2.3.4 Molecular weight analysis
The molecular weight of the GP was determined by a gel-permeation chromatography (GPC) system (ELEOS System, Wyatt Technology, USA) equipped with a Waters 515 pump and DRI detector. The columns were a Shodex OHpak SB-806 in sequence with 803 columns, which were eluted with 0.02% NaNO3 at a flow rate of one mL/min. The temperatures of the column and RI detector were maintained at 40°C. Dextran standards (Mw = 180, 2,700, 9,750, 36,800, and 135,350 Da) were used to establish a standard curve. Empower software of GPC (ASTRA5.3.4, Wyatt Technology, USA) was used for data processing (Zhang, Li, Zhang, & Li, 2019).
2. 3.5 Scanning electron microscopy (SEM) analysis of morphological properties
The micro-structures and surface morphologies of GP were characterized by scanning electron microscopy (SEM). A proper amount of polysaccharide sample powder was glued to the conductive adhesive of the experimental bench. The floating sample was blown off with a rubber suction bulb and placed in a gold conductive layer of 10 nm thickness in a vacuum. The accelerating voltage of the electron gun was 20 KV, and the sample was observed by the SEM.