JP7807969B2 - IMS analysis device and IMS analysis method - Google Patents

Description

The present invention relates to an IMS analysis device and an IMS analysis method.

Conventional IMS analyzers ionize sample gases using radiation, corona discharge, or the like. Because radiation and corona discharge have high energy, the sample gas may be chemically decomposed during ionization. In this case, the detector detects many ions generated by the decomposition of the sample gas, resulting in many peaks (detection peaks) appearing in the IMS spectrum. This makes it difficult to identify the sample gas in IMS analysis.
An IMS analyzer is known that ionizes a sample gas using low-energy electrons emitted from an electron-emitting element (see, for example, Patent Document 1). This analyzer can suppress chemical decomposition of the sample gas, making it easier to identify the sample gas. This IMS analyzer uses air as a carrier gas, and peaks corresponding to ions (primary ions) generated from the air appear in the IMS spectrum.

JP 2019-186190 A

In conventional IMS analyzers, IMS spectra may be measured in which peaks corresponding to ions generated from the target component overlap peaks corresponding to primary ions, making it difficult to identify the target component.
The present invention has been made in consideration of the above circumstances, and provides an IMS analysis device that can identify a detection target component even when a peak corresponding to ions generated from the detection target component overlaps with a peak corresponding to a primary ion.

The present invention provides an IMS analysis apparatus comprising an ion source, a collector, an electrostatic gate electrode, an electric field forming electrode, and a control unit, wherein the control unit is configured to control the potential of the electric field forming electrode so as to form a potential gradient in a drift region between the electrostatic gate electrode and the collector, the collector and the control unit are configured to measure the current waveform of a current that flows when ions reach the collector, and the control unit is configured to determine whether a second peak overlaps a first peak in the current waveform, and, if it is determined that a second peak exists, to change the potential of the electric field forming electrode so as to reduce the slope of the potential gradient and measure the current waveform.

Measuring the current waveform (IMS spectrum) by reducing the gradient of the potential gradient in the drift region increases the time it takes for ions to travel from the electrostatic gate electrode to the collector, improving the ion separation characteristics of the IMS analyzer. Therefore, if it is determined that a second peak overlaps with a first peak in the IMS spectrum obtained in the first measurement, the second peak can be separated from the first peak in the IMS spectrum obtained in the second measurement, making it possible to identify and quantify the target component using the second peak. This makes it possible to identify target components that were previously impossible to identify.

1 is a schematic cross-sectional view of an IMS analyzer according to one embodiment of the present invention. 1 is a flowchart of a method for controlling an IMS analyzer according to one embodiment of the present invention. 1 is a graph showing changes in IMS spectra.

The IMS analyzer of the present invention comprises an ion source, a collector, an electrostatic gate electrode, an electric field forming electrode, and a controller, wherein the controller is configured to control the potential of the electric field forming electrode so as to form a potential gradient in a drift region between the electrostatic gate electrode and the collector, the collector and the controller are configured to measure a current waveform of a current that flows when ions reach the collector, and the controller is configured to determine whether or not a second peak overlapping a first peak exists in the current waveform, and, if it is determined that a second peak exists, to change the potential of the electric field forming electrode so as to reduce the slope of the potential gradient, and measure the current waveform.
IMS is an abbreviation for Ion Mobility Spectrometry.
When the first peak corresponds to an ion generated from a component contained in the carrier gas, the second peak may correspond to an ion generated from a component contained in the sample gas. When a plurality of ions are generated from the component contained in the sample gas and there are peaks corresponding to these ions, both the first peak and the second peak may correspond to ions generated from the component contained in the sample gas.

Preferably, the IMS analyzer of the present invention further includes a sample gas injector, wherein the collector, the electrostatic gate electrode, and the electric field generating electrode are housed in an analysis chamber. The sample gas injector is configured to inject a mixed gas of a sample gas and a carrier gas or a carrier gas not containing the sample gas into the analysis chamber. The ion source is configured to directly or indirectly ionize components contained in the sample gas or components contained in the carrier gas. The first peak corresponds to ions generated from the components contained in the carrier gas, and the second peak corresponds to ions generated from the components contained in the sample gas. The control unit is configured to determine whether a second peak exists by comparing the current waveform measured when the carrier gas not containing the sample gas is being injected into the analysis chamber with the current waveform measured when the mixed gas is being injected into the analysis chamber. This allows for accurate determination of whether a second peak overlaps with the first peak exists.

Preferably, the ion source is an electron-emitting element, the electron-emitting element having a lower electrode, a surface electrode, and an intermediate layer arranged between the lower electrode and the surface electrode, the control unit is configured to apply a voltage between the lower electrode and the surface electrode, and the carrier gas is air containing moisture.
Preferably, the control unit and the sample gas injector are configured to adjust the voltage applied between the lower electrode and the surface electrode so that the peak height or peak area of the first peak approaches a target value when a carrier gas not containing the sample gas is injected into the analysis chamber, and are configured to inject the mixed gas into the analysis chamber and measure the current waveform after adjusting the voltage applied between the lower electrode and the surface electrode. This makes it possible to stabilize the amount of primary ions produced and quantitatively measure the components to be detected.

Preferably, the control unit is configured to determine whether a minute peak is present in the current waveform measured while the mixed gas is being injected into the analysis chamber, and if it determines that a minute peak is not present, to increase the voltage applied between the lower electrode and the surface electrode. This allows the detection sensitivity of the IMS analysis device to be increased as needed, making it possible to identify components that could not be identified with conventional IMS devices.

The present invention includes the steps of: measuring a current waveform of a current that flows when ions reach a collector while a mixed gas of a sample gas and a carrier gas is being injected into an analysis chamber;
The present invention also provides an IMS analysis method, which includes the steps of: determining whether or not a second peak overlapping a first peak exists in the current waveform; and, if it is determined that a second peak overlapping the first peak exists, changing the potential of the electric field forming electrode so that the gradient of the potential gradient in the drift region between the electrostatic gate electrode and the collector becomes smaller, and measuring the current waveform of the current that flows when ions reach the collector.
In the IMS analysis method of the present invention, it is preferable to further include a step of measuring the current waveform of the current that flows when ions reach the collector when a carrier gas that does not contain a sample gas is being injected into the analysis chamber, and to determine whether or not a second peak overlaps with a first peak by comparing the current waveform measured when the carrier gas that does not contain the sample gas is being injected into the analysis chamber with the current waveform measured when the mixed gas is being injected into the analysis chamber.

An embodiment of the present invention will be described below using the drawings. The configurations shown in the drawings and the following description are merely examples, and the scope of the present invention is not limited to those shown in the drawings or the following description.

FIG. 1 is a schematic cross-sectional view of the IMS analysis device of this embodiment.
The IMS analyzer 40 of this embodiment includes an ion source 1, a collector 6, an electrostatic gate electrode 8, electric field forming electrodes 9 a to 9 h, and a control unit 12. The control unit 12 is configured to control the potentials of the electric field forming electrodes 9 a to 9 h so as to form a potential gradient in a drift region 11 between the electrostatic gate electrode 8 and the collector 6. The collector 6 and the control unit 12 are configured to measure a current waveform of a current that flows when ions reach the collector 6. The control unit 12 is configured to determine whether or not a second peak overlapping a first peak exists in the current waveform, and, if it is determined that a second peak exists, to change the potentials of the electric field forming electrodes 9 a to 9 h so as to reduce the slope of the potential gradient, and measure the current waveform.

The IMS analysis device 40 of this embodiment is a device that ionizes a sample gas and analyzes its mobility. The analysis device 40 may be a drift tube type IMS analysis device that performs analysis using a drift tube type IMS.
The sample gas analyzed by the IMS analyzer 40 may be a gas sample or a vaporized liquid sample.

The control unit 12 is a part that controls the IMS analyzer 40. The control unit 12 may include, for example, a microcontroller having a CPU, memory, timer, input/output ports, etc. The control unit 12 may also include an electric field control unit 26, a gate control unit 27, an element voltage control unit 17, a recovery current measurement unit 19, a power supply unit, etc.

The ion source 1 is a component configured to ionize components contained in the sample gas or the carrier gas, and may be, for example, an electron emitter 2, a corona discharge electrode, or a radiation source. The following description focuses on an IMS analyzer 40 that uses an electron emitter 2 as the ion source 1.

The IMS analyzer 40 of this embodiment has an analysis chamber 30 that analyzes the target components contained in the sample gas. The analysis chamber 30 has an ionization region 10 between the electron emitter 2 and the collector 6 for ionizing the target components contained in the sample gas to generate ions (negative ions or positive ions), and a drift region 11 (ion migration region) for moving and separating the ions. An electrostatic gate electrode 8 is provided between the ionization region 10 and the drift region 11. An electron emitter 2 is disposed on the side of the ionization region 10 opposite the electrostatic gate electrode 8, with the surface electrode 4 facing the ionization region. A collector 6 is disposed on the side of the drift region 11 opposite the electrostatic gate electrode 8. The electron emitter 2, collector 6, electric field forming electrodes 9a-9h, electrostatic gate electrode 8, etc. are housed in the analysis chamber 30.

The sample gas injector 16 is a part that injects a sample gas or a carrier gas into the analysis chamber 30. The sample gas injector 16 may also be configured to inject a mixed gas of a sample gas and a carrier gas, or a carrier gas that does not contain the sample gas, into the analysis chamber 30. For example, in calibration before measuring the sample gas, the sample gas injector 16 can inject a carrier gas that does not contain the sample gas into the analysis chamber 30. When measuring the sample gas, the sample gas injector 16 can inject a mixed gas of a sample gas and a carrier gas into the analysis chamber 30. The flow rate of the gas supplied to the analysis chamber 30 by the sample gas injector 16 is, for example, 200 mL/min.
The components to be detected contained in the sample gas are analyzed by ion mobility spectrometry. When the sample is a gas, the sample gas injector 16 can be configured to continuously supply the sample gas to the analysis chamber 30. When the sample is a liquid, the sample gas injector 16 can have a vaporization chamber, and the sample gas vaporized in the vaporization chamber can be injected into the analysis chamber 30.
The carrier gas is a gas, such as air containing moisture, that is injected into the analysis chamber 30 together with the sample gas. Components contained in the carrier gas are ionized by the electrons emitted from the electron emitter 2 to generate primary ions.

The drift gas injector 15 is a component configured to inject drift gas into the analysis chamber 30. The drift gas flows in the drift region 11 in the opposite direction to the ion movement direction (from the electrostatic gate electrode 8 to the collector 6) and acts as a resistance to the ions as they move through the drift region 11. The drift gas may be purified atmospheric air (clean air), air supplied from a compressed air cylinder, or purified air exhausted from the analysis chamber 30 by the exhaust unit 20. The flow rate of the drift gas supplied by the drift gas injector 15 to the analysis chamber 30 can be greater than the flow rate of the gas supplied by the sample gas injector 16 to the analysis chamber 30. The flow rate of the drift gas is, for example, 500 mL/min.

The exhaust unit 20 is a part provided to exhaust gas from the analysis chamber 30. The exhaust unit 20 is provided to exhaust drift gas and sample gas from the analysis chamber 30. The exhaust unit 20 may be provided to forcibly exhaust gas from the analysis chamber 30 using an exhaust fan or the like, or may be provided to naturally exhaust gas from the analysis chamber 30.

The sample gas inlet 16 and exhaust 20 can be arranged so that the sample gas flows through the ionization region 10. This allows electrons emitted from the surface electrode 4 of the electron emitter 2 in the ionization region 10 to directly or indirectly ionize components contained in the sample gas, generating negative or positive ions.

The drift gas injection unit 15 and exhaust unit 20 are configured to allow the drift gas to flow from the collector side toward the electrostatic gate electrode side in the drift region 11. For example, the drift gas injection unit 15 can be configured to supply the drift gas to the drift region 11 from the collector side, and the exhaust unit 20 can be configured to exhaust the drift gas from an opening (gas outlet) in the housing 28 around the ionization region 10.

The electron emitter element 2 is an element configured to emit electrons from the surface electrode 4, and is an element that uses the emitted electrons to directly or indirectly ionize the components to be detected contained in the sample gas, thereby generating negative ions or positive ions.
The electron-emitting element 2 has a lower electrode 3 , a surface electrode 4 , and an intermediate layer 5 disposed between the lower electrode 3 and the surface electrode 4 .

The surface electrode 4 is an electrode located on the surface of the electron-emitting element 2. The surface electrode 4 preferably has a thickness of 10 nm or more and 100 nm or less. The surface electrode 4 is made of, for example, gold or platinum. The surface electrode 4 may also be made of multiple metal layers.
Even if the surface electrode 4 has a thickness of 40 nm or more, it may have a plurality of openings, gaps, or thinned portions with a thickness of 10 nm or less. Electrons that flow through the intermediate layer 5 can pass through or penetrate these openings, gaps, or thinned portions, and can be emitted from the surface electrode 4. Such openings, gaps, or thinned portions can also be formed by applying a device voltage between the lower electrode 3 and the surface electrode 4.

The lower electrode 3 is an electrode that faces the surface electrode 4 via the intermediate layer 5. The lower electrode 3 may be a metal plate, or a metal or conductive layer formed on an insulating substrate or film. Furthermore, when the lower electrode 3 is made of a metal plate, this metal plate may also be the substrate of the electron-emitting element 2. The material of the lower electrode 3 is, for example, aluminum, stainless steel, or nickel. The thickness of the lower electrode 3 is, for example, 200 μm or more and 1 mm or less.

The intermediate layer 5 is a layer in which electrons flow due to an electric field formed by applying an element voltage between the surface electrode 4 and the lower electrode 3. The intermediate layer 5 may be semiconductive. The intermediate layer 5 may contain at least one of insulating resin, insulating fine particles, and metal oxide. It is also preferable that the intermediate layer 5 contain conductive fine particles. The thickness of the intermediate layer 5 may be, for example, 0.5 μm or more and 1.8 μm or less. The intermediate layer 5 is, for example, a silicone resin layer containing dispersed silver fine particles.

The electron emitter 2 may have an insulating layer 29 between the surface electrode 4 and the lower electrode 3. This insulating layer 29 may have an opening. The opening in the insulating layer 29 is provided to define the electron emission region of the surface electrode 4. Because electrons cannot flow through the insulating layer 29, electrons flow into the intermediate layer 5 corresponding to the opening in the insulating layer 29 and are emitted from the surface electrode 4. Therefore, by providing the insulating layer 29 with an opening, the electron emission region formed in the surface electrode 4 is defined. The electron emission region can be, for example, a 5 mm square area, and can be freely designed to match the openings in the electric field forming electrodes 9a-9h and the size of the collector 6.

The surface electrode 4 and the lower electrode 3 can be electrically connected to the control unit 12 (element voltage control unit 17).
The device voltage control unit 17 is provided to control the magnitude of the device voltage (driving voltage of the electron-emitting device 2) applied between the surface electrode 4 and the lower electrode 3. When the potential of the lower electrode 3 is made substantially the same as the potential of the surface electrode 4 using the device voltage control unit 17 (the device voltage is made 0 V), no current flows through the intermediate layer 5 and no electrons are emitted from the electron-emitting device 2.

When an element voltage (driving voltage) is applied between the lower electrode 3 and the surface electrode 4 using the element voltage control unit 17 so that the potential of the lower electrode 3 is lower than the potential of the surface electrode 4, a current flows through the intermediate layer 5, and electrons that have flowed through the intermediate layer 5 pass through the surface electrode 4 and are emitted into the ionization region 10. The element voltage applied between the lower electrode 3 and the surface electrode 4 to emit electrons from the electron-emitting element 2 can be, for example, 5 V or more and 40 V or less, and preferably 10 V or more and 18 V or less.
Adjusting the magnitude of the device voltage using the device voltage control unit 17 changes the current flowing through the intermediate layer 5, thereby changing the amount of electrons emitted from the electron-emitting device 2. Therefore, increasing the device voltage increases the amount of ions generated in the ionization region 10, thereby increasing the peak intensity of the peaks appearing in the IMS spectrum measured using the recovery current measurement unit 19 (increasing detection sensitivity). However, increasing the device voltage also increases the half-width of the peaks appearing in the IMS spectrum. This is thought to be because increasing the device voltage increases the amount of ions passing through the electrostatic gate electrode 8, thereby increasing the initial half-width immediately after passing through the gate. Furthermore, increasing the half-width of the peaks appearing in the IMS spectrum increases the likelihood of multiple peaks overlapping.
Furthermore, when the magnitude of the device voltage is adjusted, the energy of the electrons emitted from the electron-emitting device 2 also changes.

When a carrier gas (air containing moisture) not containing sample gas is injected into the analysis chamber 30 using the sample gas injection unit 16 and a drift gas (air) is injected into the analysis chamber 30 using the drift gas injection unit 15, when electrons are emitted from the electron emitter 2 into the ionization region 10, the electrons immediately collide with air components to form primary ions (negative ions or positive ions). When the electrons emitted from the electron emitter 2 attach to gas components near the surface electrode 4 (electron attachment phenomenon), negative ions of the gas components are generated. When the energy of the electrons emitted from the electron emitter 2 is higher than the ionization energy of the gas components near the surface electrode 4, positive ions of the gas components are generated. These generated primary ions move toward the electrostatic gate electrode 8 due to the electric field formed in the analysis chamber 30.
The primary ions are, for example, oxygen ions obtained by ionizing oxygen gas in the air. In this case, the ionization region 10 contains primary ions in an amount corresponding to the amount of electrons emitted by the electron-emitting element 2.
The amount of the primary ions can be adjusted by adjusting the device voltage applied between the surface electrode 4 and the lower electrode 3 (by adjusting the amount of electrons emitted from the electron-emitting device 2).

When a mixed gas of sample gas and carrier gas (air containing moisture) is injected into the analysis chamber 30 using the sample gas injector 16 and drift gas (air) is injected into the analysis chamber 30 using the drift gas injector 15, electrons are emitted from the electron emitter 2 into the ionization region 10. The electrons immediately collide with air components to generate primary ions (negative or positive ions). These primary ions are then transported toward the electrostatic gate electrode 8 by the electric field formed in the analysis chamber 30. Furthermore, these primary ions transfer charge to the target component contained in the sample gas in the ionization region 10, generating negative or positive ions of the target component (ion-molecule reaction). In other words, the electron emitter 2 can be used to indirectly generate negative or positive ions of the target component contained in the sample gas in the ionization region 10. At this time, the ionization region 10 contains both ions generated from the target component contained in the sample gas and primary ions.
Furthermore, when the emitted electrons from the electron emitter 2 directly collide with the detection target component, negative or positive ions of the detection target component contained in the sample gas can be directly generated.

The electric field forming unit 7 is a part for forming a potential gradient in the region between the electron emitter 2 and the collector 6. The electric field forming unit 7 is configured to form a potential gradient that causes ions to move from the electron emitter side to the collector side. When the IMS analyzer 40 detects negative ions (negative ion mode), the control unit 12 (electric field control unit 26) applies a voltage to the electric field forming unit 7 to form a potential gradient such that the potential on the electron emitter side is lower than the potential on the collector side. When the IMS analyzer 40 detects positive ions (positive ion mode), the control unit 12 (electric field control unit 26) applies a voltage to the electric field forming unit 7 to form a potential gradient such that the potential on the electron emitter side is higher than the potential on the collector side.

The electric field generating unit 7 may include multiple electric field generating electrodes 9a-9h. The electric field generating electrodes 9a-9h may have any shape as long as they can generate a potential gradient in the region between the electron emitter 2 and the collector 6. For example, they may be ring-shaped or arch-shaped. The multiple electric field generating electrodes 9a-9h are arranged in a row so that an ionization region 10 and a drift region 11 (ion migration region) are formed inside the ring or arch. The electric field generating electrodes 9a-9h constituting the electric field generating unit 7 are electrically connected to the electric field control unit 26 of the control unit 12. Alternatively, the grid electrode 25, the collector 6, the surface electrode 4 of the electron emitter 2, or the lower electrode 3 may function as the electric field generating unit 7.
The control unit 12 can form a potential gradient in the ionization region 10 and the drift region 11 by controlling the potentials of the electric field forming electrodes 9 a to 9 h , the surface electrode 4 of the electron emitter 2 , and the like.

Two adjacent electric field forming electrodes 9a-9h included in the electric field forming unit 7 can be electrically connected across a resistor. This allows a potential difference to be generated between two adjacent electric field forming electrodes 9a-9h, and by generating this potential difference between each electrode, a potential gradient can be formed in the region between the electron emitter 2 and the collector 6.

For example, in the analysis device 40 shown in FIG. 1 , the electric field generating unit 7 includes multiple electric field generating electrodes 9a-9h, and two adjacent electric field generating electrodes 9a-9h are electrically connected via a resistor. Furthermore, the electrode 9h closest to the collector 6 among the multiple electric field generating electrodes 9a-9h is electrically connected to a grid electrode 25 via a resistor. The grid electrode 25 is connected to ground, for example, via a resistor. The potential of the electrode 9a farthest from the collector 6 can be controlled by the control unit 12. For example, the control unit 12 can apply a drift voltage so that the potential of the electrode 9a is, for example, −3000 V. The grid electrode 25 is connected to ground via a resistor, so that the potential is near 0 V. Furthermore, because two adjacent electric field generating electrodes 9a-9h are electrically connected via a resistor, the potential of the multiple electric field generating electrodes 9a-9h arranged in a row increases in a stepwise manner as they approach the collector 6. Therefore, a potential gradient can be formed in the region (ionization region 10 and drift region 11) between the electron emitter 2 and the collector 6, in which the potential gradually increases as the electrode approaches the collector 6. However, this potential gradient changes in the vicinity of the electrostatic gate electrode 8 depending on the potential of the electrostatic gate electrode 8.
Furthermore, the control unit 12 can control the potential of the electron emitter 2 in consideration of the potential gradient formed by the plurality of electric field forming electrodes 9 a to 9 h. Furthermore, the electron emitter 2 functions as a part of the electric field forming unit 7, and the plurality of electric field forming electrodes 9 a to 9 h and the electron emitter 2 can cooperate to form a potential gradient.

The electrostatic gate electrode 8 is an electrode disposed between the ionization region 10 and the drift region 11, and controls the injection of ions generated in the ionization region 10 into the drift region 11 by utilizing the electrostatic interaction between the ions and the electrostatic gate electrode 8.
The electrostatic gate electrode 8 is, for example, a grid-shaped electrode (shutter grid). The electrostatic gate electrode 8 can be arranged in a row together with the plurality of electric field generating electrodes 9a to 9h that make up the electric field generating unit 7. The electrostatic gate electrode 8 can be electrically connected to the gate control unit 27 of the control unit 12. The electrostatic gate electrode 8 is provided so as to be able to change the potential gradient generated by the electric field generating unit 7.

The gate control unit 27 changes the potential of the electrostatic gate electrode 8 to instantaneously change from low-potential-side closed (a state in which the potential of the electrostatic gate electrode 8 is low, preventing ions in the ionization region 10 from passing through the electrostatic gate electrode 8 and moving to the drift region 11) to high-potential-side closed (a state in which the potential of the electrostatic gate electrode 8 is high, preventing ions in the ionization region 10 from passing through the electrostatic gate electrode 8 and moving to the drift region 11), or to instantaneously change from high-potential-side closed to low-potential-side closed. This allows the electrostatic gate electrode 8 to be open for only a very short time, allowing ions from the ionization region 10 to be injected into the drift region 11 only during this short time. Therefore, ions from the ionization region 10 can be injected into the drift region 11 in a single pulse.

Negative or positive ions injected into the drift region 11 migrate through the drift region 11 toward the collector 6 due to the potential gradient created by the electric field generator 7, and reach the collector 6. In this process, the negative or positive ions move against the flow of the drift gas. This flow of drift gas acts as a resistance for the negative or positive ions moving from the electrostatic gate electrode 8 toward the collector 6. The magnitude of this resistance (ion mobility) varies depending on the ion species. Generally, mobility is inversely proportional to the ion's collision cross-section (ion size). Therefore, the larger the ion's collision cross-section, the longer it takes for the ion to reach the collector 6. (The larger the ion, the more frequently it collides with air molecules in the drift gas, resulting in a slower migration speed and a longer arrival time at the collector 6.) Therefore, the time (arrival time, peak position) from when the ion is injected into the drift region 11 by the electrostatic gate electrode 8 to when it reaches the collector 6 varies depending on the ion species (negative or positive ion). Therefore, it is possible to identify negative or positive ions (detection target components contained in the sample) based on this arrival time (peak position). Furthermore, ions of multiple detection target components contained in the sample gas can be separated in the drift region 11.

The collector 6 is a metal component that collects the charge of negative or positive ions. The collector 6 can be electrically connected to the recovery current measurement unit 19 of the control unit 12. The recovery current measurement unit 19 is also configured to measure, in time series, the recovery current generated when negative or positive ions transfer charge to the collector 6. This makes it possible to measure the current waveform (IMS spectrum) of the recovery current.

Multiple types of ions injected into the drift region 11 in a single pulse using the electrostatic gate electrode 8 are separated into various ions as they move through the drift region 11, and the various ions arrive at the collector 6 with a time lag. As a result, the current waveform (IMS spectrum) of the collected current exhibits peaks corresponding to the arrival times of the various ions, making it possible to calculate the mobility from the peak position (arrival time) and identify the ion components. Furthermore, because the peak height or peak area of the current waveform of the collected current corresponds to the amount of charge transferred by the various ions to the collector 6, quantitative analysis of the target components can be performed based on the peak height or peak area.

Next, an IMS analysis method using the IMS analyzer 40 of this embodiment will be described. This method can be implemented by controlling the IMS analyzer 40 using the control unit 12. Here, the case where the IMS analyzer 40 is operated in negative ion mode will be described. The drift gas is air, and the carrier gas is air containing moisture.
2 is a flowchart of the IMS analysis method of this embodiment, and FIG. 3 is a graph showing an IMS spectrum (noise removed) measured by the IMS analysis method of this embodiment. The horizontal axis of this graph represents the arrival time of ions, with the moment when the electrostatic gate electrode 8 is opened being set to 0, and the vertical axis represents the recovery current generated by the charge recovery of ions that have reached the collector 6.

In step S1, calibration is performed. Specifically, drift gas (air) is injected into the analysis chamber 30 using the drift gas injection unit 15, and carrier gas (air containing moisture) not containing sample gas is injected into the analysis chamber 30 using the sample gas injection unit 16. Then, an element voltage is applied between the lower electrode 3 and surface electrode 4 of the electron emitter 2 using the control unit 12. Electrons emitted from the electron emitter 2 attach to components contained in the air, generating negative ions (primary ions). These negative ions move near the electrostatic gate electrode 8 due to the electric field formed in the analysis chamber 30.

When the electrostatic gate electrode 8 is momentarily opened to inject negative ions (primary ions) into the drift region 11 in a single pulse, the negative ions move through the drift region 11 against the flow of the drift gas due to the electric field of the analysis chamber 30 and reach the collector 6. The charge transferred by the negative ions to the collector 6 then generates a recovery current, allowing an IMS spectrum to be measured. This IMS spectrum contains a first peak (RIP: Reactant Ion Peak) corresponding to the negative ions (primary ions) generated from components in the air. The peak height or peak area of this first peak is compared with a preset target value. If the peak height or peak area of the first peak is within ±5% of the target value, the first calibration is terminated and the process proceeds to step S2. If the peak height or peak area of the first peak is outside the ±5% range of the target value, the device voltage applied between the lower electrode 3 and the surface electrode 4 of the electron-emitting device 2 is changed, and an IMS analysis is performed to measure the IMS spectrum. This IMS analysis is repeated until the peak height or peak area of the first peak falls within ±5% of the target value, and then the process proceeds to step S2. The peak height or peak area of the first peak that appeared in the last measured IMS spectrum is calculated, and this peak height or peak area is used as the reference value. When repeating the IMS analysis of the sample gas, calibration can be performed again so that the peak height or peak area of the first peak approaches this reference value.
By performing such calibration, the amount of primary ions produced in the ionization region 10 can be stabilized, and the components to be detected can be quantitatively measured.

In step S2, drift gas (air) is injected into the analysis chamber 30 using the drift gas injection unit 15, and a mixed gas of sample gas and carrier gas (air containing moisture) is injected into the analysis chamber 30 using the sample gas injection unit 16. While this is happening, the control unit 12 applies an element voltage adjusted by calibration between the lower electrode 3 and surface electrode 4 of the electron emitter 2. Electrons emitted from the electron emitter 2 attach to components contained in the air, generating negative ions (primary ions). These primary ions then transfer charge to the target component contained in the sample gas, generating negative ions of the target component. These negative ions move near the electrostatic gate electrode 8 due to the electric field formed in the analysis chamber 30.

When the electrostatic gate electrode 8 is momentarily opened to inject negative ions into the drift region 11 in a single pulse, the electric field in the analysis chamber 30 causes the negative ions to move through the drift region 11 against the flow of the drift gas and reach the collector 6. The charge transferred by the negative ions to the collector 6 then generates a collection current, allowing an IMS spectrum to be measured. This IMS spectrum contains a first peak (RIP) corresponding to negative ions (primary ions) generated from components contained in the air and a peak corresponding to negative ions generated from the target component. For example, an IMS spectrum (1) such as the one shown by the solid black line in the graph in Figure 3 is measured. Multiple peaks corresponding to negative ions generated from the target component may appear in the IMS spectrum. If at least one of these peaks overlaps with the first peak, a so-called hidden peak is present.

Peak detection in an IMS spectrum can be performed, for example, as follows.
(1) Electrical noise due to the response of the electrostatic gate electrode 8 is removed from the measured waveform using an approximation formula based on a first-order lag element.
(2) Use a low-pass filter (cutoff = 5000 Hz) to remove high-frequency noise from the measured waveform.
(3) The peak position is detected by detecting the position of the positive to negative zero crossing point in the first derivative waveform of the measured waveform.
(4) The time of the negative to positive zero crossing point in the third derivative waveform of the measured waveform is determined, and an inflection point is detected by checking whether the second derivative waveform has a negative minimum value corresponding to the determined time.
(5) A value greater than the threshold value (noise component: 3σ) set in the original corresponding to the obtained time is acquired (separation from the noise component).
(6) Delete the overlapping points of the detected peak positions and the detected inflection points. Specifically, if there is another detected point in the vicinity (approximately 0.15 msec) of each detected point, the one with the larger value is detected as the peak, and the other one is deleted.
Such a peak detection method can be used in steps S3, S5, etc.

In step S3, the control unit 12 determines whether there is a minor peak in the IMS spectrum measured in step S2. It is also possible to determine whether there is a minor peak overlapping the first peak using a method similar to step S5 described later. This makes it possible to determine whether there is a hidden peak.
If it is determined that there is a minute peak, the process proceeds to step S5, and if it is determined that there is no minute peak, the process proceeds to step S4.
In step S4, it is determined whether the device voltage applied between the lower electrode 3 and the surface electrode 4 of the electron-emitting device 2 has reached its upper limit. If it has reached the upper limit, the process proceeds to step S5. If it has not, the process returns to step S1, where steps S1 and S2 are repeated. In step S1, a target value greater than the target value set in the previous step S1 is set. This increases the device voltage applied between the lower electrode 3 and the surface electrode 4 of the electron-emitting device 2, thereby increasing the amount of primary ions generated in the ionization region 10. As a result, a new minute peak appears in the IMS spectrum measured in step S2. This minute peak may become a new second peak. Furthermore, the peak intensities of the first and second peaks appearing in the IMS spectrum measured in step S2 increase. In step S2, for example, an IMS spectrum (2) such as that shown by the black dashed line in the graph of FIG. 3 is measured, and the peak intensities of the first and second peaks are greater than those of the IMS spectrum (1). For example, the device voltage applied between the lower electrode 3 and the surface electrode 4 of the electron-emitting device 2 can be increased from 10V to 12V.

In step S5, the control unit 12 determines whether there is a peak overlapping the vicinity of the first peak (RIP) corresponding to the primary ion in the IMS spectrum measured in the immediately preceding step S2.
First, the control unit 12 determines whether or not a second peak overlaps the first peak within a predetermined range (e.g., a range twice the half-width of the first peak) near the first peak. If it determines that a second peak overlaps the first peak, the process proceeds to step S6.
Next, the control unit 12 can determine whether a second peak overlaps with the first peak by comparing the IMS spectrum measured in the immediately preceding step S1 with the IMS spectrum measured in the immediately preceding step S2. For example, the control unit 12 can overlap the first peak of the IMS spectrum measured in step S1 with the first peak of the IMS spectrum measured in step S2 and extract portions with different peak shapes. For example, if there is a portion where the base shape or shoulder shape of the first peak is larger in the IMS spectrum measured in step S2, it can be determined that a second peak overlaps with the first peak. In this case, the process proceeds to step S6.

If there are almost no differences between the shape of the first peak in the IMS spectrum measured in step S1 and the shape of the first peak in the IMS spectrum measured in step S2, the control unit 12 determines that there is no second peak overlapping the first peak and terminates this control flow. In this case, the IMS spectrum measured in the immediately preceding step S2 can be used as the measurement result.

In step S6, the control unit 12 sets a drift voltage that changes the potential of the electric field forming electrode so as to reduce the gradient of the potential gradient formed in the drift region 11. That is, the drift voltage is set to a voltage that is lower than the drift voltage set in the previous measurement in step S2 and higher than a preset lower limit drift voltage. For example, if a drift voltage is applied in step S2 so that the potential of the electric field forming electrode 9a of the IMS analyzer 40 shown in FIG. 1 becomes −3000 V, the drift voltage is set to a voltage that is lower than 3000 V and higher than 300 V (the lower limit voltage). This reduces the gradient of the potential gradient formed in the drift region 11.
The control unit 12 can set the drift voltage with a preset change width. For example, when the change width is set to 300 V, the control unit 12 can set the drift voltage so that the potential of the electric field forming electrode 9 a becomes −2700 V.

The control unit 12 may also calculate the variation width of the drift voltage from the difference between the position (arrival time) of the first peak and the position (arrival time) of the second peak, and set the drift voltage accordingly. This makes it possible to appropriately set the drift voltage. For example, if the position of the first peak and the position of the second peak are relatively close, the variation width of the drift voltage can be increased, and if the position of the first peak and the position of the second peak are relatively far, the variation width of the drift voltage can be decreased.

In step S7, the control unit 12 determines whether the set drift voltage is within the allowable range (whether it has reached the lower limit voltage). If the drift voltage is outside the allowable range (has reached the lower limit), this control flow ends, and the IMS spectrum measured in the immediately preceding step S2 can be used as the measurement result.
If the drift voltage is within the allowable range, the process returns to step S2 and measures the sample gas. In step S2, the gradient of the potential gradient in the drift region 11 is smaller than in the previous step S2, so the time it takes for the primary ions and ions generated from the target components to reach the collector 6 after passing through the electrostatic gate electrode 8 is longer. This increases the difference between the positions (arrival times) of the first and second peaks in the IMS spectrum, allowing the second peak to be separated from the first peak. It is confirmed that the second peak identified in the previous step S5 has been separated from the first peak, and the peak is identified. Then, step S5 is performed again to determine whether there is a new peak overlapping the first peak.

For example, the IMS spectrum (3) shown by the solid gray line in the graph in Figure 3 is measured. In the IMS spectrum (3), the first peak (RIP) and the second peak (the peak corresponding to the target substance) appear later (the arrival time is longer) than in the IMS spectrum (2). In addition, the second peak is separated from the first peak.
If it is determined in step S5 that there is no peak overlapping the first peak, this control flow is terminated, and the IMS spectrum measured in the immediately preceding step S2 can be used as the measurement result.

By increasing the device voltage applied between the bottom electrode 3 and the surface electrode 4 of the electron-emitting device 2 to increase the detection sensitivity and make it possible to detect minute peaks, and then performing IMS analysis with a reduced drift voltage for overlapping peaks, the peak separation can be improved. This makes it possible to increase the detection sensitivity without reducing the peak separation.
In the above description of the embodiment, the first peak is a primary ion generated by ionizing the carrier gas, air, and the second peak is one of multiple detection peaks generated by ionizing the sample gas that overlaps with the first peak. However, the first peak may be a peak of the sample gas, and the method may be applied to separating overlapping peaks of the sample gas.

1: Ion source 2: Electron emitter 3: Lower electrode 4: Surface electrode 5: Intermediate layer 6: Collector 7: Electric field generator 8: Electrostatic gate electrodes 9a-9h: Electric field generator electrodes 10: Ionization region 11: Drift region 12: Control unit 15: Drift gas injector 16: Sample gas injector 17: Element voltage controller 19: Recovery current measurement unit 20: Exhaust unit 22: Element holder 25: Grid electrode 26: Electric field controller 27: Gate controller 28: Housing 29: Insulator 30: Analysis chamber 40: IMS analyzer

Claims (5)

The apparatus comprises an ion source, a collector, an electrostatic gate electrode, an electric field forming electrode, a control unit , and a sample gas injection unit ;
the control unit is configured to control the potential of the electric field forming electrode so as to form a potential gradient in a drift region between the electrostatic gate electrode and the collector,
the collector and the control unit are configured to measure a current waveform of a current that flows when ions reach the collector;
the control unit is configured to determine whether or not a second peak overlapping a first peak exists in the current waveform, and when it is determined that a second peak exists, to change the potential of the electric field forming electrode so that the inclination of the potential gradient becomes smaller, and measure the current waveform ;
the collector, the electrostatic gate electrode, and the electric field forming electrode are contained in an analysis chamber;
the sample gas injector is configured to inject a mixed gas of a sample gas and a carrier gas or a carrier gas not containing the sample gas into the analysis chamber;
the ion source is configured to directly or indirectly ionize components contained in the sample gas or components contained in the carrier gas;
the first peak corresponds to ions generated from a component contained in the carrier gas;
the second peak corresponds to an ion generated from a component contained in the sample gas,
the control unit is configured to determine whether or not a second peak exists by comparing the current waveform measured when a carrier gas not including the sample gas is being injected into the analysis chamber with the current waveform measured when the mixed gas is being injected into the analysis chamber;
the ion source is an electron-emitting device,
the electron-emitting element has a lower electrode, a surface electrode, and an intermediate layer disposed between the lower electrode and the surface electrode;
the control unit is configured to apply a voltage between the lower electrode and the surface electrode,
the carrier gas is air containing moisture,
The control unit and the sample gas injection unit are configured to adjust the voltage applied between the lower electrode and the surface electrode so that the peak height or peak area of the first peak approaches a target value when a carrier gas not containing the sample gas is injected into the analysis chamber, and the IMS analysis device is configured to inject the mixed gas into the analysis chamber and measure the current waveform after adjusting the voltage applied between the lower electrode and the surface electrode . 2. The IMS analysis apparatus of claim 1, wherein the control unit is configured to determine whether or not a minute peak exists in the current waveform measured while the mixed gas is being injected into the analysis chamber, and when it is determined that a minute peak does not exist, to increase the voltage applied between the lower electrode and the surface electrode. The apparatus comprises an ion source, a collector, an electrostatic gate electrode, an electric field forming electrode, a control unit , and a sample gas injection unit ;
the control unit is configured to control the potential of the electric field forming electrode so as to form a potential gradient in a drift region between the electrostatic gate electrode and the collector,
the collector and the control unit are configured to measure a current waveform of a current that flows when ions reach the collector;
the control unit is configured to determine whether or not a second peak overlapping a first peak exists in the current waveform, and when it is determined that a second peak exists, to change the potential of the electric field forming electrode so that the inclination of the potential gradient becomes smaller, and measure the current waveform ;
the collector, the electrostatic gate electrode, and the electric field forming electrode are contained in an analysis chamber;
the sample gas injector is configured to inject a mixed gas of a sample gas and a carrier gas or a carrier gas not containing the sample gas into the analysis chamber;
the ion source is configured to directly or indirectly ionize components contained in the sample gas or components contained in the carrier gas;
the first peak corresponds to ions generated from a component contained in the carrier gas;
the second peak corresponds to an ion generated from a component contained in the sample gas,
the control unit is configured to determine whether or not a second peak exists by comparing the current waveform measured when a carrier gas not including the sample gas is being injected into the analysis chamber with the current waveform measured when the mixed gas is being injected into the analysis chamber;
the ion source is an electron-emitting device,
the electron-emitting element has a lower electrode, a surface electrode, and an intermediate layer disposed between the lower electrode and the surface electrode;
the control unit is configured to apply a voltage between the lower electrode and the surface electrode,
the carrier gas is air containing moisture,
The control unit is configured to determine whether or not a minute peak exists in the current waveform measured when the mixed gas is injected into the analysis chamber, and if it determines that a minute peak does not exist, to increase the voltage applied between the lower electrode and the surface electrode . 4. The IMS analysis apparatus according to claim 1, wherein the control unit is configured to determine whether or not a second peak exists that overlaps with the first peak within a predetermined range near the first peak. a first measurement step of measuring a current waveform of a current that flows when ions generated by an ion source directly or indirectly ionizing components contained in a carrier gas that does not contain a sample gas are introduced into an analysis chamber and reach a collector;
a second measurement step of measuring a current waveform of a current that flows when ions generated by the ion source directly or indirectly ionizing a component contained in the sample gas or a component contained in the carrier gas reach a collector while the mixed gas of the sample gas and the carrier gas is being injected into the analysis chamber;
a determining step of determining whether or not a second peak overlapping the first peak exists in the current waveform measured in the second measuring step ;
a third measuring step of measuring a current waveform of a current that flows when ions reach the collector by changing the potential of the electric field forming electrode so that the gradient of the potential gradient in the drift region between the electrostatic gate electrode and the collector becomes smaller when it is determined that a second peak overlaps the first peak ;
the first peak corresponds to ions generated from a component contained in the carrier gas;
the second peak corresponds to an ion generated from a component contained in the sample gas,
In the determination step, the current waveform measured when a carrier gas not containing the sample gas is injected into the analysis chamber in the first measurement step is compared with the current waveform measured when the mixed gas is injected into the analysis chamber in the second measurement step to determine whether a second peak exists;
the ion source is an electron-emitting device,
the electron-emitting element has a lower electrode, a surface electrode, and an intermediate layer disposed between the lower electrode and the surface electrode;
the carrier gas is air containing moisture,
An IMS analysis method in which, in a first measurement step, a voltage applied between the lower electrode and the surface electrode is adjusted so that the peak height or peak area of a first peak approaches a target value while a carrier gas not containing the sample gas is being injected into the analysis chamber, and after adjusting the voltage applied between the lower electrode and the surface electrode, in a second measurement step, the mixed gas is injected into the analysis chamber and the current waveform is measured .