Optimization of (15)n detection with an atomic emission detector

Gas chromatography with atomic emission detection is a useful tool for the detection of stable isotope labels in complex samples. While papers involving the analysis of D and (13)C are numerous, little work has been done in the area of (15)N detection. For (15)N isotope detection, three reagent gases are used:  H(2), O(2), and CH(4). In this work, the reagent gas flows were varied to optimize the sensitivity of (15)N detection without sacrificing isotope selectivity. The optimal gas flows determined in this work produce the following ratios of the spectral peak areas:  O 725 area/He 728 area = 0.039 with only O(2) flowing; H 486 area/He 492 area = 12 with only H(2) flowing; C 496 area/He 502 area = 0.41 with O(2), H(2), and CH(4) flowing for C and no gases flowing for He. When using these gas settings, the (15)N sensitivity is increased by nearly 2 orders of magnitude relative to the manufacturer-recommended settings. It was also demonstrated that the presence of a compound in both the labeled and unlabeled forms in the same sample does not affect the response. The ratios of (15)N to (14)N in standards, calculated from calibration plots (which are linear for both isotopes), agree well with the actual values. A tobacco smoke sample containing various (15)N-labeled compounds was used to show the utility of the GC-AED for indicating which compounds in a complex sample contain the label. This sample also demonstrates the necessity for optimal sensitivity when dealing with samples containing small amounts of compounds with low incorporation levels.