Dual-frequency ultrasound assisted-enzyme digestion coupled with atomic fluorescence spectrometry as a green and efficient tool for cadmium detection in rice flour samples

A sample treatment technique based on a duel frequency ultrasonic device for enzymatic digestion of rice is reported. The ultrasonic device combines a high intensity ultrasonic probe and the temperature control function of ultrasonic water bath, which can effectively extract cadmium from rice within only 160 s under the optimized conditions. Compared with the traditional ultrasonic assisted enzymatic digestion, the new method not only shortens the time significantly (e.g., from the 90 min of ultrasonic water bath to the present few minutes), but also increases the extraction efficiency of cadmium (such as similar to 75% from ultrasonic probe to app. 100%). Through the optimization of ultrasonic mode, ultrasonic frequency, power and the type of enzyme, we found that enzyme played a dominant role in ultrasound assisted enzymatic digestion. Compared with a-amylase and pepsin, trypsin is more suitable for the extraction of cadmium from rice. Furthermore, ultrasound energy is beneficial to enzymatic hydrolysis of bimolecular, and this promotion is related to the frequency of ultrasound. The reliability of this method was evaluated by analyzing the content of cadmium in the certified reference materials (CRMs, GBW10045, GBW08510, GBW08511 and GBW08512) based on atomic fluorescence spectrometry combined with a modified chemical vapor generation. The proposed method has been applied satisfactorily in the determination of Cd in several rice samples.