A typical time of flight (“TOF”) mass spectrometer 101 is shown in FIG. 8.
The TOF mass spectrometer includes ion source 110, a source lens 111, an ion mirror 120 and a detector 130. Ideally, ions having a given m/z are focused by the ion mirror 120 to a planar surface of the detector 130 so that the ions having the given m/z value all reach the planar surface of the detector 130 at the same time. In other words, the ions having the given m/z value are preferably focused so that the isochronous plane of the ions having the given m/z value is angularly aligned with the planar surface of the detector 130. If however, there is an angular misalignment between the isochronous plane and the planar surface of the detector 130, e.g. by virtue of a mechanical alignment error between the detector 130 and the ion mirror 120, or to a lesser extent between the detector 130 and the ion source 110, then the flight times of ions having the m/z value will vary with location across the planar surface of the detector 130, reducing the resolving power of the TOF mass spectrometer.
Modern high resolving power (e.g. R>30K) TOF instruments are built with very tight mechanical tolerances, e.g. an alignment accuracy of 20 μm between detector and ion mirror is typically considered excessive and costly to achieve. For higher resolving powers (e.g. R>50K) as, an alignment accuracy of 10 μm may be required, which over a meter long instrument may be very difficult to achieve.
U.S. Pat. No. 5,654,544 describes how a dipole field tilts the isochronous plane of ions (having the same m/z) in a time-of-flight mass spectrometer and how this harms resolving power, with considerable explanation and theory. This patent sees this tilting effect as a problem to be solved, and describes a way of mechanically aligning the detector to account for this unwanted tilt of the ion cloud.
US 2014/0054454 (see e.g. paragraphs [0020]-[0050]) describes a TOF mass analyser comprising one or more devices arranged and adapted to correct for tilt in an isochronous plane of ions, and to adjust the isochronous plane of the ions so as to be parallel with the plane of detection in an ion detector. To achieve this effect, sequential flight and acceleration/deceleration regions separated by an angled grid are used (see FIGS. 5A and 5B). Tilting of the ion plane is based on ions across the plane spending different times in the two acceleration/deceleration regions, and only works in one dimension. Therefore two devices in series are required to realign both dimensions of the isochronous plane (see FIGS. 6A and 6B).
US 2006/0214100 discloses a multi-reflecting time of flight mass spectrometer. A small part of this patent (paragraphs [0135] to [0136]) notes that tilting of the isochronous plane can be electrostatically corrected for small alignment errors by alterations to the voltage on Matsuda plate electrodes that terminate a sector.
The present invention has been devised in light of the above considerations.