Reducing grid dispersion of ions in orthogonal acceleration time-of-flight mass spectrometry: advantage of grids with rectangular repeat cells

Abstract

In orthogonal acceleration time-of-flight mass spectrometers (oa-TOFMSs), where ion velocity in the axis of the ion source is preserved, the ions approach grids at an angle not equal to 90°. In this situation ions are expected to be dispersed more than in the case of the normal approach. This may be attributed primarily to the effect of grid wires that are not parallel with the source axis. The dispersion leads to a broadening of the flight-time distribution for ions of a given mass-to-charge ratio and hence it degrades mass resolving power. A novel 20 kV matrix-assisted laser desorption/ionisation (MALDI) oa-TOFMS instrument has been used in this study to investigate grid dispersion. The results show that the dispersions that ions experience as they pass through grids/meshes that divide regions of different electric field strength have a significant effect on the resolving power when the majority of the wires are not aligned with the source. Numerical simulations of ion motion point to an advantage in using ideal parallel wire grids with line densities in excess of one hundred lines per centimeter, orientated with the ion source axis. The orientation of parallel wire grids has previously been predicted to significantly affect resolving power in oa-TOF instruments. This article presents the first experimental data to demonstrate the effect. Parallel wire grids of such high line densities are impractical to construct and support so a grid design that approximates parallel wires has been lithographically mastered for electroforming 70% transmission grids with 120 lines per centimeter in one direction and 12 lines per centimeter at right angles to this direction. A resolution loss of ∼40% (from a resolution of m/Δ m = ∼8000 at full width at half maximum) was observed when a three-grid orthogonal accelerator constructed with this material was rotated through 90° from the predicted ideal orientation. The observed linewidths were in reasonable agreement with those predicted by ion trajectory calculations.