Emission Spectrometric Analysis Using an Okamoto-cavity Microwave-induced Plasma with Nitrogen-Oxygen Mixed Gas

Microwave-induced plasma optical emission spectrometry (MIP-OES) using Okamoto-cavity has a unique feature being suitable for in-situ analysis in steel-making industry. The Okamoto-cavity MIP can be directly loaded with organic solvents including sample solution as well as fine particles of a solid sample in the nitrogen-oxygen mixed gas plasma; furthermore, their emission intensities are much elevated by adding oxygen of up to 10% to the nitrogen matrix gas, thus contributing to better sensitivity in the MIP-OES. The reason for the intensity enhancement was investigated with a spectroscopic method that the spatial distribution of the emission intensity from the plasma was estimated with a two-dimensionally imaging spectrograph. Emission lines of chromium, which was the most important alloying element in steel materials, as well as band heads of nitrogen molecule were observed, indicating that the emission intensity of atomic chromium lines was drastically elevated whereas the intensities of ionic chromium lines and the nitrogen band heads were commonly reduced when oxygen gas was added to the nitrogen plasma. This result implies that the ionization of chromium, which dominantly occurs through collisions with nitrogen excited species, can be suppressed because the nitrogen excited species would be consumed through collisions with oxygen molecules to cause their dissociation. Optimization of the measuring parameters in the Okamoto-cavity MIP-OES was conducted to determine chromium contents with good precision, and finally the analytical performance in the MIP was compared with that in a conventional ICP-OES.