Effects of sample probe insertion on plasma conditions in direct sample insertion-inductively coupled plasma-atomic emission spectrometry

The effects of sample probe insertion on plasma excitation conditions in direct sample insertion-inductively coupled plasma-atomic emission spectrometry (DSI-ICP-AES) were investigated. Graphite sample probes in the form of a cup and a rod (with or without undercut) were used. Molybdenum rods were also used for comparison. A dark central channel is created in the plasma when a sample probe is inserted. The diameter of the central channel correlates with the diameter of the sample probe. In addition, the plasma excitation conditions were monitored using repetitive laser sampling to introduce a steady flow of Zn or Fe particles into the ICP. Changes in the relative excitation temperature and electron number density of the plasma were deduced from the ratios of the emission intensities of Fe line pair and Zn line pair, respectively. The temperature and electron number density of the plasma decreased when a sample probe was inserted. The degree of changes in the plasma conditions depends on the diameter of the sample probes, regardless of the form and material of the probe. The diameter of the sample probe instead of its mass plays a significant role in perturbing the plasma excitation conditions.