FUNDAMENTAL-STUDIES OF THE SAMPLING PROCESS IN AN INDUCTIVELY COUPLED PLASMA MASS-SPECTROMETER .1. LANGMUIR PROBE MEASUREMENTS

Langmuir probe measurements were performed in the first vacuum stage of an inductively coupled plasma mass spectrometer as source and interface conditions were adjusted. Space and floating potentials were found to increase with first-stage backing pressure, inner-gas flow rate, and solvent load. When the probe was located at the skimmer-cone-orifice position (with the skimmer cone removed), the space and floating potentials were found to be positive, which indicates that the plasma is no longer quasi-neutral. Additional experiments involved spatial mapping of the floating potentials in the expansion zone. In this zone, measured electrostatic potentials followed the geometrical features of a free-jet expansion expected for a neutral gas. This correlation indicates that neutral flow influences the electrostatic features in the shockwave structure of the expansion.