Selective removal of matrix ion peaks in plasma source time-of-flight mass spectrometry: ion deflection and detector gating

Abstract

A significant problem in plasma source time-of-flight mass spectrometry (TOFMS) is the saturation of detectors due to plasma gas and matrix ions. This paper outlines two experimental procedures to minimize the detector response to large ion currents in a glow discharge time-of-flight mass spectrometer. One novel solution is a deflection device that includes two parallel plates for ion ejection and two sleeve plates (after the deflection plates) to reduce ion and neutral scattering. Another approach is to gate the detector, by lowering the voltage on the microchannel plates, when an intense ion pack is expected. Both approaches are efficient in eliminating the plasma gas peaks and matrix ions in plasma source time-of-flight mass spectrometry. The matrix ion intensity can be reduced by several orders of magnitude, with minimal influence on the analyte ions. The detector gating technique requires a high peak current pulse with a flexible rise time. However, this approach will cause an electrical ringing to be superimposed on the analytical signal. Therefore, ion deflection is the method of choice for plasma TOFMS.