Ionic mobility spectrometer

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to an ion mobility spectrometer, particularly to a method of generating ions and sampling ion populations at different intervals as the ion-molecular reaction transitions to an equilibrium state.

【0002】

[Conventional technology]

Ionic conductivity spectrometers have long been used to determine if there are molecules of interest in the gas stream. Conventional ion mobility spectrometers have been performed by taking samples and testing for the presence or absence of molecules of interest. Some prior art ion mobility spectrometers take a sample by placing a woven or non-woven trap over the entire surface to be examined for the presence or absence of molecules of interest. Other prior art ion mobility spectrometers either generate a gas stream near the surface to be investigated for the molecule of interest or utilize an existing gas stream. The sample is carried on an inert gas stream to an ionization chamber. In these prior art ion mobility spectrometers, the sample is exposed to radioactive material in an ionization chamber. nickel⁶³Alternatively, a radioactive substance such as tritium causes β particles to collide with the flow of the sample to generate ions.

【0003】

In addition, prior art ion mobility spectrometers have a drift chamber provided near the ionization chamber. To put it simply, the drift chamber is provided with a plurality of field forming electrodes and one collecting electrode at the end opposite to the ionization chamber. The ions generated in the ionization chamber can move toward the collection electrode through the drift chamber. The collection electrode detects and analyzes the spectrum of the collected ions and generates appropriate instructions or indications when the molecule of interest is detected.

【0004】

Ionic conductivity spectrometers have many uses, including security applications used to search for and identify explosives, narcotics and other contraband. Examples of ion mobility spectrometers are those described in US Pat. No. 3,699,333 and US Pat. No. 5,027,643.

【0005】

An improved form of the above early ion mobility spectrometer is the ion mobility spectrometer developed by Ion Track Instruments, Inc., the Ion Trap Mobility Spectrometer (ITMS). It is called. This ion trap mobility spectrometer can obtain higher sensitivity and reliability than the conventional ion mobility spectrometer described above. An example of an ion trap mobility spectrometer is Anthony. It is described in US Pat. No. 5,200,614 issued to Jenkins). This prior art ion trap mobility spectrometer has achieved improved operation by improving the ionization efficiency in the reactor and the ion transport efficiency from the reactor to the collection electrode. More specifically, the ionization chamber of the ion trap mobility spectrometer is an electric field region in which the action of β particles on the carrier gas causes the accumulation of both electron and cation ion populations. The resulting high density ions have a very high ionization probability of the molecule of interest, thus resulting in a very high ionization efficiency.

【0006】

U.S. Pat. No. 5,491,337 discloses a further improved form of the ion trap mobility spectrometer. More specifically, U.S. Pat. No. 5,491,337 discloses an ion trap mobility spectrometer with higher efficiency that detects the presence of alkaloids such as narcotics.

【0007】

[Problems to be Solved by the Invention]

Despite the operational efficiencies described in the patents cited above, there is still a need for further improvements that allow higher resolution or selectivity of the spectrometer while at the same time reducing costs. In some countries, there are legal regulatory barriers to the use of radioactive materials, which hinder the use of devices with built-in radiation sources in portable applications.

【0008】

Recently, an attempt to provide an electronic ionization means is described in UK Patent Application No. 98164452. However, in this proposal, the ionic reactions are not designed to be carried out in an electric field-free state, nor are they detected during the transition to an equilibrium state. As a result, the method of the UK patent application is less sensitive and selective than the method described in the present application.

【0009】

[Means for solving problems]

The present invention is in an ion trap mobility spectrometer that uses an ion source produced by a high voltage electron pulse instead of a radioactive ion source. Ions are formed periodically in the reaction chamber to make their population as large as possible and to be thermally neutronized in a no-electric field environment, and then to react with the molecular species in the gas phase of the reaction chamber. Shortly thereafter, the ions are pulsed to the drift section of the ion trap mobility spectrometer, such as the drift section of the ion trap mobility spectrometer disclosed in US Pat. No. 5,200,614. The reaction period can be varied to sample the ion population at different intervals. This makes it possible to observe the ion-molecular reaction as the ion population approaches an equilibrium state. After that, the ion population of the molecule changes at different points when approaching the equilibrium state, so the observation results can be analyzed to know the difference between the reaction species. As a result, target identification, identification behavior, and performance are improved.

【0010】

BEST MODE FOR CARRYING OUT THE INVENTION

In FIG. 1, the ion trap mobility spectrometer (ITMS) of the present invention is represented by reference numeral 10 as a whole. The ITMS10 has a cylindrical detector 12 at one end thereof with a gas inlet 14 for receiving the sample air of interest. The sample air of interest can be transported by carrier gas.

【0011】

This carrier gas is typically clean dry air containing low concentrations of dopant substances such as ammonia, nicotinamide or other similar dopants disclosed in US Pat. No. 5,491,337. Vapor samples generated from the target material are carried on this carrier gas stream to detector 10 from a suitable inlet system such as the system described in US Pat. No. 5,491,337.

【0012】

The gas flow from the inlet 14 enters the reaction chamber 16. More specifically, the reaction chamber 16 consists of a hollow metal cylindrical cup 18 with an inlet 14 at one end. Two pin electrodes 20 and 22 are projected inward in the radial direction in the reaction chamber. The pin electrodes 20 and 22 are insulated so as to prevent discharge from a place other than the points inside the radial inside of the electrodes 20 and 22. At the end of the reaction chamber 16 opposite the inlet 14, the lattice electrode E₁Is provided. Lattice electrode E₁Is normally maintained at the same potential as the inlet side end and wall of the reaction chamber 16. Ion generation in the reaction chamber 16 will be described in more detail below. The carrier gas passes through the reaction chamber 16, evacuates around the metal cylindrical cup 18, and exits the detector through the gas outlet 24.

【0013】

The detector 10 has a grid electrode E₁A drift portion 26 is formed on the downstream side of the above. The drift portion 26 has a plurality of annular electrodes E.₂-E_NIt has. In the detector 10, clean drift gas flows through the drift portion 26 in the direction indicated by the arrow D in FIG. The drift gas merges with the carrier gas at the point where the carrier gas exits the reactor chamber 16, and exits the detector from the drift gas and carrier gas co-outlet 24.

【0014】

For most of the time, metal cylindrical cup 18, both pins 20, 22 and grid E₁The above potentials are the same, so that the reaction chamber 16 is considered an electric field-free space. However, a high voltage pulse is periodically applied between the two pin electrodes 20 and 22. To that end, the carrier gas is ionized by positive and negative corona discharges within the portion of the reaction chamber 16 between the two pin electrodes 20 and 22. In a negative DC corona, the cathode pin 20 emits electrons, which are accelerated by a very high electric field near the tip of the pin 20. As a result, secondary ions are formed by the collision of carrier gas molecules. This secondary ionization process produces mostly nitrogen cations and more electrons. The cations are pulled back to the cathode pin 20 where they emit more electrons, resulting in a sustained discharge. On the other hand, the electrons move from the pin 20 having a lower electric field strength to a part of a certain distance. Those electrons do not cause further ionization of the carrier gas. In addition, the electrons travel across the reaction chamber to the other pin electrode 22. These electrons carry much more energy than thermal energy, so most substances do not react with them to form anions. However, one notable exception is oxygen. Oxygen forms negative oxygen ions by capturing low-temperature electrons.

【0015】

The main disadvantage when considering using a single corona as an ion source for an ion mobility spectrometer is that the charge transfer process is impeded at high energies. Another disadvantage is that most of the cations are located in a small volume surrounding the tip of the cathode pin 20, so less cations are used for ion interaction than anions. However, with the detector 10 shown in FIG. 1 described above, almost the same number of cations and anions can be obtained. The ions in this pseudo-neutral plasma can react with thermal energy, thus achieving all the advantages of the ion trap mobility spectrometer described in US Pat. No. 5,200,614. This is achieved by applying a narrow high frequency high voltage pulse between the two electrodes 20 and 22. The frequency is usually above 1 MHz so that the field (electric field) can shrink very rapidly before many electrons or cations are collected on the corresponding electrodes 20 and 22, respectively. The plasma between the pins increases during the pulse period. When the pulse is switched off, the ions rapidly become thermal neutrons and react with the molecular species present in the reaction chamber 16. All charge transfer processes proceed in the direction of formation of the molecular ions with the highest charge affinity. Depending on the molecular concentration, charge can be transferred from one molecular species to another with higher charge affinity. U.S. Pat. No. 5,494,337 describes one method of modifying this process with a dopant vapor having an intermediate charge affinity between many interfering compounds and the target compound of interest, such as ammonia or nicotinamide. Has been done. Dopant vapors attract and retain charge in the presence of interfering molecules with weak charge affinity. However, the dopant vapor transfers charges to the target molecules as they appear in the reaction chamber 16. This reduces the number of different ion types present in the reaction chamber, resulting in less misidentification by the detector 10.

【0016】

The discharge pulse in the detector 10 shown in FIG. 1 is kept on for enough time to generate enough charge to ensure efficient ionization of the target molecule. Usually, the duration of the discharge pulse is several hundred microseconds, which is shorter than the time it takes for the ions to travel to the corresponding electrode. The pulse frequency is preferably 1 MHz or higher so that the required pin voltage reduction is achieved.

【0017】

After the discharge is switched off, the concentrations of the positive and negative charges are approximately equal so that little or no space charge is generated in the reactor, resulting in the maintenance of an electric field-free space. become. As a result, all charges can reach thermal equilibrium quickly (<1 ms), facilitating the optimal charge transfer process at this thermal equilibrium point. The molecule with the highest charge affinity ultimately captures the charge from all other ionic species. Given that these high-charge-affinity molecules have a concentration of only a few ppt in the reaction chamber 16, there is only 10 interactions (collisions) that transfer charge from some specific ion with lower charge affinity to the target molecule.¹²Only once in a time. At atmospheric pressure and at the temperature of detector 10, molecules are typically about 10 per second.⁸React (collision) with the frequency of times. In the reaction chamber 16, an ion concentration is created that ensures equilibrium ionization is achieved within a few milliseconds. Before reaching this equilibrium point, many ionic species associated with the target material can be observed. For example, a sample of cocaine vapor introduced into the detector from a suspicious parcel sampling may contain drug bulking agents and other alkaloids. Alkaloids other than these bulking agents and cocaine may be present in higher concentrations, but the positive charge affinity of cocaine is so high that in equilibrium all charges are cocaine-ionized, and those bulking agents and others. Alkaloids will not be detected. Similarly, in anion mode, the mixture of explosives may not be completely identified and identified because the stronger electron-negative species predominate. However, the low charge affinity compounds can be ionized and detected before the end point equilibrium is reached. In the configuration of the present invention, plasma grams (plasma aspect diagram) are obtained at various different time intervals after injection of ionic charges into the reaction chamber.

【0018】

According to the above method, in which ion populations are sampled at different time points after the discharge pulse is switched off, non-equilibrium ionization can be observed and used as another criterion for molecular species separation. Becomes possible. The change in delay time between the discharge pulse and the sampling of ions in the reaction chamber 16 allows the charge transfer process to be investigated and used to more accurately identify and identify the target material. It is a discharge pulse and a grid electrode E from a metal cylindrical cup 18.₁This is achieved by controlling and varying the time between the time point at which the high electric field is applied over the entire reaction chamber 16. This high electric field is similar to the method described in US Pat. No. 5,200,614, with just the majority of the ions on the electrode E.₁It is held throughout the reactor for a period of time sufficient to be released through the drift section of the detector. Ions are ring-shaped electrodes E₂, E₃, ..., E_NIt moves through the drift section 26 under the action of the electric field formed by. Ions pass through the guard grid 28 and are collected by the collection electrode 30. Different ionic species move longitudinally along the drift section 26 at different velocities depending on the molecular and shape. The ionic species migrate as a cluster and reach the collection electrode 30 with a Gaussian concentration distribution. This causes a current peak in the signal output. Each ion cluster is identified by amplifying the output signal and measuring the drift time.

【0019】

The two paired opposed corona discharge points in the reaction chamber 16 of the ITMS10, namely the pin electrodes 20 and 22, are driven by high voltages supplied from the two paths as shown in FIG. Normally, the high-voltage power supply 32, the high-voltage switch circuit 34, and the high-voltage regulator 36 have pin electrodes 20 and 22 as other parts of the wall of the reaction chamber 16 and the first lattice electrode E.₁Operates to keep the same high voltage (eg 1000 volts) as. This is a high resistance resistor R₁And R₂Achieved by. In the high-voltage switch circuit, the ions are emitted from the reaction chamber to the first lattice electrode E of the detector, similar to the ITMS of the prior art.₁It is configured to supply high voltage kickout pulses from time to time to drive through and further through the drift region.

【0020】

At the end of the drift period, the action of high-frequency high-pressure pulses applied to each of the two paired opposed corona pin electrodes 20 and 22 creates ions in the reaction chamber from these pin electrodes 20 and 22. The average voltage of the corona pin electrodes 20 and 22 is the high resistance resistor R₁And R₂The voltage level of the reaction chamber 16 around these electrodes is maintained through. Further, the pin electrodes 20 and 22 have a high voltage transformer T.₁From small capacity capacitor C₁And C₂High voltage (> 1MHz) is supplied via. In addition, transformer T₁Is the gate control oscillator O₁Power is supplied from. The relative polarity between the pin electrodes 20 and 22 is reversed before enough time has passed for most of the ions to reach the pin electrodes 20 and 22 and discharge, so that the plasma between the pin electrodes 20 and 22 Ions of both polarities are formed in it, and the ion population grows without discharge at the pin 20 electrode and 22 itself. Ion density increases for hundreds of microseconds, then one-shot pulse generator G₁Gate control oscillator O by the operation of₁Switches off. At this point, the voltage at the pin electrode returns to the same voltage as the wall of the reaction chamber 16. The cation population and the anion population are approximately equal, and these ion populations diffuse outward from the plasma region toward the other parts of the reaction chamber 16 where they interact with the molecule of interest.

【0021】

The variable delay circuit 38 times out after a variable period of time between tens of microseconds and milliseconds, after which the one-shot pulse generator G₁Again, the voltage of the reaction chamber 16 and the pin electrodes 20 and 22 is applied to the lattice electrode E.₁Raise to a higher voltage. This causes ions to be released from the reaction chamber 16 to the drift section 26, and the above process is restarted.

【0022】

Although the present invention has been described above in the context of one embodiment, it is possible to make various modifications and modifications without departing from the scope of the invention defined by the claims. It's obvious.

【0023】

[Effect of the invention]

According to the ion and lap mobility spectrometer of the present invention, it is possible to identify and identify the substance of interest with superior sensitivity and selectivity as compared with the prior art, and it is safe because no radioactive substance is used. And can be used for carrying.

[Simple explanation of drawings]

[Figure 1]

It is schematic cross-sectional view of the ion trap mobility spectrometer (ITMS) of this invention.

[Figure 2]

It is a block circuit diagram of the circuit for driving the electrode of ITMS shown in FIG.

[Explanation of symbols]

10 Ion trap mobility spectrometer 12 detector 14 entrance 16 Reaction chamber 18 Metal Cylindrical Cup 20, 22 pin electrodes 24 gas outlet 26 Drift section 30 Collection electrode E₁, E₂, ..., E_N Lattice electrode 32 High voltage power supply 34 High voltage switch circuit 36 High pressure regulator 38 Delay circuit G₁ Pulse generator O₁ Gate control oscillator T₁ Transformer