Nitrogen carrier gas enhancement in GC-MS via ethylene dopant improves sensitivity and preserves EI-like spectra

Helium constraints motivate renewed use of nitrogen in GC-MS. We show that adding trace ethylene (about 9%) to nitrogen restores sensitivity by up to ~20-fold while preserving canonical 70 eV electron-ionization (EI) library matches for phthalates and polycyclic aromatic hydrocarbons. The gain appears only under collision-dominated operation, characterized by a low Knudsen number (Kn ≤ 0.1), and diminishes or reverses in molecular-flow conditions (Kn > 10), providing operational evidence that collisions are essential. A collision-assisted lifetime hypothesis is consistent with the data and a phenomenological model; direct spectroscopic identification of intermediates and lifetimes remains a limitation. Cross-instrument checks confirm reproducibility, and chromatographic trade-offs intrinsic to nitrogen are unchanged. We frame the EI-compatible gain as an operational metric rather than a mechanistic claim. Importantly, this is EI-not chemical ionization (CI): all data were acquired at 70 eV under N₂ plus ethylene, and despite large enhancement the spectra remain EI-like, i.e., no softening.