Space velocity correlation in orthogonal time-of-flight mass spectrometry

Papanastasiou, Dimitris (2005) Space velocity correlation in orthogonal time-of-flight mass spectrometry. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

The idea of space-velocity correlation in continuous ion beams is proposed and tested in a prototype orthogonal Time-of-Flight Mass Spectrometer (TOF MS). Analytical solutions to the time focusing properties of ions are presented based on linear phase space distributions at the start of the TOF experiment. The analysis is extended to non-linear distributions and their associated finite arrival time spreads. The prototype is equipped with a thermal/surface ionization source and an electron impact source, providing an excellent opportunity to investigate variations in the experimental variables required for enhancing resolving power. Theory and experiment merge in an effort to describe the behaviour of ions in the source and through the optical system, in terms of their time focusing properties in the mass analyzer. Resolving powers exceeding 2000 with time spreads of the order of 1-3 ns across the atomic mass range are reported for small linear oTOF configurations. Optimum virtual sources for reflecting geometries are investigated by varying the dimensions of the mass spectrometer. The “open” orthogonal gate design is proposed as an alternative to the prevailing two-stage acceleration configuration. A low energy electron impact source is described and a triple filament thermal/surface ionization source for surface studies is constructed. The effect of a fast detection system on the overall performance of the prototype is demonstrated. Exact solutions to the space focusing properties employing static and/or time dependent uniform electric fields are developed using the “ion pair” model. A focal region with variable temporal and spatial characteristics is identified along the field-free path of the ions by considering finite initial spatial distributions. The errors associated with the truncated Taylor series are highlighted. Focusing is not an instantaneous property in TOF MS. The analysis is extended to ions desorbing from surfaces and the theory of delayed extraction is reformulated. The focusing action of the TOF analyzer relies on the inherent ability of the system to provide ions with an initial distribution in electric potential energies. Space focusing theory and the delayed extraction technique were fundamental to the development of space-velocity correlation focusing in orthogonal geometries. Further studies include the experimental determination of the work function of filaments based on their electron emission properties, measurements of the kinetic energy distribution of the emitted electrons, thermionic emission kinetics, SEM and XPS analysis of the filament surfaces and a literature survey of thermal/surface ionization and electron impact ion sources.