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

SUMMARYHelium microwave‐induced plasma (He‐MIP) for nonmetal elements analysis was generated by a 2.45 GHz microwave power (∼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 2.4 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.