A compact AC-glow discharge-optical emission spectrometer for real-time detection of N2 and Ar in O2 with sub-ppm-level sensitivity†

Microplasma is a promising source for optical emission spectroscopy (OES) capable of multiple elemental analysis with high sensitivity owing to its high density and stability. Compared with solids or liquids, the application of OES to gas samples has been rare until now owing to the limited detection sensitivity. In this work, a bright and stable microplasma with a long life span was generated by employing high AC drive voltage and used as a source for a home-built compact optical emission spectrometer with high sensitivity. A compact dual-channel spectrometer of 15 cm focal length was developed by employing custom-fabricated spectroscopy fiber bundles having a 700 μm-diameter circular input and 100 μm-width slit output. The spectral resolution of each channel was <0.5 nm at 358 and 813 nm, which is essential to the high sensitivity of the optical emission spectrometer along with a high optical throughput due to the large input cross-section. Au electrodes of 1 mm diameter separated by 1 mm distance were driven by the AC voltage of ∼7 kV and ∼30 kHz for plasma generation. From quantitative analysis experiments on N₂ and Ar in ambient O₂, where 358 and 813 nm emission lines were used for N₂ and Ar, the detection sensitivities for N₂ and Ar were found to be <1 and <0.2 ppm (3-σ), respectively, within 10 s, which implied that ppb-level detection is possible within a few minutes.