Quadrupole ion traps are used in mass spectrometers to trap ions, i.e., atoms or molecules having a charge due to the loss or gain of one or more electrons. Quadrupole ion traps use electromagnetic fields generated by applying RF signals between elongate rods (or poles) to trap ions radially within a defined volume of space that will be referred in this disclosure to as a trapping volume. Quadrupole ion traps additionally use end caps axially offset from one another to trap the ions axially within the trapping volume.
Ion traps can be used for many different purposes in mass spectrometry and other fields. For instance, they can be used to store ions temporarily while the ions are waiting to be transferred to another part of a scientific instrument, such as a measurement stage. Likewise, they can be used to temporarily store of ions after the ions are created or after the ions exit a measurement stage of a scientific instrument.
Quadrupole ion traps also are often used for separating certain ions from other ions based on the mass to charge ratio (m/z) of the ions. Specifically, the electromagnetic fields that trap ions in the ion trap can be manipulated so that ions having an m/z ratio above or below a certain m/z ratio are ejected from the trap, while other ions having different m/z ratios remain in the trap.
It also is known to use an ion trap as a fragmentation cell in which ions are fragmented into smaller pieces. In an example, an inert gas such as argon is introduced into the trapping volume. The ions trapped in the trapping volume collide with the molecules of the inert gas with sufficient force to fragment the ions. The fragments and remaining intact ions are then ejected from the trap (either selectively based on m/z ratio or in their entirety) for further processing. For instance, the fragments and ions may be ejected toward a detector for measurement. Alternatively, in a tandem mass analyzer, the fragments and ions may be ejected into another mass analyzer stage, e.g., a Fourier transform mass analyzer, RF quadrupole mass analyzer, time of flight mass analyzer, or another quadrupole ion trap mass analyzer.
As mentioned above, quadrupole ion traps use electromagnetic fields to contain the ions within the trapping volume both radially and axially. Ions can be admitted or ejected from the ion trap by altering the electric fields (e.g., turning one or more of the electric fields off or changing the amplitude and/or frequency of one or more of the electric fields) so that the ions, or at least ions having certain m/z ratios, enter or exit the trapping volume. In most quadrupole ion traps, ions enter the trapping volume travelling axially through one of the ends of the trapping volume. Many quadrupole ion traps also permit ions to exit the trap travelling axially through one of the ends of the trapping volume, typically, the end axially opposite from the entrance end. However, the ions may enter and exit the trapping volume through the same end. Other ion traps eject ions radially. Specifically, a gap may be provided in one or more of the elongate poles through which ions can exit travelling radially.
Generally, the ions are contained radially within the trapping volume by an RF containment field generated by applying an RF signal to the poles. Typically, the RF signal is a differential signal, and the in-phase component of the RF signal is applied to two opposing poles of the quadrupole and the antiphase component of the RF signal is applied to the other two opposing poles of the quadrupole.
With respect to axial containment, a quadrupole ion trap that traps ions of only a single polarity at any given instant typically axially contains the ions by applying a DC voltage to each of the axial end caps. This potential causes the ions to travel back and forth in the axial direction within the trapping volume.
However, a DC field cannot trap both positive and negative ions simultaneously because a particular axial DC field will provide an effective barrier for ions of one polarity, but would accelerate the ions of the opposite polarity axially out of the trapping volume.
U.S. Pat. No. 7,227,130 discloses a technique for generating axial RF fields that can simultaneously contain ions of both positive and negative polarities both axially and radially. Specifically, this patent discloses the application of particular RF signals between the quadrupole rods to generate a radial RF containment field in conjunction with the application of other RF signals between the end caps and the rods to generate an axial RF containment field between the end caps and the rods of the quadrupole. The axial RF containment field keeps ions of both polarities trapped and circulating between the two end caps.
One drawback of the technique described in the U.S. Pat. No. 7,227,130 is that the axial containment field and radial containment field are interdependent, i.e., they interact with each other. Consequently, one cannot be changed without affecting the other. Thus, for instance, changing the radial containment field to reduce the trapping volume radially would also change the axial containment field. To restore the axial containment field to its original state would require that the RF signals applied to the end caps be adjusted accordingly.
An ion trap comprises elongate rods, electrodes, a first circuit, and a second circuit. The rods are for defining the radial extent of a trapping volume. The first circuit is connected to the rods for applying thereto a first RF signal that generates adjacent the trapping volume a radial RF containment field that radially contains ions of different polarities within the trapping volume. The electrodes are for defining the axial extent of the trapping volume. The second circuit is connected to the electrodes for applying thereto a second RF signal that generates adjacent the trapping volume an axial RF containment field that axially contains the ions of different polarities within the trapping volume. The axial RF containment field is independent of the radial RF containment field.
The use of a quadrupole configuration for radially confining ions is merely exemplary and it should be understood that the invention can be applied to multi-pole ion traps having other numbers of poles.
The illustrated quadrupole embodiment comprises four elongate rods 101, 102, 103, and 104 that provide the poles of the ion trap. A radio frequency (RF) electrical signal is applied between the rods to generate a radial containment field for confining the ions in the radial direction, i.e., in the direction of the x-y plane shown in
Adjacent each end of the rods 101-104 is located an end cap 107 having two electrodes 109 and 111 between which an RF signal is applied to generate an axial containment field as described below. In the illustrated embodiment, the end caps 107 are within the axial ends of the volume defined by the rods 101-104. However, this is not a requirement. In the example shown in
Outer annular electrode 111 defines a second axial aperture 115 that extends through it in the z-direction and within which the inner annular electrode 109 is positioned. The inner and outer annular electrodes 109 and 111 typically are positioned at the same location in the z-direction.
A second RF signal generator 121, typically similar in structure to above-described RF signal generator 110, applies an RF signal between the electrodes 109, 111 through a second center-tapped transformer 106. The second transformer 106 also may be connected to reference potential 114, as illustrated in
In the examples shown in
Merely as an example of a typical set of dimensions for an ion trap in accordance with the invention, the quadrupole rods are about 10 mm in diameter. Opposite ones of the rods, e.g., rods 101 and 103, are spaced from each other about 19 mm center-to-center, i.e., the rods collectively define within the trapping volume 108 a cylindrical space about 9 mm in diameter. The electrodes 109, 111 constituting each end cap 107 are a thin-walled metal sleeve about 0.5 mm thick. The inner electrode 109 has an inner diameter of 5 mm and an outer diameter of 6 mm and the outer electrode 111 has an inner diameter of 7 mm and an outer diameter of 8 mm. This particular embodiment provides a radial clearance of about 1 mm between the two annular electrodes 109, 111 and about 1 mm of radial clearance between the outer electrode 111 and the rods 101-104 of the quadrupole. An exemplary amplitude and frequency of the axial containment field for an for a particular ion trap having the dimensions noted above is about 400 volts peak-to-peak at a frequency of 1 MHz.
Applying the RF signal between the electrodes 109, 111 makes the resulting axial containment field largely or entirely independent of the radial containment field. Specifically, the terminals of the axial containment field, i.e., the electrodes 109, 111 are physically and electrically independent of the terminals of the radial containment field, i.e., rods 101-104. Accordingly, there is little or no interaction between the axial and radial containment fields. This is a significant advantage since the radial containment field and the axial containment field can be controlled independently of each other without significant interdependence.
A differential RF signal can be applied between the two electrodes 109, 111, as illustrated in
The end cap electrodes 109, 111 constituting each end cap 107 may be positioned beyond the ends of the quadrupole rods 101-104, if desired. However, care should be taken that they are close enough to the ends of the rods 101-104 that there is no gap between the axial containment field and the radial containment field through which ions might be able to escape. Furthermore, placing the end cap electrodes within the axial extent of the quadrupole rods, as illustrated in
Particularly good isolation between the axial containment field and the radial containment field is achieved when the inner electrode 109 and the outer electrode 111 are substantially coplanar in the x-y plane and the outer electrode 111 is at least as long as the inner electrode 109 so that the outer electrode 111 completely occludes the inner electrode in the radial direction. The length of the electrodes is the dimension of the electrodes in the z-direction. Furthermore, particularly good containment is achieved while preserving the good isolation between the axial containment field and the radial containment field when the outer annular electrode 111 extends inwardly in the z-direction farther than the inner electrode 109. 109. For example, in one exemplary embodiment, the outer electrode 111 is longer than the inner electrode 109. Thus, if the axially outer end of the inner electrode 109 and the axially outer end of the outer electrode 111, i.e., the ends facing away from the trapping volume 108, are made even with each other (i.e., coplanar in the x-y plane), then the outer electrode 111 will extend inwardly in the z-direction farther than the inner electrode 109. This configuration slightly tilts the axial containment field toward the central longitudinal axis of the trapping volume, i.e., the axis that extends in the z-direction and is centered on the intersection of the line in the x-y plane extending between the centers of rods 101 and 103 and the line in the x-y plane extending between the centers of rods 102 and 104. This configuration actually provides better containment of ions traveling along the z-axis. It has been found that an outer electrode 111 that is longer than the inner electrode by about one half of the inner diameter of the inner electrode 109 provides particularly good isolation between the axial containment field and the radial containment field as well as good axial ion containment. In an example of this type of configuration, the inner electrode has an inner diameter of about 5 mm, the inner electrode is about 5 mm long, and the outer electrode is about 7.5 mm long.
It should be understood that, while the annular shape of the electrodes 109, 111 is particularly suitable because annular electrodes generate a containment field closely corresponding to the cross-sectional shape of the radial containment field (in the x-y plane), thereby providing a particularly uniform containment field where it is needed, this electrode shape is merely exemplary. In alternative embodiments, electrodes 109, 111 are square, rectangular, hexagonal, or irregular in shape in the x-y plane shown in
An end cap in accordance with the present invention may be applied at either or both ends of a multipole ion trap.
Ions are axially ejected from the trapping volume 108 through the aperture 113 in the inner end cap electrode 109 when the axial containment field is turned off (or decreased in amplitude). However, if the particular instrument does not require axial ejection (or entry) of ions through a particular end cap, then the inner electrode of that end cap need not have an aperture.
As in the embodiment shown in
The embodiment shown in
In one exemplary embodiment, the plates 319, 321 are each 0.5 mm thick, the apertures 313, 315 are 3 mm in diameter and the two electrodes 309, 311 are axially spaced from each other about 1 mm. The plates 319, 321 may be circular in shape to closely correspond to the radial shape of the trapping volume. However, other shapes are possible, including square, rectangular, polygonal, and irregular shapes. The two electrodes need not have the same shape, although using electrodes of different shapes will make the axial containment field more complex. In a particularly effective embodiment, the plates 319, 321 extend in the x-y plane beyond the cylindrical space collectively defined by the quadrupole rods. Thus, if the cylindrical space defined by the rods 101-104 is about 9 mm in diameter as mentioned above in connection with the embodiment of
As in the embodiment of
In the example shown, the conductive plates 319, 321 are disposed parallel to each other and orthogonal to the z-axis, and the apertures 313, 315 are centered on the z-axis. This permits ions to pass through the end cap in a straight line when the axial containment field is turned down or off to eject ions from the trap. If it is desired to cause the ions to travel off axis or otherwise take a more tortuous course as they exit the trapping volume, which usually is not desirable, but actually may be useful in some applications, then the apertures may be provided in a non-aligned configuration. In such embodiments the ions may be induced to take such course by making the shapes of the plates 319, 321 different from each other to adjust a differential DC field to a value that induces the desired trajectory through the non-aligned apertures in the plates.
In differentially driven implementations of this embodiment, there are many possible ways of supplying the differential drive voltage to the electrodes 409-411. In one embodiment, the inner electrode 409 and the middle electrode 410 are each supplied with a respective phase of the differential RF voltage, while the outer electrode 411 is connected to the reference potential. This configuration is believed to provide the most effective isolation between the axial containment field and the radial containment field.
Again, there are numerous ways to supply the RF signal for generating the axial containment field to the electrodes 509, 510, 511. In at least one single-ended embodiment, the middle electrode 510 is connected to receive the RF signal while the axially inner electrode 509 and the axially outer electrode 511 are connected to the reference potential. This is believed to provide the greatest isolation between the radial containment field and the axial containment field since the RF field is shielded on both axial sides by the electrodes 509, 511. However, this connection scheme is not a requirement.
In a differentially driven version of this type of embodiment, the axially inner electrode 509 is connected to the reference potential and the two outer electrodes 510, 511 are connected to respective phases of the differential RF signal to provide the greatest isolation between the axial containment field and the radial containment field. Other differential connection schemes may be used.
As noted above, the quadrupole configuration for providing the radial containment field in all of the illustrated embodiments is merely exemplary and the axial containment concepts disclosed herein can be applied to multipole ion traps with other numbers of rods for providing the radial containment field. The concepts can be used in connection with hexapole, octopole, dodecapole, and other radial containment field configurations. Moreover, the axial containment field can be provided using more annular electrodes or more planar electrodes than the numbers shown the above-described examples. However, each additional electrode in excess of three provides a diminishing benefit. Also, while the exemplary embodiments illustrate the use of the innovative concepts in connection with linear ion traps, this also is not a limitation.
This disclosure describes the invention in detail using illustrative embodiments. However, the invention defined by the appended claims is not limited to the precise embodiments described.
Number | Name | Date | Kind |
---|---|---|---|
4755670 | Syka et al. | Jul 1988 | A |
6121607 | Whitehouse et al. | Sep 2000 | A |
6872938 | Makarov et al. | Mar 2005 | B2 |
6888133 | Wells et al. | May 2005 | B2 |
7129478 | Baba et al. | Oct 2006 | B2 |
7227130 | Hager et al. | Jun 2007 | B2 |
20010050335 | Whitehouse et al. | Dec 2001 | A1 |
20030141449 | Wells et al. | Jul 2003 | A1 |
20050258354 | Baba et al. | Nov 2005 | A1 |
20070023648 | Baba et al. | Feb 2007 | A1 |
20090032700 | Park et al. | Feb 2009 | A1 |
Number | Date | Country | |
---|---|---|---|
20090140141 A1 | Jun 2009 | US |