## 3 Results and Discussion  
### Phase Diagram of PEG/(NH4)\({}_{2}\)SO\({}_{4}\)  
The PEG characteristics, including weight and concentration, are important factors in the properties of the phase-forming system [21]. Binodal curves were determined using the cloud point technique. A phase diagram of the PEG/(NH4)\({}_{2}\)SO\({}_{4}\) system was constructed (Figure 2) to select the appropriate ratio of phase composition. The different PEG molecular weights showed obvious changes in the phase diagram. In this system, the top phase was the denser PEG phase and the bottom phase was the (NH4)\({}_{2}\)SO\({}_{4}\)-rich aqueous phase. A curve delineated the two zones; the two-phase system could be obtained above the curve. However, there was no phase separation below the curve. Hence, the concentrations of PEG and (NH4)\({}_{2}\)SO\({}_{4}\) should be chosen appropriately to fall within the zone above the curve. It was found that the PEG6000\(-\)(NH4)\({}_{2}\)SO\({}_{4}\) ATPS provided the largest heterogeneous region, whereas PEG1500\(-\)(NH4)\({}_{2}\)SO\({}_{4}\) provided the smallest. According to Figure 2, a higher molecular weight of PEG yielded a higher two-phase region [22]. The formation of the ATPS was favored in the following order: 400 < 600 < 1500 < 4000 < 6000, and the capability of PEG for ATPS formation was directly proportional to the increase in the molecular weight. PEG with a higher molecular weight was more hydrophobic and its solubility in water decreased, leading to the polymer salting-out [23]. High-molecular-weight PEG favored the formation of the ATPS. However, the ATPS density also increased with the molecular weight of PEG, which made it difficult to separate the subsequent extraction [24]. In this work, a PEG molecular weight of 1500 was selected to carry out the ATPS extraction of _Ginkgo_ flavonoids.  
Effects of PEG1500 and (NH4)\({}_{2}\)SO\({}_{4}\) on Partition Coefficient and Recovery in ATPS  
The effects of PEG1500 and (NH4)\({}_{2}\)SO\({}_{4}\) on the phase-volume ratio and recovery of flavonoids in the ATPS were investigated. As shown in Figure 3a,b, the (NH4)\({}_{2}\)SO\({}_{4}\) concentration was constant at 15%; increasing the PEG1500 concentration resulted in a higher phase-volume ratio, and the flavonoids partitioned more to the PEG-rich top phase. A maximum yield of 92.4% was found at a PEG1500 concentration of 20%. Above this concentration, the extraction showed a slight change when the PEG concentration decreased from 20% to 35%. Figure 3c,d shows the effects of the (NH4)\({}_{2}\)SO\({}_{4}\) concentration on the phase-volume ratio and recovery of flavonoids in the ATPS (PEG1500 concentration was constant at 20%). An increase in the (NH4)\({}_{2}\)SO\({}_{4}\) concentration resulted in a decrease in the phase-volume ratio (Figure 3c), because the salt in the bottom phase could bind water molecules; thus, water would enter the bottom phase. A maximum recovery of 96% was found at an (NH4)\({}_{2}\)SO\({}_{4}\) concentration of 20%. With further increase in (NH4)\({}_{2}\)SO\({}_{4}\), the flavonoids partitioned more to the salt-rich bottom phase, resulting in a decrease in the recovery in the top phase. These results demonstrated the feasibility of  
Figure 2: Phase diagrams of different molecular weight PEG/(NH4)\({}_{2}\)SO\({}_{4}\) aqueous two-phase systems.  
achieving a high recovery using the ATPS and that the concentrations of (NH4)2 SO4 and PEG were directly related to the extraction yield of flavonoids in the two-phase system. The high recovery of flavonoids indicated that the PEG1500/(NH4)2 SO4 ATPS system was highly promising.