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Kinetic and Thermodynamic Control of Protonation in Atmospheric Pressure Chemical Ionization

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Journal of The American Society for Mass Spectrometry

Abstract

For p-(dimethylamino)chalcone (p-DMAC), the N atom is the most basic site in the liquid phase, whereas the O atom possesses the highest proton affinity in the gas phase. A novel and interesting observation is reported that the N- and O-protonated p-DMAC can be competitively produced in atmospheric pressure chemical ionization (APCI) with the change of solvents and ionization conditions. In neat methanol or acetonitrile, the protonation is always under thermodynamic control to form the O-protonated ion. When methanol/water or acetonitrile/water was used as the solvent, the protonation is kinetically controlled to form the N-protonated ion under conditions of relatively high infusion rate and high concentration of water in the mixed solvent. The regioselectivity of protonation of p-DMAC in APCI is probably attributed to the bulky solvent cluster reagent ions (SnH+) and the analyte having different preferred protonation sites in the liquid phase and gas phase.

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Acknowledgments

The authors thank Dr. Y.-P. Tu, GlaxoSmithKline, for many discussions and suggestions. This work was supported by the National Science Foundation of China (no. 21025207).

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  1. Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang, China

    Yunfeng Chai, Nan Hu & Yuanjiang Pan

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  1. Yunfeng Chai
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  2. Nan Hu
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Correspondence to Yuanjiang Pan.

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Chai, Y., Hu, N. & Pan, Y. Kinetic and Thermodynamic Control of Protonation in Atmospheric Pressure Chemical Ionization. J. Am. Soc. Mass Spectrom. 24, 1097–1101 (2013). https://doi.org/10.1007/s13361-013-0626-9

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