Mass spectrometer for gas analysis and gas analysis method

Abstract

This invention provides a mass spectrometer and gas analysis method for gas analysis that reduce the effects of coherent photons and electrons in gas analysis by mass spectrometry. [Solution] The gas analysis mass spectrometer (10) comprises a mass spectrometer housing (12) that is evacuated by a vacuum pump; an ion source (16) that ionizes gas atoms or gas molecules introduced into the mass spectrometer housing (12) and accelerates the ions in the mass spectrometer housing (12); an ion trap (22) made of a conductive material that collects the ions generated by the ion source (16); an electrical measuring device (24) that measures the potential or current of the ion trap (22) and is connected to the ion trap (22) and estimates the ions collected by the ion trap (22) from the measured potential or current; and a magnetic or electromagnetic deflection device configured to deflect or map ions of a specific mass-to-charge ratio toward the ion trap (22). An electron barrier (30) made of a conductive material having a lower potential than the ion trap (22) is placed in the ion trap (22) and is configured to form a counter-electric field for returning electrons emitted from the ion trap (22) back to the ion trap (22).

Description

Technical Field

[0001] The present invention relates to a mass spectrometer for gas analysis and a gas analysis method.

Background Art

[0002] In mass spectrometry gas analysis, the mass spectrometer housing is usually evacuated by a vacuum pump until the inside of the housing is in a high-vacuum state. Gas atoms or gas molecules to be analyzed are introduced into the high-vacuum state, and the ion source ionizes the gas atoms or gas molecules introduced into the high-vacuum state, usually by impact ionization. Similarly, especially for light atoms or molecules, it is possible to reach the high-vacuum state against the pumping effect of the high-vacuum pump. The magnetic deflection device generates a magnetic field penetrating the mass spectrometer housing, and ions of different types of gases are deflected by different degrees by the magnetic field toward the ion trap. Ions of the detected gas species are deflected toward the ion trap and captured by the ion trap. The ion trap usually forms a cathode made of a conductive material connected to the ground potential. For example, it is a plate made of a metal material. As soon as an ion collides with the ion trap and picks up an electron from the trap, the potential changes and is measured using an electrical measuring device. Based on the measured potential of the ion trap or the measured current, the number and type of the picked-up ions can be estimated, and the type and amount of the gas can be determined. The ion source accelerates the ionized particles so that only ions of a specific type of gas reach the ion trap, and other ions are accelerated after passing through the ion trap, captured by an ion shield, or collide with the high-vacuum inner surface. By changing the acceleration voltage, it is possible to affect which ions reach the ion trap according to the mass-to-charge ratio of a certain type of gas.

[0003] In mass spectrometry gas analysis, electrons can also be extracted by an effect independent of the colliding ions. For example, photons of short-wavelength light that collide with the ion trap emit photoelectrons from the ion trap surface and affect the electrical measurement signal.

[0004] The photon sources in the detection system are as follows: • Red-hot filament for electron emission in ion sources • Electrons colliding with the anode or ground potential of the ion source at a potential of several tens of eV. Ions colliding with the walls of the detection chamber Ions colliding with neutral particles Ions that collide with other ions [Overview of the project] [Problems that the invention aims to solve]

[0005] Measurement results indicate that as the operating pressure within the sector-field mass spectrometer volume increases, the signal background generated by photons also increases. Water vapor, in particular, indirectly generates a coherent background signal due to the delay before it is released from the volume. In addition, water vapor molecules with a relatively low molecular weight of M18 leak in large quantities from the test chamber or specimen into the high-vacuum state of the detection unit due to the pumping action of the high-vacuum pump.

[0006] Generally, a positive potential inhibitory potential (Wähnelt cylinder) is placed in front of the trap. This shields ions that should be shielded from the trap as an interfering potential source. However, this positive potential does not shield photons.

[0007] To date, the following are known as the main methods for avoiding or suppressing photoelectrons: The surface of the catcher is gold-plated. This coating reflects some photons, while the work function of electrons on the gold surface, which is 5.1 eV, increases the desorption barrier compared to iron, for example, which is 4.5 eV. The effect of this method is limited and insufficient. In particular, since the majority of effective photons have photon energies well above 5 eV, no barrier effect is obtained. The trap consists of a chemical element or alloy with the highest work function, and platinum is particularly suitable here. The work function of platinum electrons is approximately 5.5 eV. Alternatively, a carrier material with a thin diamond coating (e.g., stainless steel) can be selected, as the work function of diamond is 5.3 eV. The effectiveness of this method is limited and insufficient. In particular, since the majority of effective photons have photon energies significantly above 6 eV, no barrier effect is obtained. A so-called Faraday cup may be used as an ion trap, which is a cylindrical tube with an opening on the ion ingress side and a bottom / lid closure on the opposite side. The depth-to-diameter ratio must be at least 2:5. Electrons emitted within the cup are collected again by their respective opposing walls. In this way, some of the emitted photoelectrons are re-trapped by the trap. Disadvantages: Faraday cups require a large installation space, and while longer cups allow for more effective recapture of electrons emitted inside, they also increase the surface area of ​​the cup, making it increasingly disadvantageous for photons hitting the outer surface due to the photoelectrons emitted there. Another drawback is the remarkably large surface area of ​​the Faraday cup. When photons collide with the neutral surface inside the analyzer near the trap, electrons are emitted, and some of these photoelectrons collide with the surface of the Faraday cup, generating a negative offset signal. Therefore, when the background is high, a negative offset is observed throughout the entire trap system.

[0008] There are various techniques used in electron shielding for mass spectrometers to minimize photoelectron interference. Some examples include: 1. Electron Energy Blocking Filter (EEF): A filter designed to allow low-energy electrons to pass through while blocking high-energy electrons that may originate from photoelectrons. 2. Electron Energy Collector (EEC): A device that captures low-energy electrons and deflects them away from the detection area. 3. Electron beam blocking filter (ESF): A filter that blocks direct electron beams that may originate from photoelectrons and enter the system. 4. Electron beam shield: A shield used to isolate the electron path of photoelectrons from the light source in order to minimize the signal.

[0009] In vacuum leak detection, it is desirable not only to keep the leak detection limit as low as possible, but also to prepare for measurement quickly. In other words, it is desirable to start vacuum leak detection measurements as soon as possible after priming in the test chamber or on the test specimen. In particular, water vapor molecules that desorb from the inner wall surface of the test specimen with a delay slow down the decrease in pressure inside the test specimen, and consequently, the pressure inside the detection system. Mass spectrometers can generally distinguish well the mass of water molecules (mass M18) from the mass of test gases such as helium (mass M4) or hydrogen (mass M2). However, the high operating pressure of the detection system causes numerous ion collisions with the walls of the detection system. These ion collisions with the walls, as well as collisions with gas molecules and atoms remaining in the detection volume, generate high-energy photons, and the photoelectric effect in the ion trap generates an coherent background signal.

[0010] Given this background, the objective of the present invention is to reduce the influence of coherent photons and electrons in gas analysis by mass spectrometry. [Means for solving the problem]

[0011] The mass spectrometer according to the present invention is defined by the configuration of claim 1. The method according to the present invention is defined by the configuration of claim 9.

[0012] The present invention provides an electron barrier made of a conductive material, which is installed in an ion trap and is given a potential negative or lower than the potential of the ion trap so as to form a counter-electric field for returning electrons emitted from the ion trap. The emitted electrons may be photoelectrons emitted when electromagnetic radiation collides with the ion trap. The electron barrier and the counter-electric field ensure that the emitted electrons are returned to the ion trap and remain there. This prevents the permanent emission of electrons. Since the charge balance of the electrons that are initially emitted and then returned to the ion trap remains neutral, the emitted electrons do not alter the electrical measurement signal.

[0013] The electron barrier may also be called an electron repeller. The electron barrier may be positioned in front of the ion trap, or around it, as viewed from the direction of ions colliding with the ion trap. Typically, the electron barrier is a separate component from the ion trap and may be configured as a ring electrode, a cylindrical shell, or a hollow cylinder (hereinafter simply referred to as a cylinder). The height of the ring electrode, cylindrical shell, or hollow cylinder, i.e., the dimension along the longitudinal central axis, is smaller than its diameter, and the component is flat so as not to obstruct the passage of ions. The electron barrier does not trap electrons, but rather repels them into the ion trap through the generated electric field. Therefore, emitted electrons are not collected by the electron barrier or come into contact with it. Rather, it is intended to prevent emitted electrons from coming into contact with the electron barrier.

[0014] Alternatively, the electron barrier may be a grid through which ions pass on their way to the ion trap.

[0015] To return the emitted electrons as quickly as possible, it is important that an electron barrier is formed directly above the ion trap, for example, at the shortest possible distance from the ion trap.

[0016] When a shielding electrode is provided to shield an ion trap from the intrusion of unwanted positively charged ions, an electron barrier should be placed between the ion trap and the shielding electrode. These ions being shielded are typically those generated by secondary processes within the detection system, which have low energy and can cause coherent background signals. Furthermore, the shielding electrode can collect electrons generated by secondary processes within the detection system. While a positively potential shielding electrode is designed as an anode to block and deflect coherent ions or collect electrons before they collide with the ion trap, the electron barrier is intended not simply to collect emitted electrons, but to repel them with a counter-field within the trap.

[0017] In this regard, the method according to the present invention provides, for example, in an ion trap, the generation of a negative counter-field that forms a barrier against electrons emitted from the ion trap and returns the electrons into the ion trap due to the potential difference between the electron barrier and the ion trap. The negative counter-field can be formed in front of the ion trap and / or around the ion trap.

[0018] The potential of the electron barrier does not necessarily have to be constant; it can be changed over time. This allows interference effects to be determined by comparative measurements by switching the potential on and off or by changing the potential. Furthermore, the intensity of photoelectrons emitted by the ion trap can be changed by modulating or changing the potential, i.e., the (negative) counter-field voltage at the electron repeller, but the intensity of the detected ions remains unaffected. This means that the signal generated by ions in the ion trap can be distinguished from the interference effect of photoelectrons.

[0019] The electron barrier can be arranged in front of the ion trap, for example, between the shielding electrode and the ion trap, when viewed from the direction of ion passage. Alternatively or additionally, the electron barrier can be arranged around the ion trap, that is, so as to at least partially surround the ion trap. Thus, the electron barrier can be a component that is electrically separated from the ion trap and surrounds the ion trap. In all of these embodiments, it is particularly effective that the electron barrier is a component electrically separated from the ion trap.

[0020] The ion trap can be designed, for example, as a Faraday cup. The side walls of the electron repeller surrounding the Faraday cup have a repeller potential surrounding the ion trap, so as to push back the photons colliding with the outer surface of the Faraday cup from the negative repeller potential to the ion trap. For example, in the case of a cup-shaped ion trap designed as a Faraday cup, the electron barrier can be designed with a wall surrounding the outside of the ion trap, for example, a wall in the shape of a hollow cylinder, thereby forming a repeller potential surrounding the ion trap on the lateral outside and pushing back the photons colliding with the outside of the ion trap to the ion trap. The structure of the electron barrier surrounding the ion catcher preferably extends over at least a majority, particularly preferably the entire length, of the ion catcher. It is particularly preferred that the ion trap is covered by the covering surface of the electron barrier in the direction of ion passage, that is, the distal direction. The covering surface can be designed as a grid so that the incoming ions can pass through.

Brief Description of the Drawings

[0021] Two embodiments of the present invention will be described in more detail with reference to the drawings. [Figure 1] It is a cross-sectional view of the first embodiment of the mass spectrometer according to the present invention. [Figure 2] It is a schematic diagram of the second embodiment.

Modes for Carrying Out the Invention

[0022] The mass spectrometer 10 shown in Figure 1 is a sector-field mass spectrometer for gas analysis. The mass spectrometer 10 comprises a mass spectrometer housing 12 having a port 14 for a high-vacuum pump to generate a high vacuum inside the mass spectrometer housing 12. The gas to be analyzed is introduced into the mass spectrometer housing 12 through the port 14.

[0023] An ion source 16 located within the mass spectrometer housing 12 ionizes gas atoms or molecules passing through the port 14 and accelerates them with an acceleration voltage of several electron volts (eV). When detecting helium, an acceleration voltage of several hundred eV is applied.

[0024] Ions accelerated by the ion source 16 form an ion flow 18 that is deflected by a magnetic field in a direction perpendicular to the propagation direction of the ion flow 18. The electromagnetic field is generated by a magnetic deflection device (not shown). In the illustrated embodiment, the ion flow 18 is divided into two substreams 18a and 18b depending on the mass of the molecules or atoms being accelerated. Ions in substream 18a have a smaller mass and therefore a smaller deflection radius, while ions in substream 18b have a larger mass and therefore a larger deflection radius that leads them precisely to the trap. The ions in substream 18b are helium ions, which are the target of detection. Here, the acceleration voltage of the ion source 16 is designed so that the helium ions in substream 18b precisely hit the ion detector 20 located inside the mass spectrometer housing 12. For example, the orbits of ions heavier than water vapor or nitrogen, which are components of air, are not depicted, but these typically have significantly larger orbital radii and collide with the walls of the housing. When they collide with the walls of the housing, or with atoms or molecules within the volume, high-energy photons are emitted.

[0025] The ion detector 20 includes an ion trap 22 having a conductive metal plate. The ion trap 22 is connected to ground potential or a constant other potential. For photoelectrons to return, it is important that the repeller potential of the electron barrier 30 is negative, i.e., lower than the potential of the ion trap 22. When an ion collides with the ion trap 22, the ion is electrically neutralized by electrons from the material of the ion trap 22, and the potential of the surface of the ion trap 22 changes. This change in potential or the resulting current is measured by an electrical measuring device 24 electrically connected to the ion trap 22. The number of ions that collided with the ion trap 22 can be estimated from the measured potential or measured current.

[0026] A cylindrical shield electrode 26 in the shape of a Wehnelt cylinder is provided above the ion trap 22, and the central longitudinal axis 28 of the electrode extends parallel to the propagation direction of the partial ion flow 18b in the region in front of the ion trap 22. Therefore, the partial ion flow 18b penetrates the Wehnelt cylinder in its longitudinal direction. The shield electrode 26 is electrically given a positive potential, thereby forming an anode for coherent ions approaching the ion trap 22. These coherent ions may impair the measurement signal of the measuring device 24 by affecting the potential of the surface of the ion trap 22. In addition, the shield electrode 26, with its positive potential, collects coherent electrons and prevents them from colliding with the ion trap 22.

[0027] An electron barrier 30 is formed between the shield electrode 26 and the ion trap 22. The electron barrier 30 is made of a conductive material and is electrically more negatively potential than the ion trap 22 in order to form a negative electric field directly above the surface of the ion trap 22. This electric field causes electrons emitted from the surface of the ion trap 22 to be returned to the ion trap. Such electrons may be emitted from the ion trap 22 by, for example, photons.

[0028] In the illustrated embodiment, the electron barrier 30 is configured as a cylindrical ring electrode whose longitudinal extension (height) in the direction of the central longitudinal axis 28 is smaller than its diameter. Therefore, collisions between the intruding ions of the partial flow 18b and the electron barrier 30 are avoided or reduced. As the cylinder length increases, it leads to particle collisions.

[0029] Alternatively, in another embodiment, the electron barrier 30 can be configured as an electrical grid through which ions of the partial flow 18b pass, while the grid of conductive material is charged to a more negative potential than the ion trap 22 in order to push the emitted electrons back into the ion trap 22.

[0030] For example, in contrast to a closed-bottom Faraday cup, the electron barrier 30 forms an open passage in the form of a ring electrode, an open end face of a cylinder, or a gap in the case of a grid. The electron barrier 30 is a separate component spaced apart from the ion trap 22.

[0031] The embodiment shown in Figure 2 differs from the first embodiment in that the ion trap 22 electrically connected to the measuring device 24 is cup-shaped like a Faraday cup, and a hollow cylindrical shell at the same potential as the bottom surface protrudes distally from the circular bottom surface of the ion trap at its periphery. The ion trap 22 is completely enclosed by a cage-like electron barrier 30, except for a very small opening for mounting and electrical connection of the ion trap 22. The electron barrier 30 extends at least along the entire length of the ion trap 22, and the hollow cylindrical shell of the electron barrier 30 completely surrounds the ion trap 22. The leading edge of the hollow cylindrical shell of the electron barrier 30 is covered with a grid-like structure that allows the incident ions to be detected to pass through. Close to the bottom of the Faraday cup, the hollow cylindrical shell of the electron barrier 30 has an intermediate bottom that proximal covers the Faraday cup of the ion trap 22, except for concentric gaps for the mechanical mounting and electrical connection of the ion trap 22. The electron barrier 30 is electrically connected to a voltage supply device that applies an electrically negative repeller potential to the electron barrier 30.

[0032] The power supply 32 for this electron barrier 30 is not shown in Figure 1 for simplification, but it is also present in the embodiment shown in Figure 1.

Claims

1. The mass spectrometer housing (12) is evacuated by a vacuum pump, An ion source (16) that ionizes gas atoms or gas molecules introduced into the mass spectrometer housing (12) and accelerates the ions within the mass spectrometer housing (12), An ion trap (22) made of a conductive material collects ions generated by the ion source (16), An electrical measuring device (24) is connected to the ion trap (22) and measures the potential or current of the ion trap (22), and estimates the ions collected by the ion trap (22) from the measured potential or current. A gas analysis mass spectrometer (10) comprising a magnetic or electromagnetic deflection device configured to deflect or map ions of a specific mass-to-charge ratio toward the ion trap (22), A mass spectrometer (10) for gas analysis is characterized in that an electron barrier (30) made of a conductive material having a lower potential than the ion trap (22) is placed in the ion trap (22), and is configured to form a counter-electric field for returning electrons emitted from the ion trap (22) back to the ion trap (22).

2. The mass spectrometer (10) according to claim 1, characterized in that the electron barrier (30) is a component separate from the ion trap (22).

3. The mass spectrometer (10) according to claim 1 or 2, wherein the electron barrier (30) is electrically separate from the ion trap (22) and surrounds the ion trap (22).

4. A mass spectrometer (10) according to any one of claims 1 to 3, wherein the electron barrier (30) is a ring electrode or designed as a cylinder, and the dimension in the direction of the central longitudinal axis is smaller than the diameter.

5. A mass spectrometer (10) according to any one of claims 1 to 4, characterized in that the mass spectrometer (10) is a sector-field mass spectrometer or a quadrupole mass spectrometer.

6. A mass spectrometer (10) according to any one of claims 1 to 5, wherein a shield electrode (26) is provided which is connected to a positive potential and configured to shield the ion trap (22) from the intrusion of electrons, and the shield electrode (26) is preferably a Wehnelt cylinder and / or forms an anode.

7. The mass spectrometer (10) according to claim 6, characterized in that the electron barrier (30) is designed as a grid between the ion trap (22) and the shield electrode (26).

8. A mass spectrometer (10) according to any one of claims 1 to 7, characterized in that the ion trap (22) is a plate made of a metal material or includes a plate made of a metal material.

9. A gas analysis method using a mass spectrometer (10) according to any one of claims 1 to 8, A step of generating a high vacuum state inside the mass spectrometer housing (12), The process involves generating and accelerating ions by ionizing the attracted gas atoms or gas molecules using the ion source (16), A process of generating a magnetic field or electromagnetic field using a deflection device to deflect or map a specific type of gas toward the ion trap (22), A step of capturing deflected or mapped ions using the ion trap (22), The process is characterized by measuring the potential or current of the ion trap (22) using the electrical measuring device (24), and estimating the type or amount of gas of the captured ions from the measured potential or current. A gas analysis method characterized in that electrons emitted from the ion trap (22) are returned to the ion trap (22) by a negative counter-electric field generated by the electron barrier (30).

10. A gas analysis method according to claim 9, characterized by a step of shielding the ion trap (22) from coherent ions by repelling coherent ions with a positive counter-electric field generated by the shield electrode (26).

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