METHODS FOR TRANSFERRING IONS IN A MASS SPECTROMETER

Abstract

A method for transporting ions includes: providing an ion transfer tube having an axis and an internal bore having a width and a height less than the width; and providing an apparatus comprising a plurality of electrodes, each having a respective ion aperture having an aperture center, the apertures defining an ion channel configured to receive the ions from the ion transfer tube and to transport the ions to an outlet end of the apparatus, wherein at least a subset of the apertures progressively decrease in size in a direction towards the apparatus outlet end, wherein the ion transfer tube is configured such that the ion transfer tube axis is non-coincident with an axis of the ion channel or such that the width dimension of the ion transfer tube bore is parallel to a plane defined by the ion transfer tube axis and the ion channel axis.

Claims

1. A method for transporting ions from an ion source to an evacuated chamber of a mass spectrometer, the method comprising: (a) providing an ion transfer tube between the ion source and the evacuated chamber, the ion transfer tube having an axis, an inlet end configured to receive the ions and gas from the ion source, an outlet end and an internal bore between the inlet and outlet ends having a first dimension comprising a width and a second dimension comprising a height, the width being greater than the height; (b) providing an apparatus comprising a plurality of electrodes between the ion transfer tube and the evacuated chamber, wherein each electrode has a respective ion aperture having an aperture center, wherein the apertures define an ion channel configured to receive, at an inlet end of the apparatus, the ions from the outlet end of the ion transfer tube and to transport the ions therethrough to an outlet end of the apparatus, wherein the aperture centers define an axis of the ion channel and wherein at least a subset of the apertures progressively decrease in size in a direction towards the outlet end of the apparatus; and (c) providing a Radio Frequency (RF) power supply configured to provide RF voltages to the plurality of electrodes such that the RF phase applied to each electrode is different from the RF phase applied to any immediately adjacent electrodes, wherein the ion transfer tube is provided such that the ion transfer tube axis is non-coincident with the ion channel axis or such that the first dimension of the ion transfer tube bore is approximately parallel to a plane defined by the ion transfer tube axis and the ion channel axis.

2. A method as recited in claim 1, wherein the ion transfer tube is provided such that its axis is parallel to the axis of the ion channel and offset therefrom.

3. A method as recited in claim 1, wherein the axes of the ion transfer and of the apparatus are provided at an angle, β, relative to one another, wherein 0°<β≦90°.

4. A method as recited in claim 1, wherein a first portion of the ion channel of the apparatus is provided as an ion funnel adjacent to the apparatus outlet and wherein a second portion of the ion channel that is disposed adjacent to the apparatus inlet is provided with two or more of the apertures that are equal in size.

5. A method as recited in claim 1, wherein the providing of the plurality of electrodes comprises providing a plurality of ring electrodes.

6. A method as recited in claim 5, wherein the providing of the plurality of ring electrodes comprises providing support for each ring electrode on a respective one of a plurality of co-axial hollow tubes, wherein each tube is disposed parallel to the axis of the ion channel and formed of a non-electrically conducting material.

7. A method as recited in claim 5, wherein the providing of support for each ring electrode comprises providing one or more spokes disposed non-parallel to the ion channel axis, each of the spokes having an end that is physically coupled to an external housing or supporting device.

8. A method as recited in claim 1, wherein the providing of the ion transfer tube having an internal bore comprises providing an ion transfer tube having two or more parallel slots.

9. A method as recited in claim 1, wherein the providing of the ion transfer tube having an internal bore having a width and a height comprises providing an ion transfer tube wherein at least one of the width or the height of the internal bore decreases through the ion transfer tube from the inlet end of the ion transfer tube to the outlet end of the ion transfer tube.

10. A method as recited in claim 1, wherein the providing of the ion transfer tube having an internal bore having first and second dimensions comprises providing an ion transfer tube wherein the first dimension is substantially parallel to a plane defined by the ion transfer tube axis and the ion channel axis.

11. A method for transporting ions from an ion source to an evacuated chamber of a mass spectrometer comprising: (a) providing an ion transfer tube between the ion source and the evacuated chamber, said ion transfer tube having an axis, an inlet end configured to receive the ions and to receive gas from the ion source, an outlet end and an internal bore between the inlet and outlet ends having a first dimension comprising a width and a second dimension comprising a height, the width being greater than the height; (b) providing an ion transport apparatus between the ion transfer tube and the evacuated chamber, said ion transport apparatus comprising a plurality of electrodes, a plurality of surfaces of which comprise a plurality of non-co-planar rings defining a set of respective ion apertures whose diameters decrease along an axis of the ion transport apparatus from an ion input end to an ion exit aperture at an ion exit end, the set of ion apertures defining an ion channel through which the ions are transported to the evacuated chamber from the ion exit aperture; and (c) providing a Radio Frequency (RF) power supply configured to provided RF voltages to the plurality of electrodes such that the RF phase applied to each electrode is different from the RF phase applied to any immediately adjacent electrodes, wherein the providing of the ion transport apparatus comprising a plurality of electrodes comprises providing an ion transport apparatus having electrodes disposed such that a respective gap is defined between each pair of successive electrodes, wherein the gaps are oriented such that a gas flow input into the first end of the apparatus is exhausted through the gaps in one or more directions that are non-perpendicular to the axis, wherein the providing of the ion transfer tube comprises providing said ion transfer tube with an orientation, with respect to the ion transport apparatus, such that a primary zone of impingement of the gas upon the plurality of electrodes does not coincide or overlap with the ion exit aperture.

12. A method for transporting ions from an ion source to an evacuated chamber of a mass spectrometer comprising: (a) receiving the ions at an inlet end of a slotted bore of an ion transfer tube, said slotted bore having a width and a height less than the width and said ion transfer tube further having an axis and an outlet end; (b) receiving the ions from the outlet end of the ion transfer tube at an inlet end of an ion funnel having an ion funnel axis; (c) transferring the ions through the ion funnel from the inlet end to an ion exit aperture of the ion funnel; and (d) receiving the ions into the evacuated chamber from the ion exit aperture of the ion funnel, wherein the ion transfer tube is oriented, with respect to the ion funnel, such that a primary zone of impingement of the gas upon the plurality of electrodes does not coincide or overlap with the ion exit aperture.

13. A method as recited ion claim 12, wherein the ion transfer tube is oriented such that the ion transfer tube axis is non-coincident with the ion funnel axis or such that the first dimension of the ion transfer tube bore is approximately parallel to a plane defined by the ion transfer tube axis and the ion funnel axis.

14. A method for transporting ions from an ion source to a high-vacuum chamber of a mass spectrometer comprising: (a) receiving the ions at an inlet end of a slotted bore of an ion transfer tube, said slotted bore having a width and a height less than the width, said width and height defining a cross sectional area and said ion transfer tube further having an axis and an outlet end; (b) receiving the ions from the outlet end of the ion transfer tube at an inlet end of an ion funnel having an ion funnel axis, said ion funnel disposed within an intermediate-pressure chamber of the mass spectrometer that is disposed between the ion source and the high-vacuum chamber; (c) transferring the ions through the ion funnel from the inlet end to an ion outlet end of the ion funnel; and (d) transferring the ions from the intermediate-pressure chamber into the high-vacuum chamber through an aperture between the intermediate-pressure and high-vacuum chambers, wherein the ion transfer tube and the ion funnel are disposed within the intermediate-pressure chamber such that a pressure within the intermediate-pressure chamber is proportional to the cross-sectional area and a pressure within the high-vacuum chamber is essentially invariant with respect to the cross sectional area.

15. A method as recited ion claim 14, wherein the ion funnel is a portion of an ion transport device that is disposed within the intermediate-pressure chamber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The above noted and various other aspects of the present invention will become apparent from the following description which is given by way of example only and with reference to the accompanying drawings, not drawn to scale, in which:

[0035] FIG. 1A is a schematic depiction of a first mass spectrometer system in conjunction with which various embodiments in accordance with the present teachings may be practiced;

[0036] FIG. 1B is a schematic depiction of a second mass spectrometer system in conjunction with which various embodiments in accordance with the present teachings may be practiced;

[0037] FIG. 2A is a cross-sectional depiction of a stacked-ring ion guide (SRIG) ion transport device used in the mass spectrometer systems of FIG. 1;

[0038] FIG. 2B is a diagram of a single ring electrode of the SRIG ion transport device of FIG. 2A;

[0039] FIG. 3 is a pair of schematic cross sectional diagrams of a prior-art ion funnel apparatus;

[0040] FIG. 4 is a schematic illustration of a portion of a conventional round-bore ion transfer tube in both cross-sectional and perspective views;

[0041] FIGS. 5A-5B are pairs of schematic diagrams of a first ion transport apparatus in accordance with the present teachings;

[0042] FIG. 5C is a pair of schematic diagrams illustrating a variation of the ion transport apparatus depicted in FIGS. 5A-5B;

[0043] FIG. 6 is a pair of schematic diagrams of a second ion transport apparatus in accordance with the present teachings;

[0044] FIG. 7 is a pair of schematic diagrams of a generalized ion transport apparatus in accordance with the present teachings;

[0045] FIG. 8A is a pair of schematic diagrams of another ion transport apparatus in accordance with the present teachings;

[0046] FIGS. 8B-8E are respective depictions of four separate electrode structures or electrode-bearing structures included in the ion transport apparatus of FIG. 8A;

[0047] FIGS. 9A-9B are respective depictions of two separate electrode structures or electrode-bearing structures that may be included as part of an alternative set of such structures in the ion transport apparatus of FIG. 8A;

[0048] FIGS. 9C-9D are respective depictions of two separate electrode structures or electrode-bearing structures that may be included as part of a still further alternative set of such structures in the ion transport apparatus of FIG. 8A;

[0049] FIGS. 9E-9F are respective depictions of two separate electrode structures or electrode-bearing structures that may be included as part of a yet still further alternative set of such structures in the ion transport apparatus of FIG. 8A;

[0050] FIG. 10A is a cross sectional view an ion transfer tube as may be employed in accordance with various embodiments of the instant teachings;

[0051] FIG. 10B is an illustration of steps in a method for forming an ion transfer tube having a slotted bore;

[0052] FIG. 11A is a graph of gas pressure measured in an ion funnel receiving gas and ions from ion transfer tubes of two different types plotted against the tube bore cross sectional area, where diamond symbols and triangle symbols represent, respectively, measurements conducted with conventional round-bore ion transfer tubes and measurements conducted with obround- or slotted-bore ion transfer tubes;

[0053] FIG. 11B is a graph of gas pressure measured in a low pressure chamber receiving ions from the ion funnel employed in the measurements depicted in FIG. 11A, where the diamond and triangular symbols have the same meanings as in FIG. 11A.

[0054] FIG. 12 is a set of two-dimensional contour plots showing the gas flow profile of gas emerging from an ion transfer tube having a bore with an obround or slotted cross section and into an electrodynamic ion funnel in the YX (top frame) and ZX plane (bottom frame);

[0055] FIG. 13A is a schematic depiction of a system, in accordance with the present teachings, that includes an ion transfer tube having a bore with an obround or slotted cross section interfaced to a conventional electrodynamic ion funnel apparatus;

[0056] FIG. 13B is an end-on view of the various electrodes of an ion funnel showing a primary zone of impingement of gas onto the electrode surfaces when ions are supplied to the ion funnel by means of an ion transfer tube having an obround or slotted bore;

[0057] FIG. 13C is an end-on view of the various electrodes of an ion transport apparatus comprising an open geometry funnel having gas-exhaust gaps between ring electrodes or gas-exhaust apertures in electrodes showing a primary zone of impingement of gas onto the electrode surfaces when ions are supplied to the ion funnel by means of an ion transfer tube having an obround or slotted bore;

[0058] FIG. 13D is a depiction of an alternative system, in accordance with the present teachings, that includes an ion transfer tube having a bore with an obround or slotted cross section interfaced to an electrodynamic ion funnel apparatus;

[0059] FIG. 14A is a cross sectional view of a second ion transfer tube as may be employed in accordance with various embodiments of the instant teachings;

[0060] FIG. 14B is an illustration of steps in a method for forming an ion transfer tube having multiple slotted bores;

[0061] FIG. 15A is a perspective view of a third ion transfer tube as may be employed in accordance with various embodiments of the instant teachings;

[0062] FIG. 15B is a perspective view of another ion transfer tube as may be employed in accordance with the present teachings;

[0063] FIG. 16A is an example, in perspective view, of an ion transfer tube fluidically coupled to and receiving charged particles from an ion emitter array; and

[0064] FIG. 16B is a schematic illustration of an array of ion emitter capillaries fluidically coupled to a slotted ion transfer tube as may be employed in accordance with the present teachings.

DETAILED DESCRIPTION

[0065] The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the described embodiments will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiments and examples shown but is to be accorded the widest possible scope in accordance with the features and principles shown and described. The particular features and advantages of the invention will become more apparent with reference to the appended figures taken in conjunction with the following description.

[0066] FIGS. 5A-5B provide schematic illustrations of a first ion transport apparatus that may be employed in systems in accordance with the present teachings. The ion transport apparatus illustrated in FIGS. 5A-5B in the Applicant's U.S. Pat. No. 8,907,272, which is incorporated herein by reference in its entirety. The ion transport apparatus 60 illustrated in FIG. 5A comprises a plurality of nested coaxially disposed tubular circularly-cylindrical electrodes. In the example shown in FIGS. 5A-5B, four such tubular electrodes are shown comprising an outer tubular electrode 62a, a second tubular cylindrical electrode 62b disposed coaxially with and interiorly with regard to the outer tubular electrode 62a, a third tubular cylindrical electrode 62c disposed coaxially with and interiorly with regard to the second tubular electrode 62b, and an inner tubular electrode 62d disposed coaxially with and interiorly with regard to the third tubular electrode 62c. The leftmost diagram of each of FIGS. 5A-5B is a longitudinal cross sectional view through the apparatus. The rightmost diagram of each of FIGS. 5A-5B is a projected view of the apparatus along the axis 61 and in the direction of the arrow noted on that axis. Although four electrodes are shown in FIGS. 5A-5B and in other instances of the accompanying drawings, the number of electrodes is not intended, in any instance, to be restricted or limited to any particular number of electrodes.

[0067] Axis 61 is the common axis of the plurality of tubular electrodes 62a-62d and is also the axis of a convergent ion channel 67. The apparatus 60 has an entrance 63 at which gas and charged particles (primarily ions) enter the apparatus and an ion exit 69 along axis 61 at which charged particles (primarily ions) exit the apparatus in the direction of the arrow indicated on axis 61. The entrance 63 is defined by the bore of the outer electrode 62a at an end of that electrode that faces an ion source (not shown) whose position is to the left of the leftmost diagrams of FIGS. 5A-5B. Power supply 101 applies RF voltages to the electrodes and, optionally, DC voltage offsets between adjacent electrodes so as to cause the trajectories of the charged particles to converge towards the central axis 61 within an internal ion channel 67 that functions as an ion transport and convergence region. The ion channel 67 as well as its convergence region is defined by the set of ends 64a-64d of the tubular electrodes that face the ion source. Each such end, other than the end of the outer tubular electrode 62a, is recessed within the interior of the adjacent enclosing electrode as illustrated in FIGS. 5A-5B. Thus, with regard to the set of ends of the tubular electrodes that face the ion source, each such end of each progressively inward electrode is recessed with regard to the comparable end—that is, the end facing the ion source—of the immediately enclosing electrode. This configuration gives rise to a funnel shaped ion channel 67 with the diameter of the funnel narrowing in the direction from the entrance 63 to the exit 69. The exit 69 of the apparatus 60 is adjacent to and aligned with the aperture 22 of extraction lens 14 (see FIGS. 1A-1B) such that the charged particles (primarily ions) pass through the aperture into a lower-pressure chamber.

[0068] The co-axial tubular electrodes 62a-62d are nested in a fashion such that a series of annular gaps 68 exist between pairs of adjacent electrodes. Although ions and possibly other charged particles are caused to converge towards the central axis by the application of voltages applied to the electrodes, the gas jet that comprises neutral gas molecules emerging from the ion source (not shown) undergoes rapid expansion during its entry into and passage through the apparatus 60. The jet expansion causes the majority of neutral gas molecules to diverge away from the central axis 61 so as to be intercepted by and exit the apparatus through one of the annular gaps 68. The annular gaps 68 are not aligned with the aperture 22 of extraction lens 14 (see FIGS. 1A-1B) and thus gas that exits through the gaps 68 is primarily exhausted through vacuum port 31 and is thus separated from the ions.

[0069] The configuration of the electrodes of the apparatus 60 is such that most of the gas can escape through the annular gaps 68 without impinging upon an electrode surface at a high angle. Electrically insulating spacers (not shown) may be placed within the annular gaps so as to maintain the relative dispositions of the tubular electrodes. The size and positioning of such spacers may be chosen so as to minimize blocking of the gas flow through the annular gaps. Although a small amount of gas may exit together with ions through the lumen 68a of the innermost tubular electrode 62d, the quantity of gas that exits in this fashion may be minimized by maintaining a small diameter of the lumen 68a. The electrode configuration of the ion transport apparatus 60 thus inhibits buildup of gas pressure within the apparatus.

[0070] As illustrated in FIGS. 5A-5B, each one of the electrodes 62a-62d is a tube. However, it is not necessary for each tube to be wholly composed of electrically conductive electrode material. For example, in some embodiments, the electrode portions may comprise electrically conductive coatings on tubes formed of electrically insulating material. For example, in the modified ion transport apparatus 65 illustrated in FIG. 5C, electrically insulating tubes 162a-162d are disposed similarly to the disposition of tubular electrodes 62a-62d shown in FIGS. 5A-5B. Accordingly, annular gaps 68 are defined between tubes 162a-162d (FIG. 5C) in the same fashion that such gaps are formed between tubular electrodes 62a-62d (FIGS. 5A-5B), thereby allowing escape of gas through the annular gaps in the same fashion as discussed above. Note that the leftmost diagram of FIG. 5C is a longitudinal cross sectional view through the apparatus and the rightmost diagram is a projected view of the apparatus along the axis 61 in the direction of the arrow. However, the plurality of electrodes of the of the ion transport apparatus 65 comprise a plurality of electrode members 66a-66d, such as plates, rings or coatings, that are attached to or affixed to the tubes 162a-162d. Thus, the electrode members 66a-66d are supported at the ends of the tubes that face the ion source (not shown) whose position is to the left of the leftmost diagram of FIG. 5C. The tubes may support electrical leads (not shown) that are electrically coupled to the electrode members so that the appropriate RF and DC voltages may be applied to the electrode members. As in the apparatus 60 (FIGS. 5A-5B), these applied voltages cause charged particles (primarily ions) to migrate to the central axis 61 and to exit through the lumen 68a of the innermost tube 162d. The design shown in FIG. 5C produces reduced-capacitance apparatus relative to conventional ion funnel devices thereby reducing the performance requirements and cost of an RF power supply to which the apparatus is electrically coupled.

[0071] FIG. 6 provides schematic illustrations of another ion transport apparatus—ion transport apparatus 70—as taught in U.S. Pat. No. 8,907,272 and as may be employed in systems in accordance with the present teachings. Similarly to each of FIGS. 5A-5C, the leftmost diagram of FIG. 6 is a longitudinal cross sectional view through the apparatus 70 and the rightmost diagram is a projected view of the apparatus 70 along the central axis 71 of the apparatus and its associated ion channel as viewed in the direction of the arrow. In contrast to the previously-described ion transport apparatus 60 (FIGS. 5A-5B), the electrodes 72a-72d of the ion transport apparatus 70 are not in the form of cylindrical tubes but, instead, take the form of nested truncated right-circular cones, the truncated narrow portions of the cones facing the ion source (not shown) which is at the left side of the leftmost diagram of FIG. 6. More specifically, each of the electrodes 72a-72d is bounded by a respective outer surface (e.g., outer surfaces 77b and 77c as well as corresponding surfaces on other instances of the electrodes) and a respective inner surface (e.g., inner surfaces 79c and 79d as well as corresponding surfaces on other instances of the electrodes), with each of the outer and inner surfaces comprising a frusto-conical surface. The central axis 71 is the axis of radial symmetry of each of the truncated conical electrodes. Power supply 101 applies RF voltages to the electrodes and, optionally, DC voltage offsets between adjacent electrodes so as to cause the trajectories of the charged particles to converge towards the central axis 71 and the orifice 78a.

[0072] Still referring to FIG. 6, the innermost electrode 72d of the apparatus 70 has the orifice 78a at its truncated end which is centered on the axis 71 and which serves as an ion exit for the apparatus. The innermost truncated conical electrode is nested within truncated conical electrode 72c which is in the form of a similar truncated right-circular cone that is truncated so as to have an opening at its truncated end that is wider than the orifice 78a of truncated conical electrode 72d. Likewise, the truncated conical electrode 72c is nested within truncated conical electrode 72b which is itself nested within truncated conical electrode 72a. This configuration of truncated conical electrodes defines a funnel shaped ion convergence region within the interior of the apparatus that is similar to the convergent ion channel 67 shown in FIG. 5B. Further, since the cones have similar angular conical apertures, a series of gaps 78 is defined between the cones. Accordingly, expanding gas emerging from an ion source (not shown) can easily be intercepted by the gaps and exhausted from the apparatus.

[0073] As in the apparatus 60 (FIGS. 5A-5B), RF and DC voltages applied to the electrodes cause charged particles (primarily ions) to migrate to the central axis 71 and to exit through the orifice 78a of the innermost electrode 72d thereby providing efficient separation of the charged particles from the gas. Similarly to the construction of the apparatus 65 (FIG. 5C), the electrodes may alternatively be provided as conductive coatings on the truncated ends of the truncated cones, where the truncated cones are formed, in this alternative case, of electrically insulating material. In such a case, each electrode is supported on a respective one of the truncated cone structures, the supporting structure being bound by frustoconical inner and outer surfaces. The truncated cone structures may be formed by the technique of additive manufacturing (commonly known as “3D printing”) in which successive layers of material are laid down in different shapes with regard to different layers.

[0074] FIG. 7 provides a schematic illustration of a generalized apparatus having an open gas-exhaust structure that is consistent with many various different physical support structure configurations illustrated herein (e.g., FIGS. 5A-5C, 6, 7 and 8A) but that is not specifically restricted to any particular such configuration. As in the previously described drawings, the leftmost diagram of FIG. 7 is a longitudinal cross sectional view through the generalized apparatus 80 and the rightmost diagram is a projected view of the apparatus 80 along the central axis 81 of the apparatus and its associated convergent ion channel as viewed in the direction of the arrow on that axis. FIG. 7 also illustrates an ion transfer tube 15 (or, possibly, an ion source) as well as a generalized schematic pathway 85 of ions through the apparatus and a generalized schematic pathway 83 of gas through the apparatus.

[0075] The apparatus 80 of FIG. 7 is shown as comprising four electrodes—electrodes 82a, 82b, 82c and 82d—although, in a more general sense, the apparatus 80 comprises a plurality of electrodes which is not intended to be restricted or limited to any specific number of electrodes. In FIG. 7, the electrodes are shown as having a circular face or as having a circular projection in transverse cross section (e.g., such as ring electrodes or cylindrical electrodes) but the structure is not intended to be limited to such embodiments. For example, the electrodes could present a polygonal face in transverse cross section or could comprise a plurality of segments. Power supply 101 applies RF voltages to the electrodes and, optionally, DC voltage offsets between adjacent electrodes so as to cause the trajectories of the charged particles to converge towards the central axis 81 as is schematically illustrated by ion trajectories 85. The plurality of electrodes may be divided into a plurality of first electrodes (for example, electrodes 82a and 82c of FIG. 7) that are interleaved with a plurality of second electrodes (for example, electrodes 82b and 82d of FIG. 7), with the first electrodes receiving an oscillatory voltage that is opposite in phase with respect to the oscillatory voltage applied to the second electrodes.

[0076] A set of faces of the electrodes 82a-82d of the apparatus 80 are configured so as to define a funnel-shaped ion channel 67 which functions as ion transport and convergence region (see also FIG. 5B) such that the diameter of the funnel becomes narrower in the general direction from the ion entrance to the ion exit of the apparatus, i.e., in the direction of the arrow indicated on axis 81. The ion exit coincides with a lumen or aperture 88a in the electrode that is closest to the axis (electrode 82d in the illustrated example). It is understood that the lumen or aperture 88a is disposed in alignment with and adjacent to an aperture (e.g., the aperture 22 shown in FIG. 1) that leads the ions into a lower-pressure chamber after the ions pass through the lumen or aperture 88a. The electrodes are further configured such that a plurality of open gaps 88 is defined between pairs of adjacent electrodes. By contrast, the gaps 88 are not adjacent to or aligned with the aperture that leads into the lower pressure chamber.

[0077] During operation of the ion transport apparatus 80, gas comprising neutral molecules emerges from the exit end of the ion transfer tube 15 or other entrance orifice. In many situations, the ion-laden gas may emerge from the ion transfer tube or orifice as an expanding jet that generally expands outward in many directions across a range of angles. The expansion may be axisymmetric about an extension of the axis of the ion transfer tube, if the tube comprises a simple bore that is circular in cross section. However, if the tube bore comprises a different shape—such as a “letterbox” or arcuate shape—or comprises multiple such bores, then the gas expansion will be generally non-isotropic as discussed in greater detail below. Two representative gas trajectories are indicated as gas flow paths 83 in FIG. 7. As a result of this expansion and the configuration of the electrodes, most of this gas encounters one or more of the gaps 88 and is exhausted from the apparatus through these gaps. Preferably, the ion transfer tube 15 is slightly angularly mis-aligned with the apparatus axis 81 such that there does not exist a direct line of sight from the exit end of the ion transport tube 15 to the lumen or aperture 88a (note that the angular mis-alignment is exaggerated in FIG. 7). As a result of this slight mis-alignment, there is no un-impeded gas molecule trajectory from the ion transfer tube 15 to the aperture (not-illustrated) leading to the lower pressure chamber. The gas that exhausts through gaps 88 also does not directly encounter this aperture. Consequently, a very high proportion of the gas is prohibited from being transported into the lower-pressure chamber and is thus removed from the chamber containing the ion transport apparatus (e.g., chamber 13 in FIG. 1) by an evacuation port (e.g., vacuum port 31) associated with that chamber.

[0078] As similarly noted above with regard to conventional ion funnel devices, if the ion-laden gas from an ion source emerges into an ion transport apparatus as a high-velocity and rapidly expanding jet, then it is desirable to provide a minimum lateral distance between the end of the ion transfer tube or orifice 15 and the electrodes (e.g., electrodes 82a-82d as shown in FIG. 7, electrodes 62a-62d shown in FIGS. 5A-5B, electrodes 72a-72d as shown in FIG. 6, etc.) so that the initial high velocity of the emerging gas may be sufficiently dampened by collisions with background gas such that the trajectories of the ions may be manipulated independently of the gas flow. In the case of ion transfer tubes having counterbored exit ends (see for example U.S. Pat. No. 8,242,440 to Splendore et al.) where the beam velocity is greater than it would otherwise be using conventional ion transfer tubes, the minimum distance required would be correspondingly larger.

[0079] In accordance with the above considerations, the proximity of the ion transfer tube 15 to the electrodes 82a-82d as shown in FIG. 7 should be regarded as schematic only. In practice, it may be necessary to extend the distance—beyond what is depicted in the accompanying drawings—between the ion transfer tube or aperture and the illustrated electrode structures in order to satisfy a requirement for a minimum lateral distance. At the practical operating pressures of these devices in the 0.5-10 Torr range, this minimum lateral distance has found experimentally by the inventors to be in the range 55-80 mm. The extra distance may be provided by additional electrode members or electrode plates between the ion transfer tube or orifice and the illustrated electrodes. The additional electrode members or electrode plates may be formed so as to provide a passageway for the ions in which the ions may lose kinetic energy through collisions with background gas. The additional electrode members or plates may be fashioned in the form of a conventional ion transport device such as, for example, a stack of mutually-similar, apertured electrode plates (e.g., ring electrodes) wherein RF voltages of different phases are applied to the electrode members or electrode plates. Such configurations are known, for example, in conventional stacked-ring ion guides or, possibly, as are configured in the ion transport device 5 shown in FIG. 1. Note that this optional conventional set of untapered electrodes is not depicted in many of the accompanying figures.

[0080] In contrast to the generalized or average gas molecule trajectories discussed above, the ion trajectories 85 are caused to generally converge towards the central axis by the action of RF and possibly DC voltages applied to the electrodes 82a-82d. The applied DC voltages may also aid in the transport of ions in the general direction of the arrow indicated on the central axis 81. Consequently, a large proportion of the ions are caused to pass through the lumen or aperture 88a of the innermost electrode 82d. Thus, these ions are efficiently separated from neutral gas molecules and are transported into the lower-pressure chamber.

[0081] FIG. 8A illustrates another embodiment of an ion transport apparatus as taught in U.S. Pat. No. 8,907,272 and showing a specific example of the above-described general considerations. FIG. 8A provides a generalized depiction of the ion transport apparatus 90 with the leftmost diagram of FIG. 8A being a longitudinal cross sectional view through the apparatus 90 and the rightmost diagram of FIG. 8A being a projected view of the apparatus 90 along the central axis 91 of the apparatus and its associated convergent ion channel as viewed in the direction of the arrow on that axis. The apparatus comprises a plurality of ring electrodes, not limited or restricted to any particular number of electrodes, which are illustrated by the four exemplary ring electrodes 92a-92d. Power supply 101 applies RF voltages to the ring electrodes and, optionally, DC voltage offsets between adjacent ring electrodes so as to cause the trajectories of the charged particles to converge towards the central axis 91. In similarity to general nature of ring electrodes 52a-52d (e.g., see FIG. 3) of conventional ion funnel apparatuses, the ring electrodes of the apparatus 90 each have a short dimension (i.e., a thickness) that is oriented substantially parallel to the central axis 91. In other words, the long dimension (or dimensions) of the various ring 92a-92d are oriented substantially perpendicular to the central axis 91.

[0082] In similarity to the nature of ring electrodes in conventional ion funnel apparatuses, each ring electrode has a central opening that is preferably circular in shape, such that the diameters of at least a subset of the various ring electrodes progressively decrease in a general direction from the ion entry to the ion exit of the apparatus. FIGS. 8B, 8C, 8D and 8E illustrate the individual ring electrodes 92a, 92b, 92c and 92d, respectively. The respective central openings are illustrated as openings 96a, 96b, 96c and 96d. The inner faces 93 (see FIG. 8A) of these various central openings define a funnel-shaped ion channel 67 within the apparatus 90. The central opening of the first ring 92a (the largest-diameter opening) defines the ion entry of the apparatus 90 and the central opening 96d of the last ring 92d (the smallest-diameter opening) defines the ion exit of the apparatus.

[0083] Each of the ring electrodes 92a-92d of the novel apparatus 90 includes additional apertures that are separated from the respective central opening so as to define an inner ring between the central opening and the additional apertures. This configuration is illustrated in FIGS. 8B, 8C, 8D and 8E in which the additional apertures are indicated as apertures 98a, 98b, 98c and 98d, respectively and in which the central rings are indicated as central rings 95a, 95b, 95c and 95d, respectively. The presence of the apertures 98a-98d further defines outer rings which are indicated as outer rings 99a, 99b, 99c and 99d in FIGS. 8B, 8C, 8D and 8E, respectively. The central rings may be physically supported by and connected to the outer rings by spoke portions 97a, 97b, 97c and 97d. The sizes of the additional apertures 98a-98d of at least a subset of the various ring electrodes progressively increase in a general direction away from the ion entry of the apparatus. The progressive size increase of the apertures 98a-98d occurs through progressive extension of these apertures further towards the central axis 91 as ring electrodes progressively closer to the ion exit are considered and is accommodated by the simultaneous size decrease of the central openings in the same direction. This progressive size increase of the apertures 98a-98d enables these apertures to intercept a large portion of the diverging gas molecule trajectories within the apparatus.

[0084] Each ring electrode may be fabricated as a single integral piece formed of a conductive material (e.g., a metal) by drilling, cutting or punching out the central openings and additional apertures from, by way of non-limiting example, pre-existing coin-shaped circular metal blanks. Alternatively, each of the ring electrodes may be fabricated from an electrically insulating material with only certain portions having an electrically conducting coating (e.g., a metal coating) thereon. In various embodiments, the conductive coating may occupy only the central ring portions 95a-95d with additional conductive coatings on portions of the spokes 97a-97d and outer rings 99a-99d, these additional conductive coatings serving as electrical leads to the various coated central rings. Alternatively, one or more of the central ring portion, outer ring portion or spoke portions may be formed from a different material from the other portions.

[0085] In operation of the ion transport apparatus 90, RF and possibly DC voltages are applied to the center ring portions 95a-95d of the ring electrodes 92a-92d in known fashion so as to cause charged particles (primarily ions) provided from an ion source or ion transfer tube (not shown) to converge towards the central axis while also moving towards the ion exit 96d of the apparatus. The ions that pass through ion exit 96d are then focused into an aperture that leads into a lower pressure chamber, this aperture being adjacent to and aligned with the ion exit 96d. In contrast, gas comprising neutral gas molecules is intercepted by one or more of the apertures 98a-98d. This gas passes substantially unimpeded through the apertures 98a-98d so as to be exhausted from the apparatus into the chamber in which the ion transport apparatus is contained. This gas is then substantially removed by an evacuation port (e.g., vacuum port 31) associated with the chamber in which the ion transport apparatus 90 is contained. In this way ions are effectively separated from neutral gas molecules without buildup of gas pressure within the ion transport apparatus.

[0086] FIGS. 9A-9B are respective depictions of two separate electrode structures or electrode-bearing structures of an alternative set of such structures. The electrode plate structures 192a, 192b illustrated in FIGS. 9A-9B, may be considered as two examples of electrode plates which may be stacked, similarly to the stacking arrangement shown in FIG. 8A, within an ion transport apparatus as taught in U.S. Pat. No. 8,907,272 and as may be employed in systems in accordance with the present teachings. Such an ion transport apparatus will generally comprise a plurality of electrode plate structures, of which the two illustrated electrode plate structures 192a, 192b are representative. Within such an apparatus, the electrode plate structure 192a is positioned relatively closer to an ion entrance and the electrode plate structure 192b is positioned relatively closer to an ion exit. As described previously in regard to FIG. 8A, the central apertures (central apertures 196a, 196b as well as corresponding apertures in other of the associated plurality of electrode plate structures) together form an ion channel through which ions are transmitted, with the diameter of the channel decreasing from the ion entrance to the ion exit. Also, as previously described in regard to FIG. 8A, the other apertures (apertures 198a in FIG. 9A, apertures 198b in FIG. 9B as well as corresponding apertures in other of the associated plurality of electrode plate structures) are employed, in operation, to channel neutral gas molecules through the apparatus so that the gas may be exhausted from the ion transport apparatus spatially separated from the ions.

[0087] Each electrode plate structure (e.g., electrode plate structures 192a, 192b) may be formed as a single integral piece of an electrically conductive material, such as a metal. In such cases, the central apertures 196a, 196b and the other, outer apertures (other apertures 198a in FIG. 9A and 198b in FIG. 9B separated by respective spoke portions 197a and 197b and surrounded by outer rings 199a-199b, respectively) may cut out of a pre-form metal plate by any suitable mechanical, chemical, electrical, optical or electro-chemical machining technique, such as, by way of non-limiting example, by mechanical cutting, mechanical stamping, laser cutting, chemical etching, etc. As illustrated in FIGS. 9A-9B, the plates may comprise integral tab structures (or other structures) that may be used for mounting each of the plurality of electrode plates within a respective slot of a housing member (not shown) of the ion transport apparatus. The tabs may also be additionally or alternatively employed as electrical connectors. For example, assuming that the each of the plates 192a, 192b comprises a single integral piece of metal, the tabs 194a, 194b may be folded around and welded to a respective electrical contact of the housing member.

[0088] A subset of a plurality of electrode plates adjacent to the ion exit of an ion transport apparatus may comprise a set of ring electrodes (e.g. ring electrode 195b in FIG. 9B) wherein these ring electrodes adjacent to the ion exit have a constant outer diameter among the subset of the plurality of plates. Within this subset, the widths of the ring electrodes increase in a direction towards the ion exit of the apparatus as the diameter of the central apertures become smaller at the same time that the ring electrode outer diameters (defined by the inner boundaries of the other apertures such as apertures 198b) remain constant. For example, the increase in the width of the ring electrodes may be noted by comparing the width of ring electrode 195b in electrode plate 192b to that of ring electrode 195a in electrode plate 192a. Such a configuration is advantageous for optimizing the separation of ion flow (through central apertures 196a, 196b, etc.) from the flow of gas (through the other apertures 198a, 198b, etc.) and thereby minimizing the transport of gas into the lower-pressure chamber into which the ions are directed after passing through the ion exit of the apparatus.

[0089] In alternative embodiments (for example, one embodiment as illustrated in FIGS. 9C-9D and another embodiment as illustrated in FIGS. 9E-9F), the outer apertures may occupy a smaller portion of the surface area of one or more of the electrode plate. The areal extent of the electrode plates occupied by the open outer aperture sections may be designed so as to fine tune (e.g., regulate) the conductance (or even the directionality of the conductance) of the gas perpendicular to the axis. For example, in FIGS. 9C-9D, two electrode plates 292a, 292b out of a set of plates are shown and in FIGS. 9E-9F, two electrode plates 392a, 392b out of an alternative set of plates are shown. The electrode plates 292a, 292b shown in FIGS. 9C-9D respectively comprise central apertures 296a, 296b, respectively comprise outer apertures 298a, 298b, respectively comprise spoke portions 297a, 297b and respectively comprise tab sections 294a, 294b. Similarly, the electrode plates 392a, 392b shown in FIGS. 9E-9F respectively comprise central apertures 396a, 396b, respectively comprise outer apertures 398a, 398b, respectively comprise spoke portions 397a, 397b and respectively comprise tab sections 394a, 394b.

[0090] One method for reducing the areal extent of the outer apertures—through which gas flows—would be to simply retain the same number of apertures while making each aperture smaller. Another method for reducing the areal extent of the outer apertures is as shown in the example of FIGS. 9C-9D, in which the number of equally-spaced-apart outer apertures is reduced (from six apertures to five apertures per plate) but the size of the apertures remains unchanged, with respect to the outer apertures 198a, 198b shown in FIGS. 9A-9B. Yet a third method for reducing the areal extent of the outer apertures is as shown in FIGS. 9E-9F, in which the number of apertures is reduced but the apertures are not equally spaced. This latter configuration would be beneficial for cases in which the delivery of ions into ion transport apparatus having the electrode plates 392a, 392b (and others) is not axisymmetric or is not aligned with respect to the axis of the ion channel. Such would be the case, for instance, if an ion transfer tube that inputs the ions makes a small angle relative to the axis of the device (as in FIG. 7) or if the bore of the ion transfer tube is not circular in cross section or if the ion transfer tube includes multiple bores. In these situations, the relative positions of the apertured and non-apertured sections of the electrode plates would be chosen in accordance with the direction or the asymmetry of the gas jet or jets being input to the apparatus.

[0091] FIG. 10A is a cross sectional view of a portion of a slotted ion transfer tube, ion transfer tube 110, as may be employed in accordance with various embodiments of the instant teachings. Such slotted ion transfer tubes are discussed in greater detail in U.S. Pat. No. 8,309,916 in the names of inventors Wouters et al., issued on Nov. 13, 2012 and assigned to the assignee of the instant application. In this example, the slot 164 in the tube material 112 has been formed (for example, by wire-EDM, wire erosion, etching or abrasion) in diametrically outward directions, from an originally circular cross section bore of 580 μm diameter so as to create a “letterbox” like shape—with rounded corners—having a width, w (along the elongated direction), of 1.25 mm and a height, h, of 580 μm. For example, as indicated by the arrows in FIG. 10B, the slot 164 may be formed by etching or eroding (e.g., by the wire-EDM technique) auxiliary channels 144 outward from a pre-existing central circular bore hole 143 within a tube 142. A slot of the above-noted size conveniently fits within the 1/16″ outer diameter of commonly available stock tubing. The ratio, R, between the area of the novel slotted bore 164 and the standard circular bore is

[00002]$\begin{array}{cc}R=\frac{{\pi \left(580/2\right)}^2+\left(1250-580\right)\times 580}{{\pi \left(580/2\right)}^2}=2.47.Math.x.& \mathrm{Eq}..Math.2\end{array}$

[0092] The steady state chamber pressure of an evacuated chamber into which gas is introduced through an ion transfer tube may be taken as a measure of the throughput of the tube. Accordingly, the respective throughputs of three different ion transfer tubes used as inlets to a chamber were compared by observing the chamber pressures obtained with a two-stage mechanical pump having a pumping capacity of 30 m.sup.3/hr, and operated in a choked flow regime (all tubes the same length). The results show that, as expected, the chamber pressure scales in direct proportion to the bore cross-sectional area for the two tubes having circular bores. Moreover, with regard to the present discussion, it is also to be noted that, within experimental error, the ratio of pressures observed in comparison of the slotted-bore tube having bore lobe height of 580 μm to the circular-bore tube having 580 μm also scales in direct proportion to the area ratio as calculated in Eq. 2 above. To achieve throughput comparable to that of the obround-bore tube, a circular-bore tube having a bore diameter of 911 μm would be required.

[0093] One benefit of using an obround or slotted bore (i.e., a so-called “letterbox” bore shape) is that the one of the dimensions of the rectangular cross section can be kept relatively small, i.e. similar to the maximum useable diameter in case of a tube with circular inner bore so to maintain sufficient desolvation, whereas the other dimension (i.e., the width) can be much larger so as to increase the throughput of ion laden gas from the API source and thereby increasing the sensitivity of the mass spectrometer system. Some charged droplets passing through the center of such a conventional single bore tube would be as far as 455 μm away from a heat-providing tube wall as compared to the maximum distance of 290 μm experienced by droplets passing through the tube with the obround bore. The obround-bore tube is therefore expected to provide more complete desolvation than a circular-bore tube of similar length having the same bore cross-sectional area. Equivalently, the obround-bore tube is expected to, in general, provide greater throughput than and equivalent desolvation to a circular-bore tube having a diameter equal to the minimum distance across (i.e., the height of, in the present example) the obround channel.

[0094] FIGS. 11A and 11B are graphs of measured internal gas pressure plotted against the tube bore cross sectional area. FIG. 11A illustrates pressure measured within a conventional ion funnel apparatus and FIG. 11B illustrates pressure measured in a downstream analyzer chamber that is evacuated to a lower pressure than the chamber within which the ion funnel is located. In both FIGS. 11A-11B, diamond symbols relate to pressure measurements obtained in a system employing conventional round-bore ion transfer tubes and triangle symbols relate to pressure measurements obtained in a system employing obround- or slotted-bore ion transfer tubes. The x-axis in both of FIGS. 11A and 11B represents tube bore cross sectional area. In the case of round-bore tubes, a number of tubes were employed in which the inner diameter was increased stepwise from 0.58 mm to 1.4 mm. In the case of slotted tubes, the height (h) and width (w) dimensions (see FIG. 10A) of different tubes spanned the range of 0.58-0.6 mm and 1.25-2.0 mm, respectively. The gas-flow throughput of the two types of ion transfer tubes ranges from a factor 2.0 times greater to 5.6 times greater than the throughput of a standard 0.58 mm diameter round bore ion transfer tube.

[0095] The correlation between increasing the gas-flow throughput of the ion transfer tube and the internal chambers is well illustrated in FIGS. 11A-11B. The results for the round-bore ion transfer tubes exhibit an undesirable rapid increase in the analyzer chamber pressure indicating excessive overflow of gas across the multiple pumping stages (e.g., FIGS. 1A-1B) and into the analyzer chamber. One possible way of dissipating the gas pressure in the ion funnel chamber would be to increase the length of the funnel (e.g., from 54 mm to 130 mm) so as to provide sufficient distance for the jetting gas streamlines to dissipate energy and divert towards the intermediate-pressure chamber exhaust line (e.g., port 31 in FIGS. 1A-1B). With the bulk of the gas load removed in this fashion, it has been observed that the pressure in each of the pumping stages, including the analyzer region, could be restored to an optimal range.

[0096] Although increasing the length of the funnel (or other multi-electrode ion transport device such as a stacked ring ion guide) can serve as a means of maintaining pressure in each of the pumping stages in an optimal range without increasing in the number of pumping stages or pumping capacity, it is not the best solution. The increased length of the ion transport device can have the undesirable effect of increasing overall device size and footprint. Moreover, the capacitance of such a device will be increased, thus placing limitations on the magnitude of RF power that can be supplied to the device.

[0097] To address the above issues, the inventors have realized that the results shown in FIG. 11B reveal a preferable way to manage the gas load. Specifically, it is observed that the use of a slotted-bore ion transfer tube does not cause significant pressure increase in the analyzer chamber with increasing gas throughput. The pressure in the analyzer chamber is thus significantly lower than that observed for a round-bore ion transfer tube having comparable throughput. This suggests that by using an ion transfer tube with a slotted bore, the mass flux of the incoming gas is greatly reduced to downstream mass spectrometer chambers. As a result, the use of a slotted-bore ion transfer tube allows the use of a short (e.g., 54 mm) electrodynamic ion funnel.

[0098] The correlation between the shape of the bore of the ion transfer tube and the moderation of pressure increase with increasing gas throughput was treated as a novel property and was studied in detail with computational fluid dynamics. FIG. 12 shows the results of these calculations. The extended ion funnel 450 illustrated in FIG. 12 comprises a first portion 452 that includes a set of electrode plates 492 all having central apertures of the same diameter and a second portion 453 in which the diameters of the electrode plates 592 progressively decrease in diameter towards an ion exit 455. The length of the first portion 452 of the ion funnel 450 provides a minimum lateral distance between the ion transfer tube 110 and the tapering-aperture-diameter second portion 453 such that the forward velocity of the ion laden gas is sufficiently lowered by collisions with background gas within the first portion 452. When the forward velocity of the ion laden gas has been sufficiently lowered in the first portion, it becomes possible to manipulate the ions with radio frequency electric fields with low enough amplitudes to be below the Paschen breakdown limit in the second portion, such that the ions may be preferentially guided and focused towards the ion exit 455.

[0099] The top frame of FIG. 12 shows the propagation of the Mach disc within the ion funnel 450 along the x-y plane (i.e., a “side view” of the apparatus) and the bottom frame of FIG. 12 shows the propagation of the Mach disc within the x-z plane (i.e., a “top view”) as delivered into the ion funnel from a slotted-bore ion transfer tube 110. Note that, in the embodiment shown in FIG. 12, the x-direction is defined as being parallel to the central axis 451 of the ion funnel. Note also that, in the embodiment shown in FIG. 12, the long dimension or width, w, of the slot of the ion transfer tube 110 is oriented parallel to the x-y plane and that the axis 75 of the ion transfer tube is parallel to the funnel axis 451 but offset from it by a distance, a. Although the overall behavior of the gas expansion in the (upper frame of FIG. 12) was anticipated, the x-z plane (lower frame) shows a surprising radial expansion of the Mach disc which is not seen in the x-y plane. A reconstruction of the computational fluid dynamics data in three-dimensional space (not shown) reveals that the Mach disc originating from the slotted-bore ion transfer tube 110 is asymmetrical in shape. The asymmetry causes a radial dispersion of the gas streamlines away from center axis, towards the edges of the RF electrodes, and reduces the flux of gas molecules going into the downstream pumping stages through the ion exit 55. This explains the limited increase in the analyzer pressure when compared to a round bore capillary at comparable throughput. In contrast, the Mach disk arising from a round bore ion transfer tube is symmetrical (not shown) and, in that case, the bulk of the gas flux is collimated along or parallel to the center axis 51 such that an undesirable proportion of the gas may pass towards and through the ion exit 55.

[0100] FIG. 13A is a schematic depiction of a system, in accordance with the present teachings, that includes an ion transfer tube 110 having a bore with an obround or slotted cross section that is interfaced to an electrodynamic ion funnel apparatus 50. FIG. 13A may also be considered to depict a method, in accordance with the present teachings, of interfacing an ion transfer tube having a bore with an obround or slotted cross section to an electrodynamic ion funnel apparatus. For purposes of comparison, the ion funnel apparatus 50 is depicted in similar schematic fashion in both FIG. 3 and FIG. 13A. As noted previously, a central axis 51 of the funnel, which is also an ion channel axis, may be defined such that the axis 51 passes through the center of the largest aperture 54, which corresponds to an ion entrance of the funnel, and also through the smallest aperture 55, which corresponds to an ion exit. A longitudinal central axis 17 may also be defined for the ion transfer tube 110. The funnel axis 51 and the ion transfer tube axis 17 together define a geometric plane that contains both of these axes.

[0101] In the system shown in FIG. 13A, an outlet end 151 of the ion transfer tube 110 may be disposed offset from the funnel axis 51 by an offset distance, α. Alternatively, the axis 17 of the ion transfer tube may be disposed at a non-zero angle, β, from the axis 51 of the ion funnel and of the corresponding ion channel. Still further alternatively, the outlet end outlet end 151 may be offset from the funnel axis at the same time that the ion transfer tube axis makes an angle (not equal to zero) with the funnel axis. The so-defined spatial offset, angular offset, or combined spatial and angular offsets cause the trajectories of neutral gas molecules that emerge from the outlet end 151 of ion transfer tube 110 to be such that these trajectories do not project through the ion funnel exit end 55. As noted previously and as shown in FIG. 10A, the ion transfer tube has a slot 164 having a width, w, taken along the elongated direction of the slot. In the system shown in FIG. 13A, the ion transfer tube is disposed such that this elongated slot direction is within the geometric plane defined by the funnel axis 51 and the ion transfer tube axis 17.

[0102] FIG. 13B is an end-on view of the various electrodes of an ion funnel 50 showing the primary zone of impingement 18 of gas onto the electrode surfaces when ions are supplied to the ion funnel as shown in FIG. 13A. The elongated shape of the zone 18, which is a result of the asymmetric plume shape depicted in FIG. 12, has been confirmed by experiment by observing burned-on deposited films (tarnish) on used electrode surfaces. The orientation of the elongated zone of impingement within the funnel, as shown in FIG. 13B, is a result of the geometric relation between the ion transfer tube 110 and the ion funnel, as discussed above with reference to FIG. 13A. Preferably, the angle β and the offset α, as shown in FIG. 13A, are chosen such that, for a given length, L, of the ion funnel, the primary zone of gas impingement 18 does not coincide or overlap with the ion exit 55. Thus, in this fashion, the gas flow is effectively diverted away from the ion exit.

[0103] A similarly shaped primary zone of impingement is expected when such an ion transfer tube in used in conjunction with various other ion transport apparatuses taught in this document, such as those shown in FIG. 5A, FIG. 5C, FIG. 6, FIG. 7 and FIG. 8A. Accordingly, FIG. 13C shows, as but one example, the expected shape of the primary zone of gas impingement 18 when used with the apparatus 80, which was previously described in conjunction with FIG. 7. The combined effects of the plume shape and the geometric relation between the ion transfer tube and the ion funnel cause the primary zone of gas impingement 18 to fail to coincide the ion exit which, in the case of the apparatus 80 shown in FIG. 13C, corresponds to the aperture 88a in the electrode that is closest to the axis (electrode 82d). Nonetheless, the primary zone of impingement 18 overlaps several of the plurality of open gaps 88 that are provided by gas-exhaust apertures in electrodes other than electrode 82d. Gas which passes through the open gaps 88 will escape from the interior volume of the ion funnel 80 and will be purged from the chamber containing the ion funnel without causing significant pressure increase within the ion funnel or the chamber. In alternative embodiments, the gaps or gas exhaust apertures may be shaped, oriented or otherwise spatially disposed so as to align with or to spatially overlap with or match the zone of primary gas impingement so as to most efficiently divert gas flow away from the ion exit. Alternatively, the ion transfer tube may be oriented or otherwise spatially disposed such that the zone of primary gas impingement generated by gas emergent from the ion transfer tube aligns with or spatially matches a given set of gaps or apertures through which gas is exhausted. As noted in the above discussion relating to FIG. 12, the orientation of the zone of primary gas impingement is related to the angular and spatial orientation of ion transfer tubes having slotted or obround bores.

[0104] In some situations, the ion transport apparatus 80 could include at least some electrode plates having outer apertures for gas exhaust that are unevenly or asymmetrically distributed across the plane of the electrode plate, similar to the examples shown in FIGS. 9E-9F. The outer apertures may be disposed at a position and of a shape so as to strongly overlap with and or match the location and shape of the primary zone of impingement 18. Thus, a system configuration (similar to that shown in FIG. 13A) which employs both a novel ion transport apparatus as taught herein (such as those shown in FIG. 5A, FIG. 5C, FIG. 6, FIG. 7 and FIG. 8A) as well as an ion transfer tube having an obround or slotted bore is expected to be especially effective in exhausting gas load from intermediate pressure chambers and preventing buildup of gas pressure in downstream analyzer chambers.

[0105] FIG. 13D is a depiction of an alternative system, in accordance with the present teachings, that includes an ion transfer tube 110 having a bore with an obround or slotted cross section that is interfaced to an electrodynamic ion funnel apparatus 50. FIG. 13D may also be considered to depict an alternative method, in accordance with the present teachings, of interfacing an ion transfer tube having a bore with an obround or slotted cross section to an electrodynamic ion funnel apparatus. Similarly to the configuration illustrated in FIG. 13A, the longitudinal axis 17 of the ion transfer tube 110 and the central axis 51 of the ion funnel 50 define a geometric plane, where the central axis 51 can also be described as an ion channel axis, where the ion channel is identical to the funnel-shaped ion transport region. Further, the elongated direction of the slot (depicted in FIG. 13D by the width, w) of the ion transfer tube 110 is within such plane. In contrast to the system shown in FIG. 13A, the longitudinal axis 17 of the ion transfer tube 110 makes an angle, β, of substantially ninety degrees with respect to the central axis 51 of the ion funnel 50 within the alternative system shown in FIG. 13D. As a consequence, the elongated direction of the slot (depicted in FIG. 13D by the width, w) is disposed substantially parallel to the ion funnel axis 51 in the system shown in FIG. 13D. As a result of the distinctive flow characteristics associated with gas emerging from the ion transfer tube 110 (FIG. 12), the configuration shown in FIG. 13D yields a gas plume 83 that emerges from the ion transfer tube with expansion along the axis 17 as well as out of the plane of the drawing but without significant gas flow in the direction of the ion funnel. By contrast, the ions are diverted away from the gas expansion directions along ion pathways 85 by an axial field provided by the ion funnel. The ion funnel in FIG. 13D could be replaced by an alternative ion transport apparatus as taught in this document, such as those shown in FIG. 5A, FIG. 5C, FIG. 6, FIG. 7 and FIG. 8A.

[0106] FIG. 13D illustrates a ninety-degree angle between the ion funnel axis 51 and the ion transfer tube axis 110, this angle being taken as positive, for purposes of this discussion, as measured in a counterclockwise direction from the positive end of the funnel axis 51 to the positive end of the axis 17 of the ion transfer tube. For purposes of this discussion, the positive end of the funnel axis 51 is taken as being on the funnel side of the outlet end 151 of the ion transfer tube 110 and, similarly, the positive end of the axis 17 of the ion transfer tube is taken as being on the gas plume side of the outlet end 151. In practical applications, a range of angles are possible, however. Angles greater than ninety degrees are not practical, since ions emerging from the ion transfer tube 110 would initially propagate away from the ion funnel. Angles less than ninety degrees could, however, be employed. A practical minimum value for this angle, as defined above, between the ion funnel axis and the ion transfer tube axis, is, in degrees, (90°−φ°), where the angle φ is defined as being the angle at which, when subtracted from 90°, causes the extended axis 17p of the ion transfer tube to just encounter the edge of the largest aperture 54 of the ion funnel without projecting into the aperture 54 (see FIG. 13D).

[0107] FIG. 14A is a cross sectional view of another ion transfer tube that may be employed in accordance with various embodiments of the instant teachings. The ion transfer tube 140 illustrated in FIG. 14A comprises multiple distinct separated obround bores or slots 164a, 164b in tube material 142. Although two such bores are illustrated in FIG. 14A, the number of bores within a particular ion transfer tube need not be limited to any particular number. Such a multiple-bore ion transfer tube may be employed to capture charged particles emitted by a two-dimensional emitter array. The multiple-bore ion tubes may also capture charged particles emitted by separate emitter arrays—for example, two linear emitter arrays—perhaps receiving sample material from respective separate sample sources. As another example, different bores could be used concurrently in order to transport different respective analytes or substances (e.g., one obround bore may be used mainly for analyte, while a different one is used for an internal calibrant).

[0108] The multiple tube bores illustrated in FIG. 14A may be formed by wire-EDM erosion (or other erosion or abrasion technique) outward from separate pre-existing through-going circular bores of a pre-existing tube. For instance, the pre-existing tube may be a commercially available tube having multiple circular bores. If a suitable pre-existing multi-bore tube is not commercially available, then one may be fabricated by drilling multiple bore holes through a solid cylinder. Alternatively, a tube, such as the multiple-bore ion transfer tube 140 shown in FIG. 14A, may be fabricated starting with a conventional tube having a single central bore 143, as illustrated in FIG. 14B. A first step, as previously illustrated in FIG. 10B, is to etch or erode (e.g., by the wire-EDM technique) auxiliary channels 144 outward from the pre-existing central circular bore hole 143 within a tube 142, as indicated by the arrows in FIG. 10B. The ends of the auxiliary channels 144 then serve as starting points for etching or erosion of additional channels 145, as shown by the arrows in FIG. 14B. Further enlargement (if desired) of the channels 145 then yields the slots 164a, 164b as shown in FIG. 14A. The auxiliary channels 144 could be formed in some other directions than those shown.

[0109] FIG. 15A illustrates an ion transfer tube having a slotted bore having at least one inner dimension that decreases in the direction of flow of charged particles through the tube. As a result, the cross-sectional area of the bore decreases in the same direction. The ion transfer tube 111 shown in FIG. 15A comprises a single bore 164 whose bore height, decreases from h.sub.1 to h.sub.2 in the flow direction from left to right. Alternatively, the width of the bore could decrease or both the height and width could decrease. As ions or other charged particles together with entrained sheath gas travel along the bore, the average flow velocity increases as the bore cross sectional area decreases and, consequently, the flow regime tends to become laminar flow. The high ion velocity and laminar flow regime downstream tends to minimize any potential adverse effects of increasing ion space charge, tube wall charging (in the case of dielectric materials) or ion discharging against the walls (in the case of electrically conductive wall materials).

[0110] FIG. 15B is a perspective view of another ion transfer tube, ion transfer tube 114, as may be employed in systems in accordance with the present teachings. In contrast to the previously illustrated ion transfer tubes, the ion transfer tube 114 depicted in FIG. 15B comprises two separate structural members—a first tube member 113a formed of an electrically resistive material and a second tube member 113b formed of a material, such as a metal, that is an electrical conductor and that also has high thermal conductivity. The two tube members 113a, 113b are joined to one another by a leak-tight seal between the two tube members. Each of the tube members 113a, 113b has a bore. The two bores mate with one another—that is, comprise similar shapes and dimensions—at the juncture of the two tube members.

[0111] The flow within the ion transfer tube 114 is in the direction from the first tube member 113a to the second tube member 113b. Thus, the first tube member 113a and second tube member 113b are respectively disposed at the ion inlet end 151a and the ion outlet end 151b of the ion transfer tube 114. The distance from the open ion inlet of the ion transfer tube 114 to the contact between the first and second tube members 113a, 113b is represented as a length L.sub.1 which is greater than or equal to a flow transition length. The flow transition length is the distance within which the through-going flow of carrier gas changes from an initial plug flow or turbulent flow to laminar flow. The second tube member 113b has a length L.sub.2.

[0112] The resistive tube member 113a may be formed of any one of a number of materials (e.g., without limitation, doped glasses, cermets, polymers, etc.) having electrically resistive properties. It has been postulated (see Verbeck et al., US Patent Application Publication 2006/0273251) that the use of a tube comprising a resistive material enables the bleeding off of any surface charge that would otherwise accumulate on an electrically insulating tube as a result of ion impingement on the tube surface. An electrode 155, which may be a plate, a foil, or a thin film coating, is in electrical contact with an end of the first tube member. A power supply 157 whose leads are electrically connected to the electrode 155 and to the second tube member 113b is operable so as to provide an electrical potential difference between the electrode 155 and the second tube member 113b. Alternatively, the end of the first tube member 113a that faces the second tube member 113b may be provided with an electrode plate or film, such as a metalized coating together with a tab in electrical contact with the metalized coating. In such an instance, an electrical lead of the power supply 157 may be contacted to the tab, electrode plate or film, instead of directly to the second tube member.

[0113] As noted above, the length L.sub.1 of the first tube member 113a should be at least as great as the distance required for the carrier gas flow to transition from an initial plug flow or turbulent flow to laminar flow. Within this flow-transition region, collisions of ions or other charged particles with the lumen wall are minimized by the axial electric field provided by the electrical potential difference between the electrode 155 and the second tube member 113b. Since the first tube member 113a is not an electrical insulator, those charged particles which may collide with the lumen wall do not cause surface charging of the first tube member and, thus, there is no opposing electrical field at the inlet end of the ion transfer tube 114 inhibiting the flow of charged particles into the tube. Once the ions or other charged particles have passed into the second tube member 113b, the laminar gas flow prevents further collisions with the lumen wall and, thus, a resistive tube material is no longer required. Instead, it is desirable to form the second tube member 113b of a sufficient length of a material with high thermal conductivity (such as a metal) such that ions are completely de-solvated by heat while traversing the second tube member 113b. This length required for desolvation, which may be on the order of several centimeters, may comprise a significant percentage of the space available for the ion transfer tube 114. Therefore, it may be desirable to limit the length L.sub.1 of the first tube member 113a. The inventors have determined that adequate results are obtained when the length of the first tube member 113a (which may be substantially equal to L.sub.1) is approximately 5 mm.

[0114] The use of an ion transfer tube with a bore that has an elongated cross section such as a slot has the additional benefit (in addition to improved ion capture and desolvation) that it is a key element into implementing another technique that increases the sensitivity of a mass spectrometer: using arrays of electrospray emitters. Since the number of ions emitted by an array is increased with respect to that emitted by a single emitter, but the number of ions that can occupy the volume immediately in front of a conventional ion transfer tube is limited by Coulombic repulsion (the so-called space charge limit), the benefit of multiple emitters cannot be realized with a conventional ion transfer tube. FIG. 16A graphically illustrates this concept with reference to, for example, the ion transfer tube 110 for which a cross sectional view has already been provided in FIG. 10A. The elongate bore 164 may align with the long dimension of a linear array 200 of ion emitters, thereby decreasing space charge density at the tube entrance and geometrically providing a better match to the composite ion plume, both in comparison to a conventional ion transfer tube.

[0115] The ion transfer tube 160 of the system 300 (FIG. 16B) may be employed in conjunction with and so as to receive ions from a variety ion emitter array configurations and a variety of ion emitter types. The generic ion transfer tube 160 represents any of the ion transfer tubes 110, 140, 111 and 114 described elsewhere in this document or, more generally, any ion transfer tube having a slotted or obround bore. The ion transfer tube 160 may be employed in conjunction with an emitter array or may be employed in conjunction with a single ion emitter of either conventional or novel design. As one example, FIG. 16B illustrates an array of conventional ion emitter capillaries 302 fluidically coupled to the ion transfer tube 160. The emitter capillaries may be configured so as to produce ions by either the electrospray or atmospheric pressure chemical ionization techniques. As is known, an extractor or counter electrode 304 may be disposed between the plurality of ion emitter capillaries and the ion transfer tube so as to provide an electrical potential difference the assists in accelerating charged particles towards the ion transfer tube 160.

[0116] In conclusion, the use of a system that includes an ion transport tube having at least one slotted or obround bore (e.g., a “letterbox” bore) that delivers ions to an ion funnel, stacked ring ion guide, or other multi-electrode ion transport device can not only enhance overall ion throughput into a mass spectrometer analyzer chamber but, also, can advantageously permit efficient exhausting of gas in intermediate chambers such that the greater throughput does not cause significant pressure increase in the analyzer chamber. Calculations and experimental observations indicate that the limited increase in the analyzer pressure, when compared to a round bore capillary at comparable throughput, is explained by the asymmetric expansion of gas exiting the slotted or obround bore. The asymmetric expansion effect is enhanced when the axis of the ion transfer tube is either offset from or at an angle to the axis of the ion transport device and the long dimension (corresponding to the width) of the bore is oriented within the plane of the two axes. When an ion transfer tube is employed in the configuration described above, this asymmetric expansion causes a radial dispersion of the gas streamlines away from the center axis of the ion transport devices, towards the edges of the RF electrodes, and reduces the flux of gas molecules going into the downstream pumping stages. By contrast, ions are focused towards an ion exit aperture along the axis of the device by action of electric fields applied to the electrodes.

[0117] Systems in accordance with the present teachings can include an ion transfer tube having a slotted or obround bore used in conjunction with a conventional ion funnel or a stacked ring ion guide. However, the asymmetric gas expansion effect may be used to even greater advantage when the slotted-bore ion transfer tube is used in conjunction with an ion transfer apparatus having an open design, as discussed herein, to aid in exhausting gas from the apparatus. It is found that the use of an ion transport system employing an ion transfer tube having a slotted or obround bore configured as described herein enables the use of a relatively short ion funnel (54 mm) in comparison to a system employing a conventional round-bore tube having the same cross sectional area. A further advantage of having a slotted-bore ion transfer tube is that the design of the slotted bores can aid in the de-solvation of ions passing through the ion transfer tube, particularly ions arising from a high aqueous liquid chromatography stream.

[0118] The discussion included in this application is intended to serve as a basic description. Although the invention has been described in accordance with the various embodiments shown and described, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. The reader should be aware that the specific discussion may not explicitly describe all embodiments possible; many alternatives are implicit. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the scope and essence of the invention. Neither the description nor the terminology is intended to limit the scope of the invention. Any patents, patent applications, patent application publications or other literature mentioned herein are hereby incorporated by reference herein in their respective entirety as if fully set forth herein.