Ion sampling in ICP-MS: linking plasma expansion, interface physics, and ion transmission

Abstract

The ion sampling interface remains a critical bottleneck limiting sensitivity in inductively coupled plasma mass spectrometry (ICP-MS), where ∼0.1% of analyte ions survive transfer into the mass spectrometer. Here, a combined experimental and numerical investigation of plasma expansion in the first vacuum stage is reported, aimed at elucidating the gas-dynamic and plasma processes that govern ion transmission and provide key insight into the ion sampling process. High-resolution imaging resolved the complete barrel-shock/Mach-disk structure under realistic ICP-MS conditions, with Mach-disk positions extracted from centerline light intensity profiles. A systematic parametric study revealed how chamber pressure, RF power, injector gas flow, and orifice size modulate shock structure and position, providing quantitative insight into optimal skimmer placement. Complementary Computational Fluid Dynamics (CFD) and Direct Simulation Monte Carlo Method (DSMC) simulations benchmarked continuum and rarefied predictions, with DSMC showing closer agreement with experiments. Langmuir-probe diagnostics characterized the secondary RF discharge through measured electron density and temperature trends, enabling its reliable identification and mitigation through impedance matching optimization. Collectively, this unified approach establishes direct links between shock physics, plasma properties, and analyte ion transport, offering a mechanistic basis for interface optimization and informed signal tuning. The results provide actionable strategies to improve ion transmission and signal stability, suppress secondary discharges, and ultimately improve the detection limits and robustness of future of ICP-MS interface designs.