Rapid stacking of amino acids in soybean and Dendrobium officinale by on-capillary sandwich derivatization in capillary electrophoresis

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An online sandwich derivatization and stacking strategy using capillary electrophoresis was developed and successfully applied to the preconcentration of multi-amino acids in two functional food samples. Amino acids were derived with 4-Chloro-7-nitro-1, 2, 3-benzoxadiazole (NBD-Cl) to make the new compounds with chromophore in the stacking process, then the derivatives were stacked by on-column sample preconcentration and detected at 475 nm UV wavelength. The novel sandwich injection sequence was amino acids, NBD-Cl, and amino acids separately. Additionally, the running buffer was 40 mM borax buffer (pH = 9.0). A succession of derivatization and stacking conditions were optimized, including buffer concentration, pressure, NBD-Cl injection time, waiting time, matrix concentration, and sample injection time. In the appropriate range, good linearity values (R2) were obtained for nine amino acids at 0.996–0.999. Intra-day and inter-day precision with a relative standard deviation < 6.23 % (n = 5), the limit of detection in the range of 2.8–25.2 μM, and recovery ranging from 83.2 to 108.2 % were obtained. Besides, the enrichment factors were in range of 8–62 for nine AAs. The stacking approach was successfully applied to soybean and Dendrobium officinale samples, which shows great potential in the determination of free amino acids in samples containing complex matrices.