Comparison of aerosol properties and ICP-MS analytical performance of the Vulkan direct injection nebuliser and the Direct Injection High Efficiency Nebuliser

Abstract

The size and velocity distributions of the droplets in aerosols generated by a Vulkan direct injection nebuliser (Vulkan DIN) and a Direct Injection High Efficiency Nebuliser (DIHEN) were determined, at various axial and radial distances from their tips, using a light-scattering interferometric Particle Dynamics Analyser (PDA). In addition, the two nebulisers were used for sample introduction to inductively coupled plasma mass spectrometry (ICP-MS), and the resulting sensitivity and detection limits for various ions were compared. Owing to its larger critical dimensions and lower nebuliser gas kinetic energy, the Vulkan DIN produced droplets with larger average sizes and broader size distributions than the DIHEN. At 15 mm axial distance from the nebuliser tip droplets generated by the Vulkan DIN were continuously distributed across the 2–80 μm diameter size range (Sauter mean diameter 30.1 μm), whereas with the DIHEN two distinct groups of droplets were observed, with size ranges of <1–40 μm and 55–78 μm (Sauter mean diameter 10.9 μm). Both the initial acceleration of droplets after nebulisation and their subsequent deceleration (as the drag force of the nebuliser diminished) started at shorter distances from the tip for the Vulkan DIN than for the DIHEN. Consequently, the droplets reached their maximum velocity closer to the tip for the Vulkan DIN. At 15 mm from the tip the axial velocity distributions were 2–45 and 5–55 m s⁻¹, respectively, for the Vulkan DIN and the DIHEN (average 10.0 and 17.8 m s⁻¹, respectively). The higher mean velocity of the DIHEN aerosols is due to its higher nebuliser gas drag force. The residence time for droplets in the plasma should therefore be longer with the Vulkan DIN. The fast initial acceleration of droplets generated by the Vulkan DIN may be attributable to enhanced gas–liquid interactions at its tip due to the recessed positioning of the sample capillary. At 15 mm from the tip the aerosols generated by both nebulisers had higher proportions of both the largest and finest droplets 3 mm off-axis than at the axial position. For the DIHEN enrichment of large droplets was most pronounced, whereas for the Vulkan DIN enrichment of small droplets was more marked. The reason for this difference is not clear. The differences in the sensitivity and detection limits of the ICP-MS system associated with the use of the two nebulisers were moderate (1–2.4-fold and 1–3.4-fold, respectively). The DIHEN generally gave higher sensitivity, but the nebuliser that gave the lowest detection limits varied for different isotopes.