Field of the Invention
The present invention relates to an orthogonal acceleration time-of-flight (oa-TOF) mass spectrometer and, more particularly, to an oa-TOF mass spectrometer capable of preventing deterioration of the mass spectral resolution due to electrical charging of the repeller plate and grids forming the ion reservoir.
A mass spectrometer is an instrument in which ions created from a sample are made to travel through a vacuum. During the process of the flight, ions having different masses are separated and recorded as a spectrum. Known types of mass spectrometers include: magnetic mass spectrometer in which ions are dispersed according to mass using a sector magnetic field; quadrupole mass spectrometer (QMS) for sorting ions (filtering) according to mass using quadrupole electrodes; and time-of-flight mass spectrometer (TOFMS) for separating ions by making use of variations in time of flight due to different masses.
Of these mass spectrometers, magnetic mass spectrometer and QMS are adapted for ion sources that create ions continuously. On the other hand, TOFMS is suitable for ion sources that create pulsed ions. Accordingly, if one attempts to use a continuous ion source for TOFMS, some arrangement is necessary for utilization of the ion source. The orthogonal acceleration time-of-flight mass spectrometer (oa-TOFMS) is one example of TOFMS designed to emit pulsed ions from a continuous ion source.
A typical configuration of oa-TOFMS is shown in FIG. 1. This instrument has an external continuous ion source 1 (such as electron impact (EI) ion source, chemical ionization (CI) ion source, field desorption (FD) ion source, electrospray ion (ESI) source, or fast atom bombardment (FAB) ion source), differentially pumped walls 10 consisting of first and second partition chamber and a vacuum pump (not shown), a first orifice 2 formed in the first partition wall of the differentially pumped chamber 10, a ring lens 3 placed within the differentially pumped chamber 10, a second orifice 4 formed in the second partition wall forming the differentially pumped chamber 10, an intermediate chamber 11 where ion guides 5 are placed, lenses 6 consisting of focusing lenses and deflectors, a launcher 7 consisting of a repeller plate and accelerating lenses (grids), a reflector 8 for reflecting ions, and a measuring chamber 13 where components forming the ion optics such as an ion detector 9, are placed.
In this configuration, ions generated from the sample in the external ion source 1 are first introduced into the differentially pumped chamber 10 through the first orifice 2. The ions tending to diffuse within the differentially pumped chamber 10 are focused by the ring lens 3. Then, the ions are admitted through the second orifice 4 into the intermediate chamber 11, where the ions decrease in kinetic energy. The ion beam diameter is reduced by an RF electric field produced by the ion guides 5. The ions are then guided into the high-vacuum measuring chamber 13. The partition wall that partitions the intermediate chamber 11 and the measuring chamber 13 from each other is provided with a third orifice 12. This orifice 12 shapes the ions that are guided in by the ion guides 5 into an ion beam of a given diameter. The ion beam is then passed into the measuring chamber 13.
The lenses 6 including the focusing lenses and deflectors are installed at the entrance of the measuring chamber 13. The ion beam entering the measuring chamber 13 is corrected for diffusion and deflection by the lenses 6 and introduced into the launcher 7. Installed inside the launcher 7 are the ion reservoir and accelerating lenses arrayed in a direction orthogonal to the axis of the ion reservoir. In this ion reservoir, a repeller plate is disposed opposite to grids.
Referring to FIG. 2, the ion beam first travels straight toward the ion reservoir 17 that is located among the repeller plate 14, grids 15, and accelerating lenses 16. The ion beam 18 moving straight through the ion reservoir 17 and having a given length is accelerated in a pulsed manner in a direction (X-axis direction) orthogonal to the direction (Y-axis direction) along which the ion beam 18 enters, by applying a pulsed accelerating voltage to the repeller plate 14. This forms pulsed ions 19 which begin to travel toward a reflector (not shown) mounted opposite to the ion reservoir 17.
The ions accelerated in the vertical direction travel in a slightly oblique direction slightly deviating from the X-axis direction, because the velocity in the Y-axis direction assumed on entering the measuring chamber 13 and the velocity in the X-axis direction orthogonal to the Y-axis direction are combined. The latter velocity is given by the repeller plate, grids, and accelerating lenses. The ions are reflected by the reflector 8 and arrive at the ion detector 9.
When the ions are being accelerated, the same potential difference acts on every ion regardless of the masses of the individual ions. Therefore, lighter ions have greater velocities and vice versa. As a consequence, variations in ion mass appear as variations in arrival time taken to reach the ion detector 9. Variations in ion mass can be transformed into variations in ion transit time and thus ions of differing masses can be separated.
In this way, the continuous ion source can be applied to TOFMS adapted for a pulsed ion source by accelerating the ion beam created from the continuous ion source 1 in a pulsed manner by the launcher 7 consisting of the repeller plate, grids, and accelerating lenses.
In oa-TOFMS, the kinetic energy of ions made to enter the ion reservoir is normally set to a very small value of less than 50 eV. Therefore, oa-TOFMS is affected much more by charging of the electrodes than the magnetic mass spectrometer. As a result, if the electrodes forming the ion reservoir are charged at all (prior to pulsing), the ion beam introduced into the ion reservoir is deflected and tilted as shown in FIG. 3. This deteriorates the resolution and sensitivity of oa-TOFMS. Such charging can occur quite easily by adhesion of organics to the surfaces of the electrodes, the organics being residues of the sample ions.
One conventional measure for correcting this problem has been to mount a deflector immediately ahead of the ion reservoir of the oa-TOFMS instrument, along with the focusing lenses. Another measure has been to set the energy of the ion beam introduced into the ion reservoir to a larger value to reduce the effects of charging of the electrodes and other components. A further measure has been to mount a mechanism for applying an offset voltage to the repeller plate, for applying a DC voltage that cancels the charging.
If such a deflector is mounted, it is doubtless that deflection of the ion beam can be corrected. However, this is limited to cases where the ion beam is deflected ahead of the mounted deflector. If the repeller plate or accelerating lenses (grids) of the ion reservoir have been charged, correction of the deflection using the deflector is done almost unsuccessfully.
Setting the injection energy of the ion beam introduced into the ion reservoir to a larger value is more advantageous than mounting the deflector. Ions having an injection energy greater than the charging voltage at the repeller plate would be able to travel straight such that the beam is hardly deflected by the charging of the repeller plate. However, there may be a demand for decreasing the whole size of the instrument. Also, there may be a demand for space savings. Where these demands are taken into consideration, it is desired that ions introduced into the ion reservoir be accelerated in a direction as orthogonal as possible to the inlet axis of the ion reservoir. For this purpose, it is necessary to apply an accelerating voltage to the repeller plate that pushes out ions more strongly for high injection energies.
Although it can be said that increasing the injection energy is advantageous, there are practical limitations. Furthermore, the effects of charging are not always constant but liable to vary according to the degree of contamination of the instrument and with the elapse of time. In addition, there is the problem that, if a high-voltage power supply or a detector withstanding high voltages is adopted, the cost is increased accordingly.
Moreover, if a mechanism for applying an offset voltage to the repeller plate is mounted, and if a method of applying a DC voltage that cancels charging is used, an additional DC power supply capable of accurate voltage control is necessary. This incurs increased costs.