In the prior art, there are essentially two types of high-resolution reflector time-of-flight mass spectrometers, which are characterized according to the way the ions are injected.
Time-of-flight mass spectrometers with axial injection include MALDI time-of-flight mass spectrometers (MALDI-TOF MS), which operate with ionization by matrix-assisted laser desorption, but also time-of-flight mass spectrometers where stored ions are injected axially into the flight path from a storage device such as an RF quadrupole ion trap. They usually have Mamyrin reflectors (B. A. Mamyrin et al., “The mass-reflectron, a new nonmagnetic time-of-flight mass spectrometer with high resolution”, Sov. Phys.-JETP, 1973: 37(1), 45-48) in order to temporally focus ions with an energy spread. Mamyrin reflectors allow second-order temporal focusing, but not higher order focusing. Since point ion sources are used, the reflectors can be gridless, as a modification of the Mamyrin reflectors, which are operated with grids. MALDI-TOF MS are operated with a delayed acceleration of the ions in the adiabatically expanding laser plasma and with high accelerating voltages of up to 30 kilovolts; in good embodiments, with a total flight path of around 2.5 meters, they achieve mass resolving powers of R=50 000 in a mass range of around 1000 to 3000 daltons.
Time-of-flight mass spectrometers where a primary ion beam undergoes pulsed acceleration at right angles to the original direction of flight of the ions are termed OTOF-MS (orthogonal time-of-flight mass spectrometers). FIG. 1 depicts a simplified schematic of such an OTOF-MS. The mass analyzer of the OTOF-MS has a so-called ion pulser (12) at the beginning of the flight path (13), and this ion pulser accelerates a section of the low-energy primary ion beam (11), i.e. a string-shaped ion packet, into the flight path (13) at right angles to the previous direction of the beam. The usual accelerating voltages, only small fractions of which are switched at the pulser, amount to between 8 and 20 kilovolts. This forms a ribbon-shaped secondary ion beam (14), which consists of individual, transverse, string-shaped ion packets, each of which is comprised of ions having the same mass. The string-shaped ion packets with light ions fly quickly; those with heavier ions fly more slowly. The direction of flight of this ribbon-shaped secondary ion beam (14) is between the previous direction of the primary ion beam and the direction of acceleration at right angles to this, because the ions retain their speed in the original direction of the primary ion beam (11). A time-of-flight mass spectrometer of this type is also preferably operated with a Mamyrin energy-focusing reflector (15), which reflects the whole width of the ribbon-shaped secondary ion beam (14) with the string-shaped ion packets, focuses its energy spread, and directs it toward a flat detector (16). The width of the ion beam means the reflector must be operated with grids. Mass resolving powers of around R=40 000 at mass 1000 daltons are achieved in these OTOF mass spectrometers.
As these two examples suggest, time-of-flight mass spectrometers with high mass resolution are operated predominantly with Mamyrin reflectors in today's technology. Mamyrin reflectors provide second-order energy focusing, but not higher order focusing. If the energy spread of the ions is relatively large compared to the average energy, undesirable focusing errors occur. Since the kinetic energy of the ions always spreads slightly as the ions are being produced, or during their pulsed acceleration, the time-of-flight mass spectrometers must be operated with high accelerating voltages for the ions, between 5 and 30 kilovolts, for example, in order to always keep the relative energy spread as small as possible in relation to the average energy.
As a consequence of the high ion energy, the very long flight paths must be chosen in order to achieve a good temporal dispersion of ions of different masses. Since the fastest ion detectors at present offer measurement rates up to five billion measurements per second, and thus require a separation of a few nanoseconds between two ion masses which are to be resolved, the flight paths for the high mass resolutions desired must be several meters long, often far more than ten meters. If multiple reflectors are used to keep the instrument compact and to extend the flight path, the residual errors of the reflectors add up. If lower accelerating voltages are used in order to manage with shorter flight paths, the resulting higher relative energy spread, which cannot be focused in a higher order, prevents a high resolving power from being achieved.
It is known that a quadratically increasing electric potential in the reflector results in an ideal reflection with energy focusing of as high an order as desired (T. J. Cornish et al., “A curved field reflectron time-of-flight mass spectrometer for the simultaneous focusing of metastable product ions”, Rapid Commun. Mass Spectrom., 1994: 8(9), 781-785). If such a field is generated in a simple diaphragm stack by voltages which increase quadratically from aperture to aperture, the result is a defocusing effect in both lateral directions. If the kinetic energy of the ions is decreased in order to achieve long dispersive times of flight, the laterally defocusing effect increases. Further electric fields for at least “quasi-ideal” energy focusing are presented in a publication by A. A. Makarov, J. Phys. D; Appl. Phys. 24, 533 (1991).
Kingdon ion traps are generally electrostatic ion traps in which ions can orbit one or more inner electrodes or oscillate between several inner electrodes. An outer, enclosing housing is at a DC potential which the ions with a predetermined total energy (sum of kinetic and potential energy) cannot reach. In special Kingdon ion traps which are suitable for use as mass spectrometers, the inner surfaces of the housing electrodes and the outer surfaces of the inner electrodes can be designed in such a way that, firstly, the motions of the ions in the longitudinal direction of the Kingdon ion trap are completely decoupled from their motions in the transverse direction and, secondly, a symmetrical, parabolic potential profile is generated in the longitudinal direction in which the ions can oscillate harmonically in the longitudinal direction. When “Kingdon ion traps” are mentioned below, this always refers to these special designs.
In the publications DE 10 2007 024 858 A1 (C. Köster) and DE 10 2011 008 713 A1 (C. Köster), Cassini ion traps are described as special types of Kingdon ion traps which differ in the way in which several inner electrodes are arranged. The inner electrodes and the outer housing electrode (and possibly several segmented housing electrodes also) are designed here in such a way that the longitudinal motion is completely decoupled from the transverse motion, and a parabolic potential well is generated in the longitudinal direction for a harmonic oscillation.
The potential distribution φ(x,y,z) of such a Cassini ion trap can, for example, be that of a hyperlogarithmic field of the following form:
      ψ    ⁡          (              x        ,        y        ,        z            )        =                    ln        [                                                            (                                                      x                    2                                    +                                      y                    2                                                  )                            2                        -                          2              ·                              b                2                            ·                              (                                                      x                    2                                    -                                      y                    2                                                  )                                      +                          b              4                                            ai            4                          ]            ·                        U                      l            ⁢                                                  ⁢            n                                    C                      l            ⁢                                                  ⁢            n                                +                  [                                            -                              (                                  1                  -                  B                                )                                      ·                          x              2                                -                      B            ·                          y              2                                +                      z            2                          ]            ·                        U          quad                          C          quad                      +          U      off      The shape of the field can be changed by the constants a, b and B. Uln, Uquad and Uoff are potential voltages. The inner surface of the outer housing and the outer surfaces of the inner electrodes are equipotential surfaces φ(x,y,z)=const. of this potential distribution. In cross-section, the equipotential lines form approximate Cassini ovals about the inner electrodes here; two inner electrodes result in Cassini ovals of the second order, while n inner electrodes result in Cassini ovals of the nth order. For an even number of inner electrodes, there are embodiments where the ions can oscillate transversely near the center plane between at least one pair of inner electrodes. Any ratio of the longitudinal oscillation period to the transverse oscillation period can be set with the aid of form parameters.
In view of the foregoing, there is a need to provide compact time-of-flight mass spectrometers with high mass resolution, and especially to provide reflectors for time-of-flight mass spectrometers whose energy and solid angle focusing are as ideal as possible.