Mobilities of amino acid adducts with modifiers in the buffer gas of an ion mobility spectrometer depended on modifier size and modifier–amino acid interaction energy

• Buffer gas modifiers separate overlapping analytes in ion mobility spectrometry.
• Separation relies on the formation of large modifier–analyte adducts of different mobilities.
• Modifier size and modifier–analyte interaction energy explained separation.

Buffer gas modifiers have been used to separate overlapping analytes in ion mobility spectrometry (IMS); separation relies on the formation of large and slow modifier–analyte adducts with different mobilities; however, it is unknown the cause of separation and predictions about a given separation cannot be made. Therefore, we vaporized phenylethanol modifier (P) into the buffer gas of an ion mobility spectrometer coupled to a quadrupole mass spectrometer to explain the selective effect of this modifier on the mobilities of asparagine, methionine, and phenylalanine amino acids; amino acid mobilities decreased selectively due to formation of slow phenylethanol:amino acid ion adducts. Mobility reductions were asparagine (−19.4%), methionine (−19.5%), and phenylalanine (−20.8%). Then, we compared phenylalanine and methionine mobility reductions when 2-butanol (B), methyl 2-chloropropionate (M), and α-(trifluoromethyl)benzyl alcohol (F) modifiers were introduced in the buffer gas; mobility reductions were M > P > F > B for both amino acids. Parameters such as modifier size, modifier–ion interaction energies, modifier proton affinities, steric and inductive effects, and intramolecular hydrogen bond strength explained modifier effect on mobility reduction. High modifier–ion interaction energies increase adduct average lifetimes and large modifiers produce adducts with large collision cross sections and explain mobility differences between adducts. The other parameters are taken into account when calculating modifier–ion interaction energies.

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