Improved detection limits for a time-of-flight mass spectrometer with a surface ionization source

Abstract

A time-of-flight mass spectrometer was modified for use with a surface ionization (SI) source. Several pulsing methods and two source configurations were tested to optimize performance. The SI emitter was an Re ribbon perpendicular to and on-axis with the ion flight trajectories and an Re wire was positioned parallel to and 2 mm above the surface. A pulsing sequence was applied to the ion source plates that reversed the accelerating field after a time delay of 300 ns, producing a well-defined pulse of ions of relatively high signal-to-noise ratio. Application of a pulsed negative potential to the wire was believed to confine the ions and improve sensitivity threefold without any loss in resolution. The wire was also used as an SI emitter when the ribbon was heated to desorb organic compounds. The factors limiting the FWHM resolution to 100, less than half of that obtained with electron ionization, are discussed and calculated for this design. Both the thermal velocity distribution of the ions and irregularities in the spacing of the accelerating grids contributed to the loss of resolution.