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DE-112019001934-B4 - Method and apparatus for operating a vacuum interface of a mass spectrometer

DE112019001934B4DE 112019001934 B4DE112019001934 B4DE 112019001934B4DE-112019001934-B4

Abstract

Method for operating a vacuum interface (3) of a mass spectrometer (10), wherein the vacuum interface (3) comprises an evacuated expansion chamber downstream of a plasma ion source (6), the plasma ion source (6) being located at a pressure between atmospheric pressure and 100 mbar, the expansion chamber having a first aperture interface with the plasma ion source (6) to form an expanding plasma downstream of the first aperture, and a second aperture downstream of the first aperture from the plasma to skim off the expanding plasma to form a skimmed expanding plasma; wherein the expansion chamber is pumped by an interface vacuum pump (40) to provide an interface pressure in the expansion chamber; wherein the method comprises the use of a controller (50) for automatically controlling the throughput of the interface vacuum pump (40) to control the interface pressure depending on operating modes of the mass spectrometer (10), wherein the throughput of the interface vacuum pump (40) is controlled by changing the pump speed by varying the operating voltage and/or the operating current of the interface vacuum pump (40), and the throughput of the interface vacuum pump (40) is controlled depending on one or more operating conditions of the plasma ion source (6) and/or one or more elements of interest to be mass-analyzed by the mass spectrometer (10), wherein the controller (50) is connected to a visual display unit (VDU) so that the initially automatically controlled flow rate of the interface vacuum pump (40) and/or the interface pressure is displayed to a user, and wherein the user can automatically Control (50) via a graphical user interface (GUI) located on the VDU is displayed, and an input device (72) is overridden and a specific throughput of the vacuum pump (40) and/or a specific interface pressure is manually set.

Inventors

  • Christoph Wehe
  • Marcus Manecki
  • Georgina Thyssen
  • Daniel Precht
  • Sven Wohlgethan

Assignees

  • THERMO FISHER SCIENTIFIC (BREMEN) GMBH

Dates

Publication Date
20260513
Application Date
20190404
Priority Date
20180413

Claims (20)

  1. Method for operating a vacuum interface (3) of a mass spectrometer (10), wherein the vacuum interface (3) comprises an evacuated expansion chamber downstream of a plasma ion source (6), the plasma ion source (6) being located at a pressure between atmospheric pressure and 100 mbar, the expansion chamber having a first aperture interface with the plasma ion source (6) to form an expanding plasma downstream of the first aperture, and a second aperture downstream of the first aperture from the plasma to skim off the expanding plasma to form a skimmed expanding plasma; wherein the expansion chamber is pumped by an interface vacuum pump (40) to provide an interface pressure in the expansion chamber; wherein the method comprises using a controller (50) for automatically controlling the flow rate of the interface vacuum pump (40) to control the interface pressure depending on operating modes of the mass spectrometer (10), wherein the flow rate of the interface vacuum pump (40) is controlled by changing the pump speed by varying the operating voltage and/or the operating current of the interface vacuum pump (40) and the flow rate of the interface vacuum pump (40) in dependence on one or more operating conditions of the plasma ion source (6) and/or one or more elements of interest to be mass-analyzed by the mass spectrometer (10), wherein the control (50) is connected to a visual display unit (VDU) so that the initially automatically controlled flow rate of the interface vacuum pump (40) and/or the interface pressure is displayed to a user, and wherein the user overrides the automatic control (50) via a graphical user interface (GUI) displayed on the VDU and an input device (72) and manually sets a specific flow rate of the vacuum pump (40) and/or a specific interface pressure.
  2. Procedure according to Claim 1 , wherein the plasma ion source (6) is an inductively coupled plasma (ICP) ion source and the one or more operating conditions include a plasma temperature, a plasma torch position and/or a plasma gas flow.
  3. Method according to one of the preceding claims, wherein the control (50) comprises a computer and associated control electronics which has an interface with the interface vacuum pump (40).
  4. Method according to one of the preceding claims, wherein the interface vacuum pump (40) is a backing pump for a high vacuum pump which pumps a high vacuum region of the mass spectrometer (10), and the interface pressure in the expansion chamber is controlled such that it is in a range of 0.1 to 10 mbar.
  5. Method according to one of the preceding claims, comprising acquiring data from the mass spectrometer (10) at the controller (50) and using the data to adjust the throughput of the interface vacuum pump (40) in order to optimize the detection sensitivity of the mass spectrometer (10) for at least one element.
  6. A method according to one of the preceding claims, further comprising changing the operating conditions of the plasma ion source (6) from a first operating condition to a second operating condition or vice versa, and in each case automatically controlling the throughput of the interface vacuum pump (40) from a first throughput when operating the plasma under the first operating condition to a second throughput when operating the plasma under the second operating condition, wherein the first and the second throughput are different from each other.
  7. Procedure according to Claim 6 , where the first operating condition is a hot plasma condition and the second operating condition is a cold plasma condition.
  8. Procedure according to Claim 6 or 7 , wherein the first throughput provides a first interface pressure in the vacuum interface (3) to optimize the detection sensitivity for at least one element of the sample to be analyzed using the first operating condition, and the second throughput provides a second interface pressure to optimize the detection sensitivity for at least one element that is mass analyzed using the second operating condition.
  9. Procedure according to Claim 8 , wherein the at least one element whose detection sensitivity is optimized under the first operating condition differs from the at least one element whose detection sensitivity is optimized under the second operating condition.
  10. Procedure according to Claim 8 or 9 , whereby the first and second interface pressures are controlled so that they are essentially the same.
  11. Method according to one of the preceding claims, wherein more than two different operating conditions are provided, each of which has a respective vacuum pump throughput that is set by the control (50).
  12. A method according to any one of the preceding claims, further comprising providing a manometer (60) in the expansion chamber, (i) measuring the interface pressure using the manometer (60), (ii) providing signals representative of the measured pressure to the controller (50), and (iii) comparing the measured pressure with a set pressure using the controller (50), and if the controller (50) determines that there is a difference between the If the difference between the measured and the set pressure is not met, the controller (50) adjusts the flow rate of the interface vacuum pump (40) to reduce the difference between the measured pressure and the set pressure, with steps (i) - (iii) being repeated in a feedback loop to keep the interface pressure essentially at the set pressure.
  13. Device for operating a vacuum interface of a mass spectrometer (10), comprising: a plasma ion source (6) for generating a plasma between atmospheric pressure and 100 mbar; an evacuated expansion chamber downstream of the plasma ion source (6), the expansion chamber having a first aperture that interfaces with the plasma ion source (6) to form an expanding plasma downstream of the first aperture, and a second aperture downstream of the first aperture to skim off the expanding plasma to form a skimmed expanding plasma; the expansion chamber being pumped by an interface vacuum pump (40) to provide an interface pressure in the expansion chamber; and a controller (50) configured to automatically control the flow rate of the interface vacuum pump (40) depending on the operating modes of the mass spectrometer (10) by changing the pump speed through varying the operating voltage and/or the operating current of the interface vacuum pump (40), wherein the flow rate of the interface vacuum pump (40) is controlled depending on one or more operating conditions of the plasma ion source (6) and/or one or more elements of interest to be mass-analyzed by the mass spectrometer (10), in that the controller (50) is connected to a visual display unit (VDU) so that the flow rate of the interface vacuum pump (40) and/or the interface pressure is displayed to a user, and wherein the user can instruct the controller (50) to set a specific flow rate of the interface vacuum pump (40) and/or a specific interface pressure by means of a graphical user interface (GUI) displayed on the VDU and an input device (72).
  14. Device according to Claim 13 , wherein the controller (50) is configured to automatically control the throughput of the interface vacuum pump (40) depending on a plasma condition and/or a measurement mode.
  15. Device according to Claim 13 or 14 , wherein the plasma ion source (6) is an inductively coupled plasma (ICP) ion source.
  16. device according to one of the Claims 13 until 15 , wherein the control (50) comprises a computer and associated control electronics which has an interface with the interface vacuum pump (40).
  17. device according to one of the Claims 13 until 16 , wherein the throughput of the interface vacuum pump (40) can be continuously or quasi-continuously adjusted by the controller (50) over a range of throughput rates.
  18. device according to one of the Claims 13 until 17 , wherein the interface vacuum pump (40) is a backing pump for a high vacuum pump which pumps a high vacuum region of the mass spectrometer (10).
  19. device according to one of the Claims 13 until 18 , wherein the controller (50) is configured to automatically adjust the throughput of the interface vacuum pump (40) from a first throughput when operating the plasma ion source (6) under a first operating condition to a second throughput when operating the plasma ion source (6) under a second operating condition, wherein the first and second throughputs are different from each other.
  20. Device according to Claim 19 , wherein the first operating condition and the second operating condition differ in the temperature of the plasma, wherein preferably the first operating condition is a hot plasma condition and the second operating condition is a cold plasma condition.

Description

Field of invention The invention relates to the field of mass spectrometry and, in particular, to a method and an apparatus for operating a vacuum interface, especially, but not exclusively, an atmosphere-vacuum interface of a mass spectrometer. The method and the apparatus are suitable for use primarily with a plasma ion source, such as an inductively coupled plasma (ICP), microwave-induced plasma (MIP), or laser-induced plasma ion source. For illustrative purposes, the following description focuses on embodiments using inductively coupled plasma mass spectrometry (ICP-MS). background The general principles of ICP-MS are well established. ICP-MS instruments provide robust and highly sensitive elemental analysis of samples down to the parts per trillion (ppt) range and beyond. Typically, the sample is a liquid solution or suspension and is introduced into the plasma via an atomizer as an aerosol in a carrier gas, generally argon or sometimes helium. The nebulized sample enters a plasma torch, which typically comprises several concentric tubes forming corresponding channels and is surrounded at the downstream end by a spiral induction coil. A plasma gas, typically argon, flows into the outer channel, and an electrical discharge is applied to ionize some of the plasma gas. A radio frequency (RF) electric current is applied to the spiral torch coil, and the resulting alternating magnetic field causes the free electrons to be accelerated, resulting in further ionization of the plasma gas. This process continues until a stable plasma state is reached at temperatures typically between 5,000 K and 10,000 K. The carrier gas and the nebulized sample flow through the central burner channel and enter the central region of the plasma, where the temperature is high enough to cause atomization and subsequent ionization of the sample. The sample ions in the plasma must then be shaped into an ion beam for ion separation and detection by the mass spectrometer, which can be provided by, among others, a quadrupole mass analyzer, a mass analyzer with a magnetic and/or electric sector, a time-of-flight mass analyzer, or an ion trap mass analyzer. Thus, in ICP-MS, ions are generated under atmospheric pressure or relatively high pressure (e.g., above 100 mbar) outside the main vacuum system of the spectrometer. Most mass analyzers require a vacuum with a pressure of <5 x 10⁻⁵ mbar. Therefore, an interface region is provided to regulate the transfer from the atmospheric pressure ion source to the high-vacuum mass analyzer (see 1 , which is described below). This typically includes several stages of pressure reduction, ion extraction from the plasma, and ion beam generation, and may include a collision/reaction cell stage to remove potentially interfering ions from the mass analysis. The first stage of pressure reduction is achieved by sampling the plasma through a first aperture in a vacuum interface, typically provided by a sampling cone with an aperture-equipped tip, typically having an inner diameter in the range of 0.5 to 1.5 mm. The sampling cone is the typical component that is connected to the plasma source at atmospheric or relatively high (>100 mbar) pressure. The sampled plasma expands downstream of the first aperture into an evacuated expansion chamber, where the pressure is typically a few mbar (e.g., 1 to 10 mbar). The central portion of the expanding plasma then passes through a second aperture, typically provided by a skimmer cone, into a second evacuation chamber, which has a higher degree of vacuum than the expansion chamber. As the plasma expands through the skimmer cone, its density decreases sufficiently to allow the extraction of ions to form an ion beam using strong electric fields generated by ion lenses downstream of the skimmer cone. The resulting ion beam can be deflected and/or directed to the mass spectrometer by one or more ion deflectors, ion lenses, and/or ion conductors, which can operate with static or time-varying fields. A collision/reaction cell can be provided upstream of the mass spectrometer to remove potentially interfering ions from the ion beam. These are typically argon-based ions (such as Ar+, Ar2 + , ArO + ), but can also include others, such as ionized hydrocarbons, metal oxides, or metal hydroxides. The collision/reaction cell promotes ion-neutral collisions/reactions, whereby the unwanted molecular ions (and Ar + , Ar2 + ) are preferentially neutralized and, together with other neutral ions, are removed from the ion beam. Gas components are either pumped off or dissociated into ions with lower mass-to-charge ratios (m/z) and discarded in a downstream m/z discrimination (mass filter) stage. Alternatively, the analyte ions can be preferentially subjected to mass-shift reactions so that the resulting mass-shifted ions can be separated from the interfering ions in a downstream m/z discrimination stage. US 7,230,232 B2 and US 7,119,330 B2 provide examples of collision/reaction cells used