Ultrapure water with a specific resistance of 18.2 MΩ/cm—produced using an Arium 611 Ultrapure Water System (Sartorius Co., Goettingen, Germany)—was used in the experiments. Methanol (high-performance liquid chromatography [HPLC] grade), formic acid, 1 mol/L hydrochloric acid, and choline chloride were purchased from Nacalai Tesque, Inc. (Kyoto, Japan). Sodium dihydrogen phosphate and disodium hydrogen phosphate were purchased from Fujifilm Wako Pure Chemical Industries, Ltd. (Osaka, Japan). ACh chloride was purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). 2-Aminoethyl-trimethylammonium pivaloylamide (EN) was synthesized in our laboratory [10 (link)].
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