Radiant energy – Ionic separation or analysis – Cyclically varying ion selecting field means
Reexamination Certificate
1998-12-22
2001-12-11
Anderson, Bruce (Department: 2881)
Radiant energy
Ionic separation or analysis
Cyclically varying ion selecting field means
C250S292000, C250S297000, C250S288000, C250S282000, C250S281000
Reexamination Certificate
active
06329654
ABSTRACT:
BRIEF DESCRIPTION OF THE INVENTION
This invention relates generally to multipole rod assemblies used as ion guides and mass analyzers and more particularly to a mounting and a construction technique that allows the assembly of such small size multipole rods.
BACKGROUND OF THE INVENTION
Generally, four, six, eight, or more equally spaced round rods assembled in a circle are used as an ion guide in high efficiency capture, transmission, and/or storage of ions in variety of mass spectrometers. In recent years, the use of such multipole ion guides have been practiced widely, especially in mass spectrometers(MS) interfaced with atmospheric pressure ionization (API) sources. In most of the API MS systems, the ions are generally formed at atmospheric pressure and are carried into a high vacuum chamber where mass analysis of the analyte ions are performed. This process involves the removal of the neutral background gas by large capacity pumps between the ion source and the mass analyzer detector in several stages. Unfortunately, loss of valuable analyte ions between the different pumping stages to a greater extent is unavoidable. An ultimate goal for mass analysis of atmospheric pressure ions would be the removal of the background gas while retaining all of the analyte ions through all of the pumping stages.
To greater extent, the multipole ion guides serve this purpose by capturing the ions and letting the neutral gas be pumped through the rods. This purpose is served better if the ion guide is small and able to go continuously through the different pumping stages and yet minimize the gas flow between the pumping stages. The miniature ion beam guide design, construction, and assembly technique of this invention allows the enrichment of such ions with respect to the background neutral gas. Most mass spectrometers use conical interfaces with small sampling orifice to “skim” ions entrained in the neutral gas expanding into vacuum from atmospheric pressure. The small ion guide design allows the multipole rods to be inserted very close inside the cone across from the sampling orifice allowing more of the ions to be captured without distorting the alternating electric field lines.
If there are four rods per assembly, they are most often used as quadrupole mass analyzers for their ability to filter different mass-to-charge ratio ions. The ideal shape of these rods are hyperbolic; however, in most cases, circular cross sectioned rods can be approximated and are used to generate electric field lines similar to the theoretically ideal hyperbolic field lines between the rods. The Electric field lines are generated by applying AC and DC voltages between the pairs of electrodes which constitute alternating rods in the assembly. If the rod assembly is to be used as an ion guide, only AC voltage is applied to the alternating rods at 180 degrees out of phase from each other. This allows a wide range of mass-to-charge ratio of ions to be stable and transmitted within the ion guide. If a DC voltage is applied between the pair of electrodes in addition to the AC voltage, the multi-rod assemblies are used as a mass filter for a very narrow molecular weight band of ions by adjusting the ratio between the AC and the DC voltages. By keeping the ion guide design small, the electrical capacitance between the rods can be kept to a minimum consuming less power from the resonant driving circuitry.
The overall performance characteristics of an ion guide or a quadrupole mass analyzer judged by its ion transmission efficiency, mass range, sensitivity, and mass resolution is to a high degree determined by the accuracy of the multipole rod assembly. The straightness of the rods, the tolerance build up on all three dimensions of the assembly all play an important role in the accuracy of the results produced by a mass spectrometer. And as the size of the multipole rod assemblies get smaller, it gets harder to maintain the required tolerance levels. In larger rod assemblies conventional machining, welding, brazing and soldering practices can be used to fasten the rods together to keep desired tolerances. In smaller rod assemblies however, the machining becomes prohibitively more difficult and expensive due to lack of material strength, difficulty of handling, and availability of tooling. Voltage connection to the larger rod assemblies are also simpler to make with variety of fastening and brazing methods than the voltage connections to the smaller rod assemblies without distorting or bending them.
To maintain straightness of multipole rods in an assembly can be a challenging task when rod diameters of one mm and rod lengths of beyond 75 mm are being considered. Simple welding or soldering techniques can be implemented if stainless steel rods were to be considered. They are one of the most readily available, inexpensive, and easy to work with materials. Unfortunately, they are easy to bend and very hard to maintain straightness at desired diameter and length combinations. To satisfy straightness, metallic materials such as molybdenum, tungsten or gold coated quartz are commonly used in the art. However, with the desired rod diameters of one mm or less, it becomes almost impossible to fasten any support brackets or connections to the rods. Machining, welding or spot welding, brazing, or soldering of these materials to, for example, stainless steel disks as support structures would be prohibitively difficult and expensive.
Assuming one can obtain desirably straight rods, then one has to assemble them together very accurately. All six rods have to be parallel to each other from end to end. The spacings between the rods have to be equal on a circle, and the end of the rods must meet on a same plane perpendicular to the length of the rods. Once all of these requirements are met, then the complete assembly has to be aligned with the interfacing ion optic lenses and the mass analyzer.
The present invention recognizes the difficulties of having many features in a single yet a small design and be able to overcome the above mentioned design constraints.
OBJECTS AND BRIEF DESCRIPTIONS OF THE INVENTION
It is the principal object of this invention to provide an improved miniature multipole rod assembly for ion guides and mass spectrometers that will improve the ion capture, transmission efficiency, sensitivity, and mass resolution of a mass spectrometer system.
It is an object of this invention to provide an ion guide assembly that will go through different pumping stages, keep the opening between the two pumping stages as small as possible, and also have enough distance between the rods to pump out the background gas from inside the multipole rod assembly without compromising the total number of captured ions inside.
It is a further object of the present invention to keep a good mechanical dimensional tolerance between the rods in the assembly.
It is yet a further object of this invention to have a good electrical connection to the miniature rods and also to keep the capacitance of the rods to a minimal value.
It is a feature of the present invention that the entry end of the rods be very close to and shaped to accept maximum number of ions behind a conical sampling orifice and the exit end of the rods configured to be small enough to fit inside other mass analyzing devices such as a quadrupole, ion trap, and time-of-flight.
It is a further advantage of the present invention that its multipole rod assembly does not have any electrically conductive or dielectric materials that would interfere or disturb the electric field lines defined by the multipole rods and as felt by the ions.
These and further objects, features, and advantages of the present invention will become apparent from the following description, along with the accompanying figures and drawings.
REFERENCES:
patent: 5852294 (1998-12-01), Gulcicek et al.
patent: 6020586 (2000-02-01), Dresch et al.
patent: 6121607 (2000-09-01), Whitehouse et al.
Burt Allan
Catalano Clement
Gulcicek Erol C.
Sansone Michael
Whitehouse Craig M.
Analytica of Branford, Inc.
Anderson Bruce
Levisohn, Lerner, Berger & Langsam
Wells Nikita
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