Helium Microwave‐Induced Plasma at Atmospheric Pressure Generated by Okamoto Cavity for Nonmetal Analysis

Helium microwave-induced plasma (He-MIP) for nonmetal elements analysis was generated by a 2.45 GHz microwave power (similar to 1 kW) with the Okamoto cavity of the surface-wave mode at atmospheric pressure. Effects of the main parameters of the cavity such as the thickness (direction of the electric field) of the cavity, the width of the ring-slot, and the position of the discharge tube (torch) on the excitation temperature and the analytical signals were investigated. The excitation temperature and the electron density of the plasma as a parameter of the microwave power, and the plasma and carrier gas flow rates were measured. With 600-W input microwave power, an excitation temperature of 7000 K for the high energy part and an electron density of 2.4x1014/ cm 3 were obtained. An aqueous solution of NaF was introduced into the center of the annular He-MIP, the F I line (685 nm, 14.5 eV) signal was detected, and the effects of microwave power and the gas flow rates on the signal were studied. A detection limit of 60 ppb for fluorine was obtained.