As shown in Fig. 2, the ultrasound extraction time was investigated from 1 min to 20 min. The results indicated that in the range of 1–10 min the effect of ultrasound was very significant to improve the nastic movement extraction efficacies dramatically. When the time was longer than 10 min, the absorbance signals obviously decreased compared to the trend of UA-CPE. Excessive ultrasound may cause the dissolution of the extractant thus decreasing the mass transfer efficiency.