On-line analysis of nitrogen stable isotopes in NO from ambient air samples

A method was developed for the on-line analysis of nitrogen stable isotopes at the natural abundance level in NO in order to study the NO contribution to the nitrogen cycle in ecosystems and in the atmosphere. The method enab les a quick and accurate determination of N-15/N-14 ratios for NO and consi sts of the following steps: (a) accumulation of NO from air samples on a mo lecular sieve of 5 Angstrom, (b) desorption of NO from the molecular sieve during 15 min of heating at 350 degreesC tan offset of Delta delta 3.6 part s per thousand must be corrected for), (c) trapping and cryofocusation of t he desorbed NO on a PoraPlot Q matrix at -196 degreesC during heating, (d) release of the trapped NO from the PoraPlot Q matrix followed by chromatogr aphic separation, reduction to Nt!, and isotopic composition analysis. A mi nimum sample size of 125 nmol of NO is recommended. A correction function f or the calculation of the delta N-15-NO values was introduced for sample si zes from 125 to 220 nmol of NO. Measurements of NO in automobile exhaust ha ve proven the applicability of the developed method.