Optical Emission Analysis of 15 N: Pressure Effects and Identification of NO and Gaydon—Herman N 2 Bands in the Discharge

Abstract

The optical emission method, an alternative to mass spectrometry for ¹⁵N analysis, is based on measuring ratios of the intensities of the ¹⁴N₂(28), ¹⁴N ¹⁵N (29), and ¹⁵N₂ (30) lines of the 2→0 band of the C(³π_u)→B(³π_g) N₂ transition near 298 nm. However, properties of the discharge are not well understood, and interferences that affect the analysis have never been explicitly identified. An efficient low-energy radio-frequency source and home-built spectrometer were used to obtain spectra over a wide range and with multiple scans. Weak extraneous bands were identified as the overlapping β andγ series of NO, not CO or OH as previously thought. Bands identified as the third positive system of CO were only observed under conditions far removed from normal practice. As tube pressure was increased from 0.3 to 1.8 kPa, the N₂ intensity decreased relative to that of NO peaks. The 29/28 intensity ratio also varied with pressure, although not in a completely consistent way. A weak band underlying the ¹⁴N¹⁵N transition was identified as part of the Gaydon–Herman singlet series of N₂; this interference rather than peaks from other species, may be the single factor most limiting analytical performance. Implications for the analytical method are discussed.