The invention relates to multipole rod assemblies such as frequently used in mass spectrometers, for instance, quadrupoles, hexapoles, octopoles, and the like. Radio frequency (RF) quadrupole mass filters, made from four pole rods essentially aligned parallel to one another, are used to filter ions of merely one single mass-to-charge ratio m/z (frequently also from a finite m/z range), and deflect all other ions entering the mass filter radially outward to the pole rods. They are operated with the two phases of an RF voltage applied alternatively to neighboring rods, and with two anti-polar DC voltages superimposed on the two phases of the RF voltage. For filtering ions of a single mass, the DC voltages amount to about ⅙ of the peak-to-peak RF voltages. The quadrupole mass filter is generally operated in high vacuum, usually better than 10−3 Pascal, in order to avoid collisions of the ions with an omnipresent residual gas.
The mixture of ions to be analyzed by the mass filter is accelerated to low kinetic energies of about 1 to 5 electron volts, and the beam is directed, along its axis, into the quadrupole rod system of the mass filter. A low energy is needed for a long flight time inside the rod system. As a rule, a flight time is needed during which the ions are subjected to a minimum of around 100 cycles of the RF voltage to be effectively filtered.
The filtering effect is based upon defocusing by the DC voltage, and focusing by the pseudopotential generated by the RF voltage. Because the focusing strength of the pseudopotential is inversely proportional to the mass-to-charge ratio m/z, and the defocusing strength of the DC is proportional to m/z, ions with low m/z are well focused, while heavy ions are defocused. In addition, however, any quadrupole mass filter shows a low mass cut-off, at an ion mass m/z depending on the RF voltage. In this way, a quadrupole mass filter may be adjusted to just focus ions of a single m/z, while defocusing all others.
Generally, ions of low kinetic energy cannot be transferred well from outside the RF field into the superimposed fields of the mass filter. Mass filters have a narrow “acceptance profile” for ions of low energies, reflecting most of the ions outside the exact axis by the RF stray field (“fringe field”). There are several measures known to overcome this problem. The best known measure is the so-called “Brubaker pre-filter” (see, for example, U.S. Pat. No. 3,129,327 and U.S. Pat. No. 3,235,724). In some assemblies known today all rods of the quadrupole rod system are prolonged by insulated rods with a length of a few centimeters (and shorter than the main rods), and the rods of this pre-filter are supplied with an RF voltage comparable to or less than the RF voltage on the main rods. But the superimposed DC voltages are the same on all rods as an offset, as in an RF-only mode. The acceptance profile at the entrance of the pre-filter and at the entrance of the main filter are both changed in a favorable way, and ions of low kinetic energy can be more easily introduced.
In the prior art, the rods of the pre-filter and the rods of the main filter are spaced apart, and thus electrically de-coupled, by means of intermediate ceramic rings often being configured to match the outer dimensions of the main filter rods and pre-filter rods (
Therefore, the need exists to provide multipole assemblies being less impaired by the problems explained above by way of example, and, in particular, to improve performance stability of such multipole assemblies.
The following summary is included in order to provide a basic understanding of some aspects and features of the disclosure. This summary is not an extensive overview of the invention and as such it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below.
The invention pertains to a multipole assembly comprising a plurality of main mass filter rods and a plurality of auxiliary mass filter rods, each being located generally in opposing relation to one another and aligned in respect of a common ion optical axis, and a plurality of intermediate insulating pieces, such as ring-shaped washers, for example, made of ceramic or plastic, for electrically de-coupling the plurality of main mass filter rods from the plurality of auxiliary mass filter rods, wherein the plurality of main mass filter rods comprise first front faces and the plurality of auxiliary mass filter rods comprise second front faces, the first front faces and the second front faces being configured as to allow for a nesting arrangement of the plurality of intermediate insulating pieces therebetween, such that the plurality of intermediate insulating pieces is substantially screened from the ion optical axis.
In various embodiments a first front face has a circular recess, preferably having a stepped portion and a tapered portion, for neatly receiving a ring-shaped washer, and a second front face has a circular protrusion, preferably doubly stepped, being adapted to dimensions of the circular recess and the ring-shaped washer resting within the circular recess.
In other embodiments, mirrored to the aforementioned embodiment, a second front face has a circular recess, preferably having a stepped portion and a tapered portion, for neatly receiving a ring-shaped washer, and a first front face has a circular protrusion, preferably doubly stepped, being adapted to dimensions of the circular recess and the ring-shaped washer resting within the circular recess.
In some embodiments, the plurality of main mass filter rods is attached to the plurality of auxiliary mass filter rods. In particular, a first front face may have a tapped hole with an internal thread for receiving a screw with a matching external thread, and an auxiliary mass filter rod may have a bore extending longitudinally with a flange extending radially inward for providing a counter-surface for a head of the screw. In other configurations, the attachment can be brought about by the plurality of intermediate insulating pieces itself.
In further embodiments, the plurality of main mass filter rods and the plurality of auxiliary mass filter rods comprise four rods each. In this manner a quadrupole assembly is created. It is, however, equally possible to provide six rods or eight rods in order to assemble a hexapole or octopole, and the like, respectively.
In one aspect of the invention, the plurality of auxiliary mass filter rods can be attached to the plurality of main mass filter rods on an upstream and/or downstream end in relation to an ion path in a pre-filter and/or post-filter configuration, respectively.
In various embodiments, the pluralities of main mass filter rods and auxiliary mass filter rods comprise rods with a circular cross section.
According to the invention, the intermediate insulating pieces are located at and aligned with the main mass filter rods and the auxiliary mass filter rods in such a way that they are essentially hidden from a line-of-sight from the inside of the mass filter. With such a design, in particular, a higher stability of the ion signal in the mass spectrometer is achieved.
The invention benefits from the effect of the auxiliary electrodes of reducing a DC fringe field effect that ions encounter at the injection into the actual mass filter field, thereby improving the transmission efficiency, and reducing losses in performance caused, for instance, by extensive “ion burn”.
The invention can be better understood by referring to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention (often schematically). In the figures, like reference numerals designate corresponding parts throughout the different views.
While the invention has been shown and described with reference to a number of embodiments thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
A circular protrusion 210, doubly-stepped in this example, is machined on the first front faces 212 of the main mass filter rods 202, facing the observer in the illustration of
The embodiment with the screws 208, however, is given by way of example only. Other means of attaching the auxiliary mass filter rods 204 to the main mass filter rods 202, such as gluing or clamping, are also conceivable. In fact, the auxiliary mass filter rods 204 could be mounted and supported completely independent of the main mass filter rods 202, wherein then the positioning of the rods to one another would focus on the alignment of the rods and the alignment of the corresponding ion optical axes.
The auxiliary mass filter rods 204 have a longitudinal bore 216 extending over the whole axial length, in this example, giving the auxiliary mass filter rod 204 a hollow cylindrical shape. At some point along the longitudinal extension, the auxiliary mass filter rods 204 have an internal flange 218 protruding radially inward and providing a resting surface 218A for the underside of the heads of the screws 208. Thus, the flange surface facing in an axial direction may serve as a counter-surface when the screws 208 are drawn tight during the assembly of the rods.
The circular recess 222 located at the side of the flange 218 facing the main mass filter rods 202 when attached thereto comprises a stepped portion 222A and a tapered portion 222B. The difference between the two illustrated exemplary embodiments shown in
With this design as presented, no ions being diffused during their transit through the mass filter may impinge on the insulating material of the washers 206, but will rather be absorbed by the conductive surfaces of the rods 202, 204. As a result, the danger of electrostatic charging is reduced significantly.
For the sake of completeness
One of the front faces has a doubly recessed structure wherein the second inner recess 324 (resembling a tapped hole) comprises an internal thread for a mating interaction with a complementary external thread. The opposing front face has a tapped hole 326, likewise comprising an internal thread. Furthermore, the opposing front face features an axially protruding ring collar 328 being spaced apart in a radial direction from the outermost circumferential contour line 330 (being illustrated flush for both rods), so as not to interfere with an axially protruding ring collar 328*.
The intermediate insulating piece, in the example shown, comprises a disk-shaped main body 332 and two extensions 334A, 334B axially protruding therefrom, generally in the shape of cylinders having each an external thread thereon. The dimensions of the axially protruding extensions 334A, 334B are adapted on one side of the main body 332 to that of the second recess 324 and on the other side to that of the tapped hole 326, so that by screwing them together they tightly fit into second recess 324 and tapped hole 326, respectively. The dimension of the main body 332 is adapted such as to provide sufficient spacing between the axially protruding parts 328, 328* on the two opposing front faces for effecting electrical insulation therebetween.
Main mass filter rods and auxiliary mass filter rods, according to the embodiment illustrated, can be assembled by first screwing one of the axially protruding extensions 334A, 334B into the internally threaded tapped hole 326 of one of the mass filter rods, and then screwing this sub-assembly with the other axially protruding extension 334A, 334B into the internally threaded second recess 324. By such a design as displayed, the mass filter rods can be attached to one another in an electrically insulating manner while, at the same time, providing a favorable screening of the intermediate insulating piece against ion exposure originating from the interior of the mass filter assembly by means of the axially protruding collars 328, 328*.
The rods shown in the figures have a round circular cross section. It goes without saying that the principle of the invention can be extended to include rods having a different cross section, such as a section-wise hyperbolic cross section. Furthermore, the invention has been explained referring to conductive rods. However, it will be clear from the context of the disclosure that the invention is applicable also to other conductive electrode designs conceivable.
It will be understood that various aspects or details of the invention may be changed, or that different aspects disclosed in conjunction with different embodiments of the invention may be readily combined if practicable, without departing from the scope of the invention. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limiting the invention, which is defined solely by the appended claims.
Number | Date | Country | |
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61508236 | Jul 2011 | US |