In a time-of-flight (TOF) mass spectrometer with an orthogonal acceleration (‘OA-TOF’) configuration, a collimated primary ion beam is directed into a ‘pulsing region’ of the TOF orthogonal accelerator optics while the pulsing region is field-free (ideally). Typically, the pulsing region is configured as the region between two parallel, planar electrodes, with surfaces that are parallel to the primary beam axis and perpendicular to the time-of-flight direction in the TOF flight tube. The electrode farthest from the TOF flight tube is typically a solid flat plate, while the electrode between the primary beam and the TOF flight tube includes a grid with high transparency to ions. Additional gridded and/or non-gridded electrodes are additionally configured between the pulsing region gridded electrode and the TOF flight tube entrance, depending on the particular optical design employed. These electrodes form one or more constant acceleration fields in order to optimize mass resolution and transmission of ions. One or more pulsed voltages can be applied to one or more of these electrodes to generate a pulsed acceleration field in the pulsing region, directed perpendicular to the primary beam axis, which, together with any subsequent electric fields, results in the acceleration of a segment of the primary ion beam through the first pulsing region grid electrode and ultimately into the TOF flight tube.
A common orthogonal acceleration arrangement can include two stages of constant, but different, acceleration fields separated by a gridded electrode. The first acceleration stage includes the pulsing region, which is ideally field-free during the time period when the primary beam ions enter this region, but which then must abruptly provide the first acceleration stage by abruptly applying pulsed voltages to the pulsing region electrode(s). The second acceleration stage can end downstream with the TOF entrance grid electrode at a relatively large flight tube potential. The flight tube voltage is generally greater than would be practical to pulse, and so the flight tube voltage is typically applied to the TOF entrance grid continuously. Consequently, a strong electric field is present constantly in this second acceleration stage.
A gridded electrode that separates two regions of differing electric fields cannot completely isolate one electric field region from the other, at least not throughout the region bordered by the grid. In particular, a single grid cannot completely prevent the strong electric field of the second acceleration stage from generating a small electric field in the pulsing region, and thereby deflect the incoming primary ion beam and degrade TOF performance.
Various approaches can be used to mitigate this ‘field penetration’ effect in an orthogonal-acceleration arrangement. For example, a constant voltage differential is often applied between the pulse region electrodes to counteract the field penetration. However, a grid electrode in the pulsing region usually includes a solid portion to physically support the grid, between which the primary beam must pass. The constant voltage differential creates electric fields along this solid portion, which interferes with the collimation of the primary beam and, therefore, TOF performance.
Consequently, a second gridded electrode is often incorporated between the first gridded electrode that borders the pulsing region, and the second acceleration stage. This second gridded electrode further reduces field penetration from the second acceleration stage into the pulsing region. Further, a constant voltage bias can be applied to this second gridded electrode to counteract field penetration into the pulsing region without generating electric fields in the pulsing region along the solid portion of the first gridded electrode. In this configuration, the first acceleration stage is developed during the pulse acceleration period between the solid plate electrode bordering the pulsing region, and the second gridded electrode. The voltages applied to the solid plate electrode, the first gridded electrode, and the second gridded electrode are selected such that the electric field of this first acceleration stage is constant during the pulse acceleration period. The solid plate electrode is often called the ‘pusher’ electrode, since ions are accelerated away from this electrode when the acceleration pulse occurs. Similarly, the gridded electrode that separates the first and second acceleration stages is often called the ‘puller’ electrode, since ions are initially accelerated towards this electrode. Finally, the gridded electrode between the pusher and puller electrodes, which borders the primary ion beam region, can be called the ‘intermediate’ pulsing region electrode.
Limitations to TOF performance arise from the deflections of ions by electric field distortions in the vicinity of the grid openings, or apertures, in a grid that separates regions of different electric field strengths. Such deflections, sometimes called ‘grid scattering’, are especially significant at the grid that separates the first and second stages of acceleration of a two-stage orthogonal acceleration TOF analyzer, due primarily to the oblique incident angle with which pulse-accelerated ions pass through this grid, combined with their relatively low kinetic energy at this point. Such an effect can be mitigated by placing grids having parallel wires oriented along the primary beam direction. However, such grids are challenging to form into a precisely flat plane due to a lack of supporting structures orthogonal to the grid wires. An acceptable compromise was found to be grids formed by two sets of orthogonally-oriented grid wires, but in which the spacing between grid wires oriented orthogonal to the plane of ion incidence was substantially greater than the wire spacing between wires oriented parallel to the plane of ion incidence (that is, parallel to the primary beam axis). Hence, such grids comprise rectangular shaped openings, or apertures, and are now frequently deployed in OA-TOF pulse acceleration optics where the long dimension of the openings are oriented parallel to the primary beam direction, resulting in improved transmission and resolving power, compared to grids with square apertures.
One approach to optimizing TOF performance is to minimize the components of the primary ions' velocity components in the TOF direction, that is, by ensuring that the primary beam is well-collimated. It is not possible to achieve perfect beam collimation, and imperfections in apparatus such as electrostatic lens aberration errors, mechanical misalignments, ion beam space charge broadening, local electric fields due to surface charges, all contribute to cause a small flux of ions to ‘stray’ from the ideal collimated beam path within the OA-TOF pulsing region. At least some of these ‘stray’ ions have trajectories that impinge on the intermediate pulsing region electrode. Such stray ions can cause increased baseline noise and/or artifact peaks in TOF mass spectra, impacting TOF performance.