The nitrogen isotopic composition of individual amino acids in sunflower leaves after exposures to ¹⁵NO₂ in the range of ambient NO₂ concentrations (5–37 ppb) was analysed by Gas Chromatography-Combustion-Isotope Ratio Mass Spectrometry (GC-C-IRMS). Amino acids as well as the amides glutamine and asparagine were converted with MTBSTFA (N-methyl-N-(tert.-butyldimethylsilyl)-tri-fluoroacetamid) in pyridine to their corresponding TBDMS derivatives (N, O-tert.-butyldimethylsilyl) in a simple one-step silylation reaction. The derivatized amino acids were separated by gas chromatography, combusted on-line, and the products were sent continuously to an isotope ratio mass spectrometer. Accurate measurements were obtained, when more than 7 nmol N₂ were introduced into the ion source of the mass spectrometer per gas chromatographically separated and combusted compound. No interferences of the silicate and fluor containing derivatization agents on the performance of the system were observed.

In the range of ambient NO₂ concentrations sunflower leaves predominately incorporate the nitrogen derived from atmospheric NO₂ into soluble amino acids. The highest δ¹⁵N values were measured for alanine. The ¹⁵N enrichments of the detectable amino acids increased with increasing ¹⁵NO₂ concentration.