In a TOFMS, ions originating from a sample compound are made to fly over a fixed distance by being granted fixed kinetic energy, the time required for this flight is measured, and the mass-to-charge ratio of the ion is found from said flight time. For this reason, if there is variation in the position of the ion or the initial energy of the ion when the ion is being accelerated to begin its flight, this will produce variation in flight time for ions of the same mass-to-charge ratio, leading to a decline in mass resolution and mass accuracy. One known method of overcoming this issue is to use an orthogonal acceleration (also called “vertical acceleration” or “orthogonal extraction”) type TOFMS.
In an orthogonal acceleration-type TOFMS, an ion beam originating from a sample is accelerated in a pulse in a direction orthogonal to the direction of progress, and ion packets produced thereby are sent into a flight space where mass spectrometry is performed. Performing acceleration in an orthogonal direction suppresses variation in the initial speed of ions in the direction of acceleration, making it possible to significantly reduce turnaround time occurring during ion acceleration, in turn making it possible to improve mass resolution. In recent years, so-called Q-TOF devices capable of high-accuracy, high-resolution MS/MS analysis, which are equipped with a highly ion-selective quadrupole mass filter around the collision cell in the stage prior to the orthogonal acceleration-type TOFMS, with its high mass resolution and mass accuracy, have come to be widely used to perform proteome analysis, etc.
FIG. 4 is a schematic structural drawing of the orthogonal acceleration portion of a common prior-art orthogonal acceleration-type TOFMS.
Orthogonal acceleration portion 1 includes flat repeller electrode 11 provided parallel to the direction of progress of the introduced ion beam (X axis direction), extraction electrode 12 provided opposite repeller electrode 11 across from the ion beam, and a plurality of acceleration electrodes 13 (13a, 13b) that together form the acceleration area in which ions extracted by the repeller electrode 11 and extraction electrode 12 are accelerated. Among these, extraction electrode 12 and acceleration electrode 13b in the final stage of the acceleration region comprise a grid electrode in which a conductive grid is spread over the aperture traversed by ions (see Non-patent Literature 1).
In this orthogonal acceleration portion 1, an ion beam originating from a sample compound is introduced in the X axis direction into the extraction area between repeller electrode 11 and extraction electrode 12, as indicated in FIG. 4. At this time, electrodes 11 and 12 have the same potential (for example, ground potential), so there is no electric field in the extraction area or the acceleration area. At a designated point in time when an adequate quantity of ions have been introduced, a high-voltage pulse of the same polarity as the ion is applied to repeller electrode 11, and voltage serving to accelerate the ion is applied to extraction electrode 12 and acceleration electrode 13 along the Z axis direction. The magnetic field formed by the voltage applied in this way causes part of the ion beam to be deflected from the extraction area towards the acceleration area, upon which major kinetic energy applied thereto by the accelerating field causes it to traverse the grid aperture of the final-stage acceleration electrode 13b and be discharged as an ion packet. Although the accelerating field accelerates the ion in the Z axis direction, because the initial speed of the ion is in the X axis direction (drift direction), the actual direction at the start of flight will be in the direction indicated by the outline arrow in FIG. 4.
The reason for using a grid electrode for both the extraction electrode 12 and the acceleration electrode 13b is to delimit the border of the potential while ions are made to traverse at a designated transmission efficiency in order to form a uniform accelerating field in the acceleration area. However, when the ions traverse the grid electrode, a fixed proportion of the ions disappear upon coming in contact with the grid, rendering unavoidable a commensurate loss in signal sensitivity. Furthermore, diverging lens effect is produced by leaks in the electric field through microscopic openings in the grid, causing a portion of the diverging ions to not be injected, further reducing sensitivity, which runs the risk of reducing resolution or accuracy due to a decline in optical characteristics such as time convergence at the point in time of arrival at the detector.
To overcome this drawback that exists in the event of the use of a grid electrode, orthogonal acceleration-type TOFMS not using a grid electrode has also been proposed (see Patent Literature 1, 2, etc.). However, devices of this kind require the addition of hardware such as electrodes pulse-driven at a designated timing as well as advanced and complicated controls, which makes considerable cost increase unavoidable.
Another device has been proposed wherein a focusing electrode is provided in the extraction area between the repeller electrode and the extraction electrode in order to compress the ion packet in the drift direction and thereby make it possible to use a detector with a small ion detection surface (see Patent Literature 3). However, because this device, like the aforesaid prior art, uses a grid electrode in the final stage of the extraction electrode and the acceleration electrode, it is difficult to achieve a high ion transmission efficiency. Furthermore, focusing electrodes must be added anew, but in actual fact, it is difficult to provide focusing electrodes within the narrow extraction area between the repeller electrode and the extraction electrode in such a way as to exert an adequate electric field.