Results and Discussion

An external electron ionization (EI) source has been interfaced to a 7 T Fourier transform ion cyclotron resonance mass spectrometer and tested for volatile complex mixture analysis. A new Sulfinert ® deactivated inlet system provides continuous stable sample flow to the EI source, leading to stable ion signal ( ± 10 % deviation) for 2 h from a 200 nL mixture of 15 n-alkylbenzenes. Ultrahigh mass resolving power, m / m50 % ≈ 735,000, was obtained for 1-bromo-2-chlorobenzene with accurate isotopic ratio measurement. Baseline resolution was observed (see Figure) for two of the closest commonly encountered mass doublets, C3/SH4 (m2- m1 = 3.4 mDa at m/z 190) and SH313C/C4 (m2- m1 = 1.1 mDa at m/z 190), at mass resolving power, m/Δm50 % ≈ 400,000, by low-energy (18 eV) electron ionization of unprocessed diesel fuel from a 200 nL septum injection. Although hydrocarbons dominate the positive-ion 18 eV EI mass spectrum from diesel fuel, many sulfur-, nitrogen-, and oxygen-containing compounds were readily observed without prior fractionation. By comparing 18 eV EI FT-ICR mass spectra of unprocessed and processed diesel fuels, we were able to identify which heteroatom-containing specie