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Ion dispersion near parallel wire grids in orthogonal acceleration time‐of‐flight mass spectrometry: predicting the effect of the approach angle on resolution
Author(s) -
Lewin Mark,
Guilhaus Michael,
Wildgoose Jason,
Hoyes John,
Bateman Bob
Publication year - 2002
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.590
Subject(s) - ion , spectrometer , grid , mass spectrometry , acceleration , chemistry , computational physics , resolution (logic) , time of flight , energy (signal processing) , dispersion (optics) , optics , physics , geometry , classical mechanics , computer science , mathematics , organic chemistry , chromatography , quantum mechanics , artificial intelligence
Ions experience small deflections in the vicinity of grids in accelerators and ion mirrors in time‐of‐flight (TOF) mass spectrometers. Recent experiments with an orthogonal acceleration (oa) TOF instrument have verified that the effect can significantly degrade resolution when ions approach grids at an angle deviating from 90 °. The phenomenon becomes significant only when ions have components of velocity at right angles to the wires of the grids. A model is presented in this study to predict this phenomenon for parallel wire grids. The fractional energy spread of ions (calculated in the static TOF‐spectrometer frame of reference) scales directly with the approach angle of ions to the grid (as measured from normal approach). The energy spread also scales with the range of angles that is a consequence of the focusing effect in each gap between the wires of the grid. The equations imply that closely spaced parallel wire grids are best for deployment in oa‐TOF systems where non‐zero approach angles are unavoidable. Such grids are relatively impractical to manufacture and support but rectangular repeat cell grids with relatively few wires at right angles to the source axis are shown experimentally to introduce minimal energy spread. When these grids are rotated by 90 °, the resolution measured in a Q‐TOF spectrometer is degraded in approximate agreement with the parallel wire model. A practical implication of this work is that grid transmissions in oa‐TOF systems may be significantly increased without loss of resolution. Improvements of ∼200% (V‐mode) and ∼400% (W‐mode) in ion transmission were obtained in this study without compromising resolution. This was achieved with ∼73% transmission grids and greater potential improvements in transmission are being realised since this study with ∼89% transmission grids having similar geometry. Copyright © 2002 John Wiley & Sons, Ltd.