LIMIT OF DETECTION OF N-15 BY GAS-CHROMATOGRAPHY ATOMIC-EMISSION DETECTION - OPTIMIZATION USING AN EXPERIMENTAL-DESIGN

This paper deals with the optimal conditions for the detection of N-15 determined using a four-factor experimental design from [2 C-13,-1,3 N-15] caffeine measured with an atomic emission detector (AED) coupled to gas chromatography (GC). Owing to the capability of a photodiodes array, AED can simultaneously detect several elements using their specific emission lines within a wavelength range of 50 nm. So, the emissions of N-15 and N-14 are simultaneously detected at 420.17 nm and 421.46 nm respectively. Four independent experimental factors were tested Helium flow rate (plasma gas) Methane pressure (reactant gas) Oxygen pressure Hydrogen pressure It has been shown that these four gases had a significant influence on the analytical response of N-15. The linearity of the detection was determined using N-15 amounts ranging from 1.52 pg to 19 ng under the optimal conditions obtained from the experimental design. The limit of detection was studied using different methods. The limits of detection of N-15 was 1.9 pg/s according to the IUPAC method (International-Union of Pure and Applied Chemistry). The method proposed by Quimby and Sullivan gave a value of 2.3 pg/s and that of Oppenheimer gave a limit of 29 pg/s. For each determination, an internal standard : 1-isobutyl-3.7 dimethylxanthine was used. The results clearly demonstrate that GC AED is sensitive and selective enough to detect and measure N-15-labelled molecules after gas chromatographic separation.