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 -600C 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. (Auth.)