Functionalization of saturated hydrocarbons using nitrogen ion insertion reactions in mass spectrometry
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© 2016 Elsevier B.V. Ionization using field-assisted nitrogen insertion facilitates mass spectrometric characterization of saturated hydrocarbons. The technique integrates in-situ derivatization of aliphatic hydrocarbons on a dry substrate with desorption and ionization of these condensed phase samples. Nitrogen reagent ions, including N3+ generated from a micro-discharge initiated in a nitrogen atmosphere (or in lab air), react with aliphatic hydrocarbons through atomic nitrogen ion incorporation to generate iminium ions. The method is efficient in generating molecular ions of saturated hydrocarbons, particularly those of higher molecular weight. This field-assisted nitrogenation method has been optimized by systematic variation of reagent gas flow rate, tube lens voltage, and sample placement. Independent optimization of analyte ionization allows coupling to any mass analyzer with an atmospheric pressure interface. Field-assisted nitrogen ion insertion is shown by mass analysis using a linear quadrupole ion trap or using an Orbitrap to generate [M+N]+ ions predominantly. This allows the identification of the elemental compositions of alkane constituents of complex petroleum-based products. In the presence of an oxidizing agent such as benzoquinone, dehydrogenation accompanies nitrogen ion insertion under atmospheric pressure and low temperature with minimal C[sbnd]C bond rupture. This study describes the optimization of this experiment, its application to branched alkanes, the relationship to paper spray ionization and the reaction mechanism including accompanying redox reactions. Data are shown for n-alkanes, branched alkyls and aryl substituted alkanes all of which are successfully ionized. Results are also given for a variety of other functional groups; these do not give N+-addition unless long alkyl chains form part of the molecular structure.
author list (cited authors)
Li, X., Yan, X., & Cooks, R. G.