Langmuir probe potential measurements in the plasma and their correlation with mass spectral characteristics in inductively coupled plasma mass spectrometry
Abstract
A floating Langmuir probe is used to measure the apparent d.c. potential in an inductively coupled plasma (ICP) while the latter is used as an ion source for mass spectrometry (MS). The probe is swung through the plasma to provide potential measurements with some spatial resolution and to obviate cooling of the probe. The d.c. potential in the centre of the plasma is influenced by the presence of the metal sampling cone in the plasma and also by the gas flow through the orifice. In general, the potential correlates with the characteristics of the mass spectra; these parameters depend in a sensitive fashion upon the manner by which the load coil is grounded and shielded. For the load coil geometries investigated in this study, the potential and ion energies generally increase with aerosol gas flow-rate and decrease as power increases. As potential increases the abundance of doubly charged ions generally increases, ArO+/Co+ and Ar2+/Co+ decrease and CeO+ decreases slightly. The measured potential in front of the sampling orifice is generally a few volts below the mean ion energy, which indicates that both measurements are reasonable approximations to the actual d.c. plasma potential.