Patent Application
20060192112
A mass spectrometry system is an analytical device that determines the molecular weight of chemical compounds by separating molecular ions according to their mass-to-charge ratio (m/z). Ions are generated by inducing either a loss or gain of charge and are then detected. Mass spectrometry systems generally comprise an ionization source for producing ions (i.e. electrospray ionization (EI), atmospheric photoionization (APPI), atmospheric chemical ionization (APCI), chemical ionization (CI), fast atom bombardment, matrix assisted laser desorption ionization (MALDI) etc.), a mass filter or analyzer (i.e. quadrupole, magnetic sector, time-of-flight, ion trap etc.) for separating and analyzing ions, and an ion detector such as an electron multiplier or scintillation counter for detecting and characterizing ions.
The first mass analyzers introduced in the early 1900's used magnetic fields for separating ions according to their mass-to-charge ratio. Just as ionization sources have evolved so have the mass analyzers to meet the demands of various chemical molecules. One type of mass analyzer is the ion trap. Ion trap mass analyzers operate by using two or more RF ring electrodes to trap ions of a particular mass-to-charge ratio. The ion trap mass analyzer was developed around the same time as the quadrupole mass analyzer and the physics behind both of these analyzers are very similar. These mass analyzers are relatively inexpensive, provide good accuracy and resolution, and may be used in tandem for improved separations. Typical mass range and resolution for ion trap mass analyzers are (Range m/z 2000; Resolution 1500). Other advantages of ion traps include small size, simple design, low cost, and ease of use for positive and negative ions. Ion trap mass analyzers have, therefore, become quite popular. However, ion traps suffer from a few particular problems. For instance, the limited range of current commercial versions as well as low energy collisions and ion fragmentation problems.
In order to address these problems MS/MS, 2-dimensional (2D) and 3-dimensional (3D) analytical techniques and methods of fragmentation have been developed. Commonly in 3D ion trap mass spectrometry the fragmentation is achieved by setting the main RF voltage to relatively high values to increase the depth of pseudo-potential trapping wells and also by applying a supplemental field on resonance with the fundamental frequency of the ion motion. The value of the RF amplitude that is used for the fragmentation can be expressed in terms of a dimensionless parameter q. Typically, q can range from 0 to 0.908 and from various derived equations the lowest stable mass within the ion trap can be determined. The lowest stable mass within the RF field is called the fragmentation cut-off limit. All the fragment ions with masses below the fragmentation cut-off limit are unstable within the RF ion trap and are impossible to analyze. Fragmentation cut-off has been an ongoing problem for ion traps and has limited the overall potential effectiveness and flexibility of ion traps.
It, therefore, would be desirable to alleviate this problem by substantially reducing the fragmentation cut-off for ion trap systems. In addition, it would be desirable to expand the range and types of molecules that may be analyzed using ion traps. For instance, it would be desirable to decrease fragmentation cut-off so low molecular weight fragmentation information can be used and developed for sequencing and characterizing various small molecules and peptides. In addition, it would be desirable to be able to isolate, trap and scan molecules of various sizes without having to move them between mass analyzers and/or collision cells. These and other problems presented have been obviated by the present invention.
The present invention relates to an apparatus and method for providing reduced ion fragmentation problems in a mass spectrometry system. The mass spectrometry system of the present invention comprises an ionization source, a mass analyzer/filter and an ion detector. The mass analyzer of the present invention comprises an ion trap having a first electrode, a second electrode adjacent to the first electrode, a third electrode interposed between the first electrode and the second electrode, a first RF source in electrical connection with the first electrode, and second electrode and a second RF source in electrical connection with the second electrode.
The invention also provides an ion trap. The ion trap of the present invention comprises an ion trap having a first electrode, a second electrode adjacent to the first electrode, a third electrode interposed between the first electrode and the second electrode, a first RF source in electrical connection with the first electrode and second electrode, and a second RF source in electrical connection with the second electrode.
The method of the present invention comprises ionizing a sample, applying a first RF field from a first RF source, or applying a second RF field from a second RF source to trap ions in an ion trap; applying a second RF field from a second RF source to fragment the ions in the ion trap; and applying and scanning a first RF field from a first RF source to eject the fragmented ions out of the ion trap for detection.
The invention is described in detail below with reference to the following figures:
Before describing the invention in detail, it must be noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “an electrode” includes more than one “electrode”. Reference to a “ring electrode” includes more than one “ring electrode”. In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below.
The term “adjacent” means, near, next to or adjoining. Something adjacent may also be in contact with another component, surround the other component, be spaced from the other component or contain a portion of the other component. For instance, an electrode that is adjacent to a ring electrode may be spaced next to the ring electrode, may contact the ring electrode, may surround or be surrounded by the ring electrode, may contain the ring electrode or be contained by the ring electrode, may adjoin the ring electrode or may be near the ring electrode.
The term “2-dimensional (2-D) ion trap” refers to a trap in which ions are focused in space in two dimensions along a defined line. For instance, one type of 2-D ion trap is a linear trap. The definition should be interpreted broadly to include any devices in the art where ions are defined in space in a similar manner.
The term “3-dimensional (3-D) ion trap” refers to an ion trapping device that produces a trapping field that is in three dimensional space. In other words, ions are trapped to a point in space. The definition should be interpreted broadly to include any devices known or used in the art where ions can be trapped at a point in space.
The term “electrode” refers to any electrode, electrode device, or device used to create an electric field that may be used for collecting or trapping ions. The term may be interpreted broadly, however, to also include any device, or apparatus that may comprise an electrode or ring electrode. Electrodes may also comprise endcaps or other similar type devices known and used in the art in 2-D and 3-D ion traps
The term “group of electrodes” refers to two or more electrodes.
The term “detector” refers to any device, apparatus, machine, component, or system that can detect an ion. Detectors may or may not include hardware and software. In a mass spectrometer the common detector includes and/or is coupled to a mass analyzer.
The term “electrode” refers to a particular type of electrode that may be employed with the present invention.
The term “ion source” or “source” refers to any source that produces analyte ions.
The term “section” refers to one or more electrodes that may comprise a defined portion of a mass analyzer. Sections may typically comprise two or more electrodes that form a structure that may be used to create electric or magnetic fields that can be employed to manipulate or move ions in a defined direction.
The term “rod” refers to any number of solid structures that may be electrically conductive and may be used to create an electric or magnetic field for manipulating ions.
The invention is described with reference to the figures. The figures are not to scale, and in particular, certain dimensions may be exaggerated for clarity of presentation.
The ion source 3 may be located in a number of positions or locations. In addition, a variety of ion sources may be used with the present invention. For instance, electrospray ionization (EI), chemical ionization (CI), atmospheric pressure photon ionization (APPI), atmospheric pressure chemical ionization (APCI), matrix assisted laser desorption ionization (MALDI), atmospheric pressure matrix assisted laser desorption ionization (AP-MALDI) etc. or other ion sources well known in the art may be used with the invention. In particular, any source that may produce ions may be employed with the present invention. These sources may be known in the art or may be developed.
The mass analyzer 5 may comprise any number of devices known in the art for trapping ions. For instance, the mass analyzer may comprise an ion trap, a 2-D or 3-D ion trap, an ion trap or similar device in MS/MS mode or combinations of these devices capable of trapping ions.
The detector 7 is generally positioned downstream from the ion source 3 and the mass analyzer 5. The detector may comprise any number of detectors known in the art. For instance, the detector 7 may comprise any device capable of generating an output signal indicative of the analyte being studied. Detectors may include and not be limited to devices that generate secondary electrons which are amplified or which induce a current generated by a moving charge. Some of these types of detectors include the electron multiplier and the scintillation counter.
The third electrode 4 is in electrical connection with a first RF voltage source 14. The first electrode 9 and the second electrode 10 are in electrical connection with a second RF voltage source 16. An optional auxiliary waveform generator 13 may also be in electrical connection with the electrodes 9 and 10 and the second RF voltage source 16. Also, a collisional gas e.g. helium or a similar gas may be introduced into the ion trap cavity 12. Various collisonal gases or gas mixtures known in the art may be employed with the present invention. In addition, various auxiliary waveform generators may also be employed with the present invention.
It is worth noting that
In the case of a quadrupole, the equations for the fragmentation cut-offs for each of the trapping fields can be written:
Where vo and Vo are the frequency and the amplitude of the main quadrupole field; va and Va are the amplitude and the frequency of the additional second quadrupole field. The equations for the pseudo-potential well in these fields are given by:
Let us assume that the fragmentation pseudo-potential depth Da, which is defined by the second field is the same as the pseudo-potential depth D0 within the original main field to provide the same level of trapping during the fragmentation with respect to the precursor ions, i.e., D0=Da. In this case, the amplitude and the frequency of the second quadrupole field should be increased proportionally with respect to the amplitude and the frequency of the main field. Substituting the upper stability values for the q-parameters as q0=0.98; qa=0.908 in the equations above; and assuming that Va>V0, it can be determined that ma<m0, i.e., the fragmentation cut-off for the additional quadrupole field is inversely proportional to the frequency of the second field and smaller than the original fragmentation cut-off. For example, if Va=3V0, the estimated fragmentation cut-off is Mn=M0/3, which is three times lower than the original fragmentation cut-off. The present invention and techniques have application to a variety of ion traps. The second figure shows a basic idea of an ion trap of the present invention. More complex series, combinations or applications are also possible. For instance, various MS/MS, ion trap combinations, 2D and 3D ion traps may also be employed. A further extensive description of these devices is provided below.
Referring now to
It should also be note that sections 46 and 41 may comprise electrodes in the form of end-caps. Theoretically, in this type of arrangement the sections 42, 43, 44, 45 would be in the form of one or more ring electrodes. However, in this embodiment, two different trapping field RF generators would be employed. For instance, the sections 41, 42, and 43 would be connected to the first trapping RF generator 47, while sections 44, 45, 46 would be connected to the second RF generator 48. During the operation of the present invention ions would be trapped by the different fields during fragmentation and scanning/trapping operations. It is also recognized that trapping fields in many cases would be quadrupolar 2D or 3D geometry. However, this is not a requirement of the invention and other designs may be possible.
Thus, ions can be manipulated within the ion trap for storage fragmentation and scanning between sections 42 and 45 as described later. For example, ions can be transferred from one section to another using various DC voltages on the various sections 41, 42, 43, 44, 45, and 46. The appropriate DC voltages to achieve such ion transformations are shown in
Panel 53 shows the distribution of the DC potential along the central axis 50 of the 2D ion trap at the time of ion transfer from the section 45 back to the section 42, which corresponds to the new minimum of the potential. According to the present invention ions are first introduced and accumulated within the section 42. The RF generator 47 provides a trapping field within section 42. After the initial accumulation step, ions of interest can be isolated according to their mass to charge ratio by applying selection waveform by the auxiliary RF generator 47a. Then ions are transferred to the region of higher frequency field provided by the second RF voltage source 48 within section 45. Then the resonance fragmentation waveform can be applied by the auxiliary first RF voltage source 48a. After that, fragmented and remaining precursor ions can be transferred back to the section 42 and scanned out through the gap 49a within the rod 49. This is accomplished, for example, by the mass instability scanning technique. This technique is well known in the art. It is recognized that ions can also be accumulated, isolated and then fragmented within the section 45 and then transferred to the scanning section 42 for detection. Alternatively, ions can be pulsed out axially to a different tandem mass analyzer, such as a time-of-flight mass analyzer.
Other modifications of the method are possible depending upon the particular required task. For instance, one of the rods of section 45 can also have an opening for ions to exit. This can be structured similar to the rod 49 as shown in section 42. In this case, the ion detection, isolation and fragmentation can be performed in either of sections 42 or 45 depending on the mass-to-charge ratios of the analyzed ions and frequencies of the trapping fields within the sections 42 and 45. Also, the section 45 can have a distorted geometry or design (different from the pure quadrupole design). For instance, a octapole field can be created using various rod sections or designs to increase ion fragmentation efficiency with lower ion loss. Since different field regions are generally required for such fragmentations and detections, the amount of octapole field component within the fragmentation-trapping field can be optimized specifically for ion fragmentation.
In addition, it is also possible to use additional sections for designing different RF trapping fields. It is also recognized that less than six sections can also be used. For instance, dual trapping fields for a 2D ion trap. However, the homogeneity of the fields can be affected with this type of design. It is also possible that the frequencies of the trapping fields can also be designed to synchronize or be in a multiple format. The present invention not only provides improvement over fragmentation limitations, but also increases the fragmentation energy levels. Higher fragmentation energy provides additional structural information regarding analytes as well as increases the range of analytes that may be analyzed. For instance, it opens up the possibility of fragmenting more stable ions that could not be previously fragmented.
Having described the apparatus of the invention, a description of the method of operation is now in order.
In another embodiment, the Second RF voltage source 16 can maintain a field that is substantially steady at level 38 (not shown in
It is to be understood that while the invention has been described in conjunction with the specific embodiments thereof, that the foregoing description as well as the examples that follow are intended to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.
All patents, patent applications, and publications infra and supra mentioned herein are hereby incorporated by reference in their entireties.
