A high-field asymmetric waveform ion mobility spectrometer based on nested Faraday cups

Abstract

A high-field asymmetric waveform ion migration spectrometer based on a nested Faraday cage, the ion migration spectrometer includes an ion source, a migration zone and a detection zone; wherein the detection zone is composed of two parts: one part is a sensitive electrode, which is directly connected to a weak current detection circuit as a sensitive element, and the other part is a deflection electrode, which uses a DC source to provide voltage, and both parts are made of conductive materials. The length of the sensitive electrode is less than or equal to the length of the deflection electrode, the deflection electrode is closed on all sides and nested outside the sensitive electrode, and a gap is left between the deflection electrode and the sensitive electrode for airflow to pass through. The high-field asymmetric waveform ion migration spectrometer provided by the present invention can greatly reduce the interference of the external environment and the voltage applied on the deflection electrode on the sensitive electrode, significantly improve the signal-to-noise ratio of the instrument, have a good shielding effect, and at the same time have a simple structure and are easy to integrate.

Description

Technical Field

[0001] The invention relates to a high-field asymmetric waveform ion mobility spectrometer, in particular to a high-field asymmetric waveform ion mobility spectrometer using a Faraday cup as a detection part, and belongs to the technical field of biochemical substance online detection technology and equipment. Background Art

[0002] High-field asymmetric waveform ion mobility spectrometer is an online detection technology for biochemical substances that has been gradually developed in the 1990s. Its basic principle is that the ion mobility is independent of the electric field strength under low electric field conditions. When the electric field strength is greater than 10000V / cm, the ion mobility will change nonlinearly with the electric field strength. The relationship between the ion mobility and the electric field strength under high field conditions can be expressed by the following formula:

[0003] K=K0[1+α1(E / N) 2 +α2(E / N) 4 +…],

[0004] In the formula, K is the mobility of ions under high electric field, K0 is the mobility of ions under low electric field, E is the electric field strength, N is the gas molecule density, α1 and α2 are the ion mobility decomposition coefficients. Let:

[0005] α(E)=[α1(E / N) 2 +α2(E / N) 4 +…], the relationship between the mobility of ions under high field and the electric field strength can be changed to K=K0[1+α(E)]. According to the different laws of ion mobility changing with electric field strength, ions can be divided into three types: A, B, and C. When α(E / N)>0, K>K0, belonging to type A ions, K increases with the increase of electric field strength E; when α(E / N)<0, K<K0, belonging to type C ions, K decreases with the increase of E; when α(E / N)≈0, belonging to type B ions, K≈K0. That is, when the electric field strength reaches 10000V·cm -1 When the above conditions are met, the mobility of the ions presents different nonlinear change trends, which enables ions with the same or similar ion mobility under low electric field strength to be separated under high electric field strength. The electric field that meets the ion separation conditions here is called the separation electric field, and the voltage applied to the electrode to form this electric field is called the separation voltage, which is usually provided by an asymmetric high-voltage and high-frequency radio frequency power supply (RF power supply). At the same time, a compensation voltage (CV) is applied to the electrode to compensate for the ion deflection caused by the separation voltage, so that a specific ion can pass through the migration area to reach the detection area.

[0006] In recent years, with the development of high-field asymmetric waveform ion mobility spectrometers, the detection part of the detector is mostly composed of two parallel and opposite plates, one side is used as the sensitive electrode of the Faraday cage, and the other side is used to apply the deflection voltage, such as Figure 1 As shown. However, it is difficult for this parallel-plate Faraday cage to electromagnetically shield the sensitive electrode, and the applied deflection voltage will also affect the sensitive electrode. At the same time, the dispersed electric field between the two parallel plates will cause the charged particles to be lost before entering the detection part. Therefore, a large amount of noise signals will be superimposed into the spectrum of the high-field asymmetric waveform ion mobility spectrometer, and the baseline of the spectrum will also be raised as the deflection voltage increases. Summary of the invention

[0007] The purpose of the present invention is to provide a high-field asymmetric waveform ion mobility spectrometer based on a nested Faraday cup in view of the shortcomings and defects of the prior art, so as to solve the problems of difficulty in shielding the sensitive electrode and interference of the deflection electrode on the sensitive electrode in the high-field asymmetric waveform ion mobility spectrometer based on a flat-plate detector.

[0008] To solve the above problems, the technical solution of the present invention is as follows:

[0009] A high-field asymmetric waveform ion migration spectrometer based on a nested Faraday cup comprises an ion source, a migration zone and a detection zone; the detection zone comprises a Faraday cup, a direct current source and a weak current detection circuit; the characteristic is that: the Faraday cup adopts a nested structure, the Faraday cup comprises a sensitive electrode and a deflection electrode nested on the periphery of the sensitive electrode, the deflection electrode is closed on all sides, and a gap for airflow to pass through is left between the deflection electrode and the sensitive electrode; the direct current power supply is electrically connected to the deflection electrode, and the sensitive electrode is directly connected to the weak current detection circuit.

[0010] The above technical solution is characterized in that the gap between the deflection pole and the sensitive pole is greater than zero and less than or equal to 5 cm.

[0011] The technical feature of the present invention is that the length of the sensitive electrode along the sample gas flow direction is less than or equal to the length of the deflection electrode in this direction. Preferably, the length of the sensitive electrode along the sample gas flow direction is greater than zero and less than or equal to 10 cm.

[0012] Preferably, the nested structure of the Faraday cage is a rectangular sleeve type, a cylindrical sleeve type or a spherical sleeve type.

[0013] Compared with the prior art, the present invention has the following advantages and outstanding technical effects: ① In the present invention, the deflection electrode is nested outside the sensitive electrode and is closed on all sides. The peripheral deflection electrode not only plays the role of applying the deflection voltage, but also plays the role of electromagnetic shielding for the sensitive electrode, thereby reducing the interference of the outside world and the deflection voltage on the sensitive electrode, and greatly improving the signal-to-noise ratio of the instrument output signal; ② In the present invention, the length of the sensitive electrode is less than or equal to the length of the peripheral deflection electrode, which can effectively protect the sensitive electrode, while avoiding the influence of the scattered electric field on the periphery of the Faraday cage on the charged particles, reducing ion loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a structural schematic diagram of a high-field asymmetric waveform ion mobility spectrometer in the prior art.

[0015] Figure 2 It is a schematic diagram of the structural principle of the nested Faraday cage of the present invention.

[0016] Figure 3 It is a schematic diagram of the structural principle of an embodiment of the present invention.

[0017] FIG. 4( a ) and FIG. 4( b ) are comparison diagrams of experimental results of the existing flat-plate detection area and the nested Faraday cup detection area provided by the present invention respectively applied in a high-field asymmetric waveform ion mobility spectrometer.

[0018] In the figure: 1-sample gas; 2-ion source; 3-migration zone; 4-upper electrode 1 of the migration zone; 5-lower electrode 2 of the migration zone; 6-RF power supply; 7-superposition circuit; 8-compensation voltage; 9-ground electrode; 10-detection zone; 11-deflection electrode; 12-sensitive electrode; 13-weak current detection circuit; 14-DC source; 15-flat Faraday cup. DETAILED DESCRIPTION

[0019] The present invention is further described below in conjunction with the accompanying drawings and embodiments so that those skilled in the art can fully understand and implement the present invention.

[0020] Figure 3 The present invention provides a structural schematic diagram of a high-field asymmetric waveform ion migration spectrometer based on a nested Faraday cup, which includes an ion source 2, a migration area 3 and a detection area, wherein the detection area includes a sensitive electrode 12, a deflection electrode 11, a DC source 14 and a weak current detection circuit 13; a DC power supply is used to apply voltage to the deflection electrode 11. The sensitive electrode 12 is directly connected to the weak current detection circuit 13; the deflection electrode is closed all around and nested in the outer periphery of the sensitive electrode, and the deflection electrode can provide electromagnetic shielding protection for the sensitive electrode and provide a deflection voltage; a gap for airflow to pass through is left between the deflection electrode and the sensitive electrode; the gap is generally greater than zero and less than or equal to 5 cm.

[0021] Figure 2 The structural diagram of the Faraday cup embodiment provided by the present invention is that the length of the sensitive electrode along the sample gas flow direction should be less than or equal to the length of the deflection electrode in this direction. Under normal circumstances, the length of the sensitive electrode along the sample gas flow direction is greater than zero and less than or equal to 10 cm. The nested structure of the Faraday cup can be a rectangular sleeve type, a cylindrical sleeve type or a spherical sleeve type. The sensitive electrode 12 is directly connected to the weak current detection circuit, and the voltage on the deflection electrode is provided by a DC power supply.

[0022] The invention effectively solves the problem of difficult shielding of the sensitive electrode in the flat-plate detection part, and simultaneously solves the influence of the deflection voltage on the deflection electrode on the baseline of the high-field asymmetric waveform ion migration spectrum.

[0023] Embodiment 1:

[0024] Experiments were performed using existing flat plate detection areas in a high field asymmetric waveform ion mobility spectrometer, such as Figure 1 As shown. The distance between the two electrodes in the migration zone is 1.5mm. The gas sample used in the experiment is ethyl acetate with a concentration of 109ppm, the carrier gas is 99.999% nitrogen, and a 10.6eV ultraviolet lamp is used as the ionization source. The experiment is carried out without applying RF voltage but only applying compensation voltage (CV). When the deflection voltage is 1.5V, 9V and 18V respectively, the ion migration spectrum is shown in Figure 4(a).

[0025] The nested Faraday cup provided by the present invention is used in a high-field asymmetric waveform ion mobility spectrometer to perform experiments, such as Figure 3 As shown. The distance between the two electrodes in the migration zone is 1.5mm. The gas sample used in the experiment is ethyl acetate with a concentration of 109ppm, the carrier gas is 99.999% nitrogen, and a 10.6eV ultraviolet lamp is used as the ionization source. The experiment is carried out without applying RF voltage but only applying compensation voltage (CV). When the deflection voltage is 1.5V, 9V and 18V respectively, the ion migration spectrum is shown in Figure 4(b).

[0026] By comparison, it can be found that the noise of the high-field asymmetric waveform ion migration spectrometry using a flat Faraday cup is about 2pA, and the baseline of the spectrum will increase with the increase of the deflection voltage; while the noise of the high-field asymmetric waveform ion migration spectrometry using the Faraday cup in the present invention is about 0.2pA, and the baseline of the spectrum will not increase with the increase of the deflection voltage. This shows that the present invention can effectively improve the signal-to-noise ratio of the ion migration spectrometry, shield the interference of the outside world on the sensitive electrode and the interference of the deflection voltage applied to the deflection electrode on the sensitive electrode. At the same time, the shielding electrode and the deflection electrode in the detection area are of the same structure, which simplifies the detection area in the high-field asymmetric waveform ion migration spectrometer and is easy to process and integrate.

Claims

1. A high-field asymmetric waveform ion mobility spectrometer based on a nested Faraday cup, comprising an ion source (2), a migration zone (3) and a detection zone (10); the detection zone comprises a Faraday cup, a direct current power supply (14) and a weak current detection circuit (13); characterized in that: The Faraday cup adopts a nested structure, comprising a sensitive electrode (12) and a deflection electrode (11) nested on the outer periphery of the sensitive electrode, the deflection electrode is closed on the outer periphery, and a gap for airflow to pass through is left between the deflection electrode and the sensitive electrode; the DC power supply (14) is electrically connected to the deflection electrode (11), and the sensitive electrode (12) is directly connected to a weak current detection circuit (13).

2. A high-field asymmetric waveform ion mobility spectrometer based on a nested Faraday cup according to claim 1, characterized in that: The gap between the deflection pole (11) and the sensitive pole (12) is greater than zero and less than or equal to 5 cm.

3. A high-field asymmetric waveform ion mobility spectrometer based on a nested Faraday cup according to claim 1 or 2, characterized in that: The length of the sensitive electrode (12) along the flow direction of the sample gas is less than or equal to the length of the deflection electrode (11) in this direction.

4. A high-field asymmetric waveform ion mobility spectrometer based on a nested Faraday cup as claimed in claim 3, characterized in that: The length of the sensitive electrode (12) along the flow direction of the sample gas is greater than zero and less than or equal to 10 cm.

5. A high-field asymmetric waveform ion mobility spectrometer based on a nested Faraday cup according to claim 1, characterized in that: The nested structure of the Faraday cage is a rectangular sleeve type, a cylindrical sleeve type or a spherical sleeve type.

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