Fig. 3

Optimization of extraction process of IS. (a) DES/salt volume ratio, (b) salt concentration, (c) centrifuging time, (d) centrifuging speed. Concentration of spiked IS: 20 µg.mL^− 1; other experimental conditions in each step were constant (DES/salt volume ratio = 1.25, salt concentration = 0.9 g.mL^− 1, centrifuging time = 10 min, centrifuging speed = 6000, and for back-extraction process, salt concentration = 0.9 g.mL^− 1, centrifuging time = 10 min, centrifuging speed = 6000 rpm and pH = 13)