Optimization of 15N 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 C-13 are numerous, little work has been done in the area of N-15 detection. For N-15 isotope detection, three reagent gases are used: H-2, O-2, and CH4, In this work, the reagent gas flows were varied to optimize the sensitivity of N-15 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 CH4 flowing for C and no gases flowing for He. When using these gas settings, the N-15 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 N-15 to N-14 in standards, calculated from calibration plots (which are linear for both isotopes), agree well with the actual values. A tobacco smoke sample containing various N-15-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.