Patent Application
20070023646
The invention generally relates to quadrupole ion traps and more particularly to an apparatus and method for field corrections in quadrupole ion traps.
Three dimensional quadrupole ion traps (e.g., 3-D ion traps) are commercially available devices used as mass spectrometers. A 3-D ion trap can be used as single mass analyzer or as a tandem mass spectrometer. A linear quadrupole (e.g., 2-D ion trap) is another commercially available quadrupole device that can be used as a mass analyzer, and/or an ion storage component and/or as an ion collision cell for a tandem mass spectrometer. Typically, ions and/or molecules are introduced in both the 3-D and the ion trap 2-D via an aperture.
In both the 3-D ion trap and the linear quadrupole, the presence of the aperture inevitably introduces some deviation into the quadrupole field (e.g., an ideal quadrupole potential no longer exists). This deviation often negatively impacts the performance of the quadrupole. For example, the deviation may cause peak splitting, mass shifting and/or a decrease in mass resolution.
U.S. Pat. No. 6,087,658 discloses addressing this problem by modifying the hyperbolic surface by constructing a bulge around the internal end of each aperture. The bulge is intended to correct the deviation of the pure quadrupole field caused by the holes (e.g. apertures) in the end caps. It is technically difficult to add such a bulge to an ideal hyperbolic surface. Further, once the surface is modified to include the bulge, the distribution of the quadrupole potential is determined and there is typically no convenient way to change or adjust it.
U.S. Pat. No. 6,608,303 discloses the use of an aperture shim electrode placed in the aperture to correct the deviation. A shim lens power supply provides a different RF voltage for the shim electrode than for the primary electrode which permits a correction of the quadrupole field deviation caused by the presence of the aperture. A shim electrode with an additional power supply provides the possibility of altering the potential distribution including altering the potential distribution as the ion trap is used. However, placement of a shim electrode in the aperture is limited by the aperture size. Also, the shim electrode affects potential distribution in the region immediately around the aperture but has less influence elsewhere.
U.S. Pat. No. 5,650,617 also describes using an aperture shim electrode in the aperture to improve ion trapping for the externally produced ions.
U.S. Pat. No. 5,468,958 describes a sectional ion trap composed of multiple rings of cylindrical symmetry to introduce higher order multiple fields which can be tuned electronically. The sectional ion trap has the disadvantage of being difficult to make. Further, it is typically difficult to maintain the correct geometry between its sectional electrodes.
Accordingly, there is a need for reducing the deviations in the quadrupole field.
The apparatus described includes a quadrupole ion trap comprising a plurality of primary electrodes defining a trapping volume. In one embodiment, at least one of the primary electrodes has an aperture and at least one curved surface, the curved surface positioned adjacent the trapping volume. Also the apparatus includes at least one correction electrode. The correction electrode is positioned within the primary electrode having an aperture such that a portion of the primary electrode is interposed between the aperture and correction electrode.
The quadrupole ion trap may further comprise a supplemental voltage source to the correction electrode. The supplemental voltage source is operable to apply a supplemental voltage to the correction electrode. The supplemental voltage may be adjustable. One or more ring electrodes or a plurality of strip electrodes or a combination thereof may be used as the correction electrode.
A method for improving the quadrupole potential distribution is disclosed. The method comprises providing a quadrupole ion trap comprising a plurality of primary electrodes defining a trapping volume in which at least one of the primary electrodes has an aperture and at least one correction electrode positioned in the primary electrode having an aperture such that a portion of the primary electrode is interposed between the aperture and correction electrode, and applying a supplemental voltage to the correction electrode. The supplemental voltage has an adjustment means that provides for adjusting the supplemental voltage to a voltage different from a voltage applied to the primary electrode.
Typically, 3-D ion traps and linear quadrupoles are constructed with close to an ideal hyperbolic surfaces for the surfaces of the quadrupole that faces the trapping volume of the ion trap. The hyperbolic surface facilitates generation of a near ideal quadrupole potential. The ideal quadrupole potential is derived from the equations:
Φ=Φo(x2−y2)/2ro2 for a linear quadrupole, or Φ=Φo(r2−2z2)/2ro2 for a 3-D ion trap, where Φo is the potential applied to the end cap electrode or quadrupole rod surface and ro is the inner dimension of the quadrupole, respectively. The performance of the 3-D ion trap and linear quadrupole are largely determined by the potential distribution inside the quadrupole field. Thus, perturbations in the surface of the end cap or rod impact the quadrupole field and potentially the performance.
Typically, in a 3-D ion trap mass spectrometer, ions are produced in an external ion source and then the ions are brought into the ion trap. For mass analysis, the ions are ejected from the trap and detected with an ion detector also located outside the ion trap. In order to introduce and extract ions into and from the ion trap, entrance and exit aperture holes are needed in the primary quadrupole end cap electrodes. These holes are usually placed at the center of the end caps. For linear quadrupoles used as collision cells, additional ions and/or molecules are needed to facilitate ion-ion or ion molecule reactions. These ions and/or molecules are brought into the quadrupole field by using a radial injection technique. Radial injection typically requires providing an aperture by cutting a slot into one of the quadrupole rods to form an aperture for admission of ions and/or molecules.
The described apparatuses and methods reduce deviations in a quadrupole field in a quadrupole ion trap, caused by the presence of an aperture in a primary quadrupole electrode. The invention is applicable to both 3-D ion traps and linear quadrupole ion traps and provides for improved quadrupole potential distribution. (Primary quadrupole electrodes include the electrodes that generate the primary quadrupole field in a quadrupole ion trap and may include, for example, rod electrodes, end cap electrodes, ring electrodes and the like.)
More specifically, the invention includes the use of one or more correction electrodes mounted in a primary electrode having an aperture at a position such that a portion of the primary electrode is interposed between the correction electrode and the aperture. The potential deviation in a quadrupole field due to the presence of an aperture is corrected by applying a voltage to the correction electrode. In one exemplary embodiment, the correction electrode has a hyperbolic curved surface with the same curvature as the quadrupole electrode with an aperture. In this embodiment, the curvature of the correction electrode is aligned with the curvature of the primary electrode. In the other embodiment, the potential deviation is corrected using a correction electrode, which is located below or above the hyperbolic surface of the primary electrode. In this embodiment, the requirements for the correction electrode geometry and geometrical dimensions are typically more flexible then for correction electrodes which have a curved surface that is aligned with the curved surface of the primary electrode.
The correction electrode device is not limited by the size of the aperture in the primary electrode. Further, the use of a separate power supply for the correction electrode (or electrodes) provides for adjustability of the correction potential. In some embodiments, multiple correction electrodes can be placed in the primary electrode to provide additional flexibility in controlling the potential distribution. Optionally, correction electrodes can be made as a part of one or more printed circuit boards mounted within the primary electrode thus providing an economic way to correct the field.
Mass resolving power can be improved with the correction device and correction method. In some exemplary applications high mass resolving power can be achieved.
For the embodiment illustrated in
A conventional RF waveform is typically applied to the end cap electrodes 15, 16 to generate the main quadrupole potential. A separate voltage supply connected to the ring correction electrodes 20, 22 is used to create an additional correction potential. The voltage applied to the ring correction electrodes 20, 22 may be RF voltage, DC voltage or a combination thereof and may be different than the voltage applied to the primary electrodes 15, 16. The voltage and/or linear combination of voltages applied to the ring correction electrodes 20, 22 creates a correction potential which corrects the potential deviation caused by the apertures 12, 13 in the end cap electrodes 15, 16. A desirable near ideal quadrupole potential field can be achieved with suitable adjustment of the correction voltage/voltages.
For
The ring correction electrodes 20, 22 may be constructed of the same or different materials than the primary electrodes. Suitable electrode materials include for example metals, non-conducting materials with a layer of metal applied to at least one surface and the like. Conventional power supplies which provide for adjustable RF and/or DC voltage to electrodes may be employed as power supplies for supplying the supplemental voltage to the ring correction electrodes 20, 22.
As seen in
An appropriate insulating substrate can be used between the strip correction electrodes 40, 42 and the aperture electrode 45 to provide for electrical isolation of the correction electrodes 40, 42. Ceramic is exemplary of a suitable insulating substrate. This is exemplary and other insulating substance materials may be equally suitable.
RF voltage, DC voltage or a combination thereof may be applied the strip electrodes 40, 42 to provide the correction potential. Conventional power supplies which provide for adjustable RF and/or DC voltage to electrodes may be employed as power supplies for supplying the supplemental voltage to the correction electrodes 40, 42. The voltage applied to the correction electrodes 40, 42 can be controlled and can be adjusted to a voltage different than the voltage used for the primary quadrupole electrode 45. Thus, the strip electrodes 40, 42 provide for an additional potential, which may correct the potential deviation of the quadrupole field caused by the aperture 72.
As shown in
Many variations of strip correction electrodes 40, 42 may be used. For example, the surface of the strip correction electrodes 40, 42 may be curved to match the curvature of the surface of the aperture electrode and aligned to conform to the curvature of the surface of the primary electrode 45. Alternatively, as with the ring correction electrodes 20, 22 strip correction electrodes 40, 42 may be positioned above or below the surface of the aperture electrode 45. In some embodiments, the surface of the strip correction electrodes 40, 42 may be planar and accordingly may not necessarily conform to the curvature of the surface of the aperture electrode 45. The strip correction electrodes 40, 42 may be constructed from the same or different materials as the aperture electrode 45. There are various suitable materials for electrode construction such as for example metals, non conducting materials having a layer of metal on at least one surface, and the like.
The strip electrodes 40, 42 are preferably electrically isolated from the aperture electrode 45 and preferably a means is provided to supply a voltage to the strip electrodes 40, 42 different than the voltage to the aperture electrode 45. In some embodiments, it is desirable that voltage be adjustable. Optionally, the voltage to the correction electrodes 40, 42 may be adjustable as an analysis using the ion trap is in progress. Conventional equipment and methods for supplying, controlling and adjusting voltages (e.g. power supplies and the like) may be employed in the practice of the invention.
It is desirable in some embodiments to use strip electrodes 40, 42 in pairs and to arrange them in an arrangement that is symmetric with the aperture 72. An arrangement that is parallel to the aperture 72 is shown herein. This arrangement is exemplary and other arrangements in which the strip correction electrodes 40, 42 are arranged in a symmetric manner with respect to the aperture may be used.
Further, the illustrated examples show one pair of strip correction electrodes 40, 42. In some embodiments, it may be desirable to use multiple pairs of strip correction electrodes 40, 42 or strip correction electrodes 40, 42 in combination with one or more ring correction electrodes 20, 22 or one or more ring correction electrodes 20, 22. When multiple correction electrodes are used, all ring correction electrodes 20, 22 are preferably positioned to be coaxial with the aperture 12, 13 and strip correction electrodes 40, 42 are preferably positioned symmetrically with respect to the aperture 72. Multiple pairs of strip correction electrodes, multiple ring correction electrodes or combinations of ring and strip correction electrodes may be desirable for facilitating optimization of specific features.
The 3-D and linear ion traps described herein may be used as mass spectrometers. In addition to the ion trap the mass spectrometer may further comprise an ion source and a detector.
The foregoing discussion discloses and describes many exemplary methods and embodiments of the present invention. As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.
