Patent Grant
9831076
The present invention relates generally to mass spectrometry, and more particularly to a device for energetically cooling packets of ions ejected from an ion trap prior to mass analysis.
Tandem mass spectrometry, referred to as MS/MS, is a popular and widely-used analytical technique whereby precursor ions derived from a sample are subjected to fragmentation under controlled conditions to produce product ions. The product ion spectra contain information that is useful for structural elucidation and for identification of sample components with high specificity. In a typical MS/MS experiment, a relatively small number of precursor ion species are selected for fragmentation, for example those ion species of greatest abundances or those having mass-to-charge ratios (m/z's) matching values in an inclusion list. There is growing interest in the use of “all-mass” MS/MS, in which all or a substantial subset of the precursor ions are fragmented. All-mass MS/MS yields information-rich spectra and removes the need to select and isolate particular ion species prior to fragmentation. In order to simplify the interpretation of product ion spectra produced by all-mass MS/MS, the analysis may be conducted as a series of fragmentation/spectral acquisition cycles performed on different subsets or groups of the precursor ions, with each subset or group representing a different range of precursor ion m/z's. For example, if the precursor ions have m/z's ranging from 200 to 2000 Th, the first fragmentation/spectral acquisition cycle may be performed on a first packet of ions having m/z's between 200 and 210 Th, the second fragmentation/acquisition cycle may be performed on a second packet of ions having m/z's between 210 and 220 Th, and so on. U.S. Pat. No. 7,157,698 to Makarov et al., the disclosure of which is incorporated by reference, teaches a mass spectrometer architecture for implementing all-ion MS/MS with separation of the precursor ions into groups according to their m/z's. In the Makarov apparatus, an orthogonal-ejection two-dimensional ion trap is employed to eject m/z-grouped precursor ions into a collision cell, where the ions undergo fragmentation. The resultant product ions are transported to the entrance of a time-of-flight (TOF) mass analyzer for acquisition of a mass spectrum. TOF mass analyzers are particularly well-suited to all-mass MS/MS experiments due to their wide mass ranges and relatively short analysis times.
In the Makarov apparatus and similar designs employing an ion trap for mass-selective ejection, it is important to reduce the kinetic energy spread of the ejected ions, and product ions derived therefrom, prior to delivering the ions to the entrance of the mass analyzer. In TOF and other mass analyzers, high initial kinetic enlarge variations in the initial kinetic energies of the ions may significantly compromise measurement performance, particularly with respect to resolution and mass accuracy. Cooling of the ions to reduce kinetic energy and kinetic energy spread may be accomplished by directing the ions through a cooling region in which the ions lose energy via collisions with neutral gas molecules. Makarov uses an elongated collision cell structure with an axial DC gradient to provide the cooling region. The degree of energetic cooling will depend on the number of collisions experienced by the ions within the cooling region, which is governed by the product of residence time and cooling region pressure (t*P). For a cooling region held at a typical operating pressure, a total ion residence time of between 0.5-1.5 millisecond (ms) may be required to reduce ion kinetic energies to values that enable high-resolution mass analysis. This residence or cooling time may be substantially greater than the times required for ejection of an ion packet from the trap (as well as for mass analysis of an ion packet), which means that the ejection of a subsequent ion packet from the trap into the fragmentation/cooling region must be delayed until cooling of the first ion packet is completed. Differently expressed, the cooling period limits the rate at which the all-ion MS/MS analysis may be conducted and reduces the total number of analyses that may be performed during a chromatographic elution peak. Of course, the rate may be increased by employing a shorter cooling period, but doing so has a deleterious effect on resolution and/or mass accuracy.
Briefly described, a mass spectrometer constructed and configured in accordance with embodiments of the invention includes an ion trap equipped to eject a series of ion packets in temporal succession, a pulsed mass analyzer such as a TOF mass analyzer, and an ion interface device positioned in the ion path between the ion trap and the pulsed mass analyzer. The ion interface device includes a transport/collision section and a plurality of spatially separated confinement cells. A packet of ions ejected from the ion trap is received by the ion interface device and directed to a selected one of the plurality of confinement cells. The ion packet is confined and cooled within the confinement cell for a prescribed cooling period, after which it is released to the pulsed mass analyzer for acquisition of a mass spectrum. Confinement and cooling of the ion packet in the ion interface device occurs concurrently with the receipt of one or more successively ejected ion packets, each of which is directed within the ion device to another one of the confinement cells. By enabling concurrent cooling of different ion packets in spatially separated confinement cells, the ions in each ion packet may be cooled sufficiently to enable the acquisition of mass spectra at high resolution in the pulsed mass analyzer, without having to substantially delay the ejection of a subsequent packet of ions from the ion trap until cooling of the previous packet is completed.
According to more particular embodiments of the invention, the ion interface device may cause at least a portion of the ions in each received ion packet to undergo fragmentation or reaction to form product ions. The ion interface device may include at least four confinement cells. The ion interface device may include a distribution section having an array of rod electrodes oriented transversely to the longitudinal axis of the ion interface device, with the confinement cells being disposed laterally outwardly of the rod electrodes. At least some of the rod electrodes may be segmented to enable development of a transverse DC field that moves an ion packet to the selected confinement cell. The TOF mass analyzer may include first and second ion flight paths having entrance regions respectively disposed proximate to first and second sets of the confinement cells. The first and second ion flight paths of the TOF mass analyzer may terminate at a common detector assembly. The product of the cooling period and the confinement cell pressure may be a minimum of 1 ms·mTorr, and preferably in the range of 2-5 ms·mTorr.
In the accompanying drawings:
Ion interface device 105 is provided with a plurality of separate confinement cells. As will be discussed in greater detail below, ion interface device 105 receives individual packets of ions ejected from ion trap 110 and directs each ion packet to a selected confinement cell. The ion packet is held within the confinement cell for confinement period, during which time the ions undergo energetic cooling. As the ions in one ion packet cool in the associated confinement cell, one or more successively ejected ion packets are received by ion interface 105 and directed to other ones of the plurality of confinement cells. In a preferred embodiment, ion interface 105 includes a transport/collision section in which some or all of the ions in the incoming ion packet undergo fragmentation by collision activated dissociation (CAD) or other mechanism of dissociation to yield product ions.
After cooling for a predetermined confinement period, an ion packet is released from the associated confinement cell of ion interface device 105 to the inlet of TOF analyzer 115. As depicted in
The operation of the various components of mass spectrometer 100 is directed by a control and data system (not depicted in
While mass spectrometer 100 is depicted as being configured for an electrospray ion source, it should be noted that other implementations may utilize any number of pulsed or continuous ion sources (or combinations thereof), including without limitation a matrix assisted laser desorption/ionization (MALDI) source, an atmospheric pressure chemical ionization (APCI) source, an atmospheric pressure photo-ionization (APPI) source, an electron ionization (EI) source, or a chemical ionization (CI) ion source. Furthermore, while embodiments of the invention are described herein with reference to a TOF mass analyzer, those of ordinary skill will appreciate that the interface device and method described herein may be beneficially utilized in connection with other types of pulsed mass analyzers, including but not limited to Orbitrap and other electrostatic trap mass analyzers, and Fourier Transform/Ion Cyclotron Resonance (FTICR) mass analyzers.
Electrodes 205,a,b,c,d (or a portion thereof) are coupled to an RF trapping voltage source, excitation voltage source, and DC voltage source (not depicted), all of which communicate with and operate under the control of a controller that forms part of the control and data system. The RF trapping voltage source is configured to apply RF voltages of adjustable amplitude in a prescribed phase relationship to pairs of electrodes 205a,b,c,d to generate a trapping field that radially confines ions within the interior of ion trap 110. The DC voltage source is operable to apply DC potentials to electrodes 205a,b,c,d or sections thereof to, for example, generate a potential well that axially confines ions within ion trap 110. The excitation voltage source applies an oscillatory excitation voltage of adjustable amplitude and frequency across at least one pair of opposed electrodes to create a dipolar excitation field that resonantly excites ions for the purposes of isolation of selected species, collision induced dissociation, and mass-sequential scanning. During a mass-sequential scan, the RF trapping voltage amplitude is progressively increased from a first value to a second value, which respectively correspond to the lowest and highest m/z ions to be ejected, while a resonant excitation voltage is applied across electrodes 205b,d. This causes the ions to become resonantly excited and ejected from ion trap 110 (via aperture 207) in order of their m/z's. For all-mass MS/MS operation, the mass sequential scan is broken into a number of scan periods or windows, during each of which a packet of ions within a relatively narrow range of m/z's is ejected to ion transfer device 105. In one illustrative example, a mass sequential scan representing a total interval (difference between lightest and heaviest ions ejected) of 600 Th may be broken into 100 component scan windows, each representing an m/z range of 6 Th. For a typical mass-sequential scan rate of 16,000 Th/s, each scan window requires 6/16,000=375 μs to complete. Since this time may be significantly shorter than the time required for fragmentation and cooling (at typical operating pressures) of the ejected ions prior to analysis in a TOF mass analyzer, delaying the ejection of a packet of ions until the previously ejected group is fully cooled and fragmented would substantially increase the total analysis cycle time and reduce throughput. The utilization of ion interface device 105 avoids the need to delay ejection of a packet of ions pending completion of cooling and fragmentation of a previous group, as described below.
The design and operation of the ion trap described above is presented only by way of example, and should not be construed as limiting the scope of the invention. Other ion trap configurations (including two-dimensional quadrupole ion traps adapted for mass-selective axial ejection of ions through a barrier field, an example of which is described in U.S. Pat. No. 6,177,668 to Hager) may be used in place of the radial-ejection two-dimension ion trap disclosed above and depicted in the drawings.
Generally described, ion interface device 105 includes a transport/collision section 210, a distribution section 220, and four separate confinement cells 230a, 230b, 230c and 230d. An ion packet ejected from ion trap 110 enters ion interface device 105 through an inlet to transport/collision section 210. Transport/collision section 210 may be filled with a neutral collision/damping gas, such as argon, to induce fragmentation (which results from the collisions of energetic ions with atoms or molecules of the collision/damping gas, causing transfer of kinetic energy to excited vibrational modes of the ions). Concurrently, collisions remove kinetic energy from the incoming ions and product ions derived therefrom. If fragmentation of the incoming ions is desired, the conditions at which ions are resonantly ejected from ion trap 110, the DC potentials applied to electrodes of ion trap 110 and interface device 105 (as well as any intermediate lenses or other ion optics) and the composition of the collision/damping gas are selected such that the kinetic energies of the ions are sufficiently high to cause a substantial portion of the ions to undergo collisionally activated dissociation and produce product ions. In alternative implementations, product ions may be formed by filling transport/collision section 210 with reagent ions or molecules that react with sample ions in the ion packet. Typical collision/damping gas pressure within transport/collision section 210 will be in the range of 10-15 mTorr.
While
The ion packet (inclusive of any product ions) traverses transport/collision section 210 and enters distribution section 220. Movement of ions through transport/collision section 210 into distribution section 220 may be assisted by use of a longitudinal DC gradient, which may be established by the application of suitable DC potentials to electrodes of interface device 105 (including the main RF electrodes and/or any auxiliary electrodes). Within distribution section 220, ions of the ion packet are routed to an available (i.e., empty) confinement cell. Generally, routing of ions to a selected confinement cell will occur in a repeated fixed sequence. For example, a first-in-time ion packet may be routed to confinement cell 230a, a second-in-time ion packet may be routed to confinement cell 230b, a third-in-time ion packet may be routed to confinement cell 230c, and a fourth-in-time ion packet may be routed to confinement cell 230d. The timing and sequence of filling and emptying the confinement cells is discussed below in greater detail in connection with
Routing of an ion packet to the destination confinement cell may be effected by the application of suitable DC potentials to electrodes within distribution region 220 to produce DC fields in the longitudinal and transverse dimensions that urge the ions toward the confinement cell. In a particular implementation, DC potentials may be applied to electrodes of distribution section 220 to establish a longitudinal potential well that confines ions to the front portion 240a or rear portion 240b of distribution section 220. A transverse DC field may be generated to cause the ions to travel in the transverse direction leading toward the selected confinement cell. As will be discussed in further detail below in connection with
Each ion packet is confined in the corresponding confinement cell for a confinement period of adequate duration to reduce the ions' kinetic energies to values that permit acquisition of a mass spectrum at high resolution and mass accuracy. As set forth in the background section, the amount of ion cooling will be a function of the product of confinement cell pressure and confinement period. In exemplary implementations, ion interface device is operated to provide a product of confinement cell pressure and confinement period of at least 1 ms·mTorr, and more preferably in the range of 2-5 ms·mTorr. For a typical confinement cell pressure of about 1.5 mTorr, the foregoing values translate to a confinement period of at least approximately 650 μs, and more preferably in the range of about 1300-3300 μs. After an ion packet has been confined in the confinement cell for the prescribed confinement period, the ion packet is released through the confinement cell outlet to TOF mass analyzer 115. Release of an ion packet from the confinement cell may be performed by applying or changing DC potentials on electrodes associated with the confinement cell. As depicted in
While ion interface 105 is described and depicted as having four confinement cells, other implementations may utilize a lesser or greater number of confinement cells. In particular, the maximum confinement period of an ion packet in the ion interface device can be extended by increasing the number of confinement cells.
Following the emptying and refilling of confinement cell 230a, each of the other confinement cells is emptied and refilled in the sequence described above. This sequence is repeated until the analytical scanning of the ion trap is terminated (or until another specified termination point has been reached), and all ion packets have been mass analyzed in TOF mass analyzer 115.
It will be recognized that each transfer of ion packets within ion interface is not instantaneous, but instead will require a finite time to complete. However, the applicant has found (via detailed computer modeling of ion motion during transfer operations), that the aggregate transfer time is significantly shorter than the confinement period required for adequate energetic cooling, and will typically comprise about ten percent of the total residence time within interface device 105.
Another set of rod electrodes 610, oriented transversely to the major longitudinal axis of ion interface device 105, is positioned within distribution section 220. Each electrode 610 receives an RF potential of a phase opposite to the adjacent and opposing electrodes to establish the confining RF field. Certain rod electrodes 615a,b,c,d (which also receive RF potentials) are segmented to allow different DC potentials to be applied to discrete segments of each rod, such that a DC potential gradient may be created along the transverse axis defined by the dimension of elongation of the rod electrodes. The transverse DC potential gradient is controlled (by adjustment of the potentials applied to the segments) to cause an ion packet to travel in the direction of the destination confinement cell; for example, DC potentials may be applied to segments of rod electrodes 615a and 615b to produce a DC gradient that directs ions toward confinement cell 230c or 230d. Of course, the segments may all be maintained at the same DC potential if no transverse DC field is to be established; for example, in the case where an ion packet is to be directed to one of confinement cells 230a or 230b, the segments of rod electrodes 615a and 615b may be maintained at the same DC potential such that ions passing through the region defined by these rods are not transversely deflected toward confinement cell 230c or 230d.
Those skilled in the art will recognize that the transverse DC potential gradients may be controllably established using techniques other than segmentation of the rod electrodes. For example, the rod electrodes may be surface coated with a resistive material, with different DC potentials applied to opposite ends of the rod electrodes, as described in U.S. Pat. No. 5,847,386 to Thomson et al. (the disclosure of which is hereby incorporated by reference). Alternatively, as also described in the aforementioned Thomson et al. patent DC potentials may be applied to auxiliary electrodes positioned around or between the rod electrodes. In another alternative, a helical conductive path may be disposed on the surface of the rod electrodes, with different DC potentials applied to the ends of the helical path, as described in U.S. Pat. No. 7,067,802 to Kovtoun, which is also incorporated by reference.
Ions travel from distribution section 220 to the destination confinement cell through an intermediate chamber in which are disposed rod electrodes 625, which are grouped into multipole structures having central axes extending between an outlet of distribution section 220 and a corresponding confinement cell. RF potentials may be applied to rod electrodes 625 in an alternating phase pattern, such that each multipole acts as an RF ion guide and radially confine the movement as ions as they travel therethrough.
Electrostatic lenses 630, 635 and 640, which may take the form of plate lenses, are located at (respectively) the outlet apertures of distribution section 220 and the inlet and outlet apertures of confinement cells 230a,b,c,d. Suitable DC voltages may be applied to the electrostatic lenses (from the not-depicted DC source) to selectively block or permit the movement of ion packets out of distribution section 220 and into the destination confinement cell, to axially confine ions within a confinement cell, and to eject ions from the confinement cell to the mass analyzer.
Each confinement cell is provided with a set of rod electrodes 650. Ions may be axially confined within the confinement cell by applying appropriate DC potentials to the corresponding lenses located at the inlet and outlet of the confinement cell. Following completion of the prescribed confinement period, the ion packet is ejected from its confinement cell by adjusting the DC potentials applied to outlet lens 640 and/or to rod electrodes 650.
Gas is controllably supplied to the interior of ion interface device 105 from a not-depicted external source through conduit 660. The gas, which will typically comprise an inert gas such as argon, removes kinetic energy from the incoming ions via collisions and induces (if desired) collisionally activated dissociation. Ion interface device 105 is located in one or more vacuum chambers that are evacuated by means of a suitable pump. The distribution outlet apertures (at which lenses 630 are located) and confinement cell inlet and outlet apertures (at which lenses 635 and 640 are respectively located) may be conductance limiting to allow the confinement cells to be maintained at a reduced pressure relative to the transport/collision and distribution sections. In an illustrative implementation, transport/collision section 210 and distribution section 220 are maintained at a pressure of about 13 mTorr, the intermediate section (interposed between distribution section 220 and the confinement cells) is maintained at a pressure of about 6 mTorr, and confinement cells 230a,b,c,d are maintained at a pressure of about 1.5 mTorr.
It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.
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