An optimal isocratic separation condition was achieved with a mobile phase of 10 mmol/L ammonium acetate buffer (pH5.5)- acetonitrile (80/20, v/v) and the flow rate of 0.8 mL/min, based on the separation degree, peak shape, column pressure and analysis efficiency altogether. As shown in Fig. 2A, the PMP-XOS with different DP was well separated in 40 min, and the resolution of each two peaks was above 1.5. To improve the separation efficiency, a gradient condition was also achieved as follows: 0–12 min, isocratic 20% A; 12–20 min, linear gradient elution, 20%–30% A; 20–25 min, isocratic 30% A. The separation of