Density Functional Theory Simulation and Surface-Enhanced Raman Spectrometric Determination of Codeine and Falcodine

Density functional theory (DFT) was used to optimize the geometric configurations of codeine and falcodine, so as to identify and assign the vibration modes of the Raman spectra of the standard powder, and Raman spectra of standard powder was compared with the surface-enhanced Raman spectra of standard solution at low concentration levels. The detection conditions of surface-enhanced Raman spectroscopy were further optimized, and the lower limits of determination and the feasibility of quantitative analysis of codeine and falcodine were discussed. As shown by the results, the theoretical values and detection values by Raman spectra and surface-enhanced Raman spectra were basically consistent for Raman shifts of most characteristic peaks of codeine and fordrine, while there would be a certain degree of blue shift and red shift. Lower limits of determination of codeine and fordrine were both 10 mg·L^-1. Linear relationships between mass concentration and characteristic peak intensity ratio (codeine at 631.29 cm^-1 and 1 595.26 cm^-1 and fordrine at 628.58 cm^-1 and 1 251.41 cm^-1) were kept in the same range of 40-100 mg·L^-1. Test for recovery was made on blank matrix by standard addition method, giving results in the range of 99.0%-105% for codeine and 102%-104% for fordrine, with RSDs (n=5) of the determined values of 5.3% and 5.9%, respectively. The above methods could provide theoretical basis and rapid detection support for the two controlled drugs.