Fig. 4

Optimization of back-extraction process of IS, (a) salt concentration, (b) centrifuging time, (c) centrifuging speed (d) pH. Concentration of spiked IS: 20 µg.mL^− 1; other experimental conditions in each step were constant (for extraction process, DES/salt volume ratio = 1.25, salt concentration = 0.9 g.mL ^− 1, centrifuging time = 2, centrifuging speed = 8000 and for back-extraction process, salt concentration = 0.9 g.mL^− 1, centrifuging time = 10 min, centrifuging speed = 6000 and pH = 13)