A comparative study of sheathing devices to increase robustness in inductively coupled plasma optical emission spectrometry via a nitrogen flow

Abstract

Five different glass sheathing devices were used to introduce 20 mL min⁻¹ N₂ sheathing gas around the effluent from the spray chamber to see the effect of their dimensions on plasma robustness as measured using the Mg II 280.270 nm/Mg I 285.213 nm emission signal ratio in inductively coupled plasma (ICP) optical emission spectrometry (OES). A clear relationship between device dimensions and the plasma robustness profile along the ICP central channel was observed. With an aerosol inlet having an inner diameter of 1.0 cm in the sheathing device, mixing of N₂ with the aerosol was minimised and the resulting sheath increased robustness, the higher thermal conductivity of N₂versus Ar likely improving energy transfer between the bulk plasma and the central channel. The configuration of the outlet is also important, as any bottleneck favours mixing of N₂ with the aerosol, which in turn cools the plasma, thereby shifting the region of maximum robustness to higher above the load coil. Regardless of device dimensions, plasma robustness was always better with N₂ sheathing gas. An increased ion-to-atom emission signal ratio also resulted for other elements than Mg. The presence of N₂ also seems to affect excitation mechanisms, such as charge transfer between NO⁺ and Cd or Ni, which appears to take place at higher observation height, after N₂ has had enough time to diffuse into the central channel.