A simple, sensitive and selective electrochemical method was developed for the simultaneous determination of morphine and codeine using a carbon paste electrode (CPE) modified with a Zn 2 SnO 4 –graphene nano- composite by differential-pulse voltammetry (DPV). The Zn 2 SnO 4 –graphene nanocomposite was prepared via a facile and simple process. The resulting composite was characterized by scanning electron microscopy, X-ray diffraction, thermogravimetric analysis and electrochemical impedance spectroscopy. The prepared nanocomposite modified CPE showed enhanced peak currents for the oxidation of morphine and codeine as compared to those of graphene/CPE, Zn 2 SnO 4 /CPE and the bare CPE. The analytical curves for the simultaneous determination of morphine and codeine showed an excellent linear response, ranging from 0.020–15 mmol L 1 for both of them. The detection limits for determining morphine and codeine were 0.011 and 0.0090 mmol L 1 , respectively. No significant interference was seen for some interfering compounds such as Ca 2+ , glucose, lactose, sucrose, ascorbic acid, acetaminophen, ethanol, noscapine and others in the detection of analytes. The proposed method was successfully applied for the simulta- neous and individual determination of these analytes in human body fluids and pharmaceutical samples.
