Effect of hydrogen isotope on plasma impedance and thermal pinching induced by single aerosol droplets injected into an inductively coupled plasma

Abstract

The mass of the hydrogen isotope in micrometer-sized aqueous droplets introduced into an ICP has been found to affect the degree of resulting plasma perturbation. Studied plasma perturbations include local plasma cooling, plasma shrinkage due to thermal pinching, and changes in plasma impedance. The local cooling effect caused by a vaporizing aerosol droplet was found to be similar between H2O and D2O. This finding is likely a result of the similar heat capacities and vaporization rates of H2O and D2O droplets and of the comparable thermal conductivities and dissociation kinetics of H2O and D2O vapor. In contrast, a clear isotope effect was observed for plasma shrinkage and impedance change, with H2O causing a considerably stronger effect. Plasma shrinkage, gauged by the reduction in Ar emission at the edge of the plasma, was detected at all observation heights within the load-coil region, with D2O droplets causing as much as 30% less of an effect than H2O. Likewise, D2O droplets induce significantly less change in plasma impedance than H2O droplets. Depending on experimental conditions and the probing method, changes in plasma impedance caused by monodisperse D2O droplets were only 52%–80% that of H2O. The lower influence on plasma shrinkage and impedance change for D2O droplets is believed to be related to the much lower thermal conductivity of atomic deuterium, around 70% that of atomic hydrogen (protium). The time required for the plasma impedance to return to its original steady-state value was comparatively long (~10 ms) and similar for H2O and D2O droplets.