Study on the interaction between ligustrazine and 12-tungstophosphoric acid using resonance Rayleigh scattering and resonance nonlinear scattering spectra, and its analytical applications

In an HCl medium (pH 1.5), ligustrazine (2,3,5,6-tetramethylpyrazine, TMP) reacted with 12-tungstophosphoric acid (TP) to form a 3 : 1 ion-association complex. As a result, the intensities of resonance Rayleigh scattering (RRS), second-order scattering (SOS) and frequency doubling scattering (FDS) were greatly enhanced and new scattering spectra appeared. The maximum RRS, SOS and FDS wavelengths of the ion-association complexes were located at 379, 738 and 395 nm, respectively. The scattering intensity increments (ΔIRRS , ΔISOS and ΔIFDS ) were directly proportional to the concentration of ligustrazine within certain ranges. The detection limits (3σ) of RRS, SOS and FDS were 1.6, 3.2 and 2.8 ng/mL. Optimal conditions for the RRS method and factors influencing the method were discussed, and the structure of the ion-association complex and the reaction mechanism were investigated. Transmission electron microscopy (TEM) was used to characterize the structures of the ion-association complex. Based on the ion-association reaction and its spectral response, a rapid, simple and sensitive RRS method for the determination of TMP was developed. It was applied to the determination of TMP in tablet and urine samples with satisfactory results.