STABILIZATION OF AN INDUCTIVELY COUPLED PLASMA FOR INDUCTIVELY COUPLED PLASMA MASS-SPECTROMETRY WITH A FLARED TORCH EXTENSION

An audio-frequency peak in noise power spectra of signals from emission and mass spectrometers, which use an inductively coupled plasma (ICP) as a source, has been associated by a number of workers with instability at the boundary of the plasma plume where it enters the surrounding atmosphere at the mouth of the torch. It has been shown in emission spectrometry that the extension of the outer envelope of the torch beyond the region of the plume normally viewed can eliminate this peak. Because of the need to place the ion extraction aperture just downstream of the torch mouth this does not appear to be straightforward in the case of ICP mass spectrometry but this paper reports the use of a flared torch extension that provides the necessary separation of the plasma and the surrounding atmosphere. Colour photographic evidence is provided of a major improvement of plasma stability.