Ion source

Abstract

To prevent lowering of an output due to formation of an oxide film in an ion source that generates ions accompanying discharge.SOLUTION: In an ion source 100 for generating ions accompanying discharge, at least a surface of an anode 121 of at least a discharge electrode among electrodes included in the ion source is constituted by titanium. Titanium has a property of forming a film on its surface by oxidation and losing no conductivity even when oxidized. Therefore, an output of the ion source does not decrease even when an oxide film is formed on the surface of the anode, and the ion source can be used over a long period of time without performing maintenance such as polishing of the electrode.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an ion source. [Background technology]

[0002] One known ionization method used in mass spectrometers is "proton transfer reaction ionization," a type of chemical ionization (CI). In proton transfer reaction ionization (hereafter referred to as PTR ionization), plasma is generated by generating a discharge in water vapor in a specified space (primary ion source). HO contained in the plasma is then ionized. + The ions (hydronium ions) are extracted into another space (secondary ion source) and reacted with gaseous target molecules, and H is transferred from the ions to the target molecules. + (proton) to ionize the target molecule. PTR ionization is performed by transferring H3O + Because it can ionize substances with higher proton affinity than ions, it can ionize most volatile organic compounds (VOCs) except for low-molecular-weight alkanes such as methane, while not ionizing nitrogen, oxygen, carbon dioxide, nitrous oxide, etc. contained in the atmosphere. Because of these characteristics, mass spectrometers equipped with an ion source that performs the above-mentioned PTR ionization (PTR ionization mass spectrometers) have been widely used in VOC analysis in recent years (see, for example, Non-Patent Document 1).

[0003] In an ion source that performs the above-described PTR ionization (hereinafter referred to as a PTR ion source), a stainless steel electrode (Non-Patent Document 2) or an aluminum electrode is usually used as the discharge electrode. Patent Document 1 describes a PTR ion source equipped with a molybdenum electrode, but molybdenum is expensive and difficult to process, so it is not commonly used as an electrode material in PTR ion sources. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent No. 6,037,587 [Non-patent literature]

[0005] [Non-Patent Document 1] Akira Tani, "A new high-speed analysis method for volatile organic compounds - Proton Transfer Reaction Mass Spectrometer -", Journal of Atmospheric Environment, Vol. 38, No. 4, 2003, pp. A35-A46 [Non-patent document 2] Satoshi Inomata, et.al., "A novel discharge source of hydronium ions for proton transfer reaction ionization: design, characterization, and performance", Rapid Commun Mass Spectrom., Volume 20, Issue 6, 2006, p. 1025-1029, doi: 10.1002 / rcm.2405. Summary of the Invention [Problem to be solved by the invention]

[0006] However, the plasma generated in the PTR ion source contains HO + In addition to ions, various ions and radicals (O + , O.H. * ), and oxygen atoms, and these ions and the like oxidize the discharge electrodes (especially the anodes) provided in the PTR ion source, forming oxide films on the surfaces of the electrodes. In conventional general PTR ion sources, the formation of such oxide films gradually reduces output and eventually causes no discharge to occur, so maintenance such as polishing the electrodes was required to remove the oxide films or to replace the electrodes.

[0007] The present invention has been made in view of the above-mentioned points, and an object of the present invention is to prevent a decrease in output due to the formation of an oxide film on the surface of an electrode in an ion source that generates ions as a result of discharge. [Means for solving the problem]

[0008] The ion source according to the present invention, which has been made to solve the above problems, is an ion source having a discharge electrode and generating ions as a result of discharge by the discharge electrode, wherein the surface of the anode of the discharge electrode is made of titanium. [Effects of the Invention]

[0009] According to the ion source of the present invention having the above-mentioned configuration, in an ion source that generates ions as a result of discharge, it is possible to prevent a decrease in output due to the formation of an oxide film on the electrode surface. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a diagram showing the configuration of the main parts of a PTR ionization mass spectrometer according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing a configuration of a main part of an ion source in the embodiment. [Figure 3] 1 is a graph showing the change in total ion current (TIC) over time in an example and a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0012] The mass spectrometer according to this embodiment is a PTR ionization mass spectrometer (PTR-MS) that includes an ion source 100 that generates ions from a gaseous sample (sample gas) using a proton transfer reaction, and an analysis section 200 that mass-separates and detects the generated ions. Fig. 1 is a schematic diagram showing the overall configuration of the mass spectrometer according to this embodiment, and Fig. 2 is a schematic diagram showing the configuration of the main parts of the ion source 100.

[0013] The ion source 100 includes a substantially cylindrical ion source chamber 110, an anode 121, an extraction electrode 122, and a cathode 123 disposed within the ion source chamber 110, and a power supply 124 (not shown in FIG. 1) that applies a predetermined voltage to these electrodes 121, 122, and 123. The anode 121, extraction electrode 122, and cathode 123 are disposed in this order from one end (inlet end) of the ion source to the other end (outlet end). The anode 121 and the cathode 123 function as discharge electrodes for generating plasma within the ion source chamber 110. The extraction electrode 122 forms a potential gradient between itself and the anode 121, and utilizes this potential gradient to extract predetermined ions (H3O + 2, the extraction electrode 122 is shown as a plate-like electrode having a pore (aperture) for passing ions, but the shape of the extraction electrode 122 is not limited to this. Also, in the same figure, the shape of the anode 121 is shown as a cup-like electrode having a pore on the bottom surface (the surface on the left side in the figure) for passing water vapor, but the shape is not limited to this and may be, for example, a needle-like electrode. In the ion source chamber 110, the space between the anode 121 and the cathode 123 is filled with water vapor to convert H3O + It serves as a primary ion source 131 for generating ions. Also, the space between the cathode 123 and the outlet end of the ion source chamber 110 is filled with HO. + The primary ion source 131 functions as a secondary ion source 132 for generating ions derived from the sample by reacting ions with the sample gas. Furthermore, a water vapor inlet 111 is provided at the inlet end of the ion source chamber 110 for introducing water vapor (corresponding to the raw material gas in this invention) to be sent to the primary ion source 131 into the ion source chamber 110, and a sample gas inlet 112 is provided on the periphery of the ion source chamber 110 for introducing the sample gas into the secondary ion source 132.

[0014] In the ion source 100 according to this embodiment, at least the surface of the anode 121 is made of titanium. The anode 121 may be an electrode body made of stainless steel, aluminum, or the like that is plated or coated with titanium, or the entire electrode may be made of titanium. Although titanium alloys (e.g., titanium to which elements such as aluminum or nickel have been added) can be used as the titanium, pure titanium is preferred in view of ease of processing. Commercially pure titanium belonging to any of JIS Classes 1 to 4 can be used as the pure titanium, but it is preferred to use titanium of JIS Class 1 or JIS Class 2, which has a particularly high purity. When using a titanium alloy, it is preferred to use one with a titanium content of 95% by weight or more (more preferably 99% by weight or more). The extraction electrode 122 and the cathode 123 may be made of any material, but it is preferred that at least their surfaces be made of titanium, similar to the anode 121.

[0015] The analysis unit 200 includes, in order from the side closest to the ion source chamber 110, a first vacuum chamber 210, a second vacuum chamber 220, and a third vacuum chamber 230. The analysis unit 200 has a multistage differential pumping system in which the degree of vacuum is increased stepwise in this order. The first vacuum chamber 210 is evacuated by a dry vacuum pump (DP) 240. The second vacuum chamber 220 and the third vacuum chamber 230 are evacuated by a turbomolecular pump (TMP) 250 and the dry vacuum pump 240 as a roughing pump, respectively. While the analysis unit 200 is configured with three vacuum chambers here, the number of vacuum chambers can be changed as appropriate. The ion source chamber 110 and the second vacuum chamber 220, and the second vacuum chamber 220 and the third vacuum chamber 230, are connected through small ion passage openings 201 and 202. An ion transport unit 221 consisting of an octapole is disposed in the second vacuum chamber 220. A mass separator 231 consisting of a quadrupole mass filter and a detector 232 that detects ions are disposed in the third vacuum chamber 230. Here, the ion transport section 221 is configured using an octapole, but the configuration of the ion transport section 221 is not limited to this. Here, the mass separator 231 is configured using a quadrupole mass filter, but the configuration of the mass separator 231 is not limited to this.

[0016] An opening (not shown) is provided on the peripheral surface of the ion source chamber 110 to connect the ion source chamber 110 to the first vacuum chamber 210, and the inside of the secondary ion source 132 is evacuated through the opening, and neutral particles and unnecessary ions in the primary ion source 131 are discharged to the outside.

[0017] The operation of the mass spectrometer according to this embodiment during sample analysis will now be described. First, the ion source chamber 110, the first vacuum chamber 210, the second vacuum chamber 220, and the third vacuum chamber 230 are evacuated to a predetermined vacuum level using the dry vacuum pump 240 and the turbomolecular pump 250, respectively. Then, water vapor is introduced into the ion source chamber 110 through the water vapor inlet 111. Then, a predetermined voltage is applied between the anode 121 and the cathode 123 to generate a discharge between the electrodes 121 and 123, thereby generating water vapor plasma 300 in the primary ion source 131. The water vapor plasma 300 contains various ions, among which HO + Predetermined ions including ions are extracted from the water vapor plasma 300 by the extraction electrode 122 and sent to the secondary ion source 132. Furthermore, a sample gas is introduced into the secondary ion source 132 via the sample gas inlet 112. The HO sent to the secondary ion source 132 + The ions (corresponding to primary ions in the present invention) are accelerated by a potential gradient formed in the secondary ion source 132 by electrodes and a power supply (not shown), and collide with molecules of various components (sample components) contained in the sample gas. At this time, the proton affinity of the sample components is such that HO + If the proton affinity of the sample component is higher than that of the ion, a proton transfer reaction occurs due to the collision, resulting in ionization of the sample component. Examples of sample component molecules include volatile organic carbon compounds excluding lower alkanes (C≦5) such as methane, and more specifically, formaldehyde, acetaldehyde, methanol, ethanol, dimethyl ether, benzene, toluene, ethylbenzene, isoprene, methyl iodide, ethyl iodide, ethyl chloride, monoterpene, isoprene, and acetone. The generated ions derived from the sample component (corresponding to secondary ions in this invention) are guided to the second vacuum chamber 220 by the pressure difference within the device, and then focused and transported to the third vacuum chamber 230 by the ion transport unit 221 within the second vacuum chamber 220. In the third vacuum chamber 230, the ions are separated based on mass (strictly speaking, m / z) by the mass separation unit 231 and then detected by the detector 232.

[0018] As mentioned above, conventional general PTR-MS has a problem in that an insulating layer made of an oxide film forms on the electrodes included in the ion source with use, resulting in a decrease in the output of the ion source. Furthermore, this problem is particularly pronounced in the anode, which is one of the discharge electrodes for generating a discharge in the ion source. This is thought to be because oxygen-related negative ions generated by the discharge collide with the anode. As mentioned above, in the mass spectrometer according to this embodiment, at least the surface of at least the anode 121, among the electrodes included in the ion source 100, is made of titanium. Titanium forms a film on its surface by oxidation, and has the property of not losing conductivity even when oxidized. The resistivity of titanium oxide is 10 3 Ω·m~10 4 The resistance of the anode 121 is Ω·m, which is sufficiently small for an electrode in the ion source 100. As described above, the ion source 100 and mass spectrometer according to this embodiment can be used for a long period of time without maintenance such as electrode polishing, because the output of the ion source 100 does not decrease even if an oxide film forms on the surface of the anode 121. Furthermore, titanium is relatively inexpensive, and the processing difficulty of pure titanium is similar to that of stainless steel. Therefore, the ion source 100 and mass spectrometer according to this embodiment can be realized without incurring a significant increase in cost. Titanium is also suitable as an electrode material in the ion source 100 because it is nonmagnetic. An oxide film may be formed in advance on the anode 121 in this embodiment. Furthermore, if the cathode 123, the extraction electrode 122, or both are made of titanium, an oxide film may also be formed on the surfaces of these electrodes.

[0019] While specific examples of embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments and modifications are permitted within the spirit and scope of the present invention. For example, in the above-described embodiments, the present invention is applied to a PTR ion source. However, the present invention is not limited to this and may be applied to any ion source that generates ions through discharge. Specifically, the present invention can be applied to an ion source that ionizes a sample by chemical ionization other than PTR ionization, which has a primary ion source that generates primary ions (reagent ions) from a source gas (e.g., water vapor or air) through discharge by a discharge electrode, and a secondary ion source that generates secondary ions (sample molecular ions) that are ions of the sample components by reacting the primary ions with the sample components. Such an ion source may generate primary ions (HO) from air through discharge. + , NO + , O2 + , O - , O2 - , O.H. - , NO2 - , or NO3 - An example of an ion source is a multi-reaction real-time mass spectrometer (also known as a SIFT-MS; Selected Ion Flow Tube Mass Spectrometer) that generates primary ions (e.g., ions) and ionizes sample molecules through ion-molecule reactions with the primary ions. The ion source according to the present invention can also be applied to devices other than mass spectrometers, such as an ion mobility analyzer that separates and detects ions derived from sample components according to ion mobility, or an ion mobility-mass analyzer that separates the generated ions derived from sample components according to ion mobility and then further separates them according to mass-to-charge ratio. [Example]

[0020] In a PTR-MS with a configuration similar to that shown in Figure 1, the time variation of the total ion current (TIC) was measured when a sample was measured using a PTR-MS with an ion source in which the anode of the discharge electrode was made of titanium (JIS type 2 pure titanium) (Example) and a PTR-MS with an anode made of stainless steel (SUS316) (Comparative Example). In both the Example and Comparative Example, the other electrodes in the PTR ion source (i.e., the cathode and extraction electrode of the discharge electrode) were made of stainless steel (same as above). In both the Example and Comparative Example, the flow rate of water vapor introduced into the PTR ion source was 5 sccm, and the flow rate of sample gas introduced into the ion source was 15 sccm. The sample gas had the same composition. Furthermore, the discharge current in the ion source was set to 2 mA or less, and the pressure in the secondary ion source was set to 200 Pa or less.

[0021] The time-dependent changes in TIC in the example and comparative example are shown in Figure 3. Note that "Ti" in the figure indicates the measurement results for the example, and "SUS" in the figure indicates the measurement results for the comparative example. As shown in the figure, short-term fluctuations in TIC were observed in both the example and comparative example, but the comparative example showed a decrease in TIC over time, whereas the example maintained a nearly constant TIC over 600 minutes (10 hours).

[0022] [Aspect] It will be apparent to those skilled in the art that the above-described exemplary embodiments are examples of the following aspects.

[0023] (Item 1) An ion source according to one aspect of the present invention has a discharge electrode and generates ions as a result of discharge by the discharge electrode, and the surface of the anode of the discharge electrode is made of titanium.

[0024] (Item 2) The ion source according to item 2 is the ion source according to item 1, wherein the titanium is pure titanium.

[0025] (Item 3) The ion source according to item 3 is the ion source according to item 1 or 2, a primary ion source having the discharge electrode and generating primary ions from a raw material gas as a result of discharge by the discharge electrode; a secondary ion source that generates secondary ions, which are ions of the sample molecules, by reaction between the primary ions and the sample molecules; It has the following characteristics.

[0026] (Item 4) The ion source according to item 4 is the ion source according to any one of items 1 to 3, wherein the ion source generates ions by a proton transfer reaction.

[0027] (Item 5) A mass spectrometer according to item 5 is a mass spectrometer having an ion source according to any one of items 1 to 4. [Explanation of symbols]

[0028] 100...Ion source 110...Chamber 121...Anode 122...Extraction electrode 123...Cathode 200…Analysis Department 300...Water vapor plasma

Claims

1. An ion source having a discharge electrode, which generates ions as a result of discharge by the discharge electrode, wherein the surface of the anode of the discharge electrode is made of titanium.

2. 2. The ion source of claim 1, wherein said titanium is pure titanium.

3. a primary ion source having the discharge electrode and generating primary ions from a raw material gas as a result of discharge by the discharge electrode; a secondary ion source that generates secondary ions, which are ions of the sample molecules, by reaction between the primary ions and the sample molecules; 10. The ion source of claim 1, comprising:

4. 10. The ion source of claim 1, wherein the ion source generates ions by a proton transfer reaction.

5. A mass spectrometer comprising the ion source according to any one of claims 1 to 4.

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