For accurate mass calibration of the QqTOF instrument, a sodium formate solution (prepared from a mixture of 100 μL of 0.1 M sodium hydroxide solution, 200 μL of an aqueous solution of 10 % of formic acid and 20 mL of acetonitrile/water (80:20, v/v)) and a leucine–encephalin solution at 500 pg μL−1 (prepared from 500 μL of 1 mg mL−1 leucine–enkephalin solution in water with 1,000 mL of acetonitrile/water 50:50, v/v and 0.1 % formicacid) were used as calibration and lock-mass solution, respectively. All reagents were purchased from Sigma-Aldrich.