Rinsing solution (RS) (28% ethanol, v/v): 280mL of ethanol was added into 720 mL water. This solution was mixed thoroughly and stored in a polyethylene bottle.
Eluent: 0.6 mL of sulfuric acid (G.R., Guangzhou Chemical Reagent Factory, Guangdong, China) was added into 5 mL of water and then the mixture was added into 95mL of 28% ethanol, mixed thoroughly and stored in a polyethylene bottle.