ION TRAP WITH RING-SHAPED ION STORAGE CELL AND MASS SPECTROMETER

Abstract

The invention concerns an ion trap, including a first ring-shaped end cap electrode and a second ring-shaped end cap electrode, between which is formed a ring-shaped ion storage cell, as well as a plurality of radially inner disk-shaped ring electrodes and a plurality of radially outer disk-shaped ring electrodes, which delimit the ring-shaped ion storage cell. The invention also relates to a mass spectrometer that has such an ion trap as well as a control device that is designed to actuate the disk-shaped ring electrodes and the end cap electrodes for the storage, selection, excitation and/or detection of ions in the ring-shaped ion storage cell.

Claims

1. An ion trap, comprising: a first ring-shaped end cap electrode and a second ring-shaped end cap electrode, between which is formed a ring-shaped ion storage cell, characterised by a plurality of radially inner disk-shaped ring electrodes and a plurality of radially outer disk-shaped ring electrodes that delimit the ring-shaped ion storage cell.

2. The ion trap according to claim 1, in which the inner ring electrodes and the outer ring electrodes are arranged at a constant radial distance from one another.

3. The ion trap according to claim 1, in which a radial distance between the inner ring electrodes and the outer ring electrodes is smaller than a radius of the ring-shaped end cap electrodes.

4. The ion trap according to claim 1, in which in each case an inner ring electrode and an outer ring electrode are arranged on a common plane perpendicular to an axial direction.

5. The ion trap according to claim 1, in which in each case an inner ring electrode and an outer ring electrode are connected to one another in an electrically conductive manner.

6. The ion trap according to claim 1, in which for a width b of a respective disk-shaped first or second ring electrode and a distance d in the axial direction between, respectively, two adjacent first or second ring electrodes, the following applies: d/b<¼.

7. The ion trap according to claim 1, which has a number N of radially inner ring electrodes and a number N of radially outer ring electrodes, for which the following applies: 10<N<200.

8. The ion trap according to claim 1, in which the first end cap electrode and/or the second end cap electrode are divided into at least two ring-shaped segments in the circumferential direction.

9. The ion trap according to claim 1, further comprising: at least one injection device for the preferably tangential, particularly pulsed, injection of ions and/or of an electron beam into the ring-shaped ion storage cell.

10-14. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Example embodiments are shown in the schematic drawing and are explained in the following description. In the drawing,

[0042] FIG. 1 shows a schematic representation of a cross-section of a mass spectrometer with an ion trap with a ring-shaped ion storage cell,

[0043] FIG. 2 shows a top view onto the ring-shaped ion storage cell of FIG. 1 with a segmented end cap electrode, and

[0044] FIG. 3 shows a detailed representation of several outer and inner disk-shaped ring electrodes which radially delimit the storage cell.

DETAILED DESCRIPTION

[0045] In the following description of the drawings, identical reference numbers are used for components that are identical or have the same function.

[0046] FIG. 1 shows a schematic cross-sectional view of a mass spectrometer 1, which has an ion trap 2 as well as an electronic control device 3. The ion trap 2 comprises a first ring-shaped end cap electrode 4a that is at the top in an axial direction Z of an XYZ coordinate system, and a second ring-shaped end cap electrode 4b that is at the bottom in an axial direction Z, between which is formed a ring-shaped ion storage cell 5. The two end cap electrodes 4a, 4b respectively have a hyperbolically curved surface facing the ion storage cell 5, as is customary in the case of an ion trap in the form of a Paul trap.

[0047] The ring-shaped ion storage cell 5 runs radially symmetrical to the axial direction Z of the XYZ coordinate system. In the sectional illustration of FIG. 1, the radial direction corresponds to the X direction of the XYZ coordinate system. In the radial direction X, the ion storage cell 5 is bounded to the inside by a plurality or number N of radially inner disk-shaped ring electrodes E.sub.1,i (i=1, . . . , N) and to the outside by a plurality or number N of radially outer disk-shaped ring electrodes E.sub.2,i (i=1, . . . , N). The plurality N of radially inner ring electrodes E.sub.t; are arranged over one another in the axial direction Z. Accordingly, the plurality N of radially outer ring electrodes E.sub.2,i are also arranged over one another in the axial direction Z.

[0048] The ring-shaped ion storage cell 5 has a constant extension in the radial direction X, which corresponds to the distance 2 r.sub.0 between the inner and outer ring electrodes E.sub.1,i, E.sub.2,i. The radial distance 2 r.sub.0 between the inner and outer ring electrodes E.sub.1,i, E.sub.2,i here is less than the radius R of the two end cap electrodes 4a, 4b. The radius R of the end cap electrodes 4a, 4b, which corresponds to the mean radius R of the ion storage cell 5, runs in the radial direction X centrally through the two end cap electrodes 4a, 4b.

[0049] The ion storage cell 5 runs in the radial direction X between a minimum radius R−r.sub.0, which is determined by the radially outer end faces of the inner ring electrodes E.sub.1,i, and a maximum radius R+r.sub.0, which is determined by the radially inner end faces of the outer ring electrodes E.sub.2,i. Through the constant distance 2 r.sub.0 between the ring electrodes E.sub.1,i, E.sub.2,i, it is made possible for the ions 6 in the “cold, cooled” state to circulate in the ion trap 2 in an orbit that corresponds to the mean radius R of the ring-shaped ion storage cell 5.

[0050] In the example shown, the number N of inner ring electrodes E.sub.1,i corresponds to the number N of outer ring electrodes E.sub.2,i.

[0051] For the quantity N, in the example shown the following applies: 10<N<200. It has been shown that even a comparatively small number N of ring electrodes E.sub.1,i, E.sub.2,i is sufficient to allow the ions 6 in the “cold, cooled” state to circulate on stable orbits in the ring-shaped ion storage cell 5.

[0052] As can be seen in FIG. 1 as well as in FIG. 3, the ring electrodes E.sub.1,i, E.sub.2,i are arranged in pairs in common planes X, Y perpendicular to the axial direction Z. In each case two ring electrodes E.sub.1,i, E.sub.2,i arranged in a common plane X, Y are connected to one another via an electrical lead or electrical contact, not shown in the drawing, i.e. they are on the same electrical potential. Electrical contact can also be effected via the control device 3. For the width b of a respective disk-shaped first ring electrode E.sub.1,i in the radial direction X and a distance d in an axial direction Z between, respectively, two adjacent first ring electrodes E.sub.1,i, E.sub.1,i+1, the following applies: d/b<¼. Accordingly, for the (identical) width b of the second ring electrodes E.sub.2,i and for a distance d in an axial direction Z between, respectively, two adjacent second ring electrodes E.sub.2,i, E.sub.2,i+1, the following likewise applies: d/b<¼.

[0053] The distance d between the adjacent ring electrodes E.sub.1,i, E.sub.1,i+1 or E.sub.2,i, E.sub.2,i+1 can for example be between approx. 100 μm and approx. 1 mm. The width b in the radial direction X is accordingly between approx. 400 μm and 4 mm. Through the open mechanical architecture of the ion trap 2 on account of the disk-shaped ring electrodes E.sub.1,i, E.sub.2,i that are distanced from one another (“discrete”), to produce the potential wells for the ions 6, a higher vacuum conductivity of the ion trap 2 compared to conventional ion traps is ensured with a solid ring electrode.

[0054] As can be seen in FIG. 2, the first end cap electrode 4a is divided, in the circumferential direction, into four ring-shaped segments Q1, Q2, Q3, Q4, which extend in the circumferential direction in each case over an angle of 90°.

[0055] Accordingly, the second end cap electrode 4b too is divided into four ring-shaped segments Q1, Q2, Q3, Q4 (not shown in the drawing). On account of the segmentation, in the time-multiplexing operation of the control device 3, the end cap electrodes 4a, 4b can be used optionally as excitation, filtration or measurement electrodes.

[0056] The control device 3 is in signal connection with each of the four segments Q1, Q2, Q3, Q4 of the first end cap electrode 4a and with each of the four segments Q1, Q2, Q3, Q4 of the second end cap electrode 4b, in order to transmit excitation signals and to receive ion signals or measurement signals. By way of an example, FIG. 1 shows two such measurement signals S1, S2, which are produced by mirror charges of the excited ions 6 stored in the ion storage cell 5 and which originate from a respective first segment Q1 of the first or second end cap electrode 4a, 4b. On the basis of these ion signals S1, S2, which are typically evaluated differentially, and on the basis of additional ion signals recorded at the second to fourth segments Q2, Q3, Q4, a time-related dispersion (more mobile ions advance faster) of the ions 6 injected in a pulsed manner into the ring-shaped ion storage cell 5 can be established.

[0057] In this way—but also possibly where non-segmented end cap electrodes 4a, 4b are used—in the case of the ion trap 2 all customary Fourier transform measurement tools can be applied such as are used in a conventional FT ion trap, for example ion filtration during ionisation or storage, separation of ions and non-destructive detection. In particular, in the ion trap 2 SWIFT excitations can also be carried out, for example in the manner described in U.S. Pat. No. 10,141,174 B4, the entirety of which is incorporated into the content of this application by reference.

[0058] As has been described further above, the control device 3 typically actuates the end cap electrodes 4a, 4b or the respective segments Q1, Q2, Q3, Q4 of the end cap electrodes 4a, 4b for the selection, excitation and detection of the ions 6 in the ion storage cell 5.

[0059] The control device 3 is also designed for storing the ions 6 in the ion storage cell 5. To this end, the control device 3 has an HF generator 8 as well as a resistance network in order to actuate the disk-shaped ring electrodes E.sub.1,i, E.sub.2,i to produce a respective HF storage voltage V.sub.RF,i or to apply the corresponding HF storage voltage V.sub.RF,i to them. The splitting of a given e.g. harmonic HF storage voltage V.sub.RF(t)=V.sub.max sin (w t) over the individual ring electrodes E.sub.1,i, E.sub.2,i or over the respective pairs of electrodes can for example take place in the manner described in the article by M. Aliman and A. Glasmachers cited above (in this case, or if equation (2) is used, the following applies for i: −N/2<i<N/2). It is understood that the splitting of the HF storage voltage V.sub.RF(t) over the respective HF storage voltages V.sub.RF,i of the individual ring electrodes E.sub.1,i, E.sub.2,i can also take place in a manner other than that described here.

[0060] In the case of the example shown in FIG. 1 and FIG. 2, the control device 3 is arranged wholly within a volume area 7 that is surrounded by the ring-shaped ion storage cell 5. The control device 3 is arranged in the centre of the ion trap 2 or ring-shaped ion storage cell 5, and does not project beyond the ring-shaped ion storage cell 5 in the axial direction Z. The control device 3 can for example be arranged in a separate vacuum area, which for example is separated from the ring-shaped ion storage cell 5 by a seal, by differential pumping or by a housing. Through this arrangement of the control device 3, a particularly compact mass spectrometer 1 can be achieved.

[0061] The control device 3 is also designed to actuate an injection device 9 that is shown in FIG. 2, and which in the example shown, serves for the tangential, typically pulsed, injection of ions 6 into the ring-shaped ion storage cell 5. The injection device 9 can have a controllable, in particular pulsed inlet or inlet system, e.g. a controllable valve, in order to inject the ions 6 of the gas that is to be analysed, which are produced by an (external) ion source that is not shown in the drawing, into the mass spectrometer 1 or into the ring-shaped ion storage cell 5. To this end, the injection device 9 can have in particular an ion lens, not shown in the drawing, which injects the ions 6 into the ring-shaped ion storage cell 5 on a straight trajectory that is aligned tangentially to the mean radius R of the ring-shaped ion storage cell 5, as can be seen in FIG. 2.

[0062] In the example shown, the ions 6 are injected in a plane X,Y perpendicular to the axial direction Z and in an axial direction Z centrally between the end cap electrodes 4a, 4b, so that the ions 6 move in a non-excited state on a circular trajectory 11 in the middle of the ion storage cell 5.

[0063] In the example shown here, the distance d between the adjacent outer ring electrodes E.sub.2,i, E.sub.2,i+1 is chosen to be sufficiently large to inject the ions 6 between two adjacent ring electrodes E.sub.2,i, E.sub.2,i+1 into the ring-shaped ion storage cell 5, i.e. it is not necessary to provide an additional entrance for the injection of the 6 into the ring-shaped ion storage cell 5.

[0064] The mass spectrometer 1 and in particular the ring-shaped ion storage cell 5 are arranged in a housing, not shown in the drawing, which separates the mass spectrometer 1 from its surroundings, for example from a process chamber, with the gas that is to be analysed.

[0065] With the aid of vacuum pumps that are not shown in the drawing, a vacuum is produced in the ion trap 2 and in particular in the ion storage cell 5. Therefore apart from the ions 6 of the gas that is to be analysed, in the ion storage cell 5 there is only a background gas which typically has a very low pressure.

[0066] In the example shown here, the injection device 9 is designed, in addition to the injection of ions 6, for the production and injection of an electron beam 10 into the ring-shaped ion storage cell 5. The electron beam 10 too is supplied tangentially to the ion storage cell 5, as can be seen in FIG. 2. The control device 3 actuates the end cap electrodes 4a, 4b, in order to produce a Lorentz force on the electron beam 10 according to the above equation (3), and to deflect it onto a circular trajectory 11 in the ion storage cell 5. The electron beam 10 serves to produce ions 6 directly in the ion storage cell 5 through impact ionisation. The production of the ions 6 in the ion storage cell 5 can take place on a neutral gas that is to be analysed, which is supplied to the ion storage cell 5 via the injection device 9 or possibly via another entrance, before the ions 6 of the gas that is to be analysed are produced in situ in the ion storage cell 5 with the aid of the electron beam 10.

[0067] In summary, with the aid of the ring-shaped or circular ion trap 2 described above, the available ion storage cell 5 can be significantly enlarged without significant enlargement of the construction space of the ion trap 2. Through the electronic control device 3, it is also possible to force spatial, circular ion storage, in which the ions 6 move, pictorially, in a circular orbit. In this way, a circular enlargement of the ion storage cell 5 is achieved, the space charge problem is minimised, and thus the measurement resolution is increased as compared with conventional FT ion traps.

[0068] Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above.

[0069] Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are described as example forms of implementing the claims.