Field-amplified sample injection and graphene quantum dot dual preconcentration in the analysis of melamine and dicyandiamide by capillary electrophoresis

Sulfur-doped graphene quantum dots (S-GQDs) were prepared by the pyrolysis of citric acid and mercaptopropionic acid. Compared with graphene quantum dots (GQDs), the S-GQDs have improved surface state and chemical reactivity, and thus, exhibited stronger interaction with cations. Based on its excellent affinity for cations, a dual preconcentration technique combining field-amplified sample injection (FASI) and S-GQDs as multianalyte carriers was developed for the determination of melamine and dicyandiamide by capillary electrophoresis (CE). During the FASI process, a large quantity of analytes was introduced into the capillary and accumulated at the capillary inlet. Concurrently, the S-GQDs migrated to the anode and captured the analytes on its surface at the boundary of the sample and buffer solution. The use of S-GQDs allows the capture of abundant analytes, which can amplify the detection signal. This new protocol was evaluated by the quantitative determination of melamine and dicyandiamide in metformin hydrochloride preparations. The effect of volume fraction of the S-GQDs in the buffer solution, the composition and pH of the buffer, and the sample injection time on the preconcentration and separation were investigated. By controlling the pH at 4.6, the sample injection time was prolonged to 450 s. A very large amount of melamine and dicyandiamide, bearing positive electric charges, were injected into the capillary and were captured by S-GQDs. The assay using FASI preconcentration and S-GQDs as enhancer resulted in a 1.6×105-fold improved sensitivity compared with that obtained with traditional 10-kV electrokinetic injection for 10 s. The calibration curves of melamine and dicyandiamide were obtained in the concentration range from 1.0×10-14 to 1.0×10-8mol/L, with correlation coefficients (r2) >0.999. The detection limits (S/N=3) were 2.6×10-15mol/L for melamine and 5.7×10-15mol/L for dicyandiamide. The recoveries of the two analytes were 95.9%-102.4% and 92.0%-106.0%, respectively, with relative standard deviations (RSDs) of no more than 5%. The RSD values of peak height, peak area, and migration time were all less than 5.6%. This method is reliable, easy, and exhibits a good separation effect. This proves that the S-GQD-enhanced CE method could be developed into a new and sensitive technique for the determination of melamine and dicyandiamide in different preparations of metformin hydrochloride.