Application of microwave breakdown spectroscopy to detect halide impurities in the air

Abstract

In order to determine ppm orders of air pollutants, e.g., halide impurities, spectroscopy of the photons emitted from the plasma generated by a focused microwave beam in the air has been applied. The microwave beam was supplied by a pulsed magnetron (microwave frequency: 9.4 GHz; peak intensity: 100 kW; pulse width: 5 μs; pulse repetition frequency: 10 Hz), focused in a vacuum chamber, where pressure and impurities had been controlled to target values, and a plasma was generated using the air there. At the same time, the energy spectrum of the photons from the plasma was measured to determine the concentrations of chlorine and fluorine in the air by a measurement system consisting of a diffraction grating, streak camera, and charge-coupled-device camera. As a result of preliminary experiments, where the concentration of CCl2F2 was changed from 0.05 to 2 vol % in decompressed air (pressure: 10 Torr), it was demonstrated that 1700 ppm of chlorine could be determined from the background spectrum caused by the first positive band of nitrogen (B3Πg–A3Σu+), its second positive band (C3Πu–B3Πg), and first negative band (B3Σu+–X2Σg+). The experimental data demonstrated that ppm orders of halide impurities can be determined by improving pulse repetition frequency of the microwave source.