Fundamental characteristics of voltage-modulated helium plasma for multi-element analysis in low-volume samples

High-sensitivity multi-element analysis of low-volume samples remains a challenge in plasma-based analytical methods because optimal excitation conditions differ among elements and conventional steady-state plasma require relatively large sample volumes. In this study, we developed a voltage-modulated helium plasma in which the applied voltage was continuously varied over short time intervals to dynamically control the plasma energy state. The fundamental characteristics of plasma were evaluated in terms of gas temperature, emission spectrum, and excitation temperature. Under steady-state operation, plasma length, gas temperature, and excitation temperature increased with increasing applied voltage, confirming that the plasma energy state can be controlled by applied voltage. In the voltage-modulated plasma, emission intensity and excitation temperature responded rapidly to voltage changes, enabling the generation of plasma with different energy states within a short time. Notably, the excitation temperature, one of the key indicators of the plasma energy state, was dynamically varied over a wide range of approximately 1900–3600 K. In addition, voltage modulation effectively suppressed the overall increase in plasma gas temperature caused by Joule heating, while maintaining high-energy plasma states during high-voltage periods. These results demonstrate that voltage-modulated plasma generation enables rapid switching between energy states optimized for different elements, offering a practical strategy for high-sensitivity multi-element analysis with significantly reduced sample consumption. The proposed method is particularly suitable for the analysis of small-volume and heat-sensitive samples.