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

In orthogonal acceleration time-of-flight mass spectrometers (oa-TOFMSs), w here ion velocity in the axis of the ion source is preserved, the ions appr oach grids at an angle not equal to 90 degrees. 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 paral lel with the source axis. The dispersion leads to a broadening of the fligh t-time distribution for ions of a given mass-to-charge ratio and hence it d egrades mass resolving power. A novel 20 kV matrix-assisted laser desorptio n/ionisation (MALDI) oa-TOFMS instrument has been used in this study to inv estigate grid dispersion. The results show that the dispersions that ions e xperience as they pass through grids/meshes that divide regions of differen t electric field strength have a significant effect on the resolving power when the majority of the wires are not aligned with the source. Numerical s imulations of ion motion point to an advantage in using ideal parallel wire grids with line densities in excess of one hundred lines per centimeter, o rientated 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 demo nstrate the effect. Parallel wire grids of such high line densities are imp ractical to construct and support so a grid design that approximates parall el wires has been lithographically mastered for electroforming 70% transmis sion grids with 120 lines per centimeter in one direction and 12 lines per centimeter at right angles to this direction. A resolution loss of similar to 40% (from a resolution of m/Deltam = similar to 8000 at full width at ha lf maximum) was observed when a three-grid orthogonal accelerator construct ed with this material was rotated through 90 degrees from the predicted ide al orientation. The observed linewidths were in reasonable agreement with t hose predicted by ion trajectory calculations. (lnt J Mass Spectrom 206 (20 01) 201-210) (C) 2001 Elsevier Science B.V.