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

Abstract

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.

Gas Chromatography–Atmospheric Pressure Inlet–Mass Spectrometer Utilizing Plasma-Based Soft Ionization for the Analysis of Saturated, Aliphatic Hydrocarbons

“…The nature of the species responsible for hydride abstraction or protonation and subsequent hydrogen elimination in the first step is still unclear; therefore, only the following steps are presented in detail. The existence of several reactive species like H 3 O + , ozone, O 2 + , N 2 + , or nitrogen oxides, some of them capable of hydride abstraction and dehydrogenation, is known for nitrogen DBDI and related ion sources. ,,, The theory of hydride abstraction is further supported by [M–H] + and [M–3H] + signals, which are observable for small alkanes. The addition of water and subsequent dehydrogenation with the loss of one hydrogen from the alkane and one hydrogen from water is proposed as the mechanism explaining the resulting pattern of the isotope labeling experiments.…”

Comprehensive Composition, Structure, and Size Characterization for Thiophene Compounds in Petroleum Using Ultrahigh-Resolution Mass Spectrometry and Trapped Ion Mobility Spectrometry

“…The ionization mechanism of APCI for other compounds has been studied previously. Referring to the ionization process reported in other literature, − we propose the ionization mechanisms for thiophene compounds in this work (shown in the Supporting Information). [M] +• and [M + H] + could be generated through charge exchange and proton transfer, respectively.…”

Formation of Formaldehyde and Other Byproducts by TiO2 Photocatalyst Materials

“…The proton affinity of formaldehyde (712.9 kJ/mol) is only 22 kJ/mol higher than the proton affinity of water. Therefore, the formaldehyde signal depends on the humidity [72]), in contrast to most other species detected with PTRMS [73][74][75][76]. A formaldehyde calibration based on two parameters (the formaldehyde sensitivity and the humidity) was performed, arriving at the values we have reported.…”