Novel setup for investigation of the plasma conditions in direct sample insertion inductively coupled plasma atomic emission spectrometry (DSI-ICP-AES)

Abstract

Insertion of a sample probe into the inductively coupled plasma (ICP) for direct sample insertion (DSI) could change the plasma excitation conditions. In this study, a novel setup is proposed for the investigation of the probe effects. A hollow graphite tube was used as a stand-in for the DSI sample probe to exert probe effects on the plasma. Laser-ablated testing elements (Fe, Zn and Mg) were introduced separately into the central channel of the plasma via the hollow graphite tube for measurement of plasma excitation temperature and electron number density. A relatively low Ar carrier gas flow rate (0.42 L min-1) was used to minimize perturbation to the plasma due to the gas flow. The study shows that the extent of reduction in plasma excitation temperature and electron number density increases with probe insertion position at all observation heights. In addition, the probe effects strongly depend on the relative distance of the observation position to the tip of the probe. The probe effects are the strongest near the tip of the probe and reduce at observation positions further away from the tip. Signal-to-background ratios (SBR) of the testing elements also depend strongly on the probe insertion position and the relative distance of the observation position to the tip of the probe. The SBR peaks at 5-10 mm above the tip of the sample probe. The vertical profile of SBR depends on analyte diffusion in the plasma and the plasma excitation conditions.