An improved interface for inductively coupled plasma-mass spectrometry (ICP-MS)

Abstract

Adiabatic sampling of the plasma from an inductively coupled plasma (ICP) torch is a crucial first step for inductively coupled plasma-mass spectrometry (ICP-MS) to reach its full potential as a trace multi-element analysis system. Previous results in the literature are reviewed, and a series of experiments reported, which indicate that a discharge, previously called a “pinch” effect, can be present in which the plasma properties are strongly altered by a constricted current flowing between the plasma and the sampling orifices, i.e. a secondary discharge. Measurements and calculations indicate that this mechanism is consistent with previously reported deleterious effects—abnormal production of doubly ionized species, a wide kinetic energy distribution for the ions sampled into vacuum, formation of ions from metal eroded from the sampling orifices, and a decreased orifice life-time. The mechanism producing the effect was found to be an unwanted capacitive coupling between the voltage on the induction coil and the plasma. Results are included which show that by centre-grounding the coil, the “pinch” effect can be reduced to negligible proportions. The resulting improvements for ICP-MS operation are described.