Orthogonal acceleration (OA) TOFMS [1,2] allows the coupling of continuous ion sources to a TOF mass analyser while maintaining high duty cycle efficiency (sensitivity) and resolving powers (m/Delta m, FWHM) up to a few thousand without the need for an ion mirror. Crucial to both sensitivity and resolving power is the ability to generate the continuous ion beam such that it is narrow in cross-section with minimal divergence.
We have devised a method for measuring the position and divergence of the ion beam accurately. The ion beam is rastered across a diaphragm with 2 pin-hole apertures. An electron multiplier monitors the signal coming through both apertures. Beam deflection voltages (horizontal, vertical) and signal monitoring is under the control of a LabVIEW 3 virtual instrument which displays a 3-dimensional map of the ion number density across the beam. The second pin-hole generates a superimposed map and the separation of the maxima can be tied to the physical separation in distance between the two holes. This provides for an accurate calibration of the crucial distance axis (i.e., that parallel to the orthogonal acceleration axis). Measurement of the cross-sections at two points along the continuous ion beam allows the beam divergence to be estimated.
With experimental values of the initial spatial and velocity dispersions of the ions, it is possible to model the expected peak shape. Other factors affecting peak shape include (a) the temporal response of the detector, jitter of the timing electronics, and (b) deflection of ions as they pass through fine meshes in the accelerator. We are able to estimate (a) reliably but are unable to model the symmetry of the meshes. However, as the effect of the meshes is the only significant source of peak broadening that we cannot measure, it can be set as the only variable in the peak shape modeling. Comparison of model with experiment reveals the influence of the meshes. With a measure of the influence of the meshes we can predict how the incorporation of an ion mirror will enhance the resolving power of an OA TOFMS instrument.