Through the study, 10% v/v acetic acid in water was used as BGE. The sheath-liquid was composed of isopropanol-water-acetic acid (50:50:1). To ensure a large metabolome coverage for comprehensive metabolomics profiling, the sheath-liquid was composed of acetic acid (5 mM) in an isopropanol-water (50:50) solution. Purine and HP-0921 were purchased from Agilent technologies (Santa Clara, CA, USA, P/N: G1969-8001) and used as lock masses after being spiked into the sheath-liquid to yield final concentrations of 50 and 25 nM, respectively.