A pulse-synchronized microplasma atomic emission spectroscopy system for ultrasmall sample analysis

Abstract

Regenerative medicine and environmental science require the analysis of ultrasmall samples such as cells and nanoparticles. Therefore, a microplasma atomic emission spectroscopy (AES) system was developed in this study. This system combined microdroplet injection, which can introduce a small droplet of a few tens of picoliters into a plasma excitation/ionization source, with a microhollow cathode plasma source exhibiting a volume of a few microliters. When a 14 pL droplet of 100 mg L⁻¹ sodium solution was introduced into an 8 W DC microplasma, no emission was observed possibly because of an insufficient excitation of the sample at low plasma gas temperature. Therefore, a pulsed discharge, producing high-intensity electric input power, was implemented to give a pulse-synchronized microplasma AES system. A 15 μs pulsed power reaching a maximum value of 100 kW was obtained using a laboratory-built high-intensity pulsed power supply and was applied to generate the plasma. This system achieved an excitation temperature of 7000 K, exceeding that of the common inductively coupled plasma. Pulsed plasma generation and sample droplet introduction into the plasma were synchronized to provide a high-sensitivity AES analysis. To this end, the time interval before the end of the generation and the beginning of the pulsed plasma generation was adjusted using a delay circuit. The optimal delay amounted to 40 ms. A droplet comprising Na, Ca, Mg, and K at 100 mg L⁻¹ was analyzed using this system and the limits of detection equaled 300, 50, 30, and 640 fg for these analytes, respectively.