CA2270713A1 - Multipole rod assembly for spectrometers and ion transmission method - Google Patents

Abstract

A multipole rod assembly for guiding an ion stream has a large multipole rod set having relatively large dimensions, and a small multipole rod set having smaller dimensions. One end of the small multipole rod set extends within the large multipole rod set and the other end of the small multipole rod set is located outside of the large multipole rod set, so that fringing effects of fields at the ends of the large and small multipole rod sets are reduced. A second small multipole rod set can be provided axially aligned with the first small multipole rod set, and a second large multipole rod can also be provided with the second small multipole rod set extending within the second large multipole rod set. Where there is a transition between two different fields in adjacent rod sets, the invention enables the volume of the transition zone to be reduced, so as to reduce the likelihood that ions will be lost.

Description

Title: MULTIPOLE ROD ASSEMBLY FOR SPECTROMETERS AND ION
TRANSMISSION METHOD
FIELD OF THE INVENTION
This invention relates to a multipole rod assembly, for use in spectrometers or other devices for guiding an ion stream. This invention more particularly relates to transmission between adjacent stages of a spectrometer, where the stages each include a rod set.
BACKGROUND OF THE INVENTION
Quadrupoles and other multipoles are commonly used to focus, contain or filter ion beams. The quadrupole consists of four longitudinal electrodes with circular or hyperbolic cross-sections symmetrically disposed about a central axis; commonly, the electrodes comprise circular rods. Radiofrequency (RF) and Direct Current (DC) voltages are applied to the electrodes in order to create electric fields in the spaces between the electrodes which provide the desired properties of either focusing (commonly with DC voltages only), containment (usually RF
voltages only) or mass filtering or selection (with RF and DC voltages applied together).
Among the many important characteristics of these devices is the transmission efficiency. Transmission efficiency is usually expressed as a ratio of the number of desired ions which are detected at the exit (having successfully passed through the length of the device) divided by the number of desired ions which were introduced at the entrance. It is usually desirable to have a high transmission efficiency, in order to maintain a high sensitivity for the device. In many cases the transmission efficiency is not as high as 100%, and often it is much less than 100%. There are several contributing factors for reduced transmission efficiency. In some cases the ions entering a rod set are too far off axis, and strike an electrode immediately. In some cases, ions which have a stable trajectory in the device undergo excursions which are too large, causing them to strike an electrode in the middle of the device.

In other cases, the ions experience an electric field as they approach the front of the multipole which is not ideal for stable transmission, and are deflected before they can achieve a stable trajectory inside the device. This region near the entrance is called the fringing field area. Ions must pass through this fringing field before entering the device.
In this fringing field, the ions experience electric fields which are different from those experienced inside the device, and often this fringing field causes the ions to be lost or rejected. The fringing fields are the fields within the device distorted by end effects, and are experienced by ions as they enter and as they exit each rod set.
In certain complex instruments, for example, a mass spectrometer in which the ion source is at atmospheric pressures, more than one multipole or quadrupole is often used in series. For example, a first quadrupole is used to contain ions which issue into the vacuum chamber through an orifice, and to transfer the ions across a distance into a second mass filtering quadrupole in a second chamber at a lower pressure.
Thus the ions are transported through a first RF quadrupole at a pressure of a few millitorr, then through an aperture which separates the two chambers in order to keep the second chamber at a lower pressure, and then through a second mass filtering quadrupole. Ions thus pass through two entrance and two exit fringing fields, as well as two quadrupoles. All of these may cause ion losses due to the effects listed above. It is also known to provide three or more rod sets into the respective chambers or stages, which only compounds the problem.
In some gases, a quadrupole or other multipole is operated with a neutral gas to improve performance. Gas collisions of ions inside an RF quadrupole can improve the transmission by causing collisional focusing as described in U.S. Patent No. 4,963,736 by Douglas and French.
However, gas collisions experienced in regions of fringing fields may be detrimental to transmission. In addition, when ions are forced to pass through an aperture between vacuum chambers at different pressures, losses can occur if the diameter of the ion beam is larger than the diameter of the aperture. If the beam diameter is increased due to fringing fields in the region of the aperture, then the losses will be higher. Collisions with gas molecules in this region of fringing fields will cause more losses.
These problems have, to a certain extent, been recognized by others. Commonly, different stages or chambers of a spectrometer are simply separated by some sort of a wall or barrier with an aperture in it.
The aperture is sized to allow the ion beam to pass through. While an ion beam can be focused to a fairly narrow beam in an RF quadrupole, this only holds at the center of the quadrupole. As noted above, at the end of the quadrupole, due to fringing field effects, the ion beam can spread out, and often, an aperture to receive the ion beam is too small to receive the whole beam. The alternative of providing a large aperture is also often unacceptable, since this would permit large quantities of gas to flow from one chamber to another, increasing pumping requirements. Other workers in this field have recognized the possibility of using a multipole rod set extending through the aperture, as detailed below.
Thus, in an article entitled "A New Cooling and Focusing Device for Ion Guide" by H. J. Xu et al. (Nuclear Instruments and Methods in Physics Research, vol. 333, pp. 274-281 (1993)), there is disclosed a device having a hexapole rod set provided in a skimmer plate and extending between an ion source and a device for receiving the ions. The discussion simply focuses on the efficiency of this arrangement and notes that the two chambers along the side of the skimmer plate can have different pressures.
There is no suggestion or realization that this hexapole rod set could be in some way incorporated or fitted to a larger rod set on one side of the skimmer plate.
Another method of transmitting ions through an aperture is the subject of PCT Application No. PCT/US95/20378 (WO 95/23018) and U.S.
Patent No. 5,652,427. These documents describe a method of providing a continuous RF field between different pressure stages, so that ions do not have to leave the containment region of the RF field in order to pass from a higher pressure region into a lower pressure region. The electrodes for the RF multipole are constructed so as to limit the gas flow between stages in order to maintain the pressure difference.
Like the other proposal just mentioned, this published PCT
application just proposes the use of a suitably dimensioned hexapole for providing an ion guide between two chambers. Again, it nowhere suggests or mentions the possibility of combining such a hexapole of small dimensions to restrict gas flow, with a larger rod set of more conventional dimensions. Indeed, the teaching appears to be that the rod set could be used, not just to provide communication between chambers but also to provide the necessary guiding rod set within a chamber. In one embodiment, the rod set extends from one chamber, all the way through another one and then out into a third chamber.
The methods described above simply provide a containing RF
field between pressure regions. However, the problem of fringing fields between two different quadrupoles is not improved nor even discussed. An ion guide typically operates at a q value of 0.35 for best transmission, with no DC component (i.e. a value of 0). A mass resolving quadrupole operates at a q value of 0.706. An RF-only mass-resolving quadrupole operates at a q value of 0.908. As ions pass from an ion guide at a low q value into an RF/DC quadrupole at different a and q values, they experience a fringing field which can cause ion losses. As ions pass from one RF quadrupole into another at a different q value, the ion beam diameter can change, and further the focusing of the ion beam generally deteriorates in the fringing fields, resulting in spreading of the beam.
W. M. Brubaker in U.S. Patent No. 3,473,019, argues that several advantages are obtained by providing a shortened fringing field.
Shorter fringing fields yield higher sensitivity at a given resolving power, or higher resolving power at the same sensitivity. Alternatively, a shorter quadrupole can be used to provide the same resolving power, if the fringing field is shorter, since ions of lower energy can be injected without loss (or with reduced loss). In U.S. Patent No. 3,473,019, Brubaker proposes to produce a reduced length fringing field by arranging the electrodes to increase in spacing for a short distance at the entrance to the quadrupole.
However, in all of the arrangements proposed, the region in which the spacing is decreased relative to the main section of the quadrupole, has the same voltages applied as the electrodes in the main region of the device.
This is a disadvantage, since a quadrupole mass analyzer performs optimally when a and q values are near the tip of the stability region, i.e.
near a=0.167 and q=0.706. In the region where the spacing of the electrodes is decreased, the a and q values will be substantially higher. Thus even though the spatial extent of the fringing field may be smaller, because of the smaller diameter, the instability of the ions may be greater. It would be more advantageous to maintain the ideal a and q values (or even lower a and q values) through this region, which can only be done by applying lower voltages to the region of electrodes where they are located closer together.
In another U.S. Patent No. 3,129,327, W. M. Brubaker suggests the use of auxiliary electrodes inside the quadrupole, in order to produce a lower DC / RF voltage ratio (or a / q value) in the fringing field region, so that ions are more stable in this region. However, ions which approach the front of the rods from another RF quadrupole or Multipole, will still feel the fringing field effect, and still be made unstable and suffer some losses.
More specifically, Brubaker is essentially concerned with a single rod set, and all the suggested modifications, whether they be auxiliary electrodes or extensions of the base electrodes, are provided within the envelope of the basic rod set. Nowhere is there any suggestion that there could be any sort of partial overlap between the main rod set and a set of auxiliary electrodes extending into the main rod set. Additionally, the problem of providing a transition between rod sets is nowhere addressed.
It is therefore desirable to provide a method and an apparatus which match the a and q values of two quadrupoles which are in series (one after the other), and at the same time minimize the extent of any fringing field, so that ions are stable in both quadrupoles, and experience a transition in as short a region as possible in order to reduce their losses. This need for _7_ optimum matching will most commonly occur at the interface between an RF only quadrupole, and a resolving quadrupole. In the RF only quadrupole, the optimum q values is typically near 0.35. In a resolving quadrupole, the optimum a and q values are near 0.167 and 0.706 respectively, i.e. so there is a distinct transition in both a and q values between the two quadrupoles. Preferably, the apparatus provides a method of matching different diameters of quadrupoles. A large diameter ion beam, for example from a divergent ion source, can be captured by a quadrupole with larger electrode spacing, and the beam diameter can be reduced through collisional cooling (collision of the ions with the gas molecules, i n the quadrupole field). If this beam is to be fed into a resolving quadrupole, in a lower pressure region, it is desirable to maintain .the small diameter of the beam, for optimum transmission. It is also desirable to maintain the quadrupole field while ions move from one pressure region into another, and to correctly match the a and q values in the two quadrupoles independently, while still maintaining a small spatial fringing field.
It is also desirable to ensure that the region in which ions experience both gas collisions and a fringing field together, is as short as possible in order to minimize loses. Preferably, the apparatus provides a geometry which allows multipoles of different diameters to be employed and optimally matched with one another in order to minimize perturbations of the multipole field, and thus provide least disturbance to the ion beam.
SUMMARY OF THE PRESENT INVENTION
In its broadest sense, the present invention provides two multipole rod sets, one of relatively large dimensions and one of relatively small dimensions. The smaller dimensioned rod set has an end that is fitted within an end of the larger dimensioned rod set with its other end outside of the larger rod set. The term "rod set" is used here for convenience, but it is to be appreciated that, at least for the smaller dimensioned rod set, the individual elements will not necessarily be rods.

_8_ Indeed, it is preferred that they be formed generally as plates having a hyperbolic profile corresponding to the field equipotentials. Then, these hyperbolic-shaped plates readily facilitate overlapping of the two rod sets.
Also, the reference to "relatively large dimensions" and "relatively small dimensions" is with reference to the cross-sectional dimensions of the rod sets, since this determines how the rod sets fit in an end to end relationship. Generally, the larger rod sets will be longer than the smaller rod sets, but this need not necessarily be the case.
It is also desirable that a small dimensioned rod set, configured to provide the aperture between two adjacent chambers, be such as not to present surfaces tending to accumulate ions. Thus, if insulated surfaces are provided between elements of the rod set immediately around the aperture, these can tend to accumulate ions, which ions in turn will distort the field, thereby disrupting the smooth field transmission between the two chambers as intended by the present invention. Rather, where the small dimensioned rod set has hyperbolic plate-shaped elements, then these can be dimensioned large enough such that seals can be provided between the edges of the plates remote from the central axis, where there is little likelihood of any ions impinging, so as to avoid this problem.
In the more specific embodiment of the present invention, the apparatus comprises two small quadrupoles in series, which are dimensioned to fit between two larger quadrupoles which are used for ion containment or mass resolution. The small quadrupoles project into the interior of each of the larger quadrupoles, i.e. the quadrupoles overlap or are telescoped inside one another. T'he small quadrupoles are closely spaced in the axial direction. The electrodes of the smaller quadrupoles are machined in order to follow or match the shape of the equipotential inside the larger quadrupoles, and the diameter of the small quadrupoles are selected to match the beam diameter in the upstream quadrupole or multipole, and the desired beam diameters in the following (downstream) quadrupole.
The electrodes of the smaller quadrupoles may be round (cylindrical), or hyperbolic in cross section, or may be of a shape which is appropriate to block the gas flow as will be described later. The spacing of the electrodes of the smaller quadrupole is selected to be larger than the ion beam diameter, in order that ions not impinge on the electrodes and be lost.
Voltages are applied to the electrodes of the smaller quadrupole which produce equipotentials inside the space between the rods which provide a smooth transition from the equipotentials inside the larger quadrupole.
Ideally, the equipotential surfaces inside the smaller quadrupole and inside the larger quadrupole will be matched so there is no discontinuity or change in the electric field. However, where one small quadrupole is used to provide a transition between two larger quadrupoles, the equipotentials cannot be matched at both ends, and a compromise will have to be chosen which provides the best transmission between the two larger quadrupoles.
This can be selected by varying the potential on the small quadrupole independently from the potentials on the larger quadrupoles.
The only region of fringing field then is the region between the two small quadrupoles, which because of the smaller electrode diameter, is confined to a smaller region in space. Thus ions pass more quickly (in a shorter time) through the fringing field region, and have less opportunity to be rejected before entering the following quadrupole.
The small size of the quadrupoles is selected to ensure that a narrow beam diameter which issues from the larger upstream quadrupole (if it is used in a collisional focusing mode), is maintained into the second or downstream large quadrupole. In addition, if the two main or large quadrupoles are in separate vacuum chambers, the two miniature quadrupoles can provide lower gas conductance (because the electrodes are closer together) in order to limit the gas flow between the chambers. By contrast, if one continuous quadrupole is used between the vacuum chambers, the large electrode spacing needed for the quadrupole ion guide in the first chamber (in order to accept a large diameter incoming ion beam) may allow too much gas into the second chamber if the electrodes run continuously into that chamber, or else may require the use of a larger vacuum pump in order to maintain the lower pressure. Thus the two smaller quadrupoles provide a better gas barrier, while at the same time providing only a small fringing field region. In addition, transmission from an RF only quadrupole into an RF/DC quadrupole mass filter is naturally provided, whereas a continuous RF only quadrupole between vacuum chambers still leaves the requirement for a fringing field between the two quadrupoles in the lower pressure region.
Typically the small quadrupoles may project inside for a distance of 1 to 3 cm, or 10 to 20% of the length of the larger quadrupoles.
The smaller quadrupole electrodes and surrounding support structure may be used as a gas barrier to limit gas conductance between the chambers.
Preferably, the gap between the small quadrupoles is at the dividing point between the two chambers.
The electrodes are preferably machined to approximately match the shape of the equipotential inside the larger quadrupoles.
Then, the voltages applied to the small quadrupoles are adjusted to provide the best match to the electric fields on both sides, thus limiting the fringing fields to a small region between the small quadrupoles.
The advantages of this invention are:
minimizing the fringing field region, so ions spend less time in fringing fields;
the ability to match an RF quadrupole and RF/DC quadrupole in different chambers, while maintaining a pressure difference;
allowing a large RF quadrupole to be used to transport a large incoming ion beam, collisionally cooling it to a small diameter, and then transport the beam through a low conductance region consisting of the two small quadrupoles, into a second RF or RF/DC quadrupole, while avoiding the use of a plate aperture and associated fringing fields.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made by way of example, to the accompanying drawings which show preferred embodiments of the present invention and in which:
Figure 1 is a schematic view of a first embodiment of the present invention;
Figure 2a is a schematic view of a second embodiment of the present invention;
Figures 2b and 2c are schematic views of variants of the second embodiment;
Figure 3 is a schematic view of a third embodiment of the present invention;
Figure 4 is a view along the axis along line 4-4 of Figure 2;
Figure 5 shows a variant of the embodiment of Figures 2 and 4;
Figure 6 shows a cross-section along line 6-6 of Figure 5;
Figure 7 shows schematically an existing spectrometer configuration; and Figure 8 shows schematically how the present invention can be applied to the existing spectrometer configuration of Figure 7.
-DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to Figure 1, the apparatus is indicated generally by the reference 10. The apparatus 10 comprises a first set of rods Q0, a second set of rods Ql and a third set of rods Q2 axially aligned with one another. In this and the other embodiments, the rod sets are quadrupole rod sets, but it is to be appreciated that the invention has general applicability to any multipole rod set.
A wall 12 separates the first and second rod sets Q0, Ql, and is provided with an aperture 14 in known manner. Conventionally, this aperture has a diameter of about 1 mm. This size is chosen to permit most of the ion beam to pass through, even though it is recognized that outer edges of the ion beam may impinge the plate or wall 12 and be lost. The smaller size is chosen, since, commonly, there are different gas pressures on either side of the wall or plate 12, and a smaller aperture limits gas flow o r loss through the aperture 14.
Now, at the ends of each rod set, there is an end or fringing field. For the facing ends of the rod sets Q0, Ql, the end fringing fields are further distorted by the presence of the wall or plate 12, which is electrically conductive. As a consequence, while the ion beam may have a very tight focus along the axis, at the middle of each rod set, it tends to spread out o n leaving the rod set due to the end fringing fields, causing additional loss of ions around the aperture 14.
Now, in accordance with the present invention, there is provided at least one additional rod set, having smaller dimensions that the rod sets Q0, Ql and Q2. As shown in Figure 1, this can comprise two small rod sets 16, 18, axially aligned and adjacent one another; however, the invention also envisages providing just a single rod set. An important characteristic of the present invention is that at least one end of the rod set extends within the end of a larger rod set, so as to be overlapped by that rod set or to be telescoped within it.
In Figure 1, the wall 12 separates the rod set QO operating at a relatively high pressure, typically in the range from several millitorr to 5 torr for high pressure quadrupoles, from the rod sets Ql, Q2 operating at a lower pressure. The rod set Ql operates with RF only so as to simply act as an ion guide. Rod set Q2 is supplied with both RF and DC so as to act as a mass selecting quadrupole.
Now, the smaller rod sets 16, 18 are associated with the larger rod sets Ql, Q2 respectively. Thus, small rod sets 16 is supplied with the same field as the rod set Q1, so that the fields match identically. This should ensure that there are no, or negligible, end or fringing effects in a transition from Ql into the smaller rod set 16. Correspondingly, the small rod set 18 is provided with the same RF and DC fields as the larger rod set Q2, again ensuring that there is no or minimal transition effects from the end of the small rod set 18 into the larger rod set Q2.
Then, the transition between the two separate field conditions of the larger rod sets Q1, Q2 occurs solely at the interface between the small rod sets 16, 18. Due to the smaller dimensions, this should minimize ion losses, and ensure that most ions pass successfully from the small rod set 16 into the other small rod set 18.
In many mass spectrometer configurations, rod sets operating under different fields are separated by a wall or gate, and this configuration is shown in Figure 2a. Here, the two rod sets are again designated QO and Q1 for simplicity. QO operates at a pressure of 7 x 10-3 Torr, while the rod set Q1 operates at a pressure of 10 - 5 Torr. The rod set QO is operated as an RF
only rod set, while both RF and DC fields are applied to the rod set Q1 in known manner.
Now, to minimize gas conductance between the two chambers for the rod sets, a wall or barrier 20 is provided between the rod sets Q0, Ql, so as to define separate chambers for the two rod sets. The wall 20 defines an aperture 22, whose profile is discussed in greater detail below.
In accordance with the present invention, first and second rod sets 24, 26 are provided, which are small rod sets extending into an overlap by the rod sets Q0, Q1 as shown. In this embodiment, the interface or facing ends of the rod sets 24, 26 is positioned in the middle of the wall or gate 20.
The rod sets 24, 26 are spaced apart axially by a sufficient distance to provide the necessary electrical isolation. The wall 20 has a sufficient thickness that it overlaps the rod sets 24, 26, i.e. it has a thickness greater than the axial spacing between the rod sets.
The rod sets 24, 26 can then be mounted in the aperture 22.
Figure 4 shows a cross-section along the line 4-4 of Figure 2. As shown, the aperture 22 is dimensioned so as to contact just the outer peripheries of the rod sets 24, 26. The wall 20 may be made of electrically conductive material.
so as to act as a gate, and in this case the rod sets 24 and 26 would be mounted to the aperture 22 by electrically non-conductive material, the intention being to ensure that the inner surface of the aperture 22 is as remote as possible from the central axis of the aperture, indicated at 28. It will be recognised that the necessary electrical isolation can also be achieved by spacing the rod sets 24 and 26 from the aperture 22, and also that for some applications the wall 20 could be formed from non-conductive material so that the issue of electrical isolation would not arise. This is to minimize the number or amount of ions impinging on the inside surface of the aperture 22, since such ions can distort the field.
As for the first embodiment, the rod set QO and the associated small rod set 24 are both provided with an RF only field. Correspondingly, the small rod set 26 and the larger rod set Q1 are provided with corresponding fields, namely combined RF and DC fields. Then, the transition between the two field zones occurs at the interface between the rod sets 24, 26.
Reference will now be made to Figures 2b and 2c which show variants of the configuration of Figure 2a. In Figure 2a, the wall 20 is shown as having a substantial thickness and overlapping the ends of the two small rod sets 22, 24. This need not necessarily be the case. In Figures 2b and 2c, for simplicity, like components are given the same reference numeral and the description is not repeated; for modified components, these are given subscripts b, c as appropriate.
Thus, in Figure 2b, there are small multipole rod sets 24b, 26b, which are now located just on either side of the wall, now indicated at 20b, and do not extend into the aperture 22b. In this variant, an aperture 22b is smaller, and generally serves to define the minimum aperture available for ion passage between the two chambers. Clearly, the aperture 22b should not be so small as to cause any significant ion loss due to impingement of desired ions on the wall 20b.
A further variant is shown in Figure 2c. Here, the small multipole rod sets 24c, 26c are again located on either side of the wall without extending into the aperture. However, in this variant, the aperture 22c is relatively large, and importantly does not serve to limit the cross-section available for passage of ions. Rather, the cross-section available for passage of ions is defined by the two small multipole rod sets 24c, 26c. The rod sets 24c, 26c also control and minimize gas conductance between the two chambers.
The thickness of the wall 20b or 20c may be reduced so that the wall is thin as in Figure 2a. This can be achieved by making the walls 20b, 20c from thin electrical conductive material. This will then increase the RF
quadrupole field penetration along the central line 28 in the aperture 22b (22c) and thus maintain the RF quadrupole field along the ion path, to improve ion transmission. In Figure 2a, the vacuum separation wall 20a can be thick, as it is easier to attach the smaller rods to a thick wall, but in the embodiments of Figures 2b and 2c a thick wall would give less transmission than a thin wall (the same as in Figure 1). If the wall is thick, there will be no rf quadrupole field inside the aperture, or a much reduced field and ions will experience collisions with neutral gas (without rf focusing effect) which will result in worse ion transmission.
It is recognized that providing the transition between the two field zones in the aperture in a wall may not be satisfactory, since it may promote impingement of ions on the inside surface of the aperture, leading to distortion of the field and greater impingement and loss of ions.
Accordingly, a third embodiment of the invention is shown in Figure 3.
Here, the transition between the two field zones or regions is moved to a location out of the aperture and downstream from the aperture in terms of ion flow. Again, for simplicity, like components are given the same reference numerals, and the two large rod sets are identified as QO and Q1.
Referring to Figure 3, the wall or partition between the two chambers is identified at 30, and this includes an aperture 32. A small rod set 34 is mounted in the aperture 32, and the rods can be mounted as shown in Figure 4. Unlike Figure 2, the rod set 34 extends on either side of the aperture 32. The rod set 34 should be sufficiently long that the central portion of the rod set 34 provides a stable, uniform field, largely free from end or fringing effects. This has been found to ensure that the ions are tightly focused on the axis of the rod set, and the number of ions being lost or impinging on the inside of the aperture 32 will be minimized.
As before, the rod set 34 extends within the rod set Q0, and both of these two rod sets are provided with corresponding RF fields, so that ions travelling from one rod set to the other, will sense no, or minimum, field transition.
Downstream from the wall 30, there is a second small rod set 36, which extends within the inlet of the rod set Ql. Again, both of these two rod sets would be provided with corresponding RF and DC fields.
Corresponding to the upstream rod sets, the ions will then see a minimum or no field transition when exiting from the rod set 36 into the rod set Ql.
The interface between the two small rod sets 34, 36 is now located at 38, i.e. remote from the wall 30. This then ensures that any ions lost from the rod sets at this interface or junction will be lost into the chamber around the rod set Q1 and will not impinge on the wall 30.
By way of example, large rods, for the rod sets Q0, Ql would be 9.5 mm diameter, and small rods, for the rod sets 16, 18; 24, 26; and 34, 36 would be 2 mm in diameter. The rods of each rod set would be positioned so that the ratio of rod diameters to spacing between the rods (ie d/d0, where d is rod diameter and d0 is diameter of inscribed circle between the rods), is the same for both large and small rods.
While the embodiments described above in relation to Figures 1-4 have included two separate small rod sets, an important aspect of the present invention is the fact that the small rod set overlaps the larger rod set. To this end, the present invention also provides an embodiment in which there is a single small rod set extending into one or more larger rod sets. Thus, the embodiment of Figures 2 and 4 could be configured with a single small rod set extending both into larger rod sets Q0, Q1. This smaller rod set can be connected so as to have the field of either one of the rod sets Q0, Ql. For example, it could be given an RF only field corresponding to the rod set Q0, so as to minimize transition effects as ions pass into the small rod set. The exit from the small rod set into the larger rod set Q1, with the combined RF and DC fields, would then also give the transition between the two field regions.
Reference will now be made to Figures 5 and 6 which show an alternative configuration of the small rod sets. In the figures described above, the small rod sets would have circular rods, as shown in Figure 4, located at the corners of a square. However, it is also envisaged that the smaller rod sets could be in the form of plates shaped to follow the field lines. Such an arrangement is shown in Figures 5 and 6. This shows a configuration similar to Figure 2, but with two rod sets 40, 42 replacing the rod sets 24, 26. As shown in Figure 6, each rod set 40, 42 comprises plates 44 having a hyperbolic profile, so as to follow the field lines generated by the larger rod sets Q0, Q1. The advantage of this arrangement is that, as shown in Figure 6, the seals between the plates 44 can be provided, as indicated at 46, remote from the central axis, here indicated at 48. These seals 46 can, effectively, be part of a wall, separating the two rod sets Q0, Q1 and their respective chambers. This ensures that, if any ions stray too far away from the central axis 48, they are almost certain to impact either the plates 44, rather than the seals 46, thereby ensuring that the field will not be distorted by accumulated ions. Furthermore, any ions that do collect on the seals 46 will be so remote from the axis 48 as to have little effect on the field. It can also be noted that, while Figure 6 is on a relatively large scale, the total cross-section between the plates or rods is acceptable, and would still present a relatively narrow aperture, so as to minimize gas conductance. Due to the hyperbolic profile of the plates 44, as they approach the seals 46, the spacing between the plates narrows considerably, so that viscous drag would limit gas flow.
Reference will now be made to Figure 7 and 8, which show, respectfully, the rod arrangement for a conventional spectrometer, manufactured by Sciex Division of MDS Inc., and an equivalent spectrometer modified in accordance with the present invention. Thus, referring first to Figure 7, the basic configuration of the spectrometer has four rod sets Q10, Q11, Q12 and Q13. In known manner, the rod set Q11 is provided with a short set of rods, sometimes known as stubbies, indicated at Qlla, for reasons given below. The other elements of the spectrometer are largely conventional, and are briefly outlined below.

Thus, an ion source indicated generally at 50 would provide a stream of ions along an axis indicated at 52. These ions pass through a curtain plate 54. A curtain gas 56 is provided to drive off any remaining solvent. The ions then pass through an orifice 58 into a chamber which is pumped down to a pressure of 2.2 Torr by a mechanical pump indicated generally at 60. A ring electrode 62 is provided for focusing. In known manner, a skimmer 64 is provided, so as to enhance separation of the ion stream from the gas. The ions then pass into the first chamber of the spectrometer indicated at 66, containing the rod set Q10, which in known manner has one end tapered to correspond to the profile of the skimmer 64.
This chamber 66 is maintained at a pressure of 8 x 10-3 Torr by a pump 68.
This pump 68 is in turn connected to a roughing pump 70. The rod set Q10 is operated in the RF only mode so as to operate as an ion guide.
A wall or gate 72 separates the chamber 66 from the next chamber 74, containing the rod set Q11. Note that the stubbies Qlla are provided to reduce the fringing end effects, but they are less than satisfactory since the ions still have to pass through the aperture in the gate or wall 72 In this existing configuration, fringing fields exist between Qlla and Qll, between Q11 and lens 77, between lens 77 and Q12, between Q12 and lens 78 and between lens 78 and Q13. In the improved configuration of the present invention, detailed below, the fringing fields exist only between interfaces of the miniature rods, and are smaller in spatial extent.
The chamber 74 is pumped down to a pressure of 2 x 10-5 Torr, and the rod set Q11 is operated in a mass resolving mode.
From there, the ions pass through a further aperture in a housing 76 providing end walls 77 and 78 around the third rod set Q12.
This defines a chamber 80 which is maintained at a higher pressure of less than or equal to 8 millitorr, in which an inert gas is present to enable collisionally induced dissociation to take place.
The ions then pass through to the final or fourth rod set Q13 in a chamber 82, which essentially is continuous with the chamber 74. Both chambers 74, 82 are pumped down by a pump 84 to a pressure 2 x 10-5 Torr, and this pump in turn is connected to the roughing pump 70. The rod Q13 is operated as an ion guide in the RF only mode. Finally, the ions pass through an exit gate 86, into a detector, which is a channel electron multiplier or similar device for ion counting.
Now, in accordance with the present invention, the arrangement of the rods is modified as shown in Figure 8. Again, for simplicity, like rods are given the same reference numeral, and the larger rod sets are all identified as Q10, Qll, Q12, Q13. The chambers and walls are also given the same reference numerals where appropriate.
Between the rod sets Q10 and Qll, there are two coaxially aligned small rod sets indicated at 90, 91. Correspondingly, between the rod sets Q11 and Q12, there are two rod sets 92, 93. Further, between the rod sets Q12, Q13, there are two small rod sets 94, 95. These pairs of rod sets can be mounted as shown in Figure 2, i.e. with the interface between the rod sets occurring in the relevant partition or wall 72, 77 or 78. Alternatively, they can be mounted as shown in Figure 3, i.e. with the interface between the small rod sets offset axially.
As for Figures 1-6, each small rod set would be connected to a field generation device, so as to have a field that identically matches the corresponding larger rod set, so as to minimize or eliminate transition effects.

Claims (12)

1. A multipole rod assembly for guiding an ion stream comprising:
a first large multipole rod set having relatively large dimensions, and a first small multipole rod set having smaller dimensions, wherein one end of the first small multipole rod set extends within one end of the first large multipole rod set, whereby, when electric fields are generated by the first large and first small multipole rod sets, fringing effects at the ends of the first large and first small multipole rod sets are reduced, and wherein the other end of the first small multipole rod set is located outside of the first large multipole rod set.

2. A multipole rod set as claimed in claim 1, which includes means for generating an electric field connected to the first large and first small multipole rod sets, the generating means being such as to generate corresponding fields for the first large and first small multipole rod sets.

3. A multipole rod set as claimed in claim 1, which includes a second small multipole rod set having relatively small dimensions axially aligned with the first small multipole rod set, and a second large multipole rod set of relatively large dimensions, wherein the second small multipole rod set extends within the second large multipole rod set.

4. A multipole rod set as claimed in claim 3, wherein an interface between the first and second small multipole rod sets provides a field discontinuity.

5. A multipole rod set as claimed in claim 1, which includes a second large multipole rod set having relatively large dimensions, wherein the other end of the first small multipole rod set extends within the second large multipole rod set.

6. A mass spectrometer including a multipole rod set as claimed in any preceding claim.

7. A mass spectrometer including a multipole rod set as claimed in claim 5, wherein the mass spectrometer includes a barrier separating a stage operating at relatively high pressure from a stage operating at relatively low pressure, wherein the first large multipole rod set is located in the first stage and the second large multipole rod set is located in the second stage, and wherein the first small multipole rod set extends through the barrier.

8. A mass spectrometer including a multipole rod set as claimed in claim 3, which mass spectrometer includes a first stage operating at a relatively high pressure in which first stage the first large multipole rod set is located, and a second stage operating at a relatively low pressure, in which second stage the second large multipole rod set is located, wherein the first small multipole rod set is located in the first stage and the second small multipole rod set is located in the second stage, and wherein the mass spectrometer includes a barrier separating the first and second stages and providing an aperture, with the first and second small multipole rod sets located on either side of the aperture and aligned therewith.

9. A mass spectrometer as claimed in claim 8, wherein the first and second small multipole rod sets have similar cross sectional dimensions, and wherein the cross section of the aperture is substantially smaller than the free cross sections through the first and second small multipole rod sets.

10. A mass spectrometer as claimed in claim 8, wherein the first and second small multipole rod sets have similar cross sectional dimensions and wherein the cross section of the aperture is substantially larger than a free cross section through the first and second small multipole rod sets.

11. A mass spectrometer as claimed in claim 8, wherein the barrier has a substantial thickness, and extends around and contacts adjacent ends of the rods of the first and second small multipole rod sets.

12. A method of guiding an ion stream, the method comprising:
(1) providing a first large multipole rod set and a first small multipole rod set, the first large multipole rod set having relatively large dimensions and the first small multipole rod set having relatively small dimensions, and one end of the first small multipole rod set extending within the first large multipole rod set, with the other end of the first small multipole rod set extending outside of the first large multipole rod set;
(2) providing fields for the first large and first small multipole rod sets which correspond with one another;
(3) passing a stream of ions through one of the first large and first small multipole rod sets into the other of the first and second multipole rod sets, whereby the corresponding fields for the first and second multipole rod sets minimize fringing effects at the ends of the multipole rod sets.