Time-of-flight (TOF) mass spectrometers distinguish ions of different mass-to-charge ratio by the difference of there flight time from the ion source to detector. Thus TOF method essentially requires the ion source from which ions can be pulsed out having the same initial position and energy. In practice this is not possible due to inherent thermal energy spread and position spread of ions inside the ion source. Modern ToF mass spectrometers use acceleration of ions by high voltage pulse out of the pulsar region. Before ejection the ion cloud occupy comparatively wide volume and have substantial energy spread. After ejection out of the pulsar different ions of the same mass-charge ratio have different energy partly due to difference in initial position and partly due to initial velocity spread. Both factors introduce spread into the time of ion arrival to detector, thus limiting the resolution of ToF mass measurement. Energy distribution of ions, which is introduced by position spread, can be corrected by energy focusing devices like reflectron. The energy distribution, which results from velocity spread cannot be corrected by any combination of electrostatic fields and appears as a major factor limiting mass resolution of ToF. Imagine, for example, two ions of identical mass-to-charge ratio (m/z) positioned in the same point in the ion trap, but having different velocity. Both ions have the same absolute velocity V but velocity of the first ion is directed towards ToF while second ion has velocity in opposite direction. Upon application of the extraction field both ions have the same acceleration a=E/(m/z), where E—is strength of the extraction electrical field. While the first ion starts moving towards ToF, the second ion has to move in opposite direction until its velocity becomes zero and reversed. After the time δt=2V/a the second ion arrives into original position having the same velocity as first ion in the beginning of ejection. At this moment second ion is undistinguishable from the first ion. Time δt elapsed from the beginning of ejection towards reverse of second ion is called “turn-around-time”. First ion has started earlier by the time δt and will arrive at the detector earlier. Mass measurement in ToF is essentially based on the measurement of the time of ions arrival to detector, hence the two ions of identical m/z will arrive to detector within time difference of δt and this cannot be corrected by any electrostatic field configuration. In real devices ions always have thermal velocity spread δV and turn-around-time δt due to thermal spread limits the resolution of ToF spectra by theoretical value R=Ttof/2 δt, where Ttof is the total flight time. Thermal energy spread per one degree of freedom at 300 K equals 0.013 eV. For example, for singly charged ions of mass 1000 Da corresponding velocity spread equals 100 m/s. Upon application of 10 kV acceleration voltage on distance of 10 mm the total turn-around-time equals δt=1.1 ns. Assuming the total flight path of 4 m the time-of-flight equals 91 μs. Hence theoretical resolution limit due to turn-around-time in this case is 41.000.
It is known in the art of mass spectrometry, that ion traps can provide an improved ion source for TOF mass spectrometer [1]. By using momentum-dissipating collisions of ions with light buffer gas the ion cloud can be collected near the centre of the trap to a size of less than 1 mm. Kinetic energy spread of ions in such cloud is believed to be close to thermal. Modern methods of ion trapping are based on the use of harmonic periodical voltages (trapping RF) applied to one or several electrodes of the ion trap. Voltage power supply for such RF contains high-Q resonator, which keeps all the energy of the RF field during ion trapping period. Typical ion trap device with internal size of 10 mm can require voltages up to 10 kVo-p for ion trapping. In order to optimise ejection conditions the RF voltage should be switched off upon ejection of ions into TOF [2]. In practice it is extremely difficult due to huge amount of energy in RF resonator. Extraction pulse should be applied not later than few μs after RF switch off, otherwise ions will be lost on the electrodes. It follows that residual “ringing” RF is still present in the trapping volume upon application of extraction pulse. Such ringing deteriorates accuracy and resolution of TOF mass spectra by introducing hardly predictable acceleration fields during ejection. Assuming, that residual RF ringing is only 0.1% of original voltage the magnitude of oscillating voltage is several Volt. The energy spread of ions introduced by this voltage difference is of the order of several electron-volt, which is by two orders of magnitude bigger than thermal energy spread of ions at 300 K. The objective of present invention is to improve performance of TOF mass analysis in terms of resolution and mass accuracy by eliminating the energy spread of ions, which is introduced by residual RF during ejection of ions out of the ion trap.
USPTO Backgrounds
USPTO Backgrounds