Gas valve for an ion guide

Abstract

An ion optical arrangement (1) for a mass spectrometer comprises: an electrode (11) that defines an ion optical path; a housing (18) for accommodating the electrode; a voltage source for supplying a voltage to the electrode to generate an electric field; and a valve for allowing gas to enter and / or leave the housing. The valve comprises an electrostatic mechanism and / or a pneumatic mechanism. The electrostatic mechanism may comprise flexible foils (30, 31) configured to cover at least one opening (16) in the ion optical arrangement when a first voltage is applied and to be spaced apart from the at least one opening when a second voltage is applied. The pneumatic mechanism may comprise a Bourdon tube.

Description

Technical Field

[0001] The present invention relates to isotope ratio mass spectrometry (MS). More specifically, the present invention may relate to interference-resistant, high-resolution, multi-receiver isotope ratio mass spectrometry and elemental analysis, for example, in combination with a collision cell and a plasma source such as an inductively coupled plasma (ICP) source. Background Art

[0002] Multi-receiver ICP-MS is a well-established method for high-precision and accurate isotope ratio analysis. Applications involve the fields of geochronology, geochemistry, cosmochemistry, biogeochemistry, environmental science, and life science. Precise and accurate isotope ratio measurements often provide unique information to gain a deeper understanding of scientific questions that cannot be answered by any other analytical technique. However, elemental and molecular interferences in the mass spectrometer limit the achievable analytical precision and accuracy.

[0003] These interferences are present in the sample material itself, or are generated during sample preparation by sources of contamination (used chemical reagents, cleanliness of sample containers, and stepwise separation during sample purification), or even in the ion source or mass spectrometer. Such interference problems can be solved by the following means:

[0004] 1. Using a high mass resolution mass analyzer, which discriminates interferences by detecting the slight mass differences of the interferences relative to the sample ions;

[0005] 2. By performing sample preparation and chemical separation of the interferences prior to mass analysis; and / or

[0006] 3. By using a collision cell integrated into the mass analyzer.

[0007] In the collision cell, chemical interferences are removed by taking advantage of the different cross-sections of molecular and elemental species in a pressurized collision cell, either through chemical reactions or through kinetic discrimination, which results in different kinetic energy losses for molecular ions and elemental ions. By means of a high-pass energy filter behind the collision cell, lower energy molecular species can be discriminated.

[0008] A collision cell is an enclosed volume within the path of an ion beam, pressurized with a collision gas to induce interactions (i.e., collisions and / or chemical reactions between ions and gas molecules). To generate efficient collisions and chemical reactions within the collision cell, the ions are preferably at a low ion beam energy of only a few electron volts (eV). The collision cell is typically a multipole ion guide, powered by an RF field to direct the ions through the collision cell. To obtain a suitable gas pressure, the multipole ion guide is enclosed in a compact volume with a small inlet and inlet aperture, typically in the range of 1 to 3 mm in diameter. A collision cell coupled to a multi-receiver mass spectrometer is disclosed in UK patent application GB 2 546060 (Thermo Fisher Scientific (Bremen) & The University of Bristol).

[0009] Ions of different masses but with the same energy travel through the time-dependent oscillating field of the collision cell at different speeds, so the ion trajectories are mass-dependent. In other words, the trajectory depends on the mass of the ions traveling through the RF field. This effect is known as "nodding". This can particularly cause problems at the exit of the multipole structure, where ions of different masses may leave at different angles.

[0010] The mass-dependence transmitted by the collision cell can be a problem for accurate isotope ratio measurements, even when it is small. However, for some analytical applications, there is no other option but to use a collision cell to remove isobaric interferences.

[0011] For samples without interferences, it is advantageous to avoid the low-energy path of the ions through the radio frequency (RF) multipole collision cell optics and to exclude any uncertainties in the discrimination effects (i.e., chemical effects and nodding effects) induced in the collision cell.

[0012] It should be noted that the unwanted "nodding effect" is not limited to the collision cell and can also occur in other ion optical arrangements such as mass filters.

[0013] One way to solve this problem is to install a second beam path in the mass spectrometer, where the ion beam is deflected off-axis before the collision cell to bypass the collision cell and finally deflected back onto the optical axis of the mass spectrometer. Such a dual-path ion optical arrangement is described in UK patent application GB 2 535 754 (Nu Instruments). It allows switching between a low-energy collision cell beam path and an off-axis static high-energy beam path. This results in a rather complex setup with several ion beam deflectors, causing image aberrations and alignment problems. Summary of the Invention

[0014] The object of the present invention is to provide an ion optical arrangement for a mass spectrometer, such as a collision cell or a mass filter, which can largely avoid the nodding problems associated with existing RF-driven ion optical devices and is simpler and more compact than prior art dual-path arrangements. This object can be achieved by a collision cell having two operating modes: a pressurized mode in which there is a collision gas and a vacuum mode in which there is substantially no collision gas. However, this requires a rapid switch between the two modes and thus a rapid pressurization and evacuation of the collision cell. This in turn requires a valve that can be operated very quickly and efficiently.

[0015] Accordingly, a further object of the present invention is to provide a valve for an ion optical arrangement such as, but not limited to, a collision cell, which can be switched very rapidly.

[0016] Accordingly, the present invention provides an ion optical arrangement for a mass spectrometer, the ion optical arrangement comprising:

[0017] - electrodes that define an ion optical path;

[0018] - a housing for accommodating the electrodes;

[0019] - a voltage source for supplying a voltage to the electrodes to generate an electric field; and

[0020] - a valve for allowing gas to enter and / or leave the housing,

[0021] wherein the valve comprises an electrostatic mechanism.

[0022] By providing an ion optical arrangement with an electrostatic valve mechanism, the housing of the ion optical arrangement can be opened or closed quickly, thus avoiding the use of conventional valves. Since in some applications the main function of the valve is to rapidly release pressure, the valve in the ion optical arrangement can also be referred to as a pressure release mechanism.

[0023] The electrostatic mechanism can comprise a flexible foil that is configured to cover at least one opening in the ion optical arrangement when a first voltage is applied and to be spaced apart from the at least one opening when a second voltage is applied. The flexible foil, which can have a thickness of less than 1 mm, preferably less than 0.2 mm, has a very small mass and can move rapidly.

[0024] The flexible foil can comprise at least one insulating layer and at least one conductive layer. In some embodiments, the flexible foil can comprise only a conductive layer. Generally, two insulating layers can be used to insulate the conductive layer from the conductive outer wall of the housing and from a conductive support element. Each of the two insulating layers can be attached to the conductive layer or to one of the housing and the support element. A flexible foil having more than one conductive layer can also be used, such as two or three conductive layers separated by insulating layers.

[0025] In an embodiment, a flexible foil is disposed in the space between the housing and the support element. That is, the flexible foil may cover a wall portion of the housing containing one or more openings. The support element may be a plate, which preferably has a shape similar to the wall portion of the housing in order to provide a substantially uniform space. Thus, depending on the shape of the housing, the support element may be flat or curved. The support element is at least partially conductive.

[0026] The ion optical arrangement according to the present invention may further comprise a pump for pressurizing the ion optical arrangement. The pump may be used, for example, to pressurize the ion optical arrangement at least during a first operating mode in which the ion optical arrangement functions as a collision cell. The pressurizing pump may be turned off in a second operating mode in which no collision gas is used. In some embodiments, the pump may be reversed in the second operating mode.

[0027] In an embodiment, the ion optical arrangement may include a switchable pumping cross-section in the collision cell housing for establishing a higher gas pressure in a first operating mode (low cross-section) and effectively pumping the collision cell in a second operating mode (high cross-section). The first operating mode may be a low energy mode, while the second operating mode may be a high energy mode. That is, the ions passing through the collision cell have a relatively low energy in the first operating mode in the presence of gas and a relatively high energy in the second operating mode in which there is virtually no gas.

[0028] The present invention also provides an electrostatic valve for an ion optical arrangement, wherein the electrostatic valve includes a flexible foil configured to cover at least one opening in the ion optical arrangement when a first voltage is applied and to be spaced apart from the at least one opening when a second voltage is applied.

[0029] The present invention further provides an ion optical arrangement for a mass spectrometer, the ion optical arrangement comprising:

[0030] - electrodes that define an ion optical path;

[0031] - a housing for accommodating the electrodes;

[0032] - a voltage source for supplying a voltage to the electrodes to generate an electric field; and

[0033] - a valve for allowing gas to enter and / or leave the housing,

[0034] wherein the valve includes a pneumatic mechanism.

[0035] A suitable pneumatic mechanism can provide a good alternative to an electrostatic mechanism. In an embodiment, the pneumatic mechanism includes a Bourdon tube. The Bourdon tube itself is known and generally consists of a circular or coiled tube that straightens when inflated. Examples of Bourdon tubes are disclosed in US 3188 419. A Bourdon tube that can be operated by gas pressure can switch quickly.

[0036] In an embodiment, the Bourdon tube is arranged to open the housing when inflated and close the housing when deflated. The housing of the ion optical arrangement can include a hinged flap capable of closing an opening in the housing, and the hinged flap is operated by the Bourdon tube.

[0037] The ion optical arrangement of the present invention can be a collision cell or a collision / reaction cell. However, the present invention is not limited thereto, and the ion optical arrangement of the present invention can be any ion guide whose gas pressure is variable.

[0038] The present invention further provides a pneumatic valve for an ion optical arrangement, the pneumatic valve including: a hinged flap arranged to open and / or close at least one opening in the ion optical arrangement; and a Bourdon tube arranged to operate the flap.

[0039] In another embodiment, the valve mechanism can include a relay to operate the mechanism electrically. In some embodiments, the Bourdon tube and the relay can be advantageously combined.

[0040] The present invention also provides a mass spectrometer including the ion optical arrangement as described above. The mass spectrometer according to the present invention can further include at least one ion source, such as an inductively coupled plasma ion source, and at least one detector arrangement, such as a multi-receiver detector arrangement, and preferably also includes a mass filter. However, the present invention is not limited to mass spectrometers having a plasma source.

[0041] It should be noted that the ion optical axis along which ions pass through the ion optical arrangement can be straight, but this is not necessary. In some embodiments, the ion optical axis through the collision cell is straight, but the path of the ions through the ion optical arrangement can be non-straight and can be partially or completely curved, such as in the arrangement of GB 2 546 060 for example. Description of the Drawings

[0042] Figure 1 Schematically shows a multipole collision / reaction cell in which the present invention can be utilized.

[0043] Figures 2A to 2C Schematically shows an embodiment of an electrostatic pressure release mechanism that can be used with Figure 1 the collision / reaction cell or with another ion optical arrangement.

[0044] Figure 3 Schematically shows an ion optical arrangement having an electrostatic valve mechanism according to the present invention.

[0045] Figure 4A and 4B Schematically shows an embodiment of a pneumatic pressure release mechanism, which can be used in conjunction with Figure 1 a collision / reaction cell or with another ion optical arrangement.

[0046] Figure 5 Schematically shows a mass spectrometer incorporating an ion optical arrangement according to the present invention. Detailed Description

[0047] As described above, the object of the present invention is to allow an ion optical arrangement such as a collision / reaction cell or a mass filter to be rapidly switched between a pressurized state and a decompressed or vacuum state, or between a high-pressure state and a low-pressure state. Such an ion optical arrangement may also be referred to as a switchable ion guide.

[0048] The pressurized state may be, for example, a state in which a collision gas is used, which may be a state in which the ions have relatively low energy. The decompressed state may be a state in which the collision gas is not desired, which may be a state in which the ions have relatively high energy.

[0049] When operating an ion optical arrangement such as a collision cell in a pressurized mode and in a vacuum (i.e., non-pressurized) mode, it is generally required that the ion optical arrangement can be rapidly pressurized and decompressed. Specifically, a rapid and effective pressure release mechanism is desired.

[0050] Figure 1 Schematically shows an ion optical arrangement in which the present invention can be applied. The collision cell 1 is shown as including a housing 18 that houses a multipole arrangement. In the illustrated example, the multipole arrangement is a hexapole arrangement, which includes six elongated poles or rods 11 that form electrodes. The multipole arrangement has a symmetry axis that serves as the ion optical axis. A radio frequency (RF) voltage can be fed to opposite pairs of poles 11 to generate an RF electric field. Ions can enter the collision cell through an inlet hole 13 and leave the collision cell through an outlet hole 15. The RF field generated by the multipole arrangement focuses the ions on the longitudinal axis of the arrangement. This is particularly relevant when a collision gas is present in the collision cell, as collisions may cause the ions to deviate from their path.

[0051] The present invention provides a valve mechanism that is particularly suitable for a collision cell or other ion guide having pressurized and vacuum operating modes.

[0052] Figure 2ASchematically shown is an electrostatic valve mechanism that can be used, for example, in a collision cell. The exemplary collision cell 1 is shown to include a housing 18 that houses a rod 11 therein. An ion beam IB can pass through the collision cell 1 through openings in each of a front plate 12 and a rear plate 14. In the illustrated embodiment, a portion of the wall of the housing 18 has a through hole 16 that can be closed by a movable foil. The foil is located in a gap between the housing 18 and a support element 19, which is formed by a plate in this case. Both the housing 18 and the plate 19 contain conductive materials and can be made of metal or at least contain a metal layer or other conductive layer. The plate 19 that extends substantially parallel to the housing 18 can be flat, but can alternatively be curved to accommodate any curvature of the housing 18.

[0053] In the illustrated embodiment, the foil includes two layers: a conductive layer 30 and an electrically insulating layer 31. A further electrically insulating layer 32 is attached to the plate 19. In an alternative embodiment, the foil consists of a single layer: only the conductive layer 30, in which case the insulating layers 31 and 32 are permanently attached to the housing 18 and the plate 19, respectively. In yet another alternative embodiment, the foil consists of three layers: the conductive layer 30 and two insulating layers 31 and 32. Further layers can be added as long as the foil remains sufficiently flexible. Suitable materials for the insulating layers 31 and 32 are Kapton, but other materials can also be used, such as other polyimides. For example, the conductive layer can be made of copper foil.

[0054] As described above, the flexible foil is located in the gap between the housing 18 and the plate 19. One edge of the foil can be attached to the housing 18, and the opposite edge can be attached to the plate 19 such that the foil bridges the gap. By applying a DC voltage to the conductive layer, the position of the foil can be changed, as Figure 2A indicated by the arrow, which indicates the possible movement of the substantially S-shaped gap-bridging portion of the foil.

[0055] Referring Figure 2B , the housing 18 is typically connected to ground (GND). The conductive plate 19 can be connected to a high voltage indicated by HV in Figure 2B , thereby creating a voltage difference across the gap between the housing 18 and the plate 19. If the conductive layer 30 is connected to the high voltage, the foil will be repelled by the plate 19 and attracted by the housing 18. Therefore, the foil will tend to move towards the housing, and the S-shaped gap-bridging portion will move to the right (also see Figure 2A ). In other words, in Figure 2B , the electrical force F that pulls the foil towards the housing el creates a mechanical force F that is to the right m . The foil will cover the through hole 16, and the interior of the collision cell will be enclosed.

[0056] Referring Figure 2C, the via 16 can be opened by connecting the conductive layer 30 to ground instead of to a high voltage (HV). This will cause the foil to be repelled by the housing 18 and attracted to the plate 19, which in turn causes the S-shaped spacer bridging portion to move to the left (see also Figure 2A ). In other words, in Figure 2C , the electrical force F el that pulls the foil towards the plate 19 generates a mechanical force F m towards the left. The foil will no longer cover the via 16, and the interior of the collision cell will be open to the surrounding atmosphere.

[0057] Since the movement of the foil is voltage-controlled, this can be switched extremely quickly, and since the foil can have a very low mass, the movement of the foil can be very fast. Thus, the pressure within the collision cell 1 can be adjusted very rapidly, and the switching between the pressurized state and the evacuated state can be carried out almost instantaneously.

[0058] In Figure 3 , an embodiment of the electrostatic valve is shown in perspective view, as well as an ion guide and a circuit for operating the electrostatic valve. The ion guide is shown to have a housing 18 with an inlet opening 13 for ions. The flexible foil arrangement is shown to comprise a conductive layer 30, a first insulating layer 31, and a second insulating layer 32. In the embodiment shown, the insulating layers 31 and 32 are permanently attached to the housing 18 and the plate 19 respectively, and the conductive layer 30 is the only movable layer. In some embodiments, two or more conductive layers separated by additional insulating layers may be used.

[0059] In the embodiment shown, an opening 16 in the housing is also present in the first insulating layer 31. In embodiments where the first insulating layer 31 is not attached to the housing but to the conductive layer 30, the opening 16 may not be present in the first insulating layer 31.

[0060] Figure 3 A circuit for operating the electrostatic valve is also schematically shown. A voltage source 40 generates a voltage U. A switch 42 allows the conductive layer 30 to be connected to ground (as shown) or to the voltage U. The plate 19 is shown to be permanently connected to the voltage U. As shown, if the conductive layer 30 is connected to ground, it will be attracted to the plate 19 and move towards the plate. Conversely, if the conductive layer 30 is connected to the voltage source 40, the conductive layer 30 will be repelled by the plate 19 and move towards the housing 18, thus closing the opening 16.

[0061] Figure 4A and 4BAn air-operated mechanism 20 for adjusting the pumping cross-section of an ion guide, which ion guide has, for example, a collision cell housing 18 with a rod 11, is shown. The pneumatic valve mechanism 20 is shown to comprise a gate or flap 21 which is connected via a hinge 22 to the housing 18 of the collision cell 1. The flap 21 can be operated by an actuator 23, one end of which is connected to the flap 21 and the other end of which is connected to a support element 24 which is attached to the housing 18.

[0062] Figure 4A and 4B The actuator 23 shown in and is a Bourdon tube. The Bourdon tube comprises a bent tube. If the pressure difference between the inner part and the outer part of the tube increases, the bending radius of the bent tube can decrease. For this purpose, a gas tube 25, which is also connected to the support element 24, is connected to the actuator 23. In the illustrated embodiment, when the gas pressure in the gas tube 25 is higher than the gas pressure around the actuator 23, gas flows from the gas tube 25 through a channel in the support element 24 into the actuator 23. By allowing gas to flow into the actuator, the bending radius of the actuator decreases (the actuator straightens) and the flap opens. Conversely, when the gas pressure in the gas tube 25 is lower than that of the actuator 23, gas flows from the actuator 23 through the support element 24 into the gas tube 25. By allowing gas to flow out of the actuator, the bending radius of the actuator increases (the actuator bends) and the flap closes.

[0063] Thus, by providing a pressure difference between the gas tube 25 and the air (or other gas) outside the actuator 23, the flap can be quickly opened or closed, allowing the gas pressure inside the collision cell 1 to quickly adopt the gas pressure on the outside of the collision cell.

[0064] It should be noted that the collision cell 1 can be accommodated in an environment close to vacuum, while the gas tube can be connected to an environment at atmospheric pressure. The gas used to inflate the inflatable actuator can be air. Since the internal volumes of the actuator 23 and the gas tube 25 can be small, only a small amount of air or other gas is required to inflate the actuator. This air or other gas can be provided by a gas reservoir or by a pump. Thus, a small pump or valve can be sufficient to indirectly operate a relatively large flap.

[0065] By using a Bourdon tube or a similar actuator, rapid and effective pressure regulation of a collision cell or other ion guide can be achieved.

[0066] Figure 5The exemplary mass spectrometer 10 schematically shown therein includes a multipole cell 1, which may be a collision cell as described above, but may be replaced by an ion guide without a multipole arrangement. The mass spectrometer 10 may further include a plasma source 1, such as an ICP (inductively coupled plasma) source for generating an ion beam IB1. The mass spectrometer may further include a mass filter 3, such as a sector magnetic field mass filter. In the sector magnetic field mass filter, the ion beam IB1 is separated into split beams IB2 having different m / z (mass-to-charge) ratios, which may be detected by a detector assembly 4, which may be a multi-detector assembly. The mass spectrometer 10 may further include: a pump for reducing the gas pressure in the collision cell 1; a valve associated with the pump; a voltage source 5 for supplying DC and AC (RF) voltages to the collision cell 1; and a controller for controlling the various components of the mass spectrometer 10. The voltage source 5 may correspond to the voltage source 40 in Figure 3 The valve may include a foil-based valve and / or a Bourdon tube-based valve as described above.

[0067] Aspects of the present invention include:

[0068] a) An ion guide, such as a multipole collision cell, which can be rapidly switched between a first operating mode in which collision gas and / or reaction gas is present and a second operating mode in which no gas is used.

[0069] b) A mechanism for allowing rapid switching between the first operating mode and the second operating mode.

[0070] These aspects of the present invention may be used alone or in combination.

[0071] Although the present invention has been mainly described above with reference to an ion optical arrangement including a multipole such as a hexapole, the present invention is not limited thereto and may also be used for other types of ion guides.

[0072] Thus, those skilled in the art will understand that the present invention is not limited to the embodiments shown, and many additions and / or modifications may be made without departing from the scope of the present invention as defined in the appended claims.

Claims

1. An ion optical arrangement for a mass spectrometer, the ion optical arrangement comprising: - an electrode that defines an ion optical path; - a housing for accommodating the electrode; - a voltage source for supplying a voltage to the electrode to generate an electric field; and - a valve for allowing gas to enter and / or leave the housing, wherein the valve comprises an electrostatic mechanism, and wherein the electrostatic mechanism comprises a flexible foil configured to cover a plurality of openings in the ion optical arrangement to achieve a pressurized state when a first voltage is applied and to be spaced apart from the plurality of openings to achieve a decompressed state when a second voltage is applied, wherein the plurality of openings extend along the electrode.

2. The ion optical arrangement according to claim 1, wherein the flexible foil comprises at least one insulating layer and at least one conductive layer.

3. The ion optical arrangement according to claim 1 or 2, wherein the flexible foil is arranged in a gap between the housing and a support element.

4. The ion optical arrangement according to claim 1 or 2, further comprising a pump for pressurizing the ion optical arrangement.

5. The ion optical arrangement according to claim 1 or 2, which is a collision cell or a collision / reaction cell.

6. A mass spectrometer comprising the ion optical arrangement according to any one of claims 1 to 5.

7. The mass spectrometer according to claim 6, further comprising at least one ion source and at least one detector arrangement.

8. The mass spectrometer according to claim 7, wherein the at least one ion source is an inductively coupled plasma ion source.

9. The mass spectrometer according to claim 7, wherein the at least one detector arrangement is a multi-receiver detector arrangement.

10. The mass spectrometer according to claim 7, further comprising a mass filter.

Images