageously separated in such a way that at least two partial flows in the radial outlet direction compensate each other on the central axis (axial direction) of the reaction chamber in terms of radial velocity components and mix with the dry drift gas flow and further escape in the direction of the reaction chamber. The main advantage of the arrangement according to the present invention is that, due to the radial mutual flow compensation and the superposition of the sufficiently large drift gas flow perpendicular to its flow, no sample gas flow portion enters the drift chamber. Therefore, the desired dryness in the drift chamber is advantageously permanently maintained.

[0030] In another preferred embodiment of the ion mobility spectrometer according to the present invention, the sample gas inlet has an inflow ring extending circumferentially around the reaction chamber. Generally, the drift chamber and the reaction chamber of the ion mobility spectrometer are not formed in a monolithic manner, but are composed of a plurality of interconnected annular structures, so that, for example, a plurality of drift electrodes can be integrated in the drift chamber. In such an embodiment, the inflow ring is preferably shaped to be consistent with other rings of the reaction chamber, so that the inflow ring can be easily integrated into the ion mobility spectrometer. The inflow ring preferably has at least one flow channel, which connects the sample gas supply line and at least one gas inlet. The flow channel is preferably formed as the inner cavity of the inflow ring here. It is also preferred that the flow channel is formed as a recess on the axial surface of the inflow ring, so as to obtain a closed and sealed flow channel by connecting the inflow ring to the adjacent ring of the reaction chamber. The sample gas supply line refers to an inflow opening for introducing a sample gas flow (or a carrier gas flow without a sample) from an external sample gas supply device.

[0031] According to the present invention, this inflow ring solves the problem of avoiding sample gas contamination of the drift chamber and can be advantageously integrated into an ion mobility spectrometer. Preferably, the inflow ring is made of an inert material, such as stainless steel or ceramic, to ensure long-term resistance to partially corrosive sample substances. The inflow ring is preferably designed as an insulator. Alternatively, the inflow ring is preferably implemented as a voltage-conducting field support ring. Advantageously, when using this inflow ring, there is no such flow obstacle in the reaction chamber, and the sample gas may disadvantageously gather at the flow obstacle, thereby prolonging the flushing of the ion mobility spectrometer after detection.

[0032] In a particularly preferred embodiment of the ion mobility spectrometer according to the present invention, the sample gas inlet comprises gas inlets arranged in pairs opposite to each other on the inner circumference of the reaction chamber. Further preferably, each pair of oppositely disposed gas inlets has a first gas inlet and a second gas inlet. According to this embodiment, the sample gas supply line is connected to at least one first gas inlet via a first flow channel and is connected to at least one second gas inlet via a second flow channel. In other words, the flow channel is divided so as to supply the first gas inlet separately from the second gas inlet. This advantageously achieves particularly uniform flow guidance and particularly good compensation of radially opposite gas flows.

[0033] According to the present invention, a corresponding first flow channel can be assigned to each first gas inlet and a corresponding second flow channel can be assigned to each second gas inlet. This means that each gas inlet is supplied through its own flow channel. It is also preferred that a plurality of first gas inlets are connected to the sample gas inlet via the first flow channel, and a plurality of second gas inlets are connected to the sample gas inlet via the second flow channel. It is also preferred that all flow channels are connected to a single gas supply line. However, a plurality of gas supply lines can also be provided, for example one gas supply line for each flow channel or each group of (first and second) flow channels.

[0034] It is also preferred that all first gas inlets are connected to the sample gas inlet via the first flow channel, and all second gas inlets are connected to the sample gas inlet via the second flow channel. It is particularly preferred that the sample gas inlet is divided into two channels, i.e., into the first and second flow channels, in a fork-shaped symmetrical manner, and the two flow channels extend along approximately half of the circumference of the ring, respectively. Therefore, the original volume flow of the sample gas is divided into two halves of the volume flow. In the simplest case, the inflow ring obtains two symmetrically arranged circular or slotted gas inlets, which point radially to the central axis of the reaction chamber. In particular, in the case of large volume flows of sample gas or carrier gas and / or due to possible manufacturing tolerances, the small opening cross-section of the gas inlet may cause partial flows to stagger with each other, which makes flow compensation more difficult. Preferably, the inflow ring therefore has a plurality of gas inlets in the first and second flow channels, which are point-symmetrically opposite to each other with respect to the axial direction.

[0035] In another preferred embodiment of the ion mobility spectrometer according to the present invention, a flow obstacle is arranged in at least one flow channel for diverting the sample gas flow through the gas inlet. The flow obstacles are preferably designed so that they can make the sample gas flow flow into the sample gas flow as a laminar flow to the greatest extent possible and / or evenly distribute the sample gas flow in a specific flow channel between the gas inlets branching from the flow channel. Particularly preferably, a comb-shaped flow resistor is introduced downstream of each flow channel along each gas inlet so that the gas flow is distributed on multiple gas inlets guided by the comb teeth (partial flow resistors), and the gas flow therefore flows out on a larger cross section. In addition, due to the shape (height and width), number and distribution of the teeth along the channel, a uniform radial outflow of the decelerated partial flows is achieved, and these partial flows reliably meet in the center of the reaction chamber and compensate each other with respect to radial velocity.

[0036] Particularly preferably, at least one flow channel of the inflow ring has a plurality of gas inlets, as described above. According to this embodiment, a flow obstacle for diverting the sample gas flow is preferably arranged between every two gas inlets. If the flow channel is, for example, a circumferentially extending recess in the inflow ring, the gas inlet is preferably formed by a recess extending radially from the flow channel and passing through the inner wall of the inflow ring. The flow obstacle is preferably formed by the material of the inflow ring that is retained between the radially formed recesses. Particularly preferably, the circumferential recess in the region of the radial recess and the flow obstacle has a smaller width than in the region of the sample gas inlet without a branch gas inlet. Particularly preferably, the flow obstacle is designed to make the partial flow of the sample gas flowing out through the gas inlet the same in terms of volume flow and / or velocity.

[0037] In the same preferred embodiment of the ion mobility spectrometer according to the present invention, the gas outlet of the reaction chamber has a smaller flow cross section than the reaction chamber and / or the drift chamber. Particularly preferably, the flow cross section of the reaction chamber is smaller than the flow cross section of the drift chamber and the flow cross section of the gas outlet is smaller than the flow cross section of the reaction chamber. In this case, the gradual taper of the flow cross section advantageously leads to an increase in the combined gas flow at the local ionization source. The gas outlet particularly preferably has a hole or capillary with a diameter of 2mm to 10mm. According to this embodiment, it is advantageous to ensure that the flow velocity of the total gas flow formed by the sample gas flow and the drift gas flow is at least 50cm / s in the region of 2mm around the local ionization source. The drift medium is preferably air with a water concentration less than 100ppm, and the flow velocity of the drift medium is preferably 2cm / s to 10cm / s. The flow velocity is further realized by the operating parameters of the drift gas supply device and / or the sample gas supply device, such as volume flow or mass flow, pump speed, pump power or valve opening. It is preferred to ensure low water concentration by using molecular sieves and / or suitable (membrane) filters.

[0038] In a similarly preferred embodiment of the ion mobility spectrometer according to the present invention, the local ionization source is configured for dielectric barrier discharge. In other words, the local ionization source is a dielectric barrier discharge source, and all electrodes are protected by an anti-plasma dielectric. The dielectric barrier discharge ionization source is suitable and configured for generating low-temperature plasma. The use of a local ionization source to generate low-temperature plasma in an ion mobility spectrometer is known in principle from the prior art. Various embodiments of this method are known. The average temperature of the low-temperature plasma is approximately equal to the ambient temperature. The plasma is neutrally charged on average and consists of a mixture of negative charge carriers such as electrons and ions, positively charged ions and neutral particles. A prerequisite for the formation of such a plasma is a highly reduced electric field strength, which is achieved by applying a high voltage to the electrodes. Due to the reliance on the reduced electric field strength, the ignition process is both pressure-dependent and temperature-dependent. According to Paschen's law, a minimum ignition voltage is required to form a plasma, which may be different for DC voltage and AC voltage. Due to the complex dynamics that occur during the plasma formation process, the ignition voltage also depends significantly on the temperature. After the plasma is ignited, it can usually be maintained with a lower voltage.

[0039] In order to protect the electrodes of the dielectric barrier discharge source from corrosion, these electrodes are partially or completely separated from the plasma by the dielectric. Since no charge transfer occurs through this insulation, plasma can only be generated by AC voltage. In order to avoid energy losses caused by charge collisions on the insulator, high-frequency AC voltage is used, especially AC voltage with a frequency higher than 40kHz. Due to the mass inertia of ions and electrons, at such a voltage, there are fewer collisions occurring on the dielectric wall or ion migration spectrometer. This source is preferably realized by a gas-tight glass body filled with inert gases helium, krypton or argon, and a high-frequency high-voltage field is applied to the glass body to produce vacuum ultraviolet radiation. The vacuum ultraviolet radiation leaves the glass body through a thin window transparent to the spectral range and causes the photoionization of the sample gas, in which the electrode is insulated by glass and is located outside the plasma. It is also preferred to use electrodes fully protected by anti-plasma insulators, such as anti-corona polyimide or glass coatings. It is particularly preferred to use electrode wires formed by, for example, tungsten and insulated with thin glass. For example, such an electrode wire is described in the article A Gapless Micro-Dielectric-Barrier-Discharge Ion Source for Analytical Applications by Coy et al. in 2016, see 2016 ArXiv electronic version 1602, link: http: / / arxiv.org / abs / 1602.06242, the content of which is quoted in its entirety. When high frequency and high voltage are applied to such an electrode, a plasma is formed at the wire contact point. Coy et al. have observed the formation of NOx compounds on such an electrode wire. Although in the ion mobility spectrometer according to the present invention, it is also particularly advantageous to use a mesh arrangement of such electrodes, in which the wires are in contact with each other in pairs similar to single contact, the amount of undesirable by-products increases.

[0040] When such a plasma source is used, the plasma is generated under atmospheric conditions in a region of a few millimeters between the electrodes, under which the ion mobility spectrometer according to the invention is to be operated. Such an electrode arrangement is therefore a local ionization source, more specifically a local plasma source. During the formation of the plasma, its composition is determined by chemical reactions of ions and neutral gas particles starting from molecules present in large quantities in the air (molecular oxygen and nitrogen), wherein ozone and nitrogen oxides are produced as by-product ions. Depending on their charge, the ions formed locally at atmospheric pressure leave the immediate vicinity of the plasma formation region by means of an electric field superimposed on the discharge field.

[0041] In ion migration spectrometer according to the present invention, the sample gas flow mixed with drift gas flow and dried is directed to the local ionization source in the reaction chamber. Primary ions and undesirable byproducts such as ionization of NOx and ozone and formation occur near the source. The inventor unexpectedly found by experiment and comparative computer simulation that when the flow velocity of the total gas flow near the ionization source of about 2mm in diameter exceeds 50cm / s, the formation of these undesirable byproducts can be suppressed. In traditional ion migration spectrometer, the gas volume flow in the reaction chamber can not reach such high speed. In ion migration spectrometer according to the present invention, such speed can be realized by narrowing the flow channel (gradually tapering).

[0042] Preferably, the reaction chamber is funnel-shaped, wherein the flow cross section near the source is (significantly) smaller than the flow cross section near the switch gate. However, this requires the use of different segmentation elements in the reaction chamber consisting of field support rings and insulators, which increases the cost and complexity of manufacturing. Therefore, a sufficiently high flow rate is preferably achieved by placing a narrow flow channel as a capillary or an orifice of a small inner diameter of, for example, 4 mm, in the immediate vicinity of the local ionization source, for example at a distance of 2 mm. Such an orifice (or orifice plate) is preferably placed downstream of the source to avoid deposition.

[0043] Particularly preferably, the local ionization source in the ion mobility spectrometer according to the present invention is located in the gas outlet, and the flow cross section of the gas outlet is 6 mm or less. In this case, the arrangement in the gas outlet also preferably represents the arrangement of the ionization source in the reaction chamber near the gas outlet. The flow rate of the gas flow increases according to the ratio of the flow cross section of the reaction chamber to the flow cross section at the gas outlet. Advantageously, in the ion mobility spectrometer according to the present invention, the unwanted reaction products of the ionization source are therefore discharged directly through the gas outlet, and backflow or diffusion into the reaction chamber is also prevented. If the local ionization source according to Coy et al. is constructed as having crossed glass insulating wires and arranged near or in the gas outlet, the smaller diameter of the gas outlet advantageously causes the mechanical vibration stabilization of the ionization source.

[0044] In an equally preferred embodiment, the ion migration spectrometer also includes at least one potential source arranged in the reaction chamber according to the present application, for generating an electric field so that the ions formed at the local ionization source are transported away in the direction of the drift chamber. The potential source advantageously enables the primary ions formed at the ionization source to be transported away from the local ionization source, thereby preventing the ions from recombining at the local ionization source. It is also preferred that the average potential of the local ionization source is coupled with the potential of the surrounding reaction chamber to optimize the formation rate of product ions. Particularly preferably, a field support ring located in the reaction chamber next to the ionization source is used as a potential source. An axial field is generated by the field of the potential source and the ionization source as a reference potential, for transporting the primary ions into the reaction chamber. In addition, by preferably using the potential source as a field support ring, it is also possible to realize the radial field component associated with the ionization source in the reaction chamber and thus expand the primary ion beam with a smaller initial diameter. Preferably, the potential source, such as the nearest field support ring, is therefore at an increased potential compared to the average potential of the local ionization source and centrally surrounds the local ionization source at a short distance. Thus, neutral sample molecules flowing through the local ionization source at a greater radial distance are also advantageously ionized. Advantageously, this is followed by a region with reduced drift velocity, which is produced by a small or vanishing potential difference relative to the next adjacent field support ring in the direction of the drift chamber. This increases the residence time of the primary ions in the reaction chamber and thus the probability of forming product ions from the primary ions.

[0045] By electric potential source effectively remove primary ions, this also advantageously offsets the formation of ions by NOx compound hopefully.When adjusting the field intensity produced by the electric potential source, it must be taken into account that if the field intensity is too low, ions will be rushed to the gas outlet and / or ion recombination occurs at the local ionization source.Therefore, enough field intensity is needed so that ion transportation can be carried to the reaction chamber in conjunction with high gas flow, so as to suppress the formation of neutral NOx compound at the plasma source and to penetrate into the reaction chamber.By the high flow velocity (>50cm / s) at the local ionization source and the interaction of sufficiently high field intensity in the direction of the reaction chamber, the increase of primary ion flow can be advantageously realized.

[0046] Further preferably, the ion migration spectrometer according to the present application has a drift gas inlet arranged in the ion detector area. In a cylindrical drift chamber, the ion detector is preferably arranged on the top surface of the drift chamber facing away from the reaction chamber. At least one drift gas inlet is preferably arranged adjacent to the detector, for example, arranged concentrically around a circular detector. Preferably, the drift gas inlet is formed as an annular opening arranged concentrically around the detector, which advantageously can form a laminar flow of the drift flow. Also preferably, the ion migration spectrometer according to the present application includes a drift gas supply device for a controllable supply of drift gas and a sample gas supply device for a controllable supply of sample gas. The supply device is preferably arranged outside the reaction chamber and the drift chamber, wherein the sample gas supply device supplies the sample gas to the sample gas inlet (with or without a carrier gas of a sample material) through a sample gas supply line, and wherein the drift gas supply device supplies the drift gas to the drift gas inlet. To this end, each supply device can have one or more pumps and / or one or more valves, and can have a gas supply line designed as, for example, a hose or a tube. The design of the gas supply line is known to the person skilled in the art and is not the subject of the present application.

[0047] Another aspect of the present application relates to a method for operating an ion mobility spectrometer, which is used to determine the ion mobility spectrum of a sample, and in particular to a method for operating an ion mobility spectrometer as described above according to the present application. In this regard, the method according to the present invention includes the method steps described below. In the first method step of the method according to the present invention, a first operating parameter is set. The first operating parameter relates to the operation of a local ionization source of the ion mobility spectrometer, a drift gas supply device for supplying air as a drift gas, and / or a sample gas supply device, so as to generate NOx ions at the local ionization source and introduce the generated NOx ions into the drift chamber of the ion mobility spectrometer. The first operating parameter is selected in this way so that the formation of NOx compounds at the local ionization source and their entry into the drift chamber are affected in this way so that the concentration of NOx compounds in the drift chamber can achieve the determination of a unique NOx peak in the IMS spectrum. In a further step of the method according to the present invention, the first drift time t of the NOx ions passing through the drift chamber of the ion mobility spectrometer is measured. NOx The drift time measurement is performed while maintaining the first operating parameter and the current drift chamber conditions, for example the current drift chamber temperature T.

[0048] In a further step of the operating method according to the invention, a second operating parameter of the local ionization source, the drift gas supply device and / or the sample gas supply device is set, wherein the second operating parameter involves or results in the generation of sample ions and / or avoidance of the introduction of NOx ions into the drift chamber when the local ionization source does not form NOx ions. In other words, the second operating parameter is selected so that such a low concentration of NOx compounds is present in the drift chamber of the ion mobility spectrometer that no NOx peak is present in the IMS spectrum. In a further step of the method according to the invention, a second drift time t of the sample ions passing through the drift chamber of the ion mobility spectrometer is measured. P The drift time measurement is performed while maintaining the second operating parameter and the current drift chamber conditions, for example the current drift chamber temperature T.

[0049] In the method according to the invention, based on the measured first drift time t NOx , the measured second drift time t P and the normalized ion mobility K0 of NOx ions as reference R (T) According to K0 P (T) = t NOx / t P *K0 R (T) Determine the normalized ion mobility K0 of the sample P (T). Normalized ion mobility K0 of NOx ions R (T) is preferably taken from a reference table containing known values ​​of the normalized ion mobility of NOx ions, which values ​​have preferably been determined on a test bench for a plurality of drift chamber temperatures for the ion mobility spectrometer used in the method. Particularly preferably, K0 R (T) can be calculated by a mathematical interpolation function, and its interpolation parameter is derived from the measured value determined in the test bench. The normalized ion mobility of NOx ions is often used as a reference value in the ion mobility spectrum and therefore a corresponding value can be obtained. However, the method according to the present invention can internally generate NOx ions as a reference (calibrant) in the first operating mode of the ion mobility spectrometer, and can realize an IMS spectrum based on a reference measurement sample in the second operating mode of the ion mobility spectrometer.

[0050] In a preferred embodiment of the method according to the invention, the first operating parameter stops or reduces the sample supply. If necessary, a carrier gas without sample material is also introduced in order to achieve approximately similar flow conditions in all operating modes. It is also preferred that the first operating parameter adjusts the flow rate of the sample gas supply and / or the drift gas supply in such a way that the flow rate at the local ionization source is set to preferably less than 50 cm / s. The flow rate here relates to the total gas flow at the source.

[0051] Also preferably, the first operating parameter sets the supply voltage of the dielectric barrier discharge source as the local ionization source to be higher than a predetermined limit voltage. The limit voltage is preferably determined empirically for the ion mobility spectrometer used in the method according to the invention, and the limit voltage preferably depends on the ambient conditions of pressure and temperature at the discharge source and is related to a voltage above which the concentration of NOx ions is higher than a predetermined limit value and below which the concentration is lower than the limit value.

[0052] Preferably, the second operating parameter sets the flow velocity of the sample gas supply and the flow velocity of the drift gas supply so that the flow velocity of the total flow at the local ionization source is preferably higher than 50cm / s. Also preferably, the second operating parameter sets the supply voltage of the dielectric barrier discharge source to be lower than a predetermined limit voltage. In addition to the above-mentioned first and second operating parameters, it is also possible to determine the operating parameters of the presence of other significant effects of the ion migration spectrometer in the drift chamber according to the method of the present invention.

[0053] For the detection of many substances in ion mobility spectrometers, NOx compounds produced together with plasma are disadvantageous. Through the above measures, the formation of such compounds can be suppressed in a targeted manner or their entry into the reaction chamber and drift chamber. On the other hand, the ions of NOx compounds in dry, clean air have a characteristic normalized ion mobility K0 R (T), i.e., the position (peak) in the drift time spectrum. In the first operating state of the ion mobility spectrometer, preferably in the absence of an external sample supply, these positions can be used as an internal reference for calibration. Preferably, the first operating state is established for a short period of time, for example, less than 1 s. Also preferably, the first operating state is achieved by increasing the high voltage amplitude and reducing the total gas flow around the local ionization source, wherein only the spectral intensity required for reliable evaluation is generated.

[0054] Once calibration is completed, it is necessary to ensure that the NOx compound leaves the reaction chamber and establishes the original measurement state. Therefore, the method according to the present invention preferably further comprises the step of discharging NOx ions from the reaction chamber. Also preferably, the method according to the present invention comprises the step of reducing the high voltage amplitude of the ionization source at least to below the limit voltage or closing the high voltage amplitude in parallel with the step of discharging NOx ions. By closing high voltage or reducing high voltage to below the voltage required for maintaining plasma and by flushing with the dry drift gas flow increased, the time for recovering the original measurement state can be shortened. If the ionic strength of the NOx compound is lower than a predetermined threshold value after discharging, the original measurement state is thus recovered. In order to verify, the second measurement state for measuring the sample must be briefly established, so that the drift time spectrum formed about the NOx ions can be evaluated.

[0055] As mentioned above, by using the lowest possible high voltage amplitude to ignite and maintain plasma, the ions formed by NOx compounds can be suppressed. Meanwhile, the minimum ignition voltage of plasma depends on the gas density and therefore depends on the temperature and absolute pressure of the gas according to the ideal gas law, but these values ​​may change during operation. According to the present invention, the ignition of plasma and maintenance are controlled by a control system according to the temperature and pressure of the gas flowing through, and the control system is provided with the best high voltage parameters for amplitude and frequency and adjusts these parameters when changing. In addition, the control system controls the gas flow. In order to ensure the ignition of plasma under the minimum voltage and maintenance, it is further preferred to measure the gas temperature and gas pressure near the ionization source by a sensor, and measure the drift time spectrum. The integral on the entire drift time spectrum or the selected range is an additional control variable, which is a measure of ion generation. By the above-mentioned control system, the first operating mode and the second operating mode are also preferably realized.

[0056] The ion mobility spectrometer according to the invention preferably operates with dry air (water content less than 100 ppm) as drift gas and also with dry carrier gas. Under these conditions, the ignition of the plasma is particularly difficult. On the other hand, a higher water content generally promotes the ignition of the plasma. Therefore, a higher ignition voltage is generally required to ignite the plasma in dry air.

[0057] The ion mobility spectrometer according to the invention preferably has a radiation source which is designed and arranged for irradiating the local ionization source with light in the wavelength range of 480 to 280 nm. Surprisingly, it has been shown that by such irradiation of the local ionization source in dry air with a water content of less than 100 ppm, the ignition voltage of the plasma can be reduced and the ignition time of the plasma can be shortened. Advantageously, in the wavelength range mentioned, a light radiation power of less than 100 mW is sufficient to significantly support the ignition of the plasma.

[0058] Particularly preferably, the radiation source is a light emitting diode with a wavelength of 480 to 280 nm. This wavelength range corresponds to the second positive order of molecular nitrogen. Particularly preferably, the radiation source is designed to irradiate the local ionization source with a wavelength of 405 nm. Particularly preferably, the radiation source is a laser diode with a corresponding wavelength, such as a laser diode commonly used in a Blu-ray player. The local plasma source is preferably irradiated directly, indirectly and / or in a focused manner via an optical cable by at least one radiation source. The irradiation is preferably performed for a short time until the ionization begins, which can be identified in the drift time spectrum based on, for example, the intensity of the reactant ions. The minimum value of the intensity or the integral within a predetermined interval can be used as a criterion for turning off the radiation source, and being lower than the minimum value causes the radiation source to be turned on again. Pulse continuous operation of the radiation source is also preferred. The pulse width is preferably adapted to environmental conditions, such as humidity, pressure and temperature of the local ionization source.

[0059] The method steps according to the method of the present invention can be realized by electrical or electronic components or components (hardware) through firmware (ASIC) or by executing a suitable program (software). Also preferably, the method according to the present invention is realized or implemented by a combination of hardware, firmware and / or software. For example, the various components for executing the various method steps are designed as separate integrated circuits or are arranged on a common integrated circuit. The various components for executing the various method steps are also preferably arranged on a (flexible) printed circuit board (FPCB / PCB), a tape carrier package (TCP) or other substrates.

[0060] The individual method steps of the method according to the present invention are further preferably designed as one or more processes, which are run on one or more processors in one or more electronic computing devices and are generated during the execution of one or more computer programs. The computing device is preferably designed to cooperate with other components, such as a communication module and one or more sensors, in order to implement the functions described herein. The instructions of the computer program are preferably stored in a memory, such as a RAM element. However, the computer program can also be stored in a non-volatile storage medium, such as a CD-ROM, a flash memory, etc.

[0061] It is further obvious to the person skilled in the art that the functions of several computers (data processing devices) can be combined or combined in a single device, or that the functions of a specific data processing device can be distributed to multiple devices in order to perform the steps of the method according to the invention without deviating from the method described according to the invention. Preferred embodiments of the method according to the invention correspond to preferred embodiments of the ion mobility spectrometer according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] The embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the accompanying drawings:

[0063] Figure 1 is a schematic cross-sectional view of an ion mobility spectrometer according to one embodiment;

[0064] Figure 2 is a schematic perspective cross-sectional view of a reaction chamber and a portion of a drift chamber of an ion mobility spectrometer according to one embodiment;

[0065] Figure 3 is a schematic perspective view of an ion mobility spectrometer according to one embodiment;

[0066] 4 is a schematic diagram of (A) a sample gas flow, (B) a drift gas flow, and (C) a total flow formed by the sample gas flow and the drift gas flow in an ion mobility spectrometer according to one embodiment;

[0067] Figure 5is a schematic cross-sectional view of the flow rate of the total flow around the local ionization source at the gas outlet of an ion mobility spectrometer according to one embodiment; and

[0068] FIG. 6 shows IMS mass spectra of negatively charged ions in the absence (A) and presence (B) of contaminating NOx ions in the drift chamber. DETAILED DESCRIPTION

[0069] The ion mobility spectrometer 100 according to one embodiment of the present application is Figure 1 is shown in a schematic side view and in Figure 2 1 is shown in a schematic perspective cross-sectional view. The ion migration spectrometer 100 has a cylindrical drift chamber 10 and a cylindrical reaction chamber 20 adjacent thereto along the axial direction 1. The drift chamber 10 is defined by a switch gate 11 on the side facing the reaction chamber 20. On the side opposite to the switch gate 11, the drift chamber 10 is defined by an ion detector 12, and the reaction chamber 20 is defined by a gas outlet 23. The drift gas inlet 14 is arranged in an annular manner around the ion detector 12. The sample gas inlet 21 is arranged in the reaction chamber 20 adjacent to the switch gate 11, and has gas inlets 25 arranged in pairs opposite to each other on the inner circumference of the reaction chamber 20. A local ionization source 24 configured for dielectric barrier discharge is further arranged in the reaction chamber 20 near the gas outlet 23.

[0070] During operation of the ion mobility spectrometer 100, a drift gas 13, in particular dry air with a water content of less than 100 ppm, is introduced into the drift chamber 10 via a drift gas supply (not shown) through a drift gas inlet 14 at a defined flow rate of, for example, 4 cm / s. The drift gas 13 flows through the drift chamber 10, the switching grid 11 and the reaction chamber 20 and leaves the ion mobility spectrometer 100 via a gas outlet 23. Furthermore, during operation of the ion mobility spectrometer 100, a sample gas 22 consisting of a sample material contained in a carrier gas can be introduced into the reaction chamber 20 via a sample gas inlet 21. The sample gas 22 mixes with the drift gas flow 13 in the axial direction downstream of the sample gas inlet 21, and the total gas flow formed by the sample gas 22 and the drift gas flow 22 flows to the gas outlet 23, wherein the total gas flow is increased due to the smaller flow cross section of the gas outlet 23. Before the increased total gas flow leaves the ion mobility spectrometer 100 through the gas outlet 23, the total gas flow flows around the local ionization source 24, reaching a flow rate of more than 50 cm / s depending on the flow rate of the drift gas 13. Figure 5 Flow distribution in a cross section of the ion mobility spectrometer 100 around the local ionization source 24 is shown in FIG. The contour lines represent the gas flow rate in cm / s. Thus, a flow rate of the total gas flow of about 70 cm / s is achieved in the region of the local ionization source 24.

[0071] At the local ionization source 24, the sample material is ionized by a dielectric barrier discharge. In addition, air components used as drift gas 13 can be ionized. In the vicinity of the local ionization source 24, an LED is arranged as a radiation source 40, by which the local ionization source 24 can be directly illuminated. When switched on, the LED 40 can be used to ensure ignition of the plasma under dry ambient conditions, in particular when both the drift gas and the carrier gas or sample gas are dry, for example the water concentration of the total gas flow is less than 100 ppm.

[0072] The reaction chamber 20 also has a local potential source 30, through which a voltage relative to a reference source 32 (ground) can be established. According to the potential difference thus generated between the ionization source 24 and the potential source 30, ions of a specific polarity move in the direction of the switch gate 11 and against the total gas flow. This movement is further mediated by other potential support rings (field support rings) 31 arranged in the reaction chamber 20 and the drift chamber 10. In this case, mainly ions with a ratio of ion velocity to flow velocity greater than 1 move against the total gas flow toward the switch gate 11, while other ions are guided to the gas outlet 23 by the high flow rate of the total gas flow and are derived from the ion mobility spectrometer 100 through the gas outlet. The ions that reach the switch gate 11 move in the direction of the ion detector 12 in a defined manner in the drift chamber against the drift gas flow 13 through the potential support rings 31 arranged therein. The ion mobility of the ions can be determined based on the measured drift times, which are required for the distance of the ions from the switch gate 11 to the ion detector 12. It has been shown that the design of the ion mobility spectrometer 100 according to the present application advantageously allows separation of ionized sample material and neutral particles formed by the ionization source. Therefore, the ion mobility spectrometer 100 of the present application can be used to determine ion mobility spectra with as little interfering peaks as possible, such as NOx ions.

[0073] According to one embodiment, the sample gas inlet 26 has an inflow ring 21, such as in Figure 3 In this case, the inflow ring 21 has a sample gas supply line 27, which is designed to be connected to an external sample gas supply device (with a pump and / or a valve, not shown). Figure 3 The sample gas supply line 27 is formed as a hole passing through the outer circumference of the inflow ring 21. In addition, the inflow ring 21 has a flow channel 28, according to Figure 3 The flow channel is formed as a recess configured on the axial surface of the inflow ring 21 and connected to the sample gas supply line 27. The flow channel 28 is then sealed by installing the inflow ring 21 in the ion mobility spectrometer 100 using a suitable adhesive, welding or elastomeric seal between the inflow ring 26 and the adjacent ring segment of the ion mobility spectrometer 100.

[0074] Figure 3The inflow ring shown in the figure has in particular a first flow channel 281 branching off from the sample gas supply line 27 in a first direction and a second flow channel 282 branching off from the sample gas supply line 27 in a second direction. The first flow channel 281 connects the sample gas supply line 27 to a plurality of first gas inlets 251 for introducing a sample gas 22 having a radial flow component, i.e., at least partially introducing the sample gas in a radial direction 2 (see Figure 1 ). The gas inlet 251 can be oriented perpendicular to the axial direction 1 and directly point to the rotational symmetry axis of the ion migration spectrometer 100. However, the gas inlet 251 can also surround a slight acute angle with the axial direction 1, so that the sample gas flow 22 is introduced with an axial velocity component and a radial velocity component in the direction of the ionization source 24. The second flow channel 282 also connects the sample gas supply line 27 to a plurality of second gas inlets 252 for introducing the sample gas 22. The first gas inlet 251 and the second gas inlet 252 are arranged opposite to each other in pairs. In particular, the first gas inlet 251 and the second gas inlet 252 are arranged point-symmetrically with respect to the rotational symmetry axis pointing to the axial direction 1 of the ion migration spectrometer 100. Advantageously, this allows the radial velocity component of the sample gas flow 22 introduced by the first gas inlet 251 and the radial velocity component of the sample gas flow 22 introduced by the opposite second gas inlet 252 to compensate each other. Advantageously, this causes the radial flow velocity of the sample gas flow 22 in the rotational symmetry axis region to be almost zero, so that the sample gas flow 22 will not flow to the opposite wall of the reaction chamber 20. This results in an advantageous delivery of the sample to the local ionization source 24, in particular to a confined spatial region around the local ionization source 24. The ion mobility spectrometer 100 of the present application is therefore capable of achieving a high yield of ions generated at the local ionization source 24 of the sample material.

[0075] 4(A), (B) and (C) schematically illustrate the sample gas flow 22, the drift gas flow 13 and the total gas flow 13, 22 in the ion migration spectrometer 100. As shown in FIG4(A), the sample gas 22 is first introduced into the inflow ring 26 through the sample gas supply line 27, and is particularly introduced into the flow channel (not shown) arranged inside the inflow ring. Via the flow channel, the sample gas 22 reaches the first gas inlet 251 and the second gas inlet 252 arranged oppositely and is introduced into the reaction chamber 20 through these gas inlets. By being introduced through the oppositely arranged inlets 251, 252, the radial flow components of the sample gas 22 effectively compensate each other. By superimposing with the drift gas flow not shown in FIG4(A), the sample gas 22 is also moved toward the local ionization source 24 and is increased and discharged downstream through the gas outlet 23.

[0076] FIG. 4 (B) shows the drift gas flow 13 in the ion mobility spectrometer 100 separately. The drift gas flow 13 is introduced into the drift chamber 10 through a gas inlet (not shown) and flows in a laminar manner on a large scale in the direction of the reaction chamber. After the transition into the reaction chamber 20, the drift gas flow 10 undergoes a local contraction in the region of the sample gas inlet 21 due to the introduction of a sample gas 22 having a radial flow component. Thereafter, the drift gas flow 13 continues to flow in the direction of the gas outlet 23, in which the local ionization source 24 is arranged. Since the flow cross section of the gas outlet 23 is significantly smaller than the flow cross section of the reaction chamber 20, the drift gas flow in the gas outlet 23 and around the local ionization source 24 is greatly increased. According to the Bernoulli effect, the increase is caused by the ratio of the flow cross sections of the reaction chamber 20 and the gas outlet 23, and preferably, the flow cross section of the reaction chamber 20 is at least 2 times, preferably 5 times and particularly preferably 10 times the flow cross section of the gas outlet 23. FIG. 4(C) shows the total gas flow formed by the sample gas flow 22 of FIG. 4(A) and the drift gas flow 13 of FIG. 4(B). This also shows the flow pattern characteristics of the ion migration spectrometer 100 of the present application, especially the lack of the portion of the sample gas flow 22 that radially exceeds the symmetry axis of the ion migration spectrometer 100 and the increase of the total gas flow due to being discharged through the gas outlet 23 around the local ionization source 24. Therefore, the ion migration spectrometer 100 according to the present application achieves a high ion yield of the sample material at the source 24. Similarly, the ionized sample material is effectively transported to the drift chamber 10, and on the other hand, the non-ionized components of the air used as the drift gas are discharged from the ion migration spectrometer 100 through the gas outlet 23. The inflow of the sample gas through the sample gas inlet 21 also effectively prevents the non-ionized sample gas, especially water, from entering the drift chamber 10.

[0077] The ion migration spectrometer 100 of the present application can further advantageously operate in two different operating modes. The first operating mode advantageously can use the internally generated NOx ions to calibrate the ion migration spectrometer 100, while the second operating mode can use the temperature-dependent normalized ion mobility of the previously known NOx ions to determine the ion migration spectrum of the sample material based on the reference. Fig. 6 (A) and (B) show the drift time spectrum determined by the ion migration spectrometer 100 according to the present invention when the sample gas is contaminated by NOx ions (Fig. 6 (A)) and uncontaminated (Fig. 6 (B)). Based on these figures, the two operating modes of IMS100 are briefly described below.

[0078] In the first operating mode of the ion migration spectrometer 100 according to the present application, it is operated with the first operating parameters. These first parameters result in only or at least mainly forming NOx ions, such as NO2 ions and / or NO3 ions, at the local ionization source 24. For this reason, the sample gas supply is particularly stopped or throttled, and the carrier gas supply can also be stopped or continue to operate. When using drift gas, such as dry air as carrier gas, the latter option is particularly useful. In addition, the sample gas supply device and / or the drift gas supply device for introducing the drift gas through the drift gas inlet 14 can be operated so that the flow rate of the total gas flow at the local ionization source 24 is less than 50cm / s. In addition, the operating voltage of the local ionization source can be set to be higher than a predetermined limit voltage, which is preferably determined empirically for the ion migration spectrometer 100 in the test bench, and when it is higher than the limit voltage, NOx ions are formed by the drift gas in particular with a predetermined yield. By setting the first operating parameters, NOx ions are therefore mainly formed at and around the local ionization source 24, and the ionized sample material is hardly formed.

[0079] FIG6(B) shows two characteristic peaks 52 and 53 of NOx ions in the ion mobility spectrum. The second IMS peak 52 corresponds to 2.18 cm 2 Normalized ion mobility K0 of Vs 52 and the third IMS peak 53 corresponds to 2.00 cm 2 Normalized ion mobility K0 of Vs 53 Also shown is a value of 2.126 cm 2 Ion mobility K0 / Vs 51 The first IMS peak 51 of the exemplary ionized sample material. For example, this is the peak of the primary ion or the negative reactant ion. As can be seen from FIG. 6 (B), the IMS peaks 52 and 53 of the NOx ions significantly hinder the resolution of the first IMS peak 51. Therefore, the measurement of the IMS peak 51 should be carried out in the absence of NOx ions as much as possible. At the same time, by comparing with the previously known value of the normalized ion mobility K0 (T) of NOx, the IMS peaks 52 and 53 of the NOx ions can be used to measure the ion mobility of the sample material based on the reference. The IMS peak 53 is particularly suitable for this due to its separated position. For this reason, under the current measurement conditions of the ion mobility spectrometer 100, such as the current temperature, only the IMS peaks 52 and 53 of the NOx ions are determined in the first operating mode and the drift time of the NOx ions is determined. Together with the resolved IMS peak 51, that is, the drift time of the sample material, the ion mobility of the sample material can be determined based on the drift time ratio of the sample material and the NOx ions.

[0080] In the second operation mode of the ion mobility spectrometer 100, the second operation parameters are therefore set so as to set a non-zero sample gas supply flow rate and further increase the drift gas supply flow rate so that the total gas flow rate at the local ionization source is 50 cm / s or higher. Figure 5 The corresponding flow distribution of the total gas flow in the second operating mode is shown in the cross section of the ion migration spectrometer 100 around the local ionization source 24 in the middle, wherein the flow velocity of the total gas flow is about 70cm / s and is realized in the region of the local ionization source 24. Therefore, the NOx compounds are advantageously discharged through the gas outlet 23. At the same time, the supply voltage of the dielectric barrier discharge source as the local ionization source 24 is set to be lower than the predetermined limit voltage, so that the NOx ions formed at the local ionization source 24 are significantly less. Therefore, in the second operating mode, it is advantageous that almost no NOx ions enter the drift chamber 10, and the drift time spectrum of Figure 6 (A) is thus determined. Together with the drift time of the NOx ions determined in the drift time spectrum similar to Figure 6 (B) in the first operating mode, the ion mobility of the sample material is then determined based on the drift time ratio of the sample material and the NOx ions and the previously known ion mobility K0 (T) of the NOx ions. Preferably, the ion migration spectrometer 100 is purged between the first and second operating modes to discharge the formed NOx ions as completely as possible.

[0081] Reference numerals list

[0082] 1 Axial

[0083] 2 Radial

[0084] 10 Drift Room

[0085] 11 Switching gate

[0086] 12 Ion detector

[0087] 13 Drift Air

[0088] 14 Drift gas inlet

[0089] 20 Reaction Chamber

[0090] 21 Sample gas inlet

[0091] 22 Sample gas

[0092] 23 Gas outlet

[0093] 24 Local ionization source

[0094] 25 Gas inlet

[0095] 251 First gas inlet

[0096] 252 Second gas inlet

[0097] 26 Inflow Ring

[0098] 27 Sample gas supply line

[0099] 28 Flow Channel

[0100] 281 First Flow Channel

[0101] 282 Second flow channel

[0102] 29 Flow Obstructions

[0103] 30 Potential Source

[0104] 31 Field support ring

[0105] 32 Reference potential

[0106] 40 Radiation source (LED)

[0107] 51 First IMS peak

[0108] 52 Second IMS peak

[0109] 53 Third IMS peak

[0110] 100 Ion Mobility Spectrometer

Claims

1. An ion mobility spectrometer (100) for determining the ion mobility of a sample, the ion mobility spectrometer comprising: A cylindrical drift chamber (10) designed to transport ions from a switch grid (11) to an ion detector (12) against an axial drift gas flow (13); A cylindrical reaction chamber (20) is adjacent to the drift chamber in the axial direction (1) and has a sample gas inlet (21) adjacent to the switch grid (11) for introducing a sample gas (22), a gas outlet (23) opposite to the switch grid (11) for discharging drift gas and sample gas (22), and a local ionization source (24) arranged at the gas outlet (23). The sample gas inlet (21) has an opposite gas inlet (25) on the inner circumference of the reaction chamber (20).

2. The ion mobility spectrometer (100) according to claim 1, characterized in that: The drift chamber (10) and the reaction chamber (20) are designed to be cylindrical, and the sample gas (22) is supplied via a gas inlet (25) with a radial flow component.

3. The ion mobility spectrometer (100) according to claim 2, wherein: The radial components of the flow velocities of the partial flows of the sample gas (22) introduced via opposite gas inlets (25) compensate for one another.

4. The ion mobility spectrometer (100) according to any one of the preceding claims, wherein: The sample gas inlet (21) comprises an inflow ring (26) extending circumferentially around the reaction chamber (20) and having a flow channel (28) connecting at least one sample gas supply line (27) with at least one gas inlet (25).

5. The ion mobility spectrometer (100) according to claim 4, wherein: The sample gas inlet (21) includes gas inlets (25) arranged in pairs opposite to each other on the inner circumference of the reaction chamber (20), wherein each pair of oppositely disposed gas inlets (25) has a first gas inlet (251) and a second gas inlet (252), and the sample gas supply line (27) is connected to at least one of the first gas inlets (251) via a first flow channel (281) and is connected to at least one of the second gas inlets (252) via a second flow channel (282).

6. The ion mobility spectrometer (100) according to claim 4, wherein: A flow obstacle (29) is arranged in at least one flow channel (28) for deflecting the sample gas flow through the gas inlet (25).

7. The ion mobility spectrometer (100) according to claim 4, wherein: At least one flow channel (281, 282) has a plurality of gas inlets (251, 252).

8. The ion mobility spectrometer (100) according to claim 7, wherein: A flow obstacle (29) for deflecting the sample gas flow is arranged between every two gas inlets (25).

9. The ion mobility spectrometer (100) according to claim 8, wherein: The flow obstacle (29) is designed to equalize the partial flow of the sample gas flowing out through the gas inlet (25).

10. The ion mobility spectrometer (100) according to claim 1, wherein: The gas outlet (23) has a smaller flow cross section than the reaction chamber (20) and / or the drift chamber (10).

11. The ion mobility spectrometer (100) according to claim 1, wherein: The local ionization source (24) is configured for dielectric barrier discharge, and / or the ion mobility spectrometer comprises a radiation source (40) for irradiating the local ionization source (24) with light having a wavelength of 280 nm to 480 nm.

12. The ion mobility spectrometer (100) according to claim 1, further comprising at least one potential source (30) arranged in the reaction chamber (20), the potential source being used to generate an electric field for transporting ions formed at the local ionization source (24) toward a drift chamber (10).

13. The ion mobility spectrometer (100) according to claim 1, further comprising a drift gas inlet (14) arranged in the area of ​​the ion detector (12) and a drift gas supply device for controllably supplying drift gas and a sample gas supply device for controllably supplying sample gas.

14. A method for operating an ion mobility spectrometer (100) for determining the ion mobility of a sample, comprising the following method steps: Setting first operating parameters of a local ionization source (24) of an ion mobility spectrometer, a drift gas supply device for supplying air as a drift gas, and / or a sample gas supply device so as to generate NOx ions at the local ionization source (24) and introduce the generated NOx ions into a drift chamber (10) of the ion mobility spectrometer (100); The first drift time t of NOx ions is measured at the drift chamber temperature T. NOx ; Setting second operating parameters of the local ionization source, the drift gas supply device and / or the sample gas supply device so as to generate sample ions without forming NOx ions at the local ionization source (24) and / or avoid introducing NOx ions into the drift chamber (10); The second drift time t of the sample ion is measured at the drift chamber temperature T. P ; Using the measured first drift time t NOx , the measured second drift time t P and the normalized ion mobility K0 of NOx ions as reference R (T) According to K0 P (T) = t NOx / t P *K0 R (T) Determine the normalized ion mobility K0 of the sample P (T).

15. The method according to claim 14, wherein: The first operating parameter sets the flow rate of the sample gas supply device and / or the flow rate of the drift gas supply device so as to limit the flow rate at the local ionization source to less than 50 cm / s and / or to set the supply voltage of the dielectric barrier discharge source as the local ionization source (24) to be higher than a predetermined limit voltage, and wherein the second operating parameter sets the flow rate of the sample gas supply device and / or the flow rate of the drift gas supply device so as to set the flow rate at the local ionization source (24) to more than 50 cm / s and / or to set the supply voltage of the dielectric barrier discharge source as the local ionization source (24) to be lower than a predetermined limit voltage.

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