Characterization of the central channel and its flow velocity distribution in an analytical Inductively-Coupled-Plasma source

Abstract

In this study, we extend our previous high-speed photographic studies (Xie et al. 2022, Han et al. 2025) to direct characterization of the central channel and its flow velocity distribution in an analytical Inductively-Coupled-Plasma source. The strong plasma emission was successfully suppressed by optimising the high-speed camera's aperture size and shutter duration, allowing us to directly observe the central channel and the discrete ion cloud. The width of the central channel was determined from the luminance distribution rather than the analyte ion distribution, unlike the commonly used method. The effect of the addition percentage of oxygen and nitrogen to the sample flow on the channel width is investigated. By injecting and tracking five types of suspension particles with various inertial properties, the axial flow velocity in the central channel is determined using the time-of-flight method. The axial flow velocity distributions under varying r.f. power, sample flow rate, and the addition fraction of oxygen and nitrogen in the sample flow were finally experimentally determined. Results show that the width of the central channel in pure Ar-ICP is 7.2 mm under the investigated operating conditions. The presented width value is comparable with the peak-to-peak value of the plasma parameter distributions, but about three times that determined from the analyte ion distribution. In the mixed-gas ICP, the width increases with increasing nitrogen addition percentage in the sample flow, but is insensitive to the oxygen addition fraction. Compared with the flow velocity, no particle slipping or dragging was observed, indicating that the flow velocity was well represented by that of the suspension particles used. In pure Ar-ICP, Ar-O2 ICP and Ar-N2 ICP, the axial flow velocity tends to increase and then decrease with the axial position with respect to the torch outlet. In pure Ar-ICP, a cubic polynomial fitting of the axial flow velocity with the quadratic root of the axial position is proposed for velocity estimation. A velocity plateau is clearly observed in the NAZ. The present work provides detailed information on the central channel for pure argon, Ar-O2 ICP, and Ar-N2 ICP and experimental data on the axial flow velocity distribution.