Patent Number: 063103532
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With respect to FIG. 1, a prior art REFLEX TOFMS 1 is shown, with a laser system 2, ion source 3, Einsel lens 4, deflector 5, ion gate 6, reflectron 7, linear detector 8, reflector detector 9 and a data acquisition unit 10. In FIG. 1, the radiation from laser system 2 generates ions from a solid sample. Ions are accelerated through, and out of, the ion source 3 by an electrostatic field. Lens 4 is used to reduce the divergence of the ion beam exiting source 3. Here, some unwanted ions can be removed from the ion beam using blanking plates 5. The remaining ions may drift through the spectrometer until they arrive at ion gate 6. At ion gate 6, ions of interest are selected for further analysis. Selected ions continue to drift through the spectrometer until arriving at linear detector 8. Alternatively, the reflectron 7 may be used to reflect the ions so that they travel to the reflector detector 9. The mass and abundance of the ions is measured via the data acquisition system 10 as the flight time of the ions from the source 3 to one of the detectors 8 or 9 and the signal intensity at the detectors respectively. With respect to FIG. 2, a diagram of an ion source 3 as used with the prior art REFLEX TOFMS of FIG. 1 is shown. Ions are generated at the surface of the sample plate 11 which is biased to a high voltage (e.g. 20 kV). Ions are accelerated by an electrostatic field toward the extraction plate 12 which is held at ground potential. Ions are focused by the electrostatic lens system 4, and steered in two dimensions by the deflection plates 13. Finally, some types of unwanted ions are removed from the ion beam by blanking plates 5. Prior art electrostatic lens system 4 operates on the same principle as a prior art Einsel lens. As depicted in FIG. 3, a prior art Einsel lens consists of three elements which are used to form an electrostatic field. Each lens element 14, 15, and 16 is composed of conducting material and has an inner surface which is cylindrically symmetric about the primary direction of ion motion. The depiction of FIG. 3 is thus a cross sectional view. The two outer cylinders 14 and 16 are held at a ground potential whereas inner cylinder 15 is energized to a high voltage when the lens is in operation. Energizing electrode 15 produces an electrostatic field with equipotential lines 20. Equipotential lines 20 were calculated numerically assuming that electrode 15 was energized to a potential of 1000 V. Equipotential lines 20 appear at 100 V intervals between 100 V and 900 V inclusive. FIG. 3 also shows example ion trajectories 17, 18, and 19 through the energized lens. To calculate these trajectories, 1,500 eV kinetic energy ions were assumed, entering the page from the left and traveling directly toward the right. The direction of the force on the ions at any given point along their path is always towards a lower potential and always perpendicular to the tangent of the equipotential lines. Because path 18 is along the symmetry axis of the lens, the force on an ion on this path will always be on axis. However, on any other path the force on the ion will have some radial component. Thus an ion on path 17 or 19 will experience three regions of force in the radial direction and two regions of force in the axial direction. When an ion is in cylinders 14 or 16 it will experience an outward radial force and a decelerating axial force. The decelerating axial force continues until the ion reaches the midpoint of the lens in electrode 15. This decelerating axial force tends to increase the flight time of the ion. The outward radial force tends to defocus the ion beam. In electrode 15, the ion experiences an inward radial force. This focuses the ion beam. While in electrode 16, the ion again experiences an outward radial force which defocuses the ion beam. Between the center plane of electrode 15 and the exit of the lens, the ions are accelerated to their original kinetic energy. Even though the net result of the electrostatic field is to focus the ion beam, the defocusing portions of the field cause the operating voltage to be a relatively high 1,500 V. The deceleration of the ions by this high strength field and the resultant increase in ion flight times is correspondingly high compared to that of a shielded lens of the present invention. With respect to FIG. 4, a graph of the mass spectrum of angiotensin II showing the molecular ion at mass 1047 amu, using the prior art REFLEX TOFMS, is shown. This spectrum was recorded using lens 4, reflectron 7, and detector 9. Because reflectron 7 was used, it is possible to observe some ions (at apparent masses 902, 933, and 1030 amu) which are products of the dissociation of the molecular ions. With respect to FIG. 5, a shielded lens according to the present invention is shown. The shielded lens includes two planar grids 21 and 22 and a cylinder 23. Elements 21, 22, and 23 are composed of conducting material. Grids 21 and 22 are fine mesh conducting grid--e.g. 95% transmission, 8 lines per centimeter (or 20 lines per inch), Ni grid. Grids 21 and 22 are held at ground potential during normal operation, while cylinder 23 is energized to some high voltage. For example, to obtain the same focusing as that obtained by the prior art lens of FIG. 3 under the same conditions, cylinder 23 of the shielded lens is biased to 100 V. Energizing cylinder 23 to 100 V produces an electrostatic field with equipotential lines 24. Equipotential lines 24 were calculated numerically and occur at 10 V intervals between 10 and 90 V inclusive. FIG. 5 also shows example ion trajectories 25, 26, and 27 through the energized lens. To calculate these trajectories, 1,500 eV kinetic energy ions were assumed, entering the page from the left and traveling directly toward the right. The direction of the force on the ions at any given point along their path is always towards a lower potential and always perpendicular to the tangent of the equipotential lines. Because path 26 is along the symmetry axis of the lens, the force on an ion on this path will always be on axis. However, on any other path the force on the ion will have some radial component. Thus an ion on path 25 or 27 will experience forces in the radial and axial directions. Unlike the case of the prior art Einsel lens, the radial force on an ion in a shielded lens of the present invention will always be inward. As a result, the operating voltage of a shielded lens is typically a factor of 10 less than that of a prior art Einsel lens. In the above examples, the prior art Einsel lens required a voltage of 1,000 V whereas under the same conditions the shielded lens of the present invention required only 100 V. Note that in alternate embodiments, electrode 23 may have some shape other than cylindrical. In fact, to a good approximation, the results of FIG. 5 are valid for a planar symmetric shielded lens. That is if the electrodes represented in FIG. 5 are extended indefinitely into and out of the page, the results in terms of equipotential lines 24 and ion trajectories 25, 26, and 27 would not change much from the cylindrically symmetric device actually simulated. Thus, electrode 23 may be replaced by two planar electrodes of the same (or slightly different) potentials placed on opposite sides of the ion beam. With respect to FIG. 6, a diagram of ion source 3 modified to include shielded lens 28 according to the present invention is shown. Ions are generated at the surface of the sample plate 11 which is biased to a high voltage (e.g. 20 kV). Ions are accelerated by an electrostatic field toward the extraction plate 12 which is held at ground potential. Ions are focused by shielded lens system 28 according to the present invention, and steered in two dimensions by the deflection plates 13. Finally, some types of unwanted ions are removed from the ion beam by blanking plates 5. As in prior art Einsel lenses, ions in a shielded lens will also experience an acceleration in the axial direction. Ions entering the lens will be decelerated until they are half way through the lens. Then the ions will be accelerated back to their original kinetic energy. As with the Einsel lens this deceleration followed by reacceleration results in a net increase in the total flight time of the ions from sample plate 11 to one of detectors 8 or 9. This effect is more clearly demonstrated in FIG. 7. FIG. 7 is a plot of ion intensity as a function of ion flight time from sample plate 11 to detector 8 as determined by numerical calculation. The three main features of interest in this plot are flight time 29, flight time distribution 30, and flight time distribution 31. Flight time 29 is the flight time of the ions which would be observed assuming no lens were used. Flight time distribution 30 is that calculated assuming a shielded lens were used. Flight time distribution 31 was calculated assuming a prior art Einsel lens was used. The shielded lens of the present invention clearly causes less change in the flight time of the ions than the prior art lens. The difference in flight time distribution 30 and original flight time 29 is roughly 5 ns whereas that of flight time distribution 31 and flight time 29 is about 50 ns. The factor of 10 smaller influence on ion flight times by the shielded lens versus the prior art lens is largely the result of the factor of 10 lower operating voltage. Also, note that the use of the shielded lens of the present invention results in a more Gaussian flight time distribution, 30, than does the use of the prior art lens, 31. Many ions of distribution 31 are lost from the main peak and form a tail at longer flight times. Such a tail does not occur in shielded lens distribution 30. Finally, taking into account the loss of ions to the tail of distribution 31 and the fact that 10% of the ions would be lost by collision with grids when using a shielded lens, the shielded lens had a transmission efficiency of 85% whereas the prior art Einsel lens had a transmission efficiency of 66%. That is, more ions went undetected when using the prior art lens than when using the shielded lens. FIG. 8 is a depiction of an alternative embodiment of a shielded lens according to the present invention wherein only one grid electrode is used. As in FIG. 5, the depiction represents a cross section of a cylindrically symmetric device. However, similar results would be obtained from a planar symmetric geometry. As in the preferred embodiment (FIG. 5), the geometry of the alternate embodiment depicted in FIG. 8 consists of conducting cylinder 23, and one of grids 22. However, one of grids 22 has been replaced by cylinder 32 which is virtually identical to cylinder 23. In this embodiment, cylinder 32 and grid 22 are both held at some first potential (e.g. ground potential) whereas cylinder 23 is held at some second potential (e.g. 100 V). By selecting the correct potentials ions may be focused in a manner similar to that depicted in FIG. 5. Because only one grid is used in this alternate embodiment, the ion transmission efficiency through this device is higher. That is, if the grid material has a 95% transmission efficiency, then passing the ions through two such grids results overall in a transmission efficiency of 90%. Thus, having two grids as in the preferred embodiment results in a lower transmission efficiency device than a device such as the alternate embodiment of FIG. 8 having only one grid. Even though the device has only one grid, instead of two, the relatively low operating voltage is maintained. The ions will be focused in the region near cylinders 32 and 23 in a manner similar to that described for an Einsel lens (FIG. 3). However, the strongest focusing of the ions occurs in the region near cylinder 23 and grid 22 as described in association with FIG. 5. This embodiment operates at about one fifth of the operating voltage of an Einsel lens of similar geometry. 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.