US RE38861 E1
A method and apparatus to direct ions away from their otherwise intended or parallel course. Deflectors are used to establish electric fields in regions through which ions are to pass. With such electric fields, ions may be deflected to a desired trajectory. According to the present invention, a multideflector, in the form of a series of bipolar plates spaced evenly across the ion beam path, is used as an ion deflector.
1. An improved time of flight mass spectrometer comprising:
a deflector for deflecting an ion from an ion path consisting of more than two plates arranged across said ion path in such a way that, during a given passage through said deflector, said ion must pass between two and only two adjacent plates; and
a detector for detecting said ion;
wherein each of said plates is energized to a potential.
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10. A multideflector for analyzing ions in a time of flight mass spectrometer comprising:
an ion source;
an ion detector;
a flight tube for transporting ions formed within said ion source; and
a gate disposed along said flight tube;
wherein said ion source produces ions capable of travel along said flight tube, and wherein said detector detects the presence of said ions; and
wherein said gate is formed by a series of metal plates arranged across said flight tube in such a way that, during a given passage through said multideflector, said ions must pass between two and only two adjacent plates, said plates being aligned to deflect substantially all ions away from the direction of ion propagation along said flight tube.
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19. A mass selector for use in a time of flight instrument comprising:
a flight tube;
a gate; and
an ion source;
wherein said ion source produces ions that travel through said flight tube, and wherein said gate impedes the travel of said ions by deflecting said ions into at least two directions.
20. A mass selector according to
21. A mass selector according to
22. A mass selector according to
23. An improved time of flight mass spectrometer comprising:
a multideflector for deflecting ions from an ion path consisting of more than two bipolar deflection plates each comprising a pair of metal plates separated from one another by an insulator, said bipolar deflection plates being arranged across said ion path in such a way that, during a given passage through said multideflector, each of said ions must pass between two and only two adjacent bipolar deflection plates; and
a detector for detecting said ions;
wherein each of said metal plates is energized to a potential and the potentials of the metal plates of each pair have opposite polarities.
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This invention relates generally to ion beam handling and more particularly to a deflector for use in time-of-flight mass spectrometry.
This invention relates in general to ion beam handling in mass spectrometers and more particularly to ion deflection in time-of-flight mass spectrometers (TOFMS). The apparatus and method of mass analysis described herein is an enhancement of the techniques that are referred to in the literature relating to mass spectrometry.
The analysis of ions by mass spectrometers is important, as mass spectrometers are instruments that are used to determine the chemical structures of molecules. In these instruments, molecules become positively or negatively charged in an ionization source and the masses of the resultant ions are determined in vacuum by a mass analyzer that measures their mass/charge (m/z) ratio. Mass analyzers come in a variety of types, including magnetic field (B), combined (double-focusing) electrical (E) and magnetic field (B), quadrupole (Q), ion cyclotron resonance (ICR), quadrupole ion storage trap, and time-of-flight (TOP) mass analyzers, which are of particular importance with respect to the invention disclosed herein. Each mass spectrometric method has a unique set of attributes. Thus, TOFMS is one mass spectrometric method that arose out of the evolution of the larger field of mass spectrometry.
The analysis of ions by TOFMS is, as the name suggests, based on the measurement of the flight times of ions from an initial position to a final position. Ions which have the same initial kinetic energy but different masses will separate when allowed to drift through a field free region.
Ions are conventionally extracted from an ion source in small packets. The ions acquire different velocities according to the mass-to-charge ratio of the ions. Lighter ions will arrive at a detector prior to high mass ions. Determining the time-of-flight of the ions across a propagation path permits the determination of the masses of different ions. The propagation path may be circular or helical, as in cyclotron resonance spectrometry, but typically linear propagation paths are used for TOFMS applications.
TOFMS is used to form a mass spectrum for ions contained in a sample of interest Conventionally, the sample is divided into packets of ions that are launched along the propagation path using a pulse-and-wait approach. In releasing packets, one concern is that the lighter and faster ions of a trailing packet will pass the heavier and slower ions of a preceding packet. Using the traditional pulse-and-wait approach, the release of an ion packet as timed to ensure that the ions of a preceding packet reach the detector before any overlap can occur. Thus, the periods between packets is relatively long. If ions are being generated continuously, only a small percentage of the ions undergo detection. A significant amount of sample material is thereby wasted. The loss in efficiency and sensitivity can be reduced by storing ions that are generated between the launching of individual packets, but the storage approach carries some disadvantages.
Resolution is an important consideration in the design and operation of a mass spectrometer for ion analysis. The traditional pulse-and-wait approach in releasing packets of ions enables resolution of ions of different masses by separating the ions into discernible groups. However, other factors are also involved in determining the resolution of a mass spectrometry system. “Space resolution” is the ability of the system to resolve ions of different masses despite an initial spatial position distribution within an ion source from which the packets are extracted. Differences in starting position will affect the time required for traversing a propagation path. “Energy resolution” is the ability of the system to resolve ions of different mass despite an initial velocity distribution. Different starting velocities will affect the time required for traversing the propagation path.
In addition, two or more mass analyzers may be combined in a single instrument to form a tandem mass spectrometer (MS/MS, MS/MS/MS, etc.). The most common MS/MS instruments are four sector instruments (EBEB or BEEB), triple quadrupoles (QQQ), and hybrid instruments (EBQQ or BEQQ). The mass/charge ratio measured for a molecular ion is used to determine the molecular weight of a compound. In addition, molecular ions may dissociate at specific chemical bonds to form fragment ions. Mass/charge ratios of these fragment ions are used to elucidate the chemical structure of the molecule. Tandem mass spectrometers have a particular advantage for structural analysis in that the first mass analyzer (MS1) can be used to measure and select molecular ion from a mixture of molecules, while the second mass analyzer (MS2) can be used to record the structural fragments. In tandem instruments, a means is provided to induce fragmentation in the region between the two mass analyzers. The most common method employs a collision chamber filled with an inert gas, and is known as collision induced dissociation CID. Such collisions can be carried out at high (5-10 keV) or low (10-100 eV) kinetic energies, or may involve specific chemical (ion-molecule) reactions. Fragmentation may also be induced using laser beams (photodissociation), electron beams (electron induced dissociation), or through collisions with surfaces (surface induced dissociation). It is possible to perform such an analysis using a variety of types of mass analyzers including TOF mass analysis.
In a TOFMS instrument, molecular and fragment ions formed in the source are accelerated to a kinetic energy:
Generally, TOF mass spectrometers have limited mass resolution. This arises because there may be uncertainties in the time that the ions were formed (time distribution), in their location in the accelerating field at the time they were formed (spatial distribution), and in their initial kinetic energy distributions prior to acceleration (energy distribution).
The first commercially successful TOFMS was based on an instrument described by Wiley and McLaren in 1955 (Wiley, W. C.; McLaren, I. H., Rev. Sci. Instrumen. 26 1150 (1955)). That instrument utilized electron impact (EI) ionization (which is limited to volatile samples) and a method for spatial and energy focusing known as time-lag focusing. In brief, molecules are first ionized by a pulsed (1-5 microsecond) electron beam. Spatial focusing was accomplished using multiple-stage acceleration of the ions. In the first stage, a low voltage (−150 V) drawout pulse is applied to the source region that compensates for ions formed at different locations, while the second (and other) stages complete the acceleration of the ions to their final kinetic energy (−3 keV ). A short time-delay (1-7 microseconds) between the ionization and draw out pulses compensates for different initial kinetic energies of the ions, and is designed to improve mass resolution. Because this method required a very fast (40 ns) rise time pulse in the source region, it was convenient to place the ion source at ground potential, while the drift region floats at −3 kV. The instrument was commercialized by Bendix Corporation as the model NA-2, and later by CVC Products (Rochester, N.Y.) as the model CVC-2000 mass spectrometer. The instrument has a practical mass range of 400 daltons and a mass resolution of 1/300 and is still commercially available.
There have been a number of variations on this instrument Muga (TOFTEC, Gainsville) has described a velocity compaction technique for improving the mass resolution (Muga velocity compaction). Chatfield et al. (Chatfield FT-TOF) described a method for frequency modulation of gates placed at either end of the flight tube, and Fourier transformation to the time domain to obtain mass spectra. This method was designed to improve the duty cycle.
Cotter et al. (VanBreeman, R. B.: Snow, M.: Cotter, R. J., Int. J. Mass Spectrom. Ion Phys. 49 (1983) 35.; Tabet, J. C.; Cotter, R. J., Anal. Chem. 56 (1984) 1662; Olthoff, J. K.; Lys. I.: Demirev, P.: Cotter, R. J.,Anal. Instrumen. 16 (1987) 93, modified a CVC 2000 time-of-flight mass spectrometer for infrared laser desorption of involatile biomolecules, using a Tachisto (Needham, Mass.) model 215G pulsed carbon dioxide laser. This group also constructed a pulsed liquid secondary time-of-flight mass spectrometer (liquid SIMS-TOF) utilizing a pulsed (1-5 microsecond) beam of 5 keV cesium ions, a liquid sample matrix, a symmetric push/pull arrangement for pulsed ion extraction (Olthoff, J. K.; Cotter, R. J., Anal. Chem. 59 (1987) 999-1002.; Olthoff, J. K.; Cotter, R. J., Nucl. Instrum. Meth. Phys. Res. B-26 (1987) 566-570. In both of these instruments, the time delay range between ion formation and extraction was extended to 5-50 microseconds, and was used to permit metastable fragmentation of large molecules prior to extraction from the source. This in turn reveals more structural information in the mass spectra.
The plasma desorption technique introduced by Macfarlane and Torgerson in 1974 (Macfarlane, R. D.; Skowronski, R. P.; Torgerson, D. F., Biochem. Biophys. Res Commoun. 60 (1974) 616.) formed ions on a planar surface placed at a voltage of 20 kV. Since there are no spatial uncertainties, ions are accelerated promptly to their final kinetic energies toward a parallel, grounded extraction grid, and then travel through a grounded drift region. High voltages are used, since mass resolution is proportional to U o/;eV, where the initial kinetic energy, U O/is of the order of a few electron volts. Plasma desorption mass spectrometers have been constructed at Rockefeller (Chait, B. T.; Field, F. H., J. Amer. Chem. Soc. 106 (1984) 193), Orsay LeBeyec, Y.; Della Negra, S.; Deprun, C.; Vigny, P.; Giont, Y. M., Rev. Phys. Appl. 15 (1980) 1631), Paris (Viari, A.; Ballini, J. P.; Vigny, P.; Shire, D.; Dousset, P., Biomed. Environ. Mass Spectrom, 14 (1987) 83), Upsalla (Hakansson, P.; Sundqvist B., Radial Eff. 61 (1982) 179) and Darmstadt (Becker, O.; Furstenau, N.; Krueger, F. R.; Weiss, G.; Wein, K., Nucl. Instrum. Methods 139 (1976) 195). A plasma desorption time-of-flight mass spectrometer has bee commercialized by BIO-ION Nordic (Upsalla, Sweden). Plasma desorption utilizes primary ion particles with kinetic energies in the MeV range to induce desorption/ionization. A similar instrument was constructed at Manitobe (Chain, B. T.; Standing, K. G., Int. J. Mass Spectrum. Ion Phys. 40 (1981) 185) using primary ions in the keV range, but has not been commercialized.
Matrix-assited laser desorption, introduced by Tanaka et al. (Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshica, T., Rapid Commun. Mass Spectrom. 2 (1988) 151) and by Karas and Hillenkarnp (Karas, M.; Hillenkarnp, F., Anal. Chem. 60 (1988) 2299) utilizes TOFMS to measure the molecular weights of proteins in excess of 100,000 daltons. An instrument constructed at Rockefeller (Beavis, R. C.; Chait, B. T., Rapid Commun. Mass Spectrom. 3 (1989) 233) has been commercialized by VESTEC (Houston, Tex.), and employs prompt two-stage extraction of ions to an energy of 30 keV.
Time-of-flight instruments with a constant extraction field have also been utilized with multi-photon ionization, using short pulse lasers.
The instruments described thus far are linear time-of-flights, that is: there is no additional focusing after the ions are accelerated and allowed to enter the drift region. Two approaches to additional energy focusing have been utilized: those which pass the ion beam through an electrostatic energy filter.
The reflectron (or ion mirror) was first described by Mamyrin (Mamyrin, B. A.; Karatajev. V. J.; Shmikk, D. V.; Zagulin, V. A, Sov. Phys., JETP 37 (1973) 45). At the end of the drift region, ions enter a retarding field from which they are reflected back through the drift region at a slight angle. Improved mass resolution results from the fact that ions with larger kinetic energies must penetrate the reflecting field more deeply before being turned around. These faster ions than catch up with the slower ions at the detector and are focused. Reflectrons were used on the laser microprobe instrument introduced by Hillenkamp et al. (Hillenkamp, F.; Kaufmann, R.; Nitsche, R.; Unsold, E., Appl. Phys. 8 (1975) 341) and commercialized by Leybold Hereaus as the LAMMA (LAser Microprobe Mass Analyzer). A similar instrument was also commercialized by Cambridge Instruments as the IA ( Laser Ionization Mass Analyzer). Benninghoven (Benninghoven reflectron) has described a SIMS (secondary ion mass spectrometer) instrument that also utilizes a reflectron, and is currently being commercialized by Leybold Hereaus. A reflecting SIMS instrument has also been constructed by Standing (Standing, K. G.; Beavis, R.; Bollbach, G.; Ens, W.; Lafortune, F.; Main, D.; Schueler, B.; Tang, X.; Westmore, J. B., Anal. Instrumen. 16 (1987) 173).
Lebeyec (Della-Negra, S.; Lebeyec, Y., in Ion Formation from Organic Solids IFOS III, ed. by A Benninghoven, pp 42-45, Springer-Verlag, Berlin (1986)) described a coaxial reflectron time-of-flight that reflects ions along the same path in the drift tube as the incoming ions, and records their arrival times on a channelplate detector with a centered hole that allows passage of the initial (unreflected) beam. This geometry was also utilized by Tanaka et al. (Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, T., Rapid Comun. Mass Spectrom. 2 (1988) 151) for matrix assisted laser desorption. Schlag et al. (Grotemeyer, J.; Schlag, E. W., Org. Mass Spectrom. 22 (1987) 758) have used a reflectron on a two-laser instrument. The first laser is used to ablate solid samples, while the second laser forms ions by multiphoton ionization. This instrument is currently available from Broker. Wollnik et al. (Grix., R.; Kutscher, R.; Li, G.; Gruner, U.; Wollnik, H., Rapid Commun. Mass Spectrom. 2 (1988) 83) have described the use of reflectrons incombination with pulsed ion extraction, and achieved mass resolutions as high as 20,000 for small ions produced by electron impact ionization.
An alternative to reflectrons is the passage of ions through an electrostatic energy filter, similar to that used in double-focusing sector instruments. This approach was first described by Poschenroeder (Poschenroeder, W., Int. J. Mass Spectrom. Ion Phys. 6 (1971) 413). Sakurai et al. (Sakuri, T.; Fujita, Y; Matsuo, T.; Matsuda, H; Katakuse, I., Int. J. Mass Spectrom. Ion Processes 66 (1985) 283) have developed a time-of-flight instrument employing four electrostatic energy analyzers (ESA) in the time-of-flight path. At Michigan State, an instrument known as the ETOF was described that utilizes a standard ESA in the TOF analyzer (Michigan ETOF).
Lebeyec et al. (Della-Negra, S.; Lebeyec, Y., in Ion Formation from Organic Solids IFOS III, ed. by A. Benninghoven, pp 42-45, Springer-Verlag, Berlin (1986)) have described a technique known as correlated reflex spectra, which can provide information on the fragment ion arising from a selected molecular ion. In this technique, the neutral species arising from fragmentation in the flight tube are recorded by a detector behind the reflectron at the same flight time as their parent masses. Reflected ions are registered only when a neutral species is recorded within a preselected time window. Thus, the resultant spectra provide fragment ion (structural) information for a particular molecular ion. This technique has also been utilized by Standing (Standing, K. G.; Beavis, R.; Bollbach, G.; Ens, W.; Lafortune, F.; Main, D.; Schueler, B.; Tang, X.; Westmore, J. B., Anal. Instrumen. 16 (1987) 173).
Although TOF mass spectrometers do not scan the mass range, but record ions of all masses following each ionization event, this mode of operation has some analogy with the linked scans obtained on double-focusing sector instruments. In both instruments, MS/MS information is obtained at the expense of high resolution. In addition correlated reflex spectra can be obtained only on instruments which record single ions on each TOF cycle, and are therefore not compatible with methods (such as laser desorption) which produce high ion currents following each laser pulse.
New ionization techniques, such as plasma desorption (Macfarlane, R. D.; Skowronski, R. P.; Torgerson, D. F.; Biochern. Bios. Res. Commun. 60 (1974) 616), laser desorption (VanBreemen, R. B.; Snow, M.; Cotter, R. J., Int. J. Mass Spectrom. Ion Phys. 49 (1983) 35; Van der Peyl, G. J. Q.; Isa, K.; Haverkamp, J.; Kistemaker, P. G., Org. Mass Spectrom. 16 (1981) 416), fast atom bombardment (Barber, M.; Bordoli, R. S.; Sedwick, R. D.; Tyler, A. N., J. Chem. Soc., Chem. Commun. (1981) 325-326) and electrospray (Meng, C. K.; Mann, M.; Fenn, J. B., Z. Phys. D10 (1988) 361), have made it possible to examine the chemical structures of proteins and peptides, glycopeptides, glycolipids and other biological compounds without chemical derivatization. The molecular weights of intact proteins can be determined using matrix assisted laser desorption ionization (MALDI) on a TOF mass spectrometer or electrospray ionization. For more detailed structural analysis, proteins are generally cleaved chemically using CNBr or enzymatically using trypsinor other proteases. The resultant fragments, depending upon size, can be mapped using MALDI, plasma desorption or fast atom bombardment. In this case, the mixture of peptide fragments (digest) is examined directly resulting in a mass spectrum with a collection of molecular ion corresponding to the masses of each of the peptides. Finally, the amino acid sequences of the individual peptides which make up the whole protein can be determined by fractionation of the digest, followed by mass spectral analysis of each peptide to observe fragment ions that correspond to its sequence.
It is the sequencing of peptides for which tandem mass spectrometry has its major advantages. Generally, most of the new ionization techniques are successful in producing intact molecular ions, but not in producing fragmentation. In the tandem instrument the first mass analyzer passes molecular ions corresponding to the peptide of interest. These ions are fragmented in a collision chamber, and their products extracted and focused into the second mass analyzer which records a fragment ion (or sequence) spectrum.
A tandem TOFMS consists of two TOF analysis regions with an ion gate between the two regions. As in conventional TOFMS, ions of increasing mass have decreasing velocities and increasing flight times. Thus, the arrival time of ions at the ion gate at the end of the first TOF analysis region is dependent on the mass-to-charge ratio of the ions. If one opens the ion gate only at the arrival time of the ion mass of interest, then only ions of that mass-to-charge will be passed into the second TOF analysis region.
However, it should be noted that the products of an ion dissociation that occurs after the acceleration of the ion to its final potential will have the same velocity as the original ion. The product ions will therefore arrive at the ion gate at the same time as the original ion and will be passed by the gate (or not) just as the original ion would have been.
The arrival times of product ions at the end of the second TOF analysis region is dependent on the product ion mass because a reflectron is used. As stated above, product ions have the same velocity as the reactant ions from which they originate. As a result, the kinetic energy of a product ion is directly proportional to the product ion mass. Because the flight time of an ion through a reflectron is dependent on the kinetic energy of the ion, and the kinetic energy of the product ions are dependent on their masses, the flight time of the product ions through the reflectron is dependent on their masses.
As TOFMS is a pulsed technique, one of the difficulties in its use is in interfacing it with continuous ion sources such as electrospray ionization. One common method for interfacing such a source with TOFMS is referred to as orthogonal acceleration. In this method, the TOF analysis is performed in a direction which is roughly orthogonal to the direction of motion of the ion beam produced by the source. The beam from the source passes into and through an interface region at the beginning of the TOF mass spectrometer. In the interface region, the ion beam passes between accelerating electrodes. By energizing the accelerating electrodes, the portion of the ion beam which is between the accelerating electrodes is accelerated such that a TOF mass analysis can be performed on these ions. Ideally, the accelerating electrodes are energized at regular intervals such that all the ions from the source are accelerated and analyzed.
The difficulty with the orthogonal acceleration method is that if the TOF direction is to be truly orthogonal to the direction of motion of the ion beam, the ions must be deflected using a deflector or similar device. This causes a distortion in the flight times of the ions and thus decreases the mass resolution of the spectrometer.
The purpose of the present invention is to achieve truly orthogonal TOFMS while maintaining a higher mass resolution than can otherwise be achieved in similar instruments.
Several references relate to the technology herein disclosed. For example, F. Hillenkamp, M. Karas, R. C. Beavis, B. T. Chait, Anal. Chem. 63(24), 1193A(1991); Wei Hang, Pengyuan Yag, Xiaoru Wang, Chenglong Yang, Yongxuan Su, and Benli Huang, Rapid Comm. Mass Spectrom. 8, 590(1994); A. N. Verentchikov, W. Ens, K. G. Standing, Anal. Chem. 66, 126(1994); J. H. J. Dawson, M. Guilhaus, Rapid Comm. Mass Spectrom. 3, 155(1989); M. Guilhaus, J. Am. Soc. Mass Spectrom. 5, 588(1994); E. Axelsson, L. Holmlid, Int. J. Mass Spectrom. Ion Process. 59.231(1984); O. A. Mirgorodskaya, et al., Anal. Chem. 66, 99(1994); S. M. Michael, B. M. Chien, D. M. Lubman, Anal. Chem. 65, 2614(1993).
One of the major considerations in the design of TOF mass spectrometers is that of ion deflection. Ion deflection serves the purpose of both steering the ion beam onto a desired path and for selecting/rejecting ions during the course of the mass spectroscopic analysis. In conventional spectrometers, ion deflection is typically achieved via deflection plates. A conventional deflector consist of two metal plates which are placed parallel to one another on opposite sides of the expected path of the ion beam in planes which are perpendicular to the direction in which the ion beam is to be deflected. These deflection plates are biased to an electrical potential which produces the desired deflection. The difficulty with such a deflection system in a TOF mass spectrometer is that its use results in distortions in the flight times of the deflected ions.
The distortions in ion flight times caused by the use of deflection plates is the result of 1) differences in the flight times of ions through the deflection region and 2) changes in the velocities of the ions in the time-of-flight direction resulting from deflection. The present invention reduces the differences in the flight times of ions through the deflection region to negligible values by reducing the length of the deflection region and by decreasing the potentials on the deflecting elements.
In the multideflector, an array of special bipolar deflection plates is used to induce ion deflection. Each multideflector deflection plate is composed of two metal plates separated by an insulator. When active the two metal plates are biased to the same electrical potential but with opposite polarities. The bipolar deflection plates are placed adjacent, and parallel to one another, and approximately parallel to the ion beam path such that when the multideflector is deenergized, the vast majority of the ion beam passes unperturbed through the device. Further, the bipolar plates are assembled into the multideflector such that each side of each deflection plate is facing the opposite polarity side of the adjacent deflection plate.
The invention is a specific design for an Orthogonal TOF mass spectrometer incorporating Einsellens focusing, and a single stage grided reflector. Other objects, features, and characteristics of the present invention, as well as the methods of operation and functions of the related elements of the structure, and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following detailed description with reference to the accompanying drawings, all of which form a part of this specification.
With respect to
The TOF mass analysis takes place in a plane which is orthogonal to path 7. An example ion path 9 through the spectrometer in this plane is depicted in FIG. 1B. The TOF mass analysis begins in interface 3 where ions are accelerated by an electric field and deflected onto a proper trajectory. Ions pass out of the interface and drift through the spectrometer until arriving at reflectron 4. If the reflectron is deenergized, the ions will drift through the reflectron and strike detector 5. If the reflectron is energized, however, the ions will be reflected and eventually strike detector 6 according to path 9. By measuring the time required for the ions to move from their starting point in the interface to one of the detectors, the mass to charge ratio of the ions can be determined. The mass and relative abundance of the ions is determined by measuring the time required for the ions to travel from their starting point in the interface to one of the detectors and the signal intensity at the detectors respectively.
With respect to
When elements 10 and 11 are deenergized—that is when elements 10 and 11 are held at ground electrical potential—ions from source 2 may pass freely through the interface according to path 12. When energized, a potential difference is imposed between elements 10 and 11 and between elements 11 and 14. Those ions which are between elements 10 and 11 when the potentials are applied are accelerated by the resulting electric fields along paths which are parallel to example ion path 13. Even though the electric fields between elements 10 and 14 accelerate the ions in a direction which is orthogonal to path 12, the ions retain their initial velocity in the axial direction (i.e. in the direction given by path 12). As a result, the ions enter deflection system 15 moving in a direction which is not exactly orthogonal to path 12. Typically, ions enter deflection system 15 moving in a direction which is 3 to 6 degrees from the orthogonal direction. Because the TOF mass analysis occurs in the orthogonal direction, the deflection system must turn the ions onto a path which is orthogonal to path 12.
With respect to
Some of the advantages of the multideflector of the present invention over conventional deflection plates are demonstrated in
Note that the scale of
One of the primary considerations in choosing the distance between the plates is that of transmission efficiency. In a first approximation, if the plates are 0.1 mm thick and the distance between the plates is 3 mm then about 3% of the ion beam will collide with the plates while 97% of the beam will pass through the device and be analyzed.
A second consideration in selecting the distance between the plates in the multideflector is that of operating voltage. In accordance with equations 4 and 5, lower voltages are desirable in order to maintain a high mass resolution. Consequently, a small intexplate distance is desirable. The selection of the interplate distance is thus a trade-off of transmission efficiency and mass resolution.
The results of the simulation as shown in
Another advantage of the multideflector over conventional deflection plates is depicted in
This characteristic also makes the multideflector more predictable than the conventional deflector particularly in regard to the relationship between applied voltage and deflection angle. In accordance with equation ±315 V should be applied to the conventional deflector of
In contrast, because the multideflector is self shielding, the effective length is nearly the same as the length, L, of the plates. Thus, the required deflection voltage of ±95 V predict using equation 5 is in close agreement with the ±100 V determined using the numerical calculation. In this manner, the predictability of the multideflector makes it a more practical device.
In this situation, the multideflector has an additional advantage over conventional deflectors because of its smaller size in the orthogonal direction. The ion beam produced by source 2 is typically composed of a variety of mass-to-charge ratio ions. Often, the kinetic energy of these ions differs and is typically a function of mass. In the case of the Bruker source, the kinetic energy of the ions is a linear function of mass. A conventional deflection system cannot be adjusted to simultaneously deflect all of these ions onto an orthogonal trajectory. However, by varying the voltage on the multideflector during the ion analysis, ions of every mass can be deflected onto an orthogonal path simultaneously.
As depicted in
A conventional deflector cannot be used in this way because the size of the electric field in the orthogonal direction is too large. The flight time of an ion through the multideflector is about one sixth of that through the effective length of the conventional deflector discussed in
One disadvantage of using the bipolar plates as described thus far is that they are planar and thus can deflect the ion beam through only a limited angle before the ions are deflected into collisions with the deflection plates themselves. Thus, to accomplish large angles of deflection, for example 180°, curved deflection plates would be useful.
The multideflector may be used to focus or defocus ions in the deflection dimension.
In a similar manner, the ion beam may be focused by increasing the electric field strength as a function of position.
While the foregoing embodiments of the invention have been set forth in considerable detail for the purposes of making a complete disclosure of the invention, it will be apparent to those of skill in the art that numerous changes may be made in such details without departing from the spirit and the principles of the invention.