ION KINETIC-ENERGY DETERMINATIONS IN RADIOFREQUENCY GLOW-DISCHARGE MASS-SPECTROMETRY

Radio-frequency glow discharge (rf-GD) sources produce an abundance of both atoms and ions. For the mass spectrometric application of the gl ow discharge technique, knowledge of the ion kinetic energies is requi red to optimize extraction and focusing of ions from the source region into the analyzer. This paper details kinetic energies experimentally determined with the use of the ''retarding potential'' method. For th is study, the analyzer quadrupole of a double-quadrupole mass spectrom eter was positively biased to act as a repeller. Ion kinetic energies (IKEs) determined for a variety of discharge and analyzer operating co nditions ranged from 12.5 eV to 25.0 eV for (CU+)-C-63. Kinetic energy measurements were confirmed from ion trajectory simulations and follo w closely the experimental values for identical analyzer conditions an d initial IKEs. Results of this study indicate that the conditions und er which ions are formed (plasma conditions) affect IKEs and energy sp reads to a greater extent than analyzer parameter variations. Differen t from atmospheric plasma sources, IKEs for rf-GD species do not vary as a function of ion mass/identity. Evidence is also given in support of a slight mass biasing owing to the transmission properties of doubl e-quadrupole analyzers. The findings detailed herein demonstrate the e ffects of rf modulation on both ion kinetic energy values and distribu tions.