Fragmentation of Saturated Hydrocarbons upon Atmospheric Pressure Chemical Ionization Is Caused by Proton-Transfer Reactions.

Chemical characterization of complex mixtures of large saturated hydrocarbons is critically important for numerous fields, including petroleomics and renewable transportation fuels, but difficult to achieve. Atmospheric pressure chemical ionization (APCI) mass spectrometry has shown some promise in the analysis of saturated hydrocarbons. However, APCI causes extensive fragmentation to these compounds, which impedes its effectiveness. To prevent this fragmentation, its causes were examined via gas-phase ion-molecule reactions in vacuum in a linear quadrupole ion trap mass spectrometer. The results demonstrate that the mechanism proposed previously for ionization of saturated hydrocarbons upon APCI, hydride abstraction by carbocation reagent ions, is not correct. Instead, the fragmentation is caused by ionization of saturated hydrocarbons via exothermic proton-transfer reactions involving highly acidic, protonated atmospheric molecules, such as nitrogen and water. Accordingly, the extent of fragmentation was found to correlate with the proton affinities of the atmospheric molecules studied. Remarkably, controlled experiments involving isolated atmospheric ions and neat saturated hydrocarbons in vacuum yielded almost identical mass spectra as APCI involving atmospheric pressure conditions, the presence of many different chemicals, and an electrical discharge. In order to prevent or reduce the extent of fragmentation of saturated hydrocarbons upon APCI, and therefore enable accurate mass spectrometric characterization of complex mixtures of saturated hydrocarbons, the ion source should be purged of air to remove nitrogen and water and fill it with an inert gas with a substantially lower proton affinity.