A pulsed ion deflection system for background reduction in (252)CF-plasma desorption mass spectrometry

The use of an electrostatic particle guide (EPG) for background reduction in a time-of-flight mass spectrometer is described. Operating with reverse polarity, the EPG deflects ions radially from the beam axis, separating the ionic and neutral components of the beam. Use of the deflection EPG in a synchronized pulsed mode with a barrier disk aligned in the center of the beam axis eliminates up to 80% of the spectral background in the molecular ion region of an insulin spectrum obtained by (252)Cf-plasma desorption mass spectrometry. The background eliminated is due to the neutral products of metastable fragmentation and to uncorrelated events. Although peak intensities are reduced when the pulsed deflection EPG system is used, the reduction in background achieved is greater, resulting in an overall improvement in peak-to-background ratios of up to a factor of three. A new large-area stop detector designed for use with the pulsed deflection EPG is described. The new hybrid detector, which utilizes the combination of a large (75-mm active diameter) microchannel plate (MCP) and a scintillation detector, provides greater sensitivity for high-mass ions than a conventional MCP chevron detector.