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Eur. J. Mass Spectrom. 6, 515 - 522 (2000)

Time-of-flight analysers with sector fields: advances and prospects

Jun Zhang and Christie G. Enke*
Department of Chemistry, University of New Mexico, Albuquerque, NM 87131, USA

ABSTRACT:
A novel tubular ion mirror has been designed for reflectron time-of-flight mass spectrometry (TOF-MS). This design consists of only three cylindrical elements. With this mirror, an electric field is achieved that is non-linear along the mirror axis and has improved off-axis homogeneity compared with conventional simple geometry ion mirrors. The practical operation of this mirror is simpler than that of conventional diaphragm ion mirrors. Design parameters, including the relative lengths and voltages of the three elements, can be determined empirically. Once the mirror depth dimensions have been implemented, the voltages of the middle and rear elements are the only adjustable parameters. The electric field achieved by the mirror deviates only slightly from a linear mirror with constant field strength. The second-order deviation curve reveals two regions of curvature; all ions are turned around in the second curvature region. A plot of arrival time vs ion kinetic energy has a flattened S shape. The aspects of this shape are affected by the voltages of the middle and rear elements. SIMION 7 simulation demonstrates that this new non-linear ion mirror can perform second-order kinetic energy correction for large kinetic energy variation (33%) and substantial beam dimensions. Both the ion kinetic energy variation and the mirror element voltages affect the mass resolution obtained with the mirror. Ion trajectory simulations with a 10 mm diameter beam from a standard, two-field source predict that baseline resolution can be 16,000, 7600 and 1200 for 15%, 26% and 33% kinetic energy variations, respectively.

Keywords: ion mirror, time-of-flight, mass spectrometry, focusing, simulation, reflectron, high resolution

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