Potential of GC-Combustion-MS as a Powerful and Versatile Nitrogen-Selective Detector in Gas Chromatography

Here, we show the potential and applicability of the novel GC-combustion-MS approach as a nitrogen-selective GC detector. Operating requirements to achieve reproducible and compound-independent formation of volatile NO species as a selective N-signal during the combustion step are described. Specifically, high temperatures (≥1000 °C) and post-column O₂ flows (0.4 mL min^-1 of 0.3% O₂ in He) turned out to be necessary when using a vertical oven without makeup flow (prototype #1). In contrast, the use of a horizontal oven with 1.7 mL min^-1 He as an additional makeup flow (prototype #2) required milder conditions (850 °C and 0.2 mL min^-1). A detection limit of 0.02 pg of N injected was achieved, which is by far the lowest ever reported for any GC detector. Equimolarity, linearity, and peak shape were also adequate. Validation of the approach was performed by the analysis of a certified reference material obtaining accurate (2% error) and precise (2% RSD) results. Robustness was tested with the analysis of two complex samples with different matrices (diesel and biomass pyrolysis oil) and N concentration levels. Total N determined after the integration of the whole chromatograms (524 ± 22 and 11,140 ± 330 μg N g^-1, respectively) was in good agreement with the reference values (497 ± 10 and 11,000 ± 1200 μg N g^-1, respectively). In contrast, GC-NCD results were lower for the diesel sample (394 ± 42 μg N g^-1). Quantitative values for the individual and families of N species identified in the real samples by parallel GC-MS and additional GC × GC-MS analyses were also obtained using a single generic internal standard.