Ammonia-filled discharge tubes for optical emission spectrometric determination of 15N : 14N isotope ratios

A procedure is described for filling a discharge tube, permanently attached to a vacuum line, with ammonia in the pressure range 1.5–5 Torr, without any carrier gas. The ¹⁵N : ¹⁴N isotope ratio is determined from the N₂ spectrum emitted when the tube is excited by a 2450-MHz microwave source. Cooling one end of the discharge tube to —60°C enables the N₂ (1,0) bandheads at 316 nm to be used for ¹⁵N contents down to about 0.04 atom-%. Unidentified interfering bands and emission from NH, OH, H₂ and H are discussed. Samples containing 1–30 mg of nitrogen can be analysed with an accuracy and precision suitable for most soil—plant studies employing ¹⁵N as a tracer.