TIME-OF-FLIGHT MASS SPECTROMETER WITH MULTIPLE REFLECTION

Abstract

The invention provides (a) a time-of-flight mass spectrometer with an acceleration region, a single-stage or multi-stage reflector, and an ion detector, further comprising an additional reflector whose potential has, at least in a subregion, a two-dimensional logarithmic potential component and a two-dimensional octopole potential component, and (b) methods for operating the time-of-flight mass spectrometer.

Claims

1. A time-of-flight mass spectrometer comprising: an acceleration region; a single-stage or multi-stage reflector; an ion detector; and an additional reflector whose potential has, at least in a subregion, a two-dimensional logarithmic potential component and a two-dimensional octopole potential component.

2. The time-of-flight mass spectrometer according to claim 1, wherein the multi-stage reflector is a two-stage grid reflector.

3. The time-of-flight mass spectrometer according to claim 1, wherein the two-dimensional logarithmic potential component is given by: $U_{\log }\left(\Delta x,\Delta y\right)=U_l\log \left[\frac{{\left(\Delta x^2+\Delta y^2\right)}^2-2b^2\left(\Delta y^2-\Delta x^2\right)+b^4}{a^4}\right]$ where Δx and Δy are relative coordinates in the additional reflector, the Δx-direction is the direction of reflection, U.sub.1 defines the strength of the logarithmic potential component of the reflector potential, and a and b are constants of the two-dimensional logarithmic potential.

4. The time-of-flight mass spectrometer according to claim 1, wherein the two-dimensional octopole potential component is given by: $U_{\mathrm{oct}}\left(\Delta x,\Delta y\right)=U_o\left(\frac{\Delta x^4-6\Delta x^2\Delta y^2+\Delta y^4}{r^4}\right)$ where Δx and Δy are relative coordinates in the additional reflector, the Δx-direction is the direction of reflection, U.sub.o defines the strength of the octopole potential component of the reflector potential, and r is a constant of the octopole potential.

5. The time-of-flight mass spectrometer according to claim 1, wherein relative coordinates of the logarithmic potential component and the octopole potential component are identical.

6. The time-of-flight mass spectrometer according to claim 1, wherein the reflector potential of the additional reflector is substantially a superposition of the logarithmic potential component and the octopole potential component.

7. The time-of-flight mass spectrometer according to claim 1, wherein the additional reflector has two inner electrodes at a potential which attracts ions and a plurality of outer electrodes, where a cross-section of the inner electrodes is convex in shape, at least toward the inside of the reflector, and where the outer electrodes are arranged in a direction of reflection between the inner electrodes and a rear end of the additional reflector, and have a continuously increasing reflection potential, starting from the inner electrodes.

8. The time-of-flight mass spectrometer according to claim 1, wherein the additional reflector has a shielding electrode at its entrance, said electrode having a gridless slit-shaped opening and shielding the electric field of the additional reflector from an adjacent field-free flight region.

9. The time-of-flight mass spectrometer according to claim 1, additionally having a device which is located upstream of the acceleration region and is set up such that ions are transferred into the acceleration region perpendicularly to the direction of acceleration.

10. The time-of-flight mass spectrometer according to claim 1, wherein the acceleration region has an RF ion trap or an ion source.

11. The time-of-flight mass spectrometer according to claim 1, wherein the acceleration region, the single-stage or multi-stage reflector, the additional reflector, and the ion detector are preferably arranged and set up such that ions that are accelerated in the acceleration region only pass through the two reflectors once before being detected at the ion detector.

12. A method for operating a time-of-flight mass spectrometer, comprising: accelerating ions in an acceleration region; passing the ions through a first reflector after a first field-free flight region; passing the ions through a second reflector after a second field-free flight region; and detecting the ions in an ion detector after a third field-free flight region, where one of the two reflectors is a single-stage or two-stage reflector and the other is a reflector whose potential has, at least in a subregion, a two-dimensional logarithmic potential component and a two-dimensional octopole potential component.

13. The method according to claim 12, wherein the ions pass through the reflectors only once.

Description

BRIEF DESCRIPTION OF THE ILLUSTRATIONS

[0035] FIG. 1 shows a schematic representation of an OTOF-MS with a two-stage grid reflector, as known from the Prior Art.

[0036] FIG. 2A shows a schematic representation of the mass-dispersive part (200) of a first example embodiment, which comprises an orthogonal acceleration region (210), a two-stage grid reflector (240), an ion detector (250), and an additional reflector (230) with a two-dimensional logarithmic and octopole potential component, once in an x-z plane (top), once in an x-y plane (bottom).

[0037] FIG. 2B shows a schematic representation of the additional reflector (230) in the x-y cross-section with the relative coordinates of the additional reflector (230).

[0038] FIG. 2C shows the electric potential of the additional reflector (230) along the direction of reflection.

[0039] FIG. 3A shows a schematic representation of the mass-dispersive part (300) of a second embodiment, which comprises an orthogonal acceleration region (310), a two-stage grid reflector (340), an ion detector (350), and an additional reflector (330) with a two-dimensional logarithmic and octopole potential component, once in an x-z plane (top), once in an x-y plane (bottom).

[0040] FIG. 3B shows a schematic representation of the additional reflector (330) in the x-y cross-section with the relative coordinates of the additional reflector (330).

[0041] FIG. 4 shows a schematic representation of a preferred embodiment of a reflector (400) with a two-dimensional logarithmic and octopole potential component in the x-y cross-section with the relative coordinates of the additional reflector (400).

[0042] FIG. 5 shows a schematic representation of a third embodiment, which comprises an acceleration region (510) with a desorbing ion source, a two-stage grid reflector (540), an ion detector (550), and an additional reflector (530) with a two-dimensional logarithmic and octopole potential component.

[0043] FIG. 6 shows a schematic representation of the mass-dispersive part (600) of a fourth embodiment, which comprises an orthogonal acceleration region (610), a two-stage grid reflector (640), an ion detector (650), and an additional reflector (630) which has a two-dimensional logarithmic and octopole potential component, once in an x-z plane (top), once in an x-y plane (bottom). The additional reflector (630) is elongated in the z-direction compared to the second embodiment, and the accelerated ions pass through it twice so that a W-shaped ion trajectory (660) results.

DETAILED DESCRIPTION

[0044] The disclosure can be better understood by referring to the following illustrations. The elements in the illustrations are not necessarily to scale, but are primarily intended to illustrate the principles of the disclosure (mostly schematically).

[0045] FIG. 2A shows a schematic representation of the mass-dispersive part (200) of a first embodiment, which comprises an orthogonal acceleration region (210), a two-stage grid reflector (240), an ion detector (250), and an additional reflector (230) with a two-dimensional logarithmic and octopole potential component. The top part of the figure shows how the components are arranged in the x-z plane. The bottom part of the figure shows the components in the x-y plane.

[0046] In contrast to FIG. 1, the first embodiment has an additional reflector (230) in addition to a two-stage grid reflector (240). As already shown in FIG. 1, an ion beam from an upstream device (not shown) with a velocity component in the z-direction is transferred into the acceleration region (210). The acceleration region (210) periodically pulses out a string-shaped section of the ion beam orthogonally into a field-free region (220). The ions which are pulsed out are first deflected in the additional reflector (230) and pass through the field-free region (220) a second time, before their direction is reversed a second time in the two-stage grid reflector (240). After the two-stage grid reflector (240), the ions pass through the field-free region (220) a third time and are detected in the ion detector (250). The ion trajectory (260) in the mass-dispersive part (200) is N-shaped and comprises three field-free subregions.

[0047] FIG. 2B shows a schematic representation of the additional reflector (230) in the x-y cross-section with the relative coordinates of the additional reflector (230). The additional reflector (230) has two inner electrodes (231), a plurality of outer electrodes (232, 233), a terminating electrode (234), and a shielding electrode (235). The electrodes of the additional reflector (231, 232, 233, 234, 235) are arranged in mirror symmetry with respect to the x-z plane.

[0048] The cross-sections of the two inner electrodes (231) correspond to a Cassini curve, with a slightly egg-shaped appearance. The inner electrodes (231) are at a potential that attracts the ions. The additional reflector (230) is bounded toward the end by a slightly curved terminating electrode (234), which is at a potential that repels the ions.

[0049] The outer electrodes (232, 233) of the additional reflector (230) consist of curved metal sheets which follow the shape of the equipotential surfaces of the reflector potential at their respective positions. The outer electrodes (233) have a continuously increasing potential from the inner electrodes (231) through to the terminating electrode (234). The outer electrodes (232) are at a potential that attracts the ions, as are the two inner electrodes (231), but their potential attracts ions less than the potential of the two inner electrodes (231). The curved outer electrodes (233) become flatter and flatter, following the shape of the equipotential surfaces of the superimposed logarithmic and octopole potential components, until the last outer electrode before the terminating electrode (234) is essentially flat. The last outer electrode is at the potential of the ion beam before the acceleration in the acceleration region (210) so that the points of reversal of the ions are located here.

[0050] The shielding electrode (235) has a slit-shaped opening along the z-direction at the entrance of the additional reflector (230), and shields the electric field of the reflector from the adjacent field-free region. The shielding electrode (235) follows the shape of the slit-shaped opening of the equipotential surface of the reflector potential and encloses the two inner electrodes (231), the outer electrodes (232, 233), and the terminating electrode (234). The dashed line (236) marks the transition from the field-free flight region to the reflector potential. The additional reflector (230) has no shielding grid at the entrance, so the ion losses of the time-of-flight mass spectrometer according to the invention essentially correspond to those of the time-of-flight mass spectrometer from FIG. 1.

[0051] FIG. 2C shows the electric potential UR of the additional reflector (230) in the x-z plane along the direction of reflection. The dashed line (236) again marks the transition from the field-free flight region to the reflector potential UR. The reflector potential UR has a local minimum at the x-position of the inner electrodes (231) and a point of inflection at the position (237).

[0052] FIG. 3A shows a schematic representation of the mass-dispersive part (300) of a second embodiment, which comprises an orthogonal acceleration region (310), a two-stage grid reflector (340), an ion detector (350), and an additional reflector (330) with a two-dimensional logarithmic and octopole potential component. The top part of the figure shows how the components are arranged in the x-z plane. The bottom part of the figure shows the components in the x-y plane.

[0053] As shown for the first embodiment in FIG. 2A, here too an ion beam from an upstream device (not shown) with a velocity component in the z-direction is transferred into the acceleration region (310). The acceleration region (310) periodically pulses out a string-shaped section of the ion beam orthogonally into a field-free region (320). The ions that are pulsed out are first deflected in the additional reflector (330) and pass through the field-free region (320) a second time, before their direction is reversed a second time in the two-stage grid reflector (340). After the two-stage grid reflector (340), the ions pass through the field-free region (320) a third time and are detected in the ion detector (350).

[0054] In contrast to FIG. 2A, the first field-free subregion between the acceleration region (310) and the additional reflector (330) is shorter than in FIG. 2A. In a further embodiment, the first field-free subregion can even be dispensed with completely. In addition, the ions in the acceleration region (310) are accelerated in the opposite direction to that of the acceleration region (210). The potential of the additional reflector (330) has a local minimum along the direction of reflection, i.e., the ions are first accelerated on entering the additional reflector and only decelerated afterward, and has a point of inflection after the local minimum. The potential makes it possible for the spatial spread of the ions and their divergence in the y-direction on entering and exiting to be essentially the same, or for spatial focusing or parallelization of a divergent ion beam in the y-direction to be achieved. The focusing in the additional reflector (330) can be designed such that the spatial distribution of the ions at the ion detector (350) corresponds to its dimension in the y-direction, and can replace a focusing ion lens, which is often part of an acceleration region of an OTOF-MS according to the Prior Art.

[0055] FIG. 3B shows a schematic representation of the additional reflector (330) with a two-dimensional logarithmic and octopole potential component in the x-y cross-section with the relative coordinates of the additional reflector (330). The additional reflector (330) has two inner electrodes (331), a plurality of outer electrodes (332, 333), a terminating electrode (334), and a shielding electrode (335). The electrodes of the additional reflector (331, 332, 333, 334, 335) are arranged in mirror symmetry with respect to the x-z plane.

[0056] The additional reflector (330) shown in FIG. 3B is simplified in regard to the outer and inner electrodes, compared to the additional reflector (230) from FIG. 2B. The outer electrodes (332, 333) are flat metal sheets here. The two inner electrodes (331) have a circular cross-section and are at a potential which attracts the ions, like those in FIG. 2B.

[0057] The additional reflector (330) is bounded toward the end by a slightly curved terminating electrode (334), which is at a potential which repels the ions. The shielding electrode (335) has a slit-shaped grid-free opening along the z-direction at the entrance to the additional reflector (330), and shields the electric field of the reflector from the adjacent field-free region.

[0058] The outer electrodes (333) have a continuously increasing potential from the inner electrodes (331) through to the terminating electrode (334). The outer electrodes (332) are at a potential which attracts the ions, as are the two inner electrodes (331), but their potential attracts ions less than the potential of the two inner electrodes (331). The separations of the outer electrodes along the region (337) are chosen such that the same potential difference ΔU is always applied between the outer electrodes. Twice the potential difference 2ΔU is applied between the outer electrodes along the region (336), including the inner electrode (331). The potential differences can easily be generated from a single operating voltage by means of a voltage divider with precision resistors or equivalent electric circuits. In simulations, it was possible to show that this geometrically simplified form generates a potential distribution that has the same favorable spatial and temporal focusing properties as the reflector (230) in FIG. 2B, since the slight distortion of the potentials produces only harmless higher multipole components.

[0059] FIG. 4 shows a schematic representation of a preferred embodiment of an additional reflector (400) with a two-dimensional logarithmic and octopole potential component in the x-y cross-section with the relative coordinates of the additional reflector (400).

[0060] The additional reflector (400) has a vacuum housing (410), in which two ceramic plates (435) and (436) are secured. Flat outer electrodes (432, 433) and a slightly curved terminating electrode (434) are inserted in milled gaps in the ceramic plates (435, 436). The outer electrodes are bent once and folded over to form a small protective shield (437), to give the outer electrodes more hold in the milled gaps and to prevent leakage currents between the outer electrodes on the surface of the ceramic plates (435, 436). The protective shields (437) cover a part of the ceramic plates (435, 436) in such a way that there is no electric field along the surface under the shields, although high voltages of one to two kilovolts can be present between adjacent outer electrodes. The shielding electrodes (438) and (439) continue into an envelope of the field-free flight region and generate the constant potential which prevails there. The reflector additionally has two inner electrodes (431) with circular cross-section.

[0061] FIG. 5 shows a schematic representation of a third embodiment, which comprises an acceleration region (510) with a desorbing ion source, a two-stage grid reflector (540), an ion detector (550), and an additional reflector (530) with a two-dimensional logarithmic and octopole potential component.

[0062] In contrast to the first two embodiments, the ions here are first produced in the acceleration region (510) itself, e.g., by means of a MALDI ion source or other types of desorbing ion source. The ions are accelerated in the acceleration region in the x-direction as well as in the z-direction and formed into a slightly divergent ion beam (560). A further difference compared to the first embodiment consists in the fact that, after a first field-free subregion, the ions first pass through the two-stage grid reflector (540) and only then through the additional reflector (530).

[0063] FIG. 6 shows a schematic representation of the mass-dispersive part (600) of a fourth embodiment, which comprises an orthogonal acceleration region (610), a two-stage grid reflector (640), an ion detector (650), and an additional reflector (630), which has a two-dimensional logarithmic and octopole potential component. The top part of the figure shows how the components are arranged in the x-z plane. The bottom part of the figure shows the components in the x-y plane.

[0064] An ion beam is transferred from an upstream device (not shown) with a velocity component in the z-direction into the acceleration region (610). The acceleration region (610) periodically pulses out a string-shaped section of the ion beam orthogonally into a field-free region (620). The ions that are pulsed out are first deflected in the additional reflector (630) and pass through the field-free region (620) a second time, before their direction is reversed a second time in the two-stage grid reflector (640). The additional reflector (630) is elongated in the z-direction compared to the second embodiment in FIG. 3B, and the accelerated ions pass through it twice. After their direction is reversed a second time in the additional reflector (630), the ions pass through the field-free region (620) a fourth time and are detected in the ion detector (650). Overall, an approximately W-shaped ion trajectory (660) results, which allows the dimension of the mass-dispersive part (600) in the x-direction to be reduced compared to the previous embodiments, while the overall length of the field-free flight regions stays the same.

[0065] The invention has been described above with reference to different, specific example embodiments. It is to be understood, however, that various aspects or details of the embodiments described can be modified without deviating from the scope of the invention. Furthermore, the features and measures disclosed in connection with different embodiments can be combined as desired if this appears practicable to a person skilled in the art. Moreover, the above description serves only as an illustration of the invention and not as a limitation of the scope of protection, which is exclusively defined by the appended claims, taking into account any equivalents which may possibly exist. The person skilled in the art will find it easy to develop further embodiments of a time-of-flight mass spectrometer according to the invention on the basis of the potential distributions according to the invention in the additional reflector.