Fuel, Vol.225, 632-645, 2018
Evaluating the effect of ion source gas (N-2, He, and synthetic air) on the ionization of hydrocarbon, condensed aromatic standards, and paraffin fractions by APCI(+)FT-ICR MS
This study aimed to use different types of ion source gases [synthetic air, nitrogen (N-2), and helium (He)] to compare the ionization efficiency of linear, branched, and cyclic hydrocarbon (HC) standards (i.e., hexatriacontane, squalene, and 5-alpha-cholestane, respectively), condensed aromatics (CA) standards (i.e., coronene, benz[a]anthracene, and n,n'-bis(3-pentyl)perylene-3,4,9,10-bis(dicarboximide)), one n-paraffin standard, containing carbon numbers ranging from C-5 to C-120, and two saturated HC fractions. In all cases, a positive-ion mode atmospheric pressure chemical ionization coupled to a Fourier transform ion cyclotron resonance mass spectrometer was used. Isooctane reagent was used to facilitate the ionization of n-paraffins. Three ionization mechanisms were observed: electron transfer ([M](rad+)), proton transfer ([M + H+]), and hydride abstraction ([M- H](+)). For the ionization of HC standards, synthetic air and He gases presented better ionization efficiency and produced mass spectra with greater mass accuracy and signal-to-noise rate. Moreover, linear HCs were preferentially ionized through hydride abstraction (production of [M- H](+) ions), whereas the ionization of unsaturated and cyclic HCs mostly occurred through the production of [M +H](+) and [Mrad+] species. The unique exception is related to 5-alpha-cholestane, which is ionized as [M- H](+) in the presence of synthetic air. For the CA standards, N-2 and synthetic air promoted the detection of CA standards mainly by electron transfer mechanism, [M](rad+) species. Conversely, He favored the proton transfer ionization ([M + H](+)) with minimal fragmentation or oxidation of the analyte. In all cases, synthetic air provided mass spectra with excellent signalto-noise ratio. This performance was attributed to the high reactive-ionizing power of O-2 gas over the HC and CA molecules. For the n -paraffin samples, synthetic air and He provided better ionization performance through hydride abstraction ([M - H](+)). N-2 favored the production and ionization of heteroatomic compound classes (O-x and NO).