For the MS analysis, a time-of-flight (TOF) MS scan was performed. The mass spectrometer was set to scan a mass range from 300 to 1200 Da in both positive and negative electrospray (ESI) ionization modes. To ensure accurate mass measurement, 0.1 mg/L leucine-enkephalin in water:MeOH:formic acid (50:50:0.1, v/v/v) was used as a lock-mass calibrant with the infusion flow rate of 10 μL/min. The mass spectrometer was operated with the following parameters: the capillary voltage was set at 2.50 kV in both positive and negative mode of ionization; the sampling cone voltage was set to 30 V and the source offset voltage was 100 V.