Abstract: A new method for the determination of cysteine by single-wavelength and dual-wavelength resonance light scattering spectroscopy with malachite green was established. The reaction conditions of the cysteine-malachite green system were optimized. The results showed that in borax-sodium hydroxide buffer solution, malachite green combined with boric acid to produce resonance light scattering, and two strong resonance light scattering peaks appeared at wavelengths of 285 and 338 nm; after adding cysteine, the resonance light scattering became more intense, and the intensity of resonance light scattering increased with the increase of cysteine concentration. The difference between the intensity of resonance light scattering at 285 and 338 nm displayed a linear relationship with cysteine concentration in the range of 0.10–0.60 mg/L, with limits of detection of 10.7 and 13.3 μg/L, respectively. The limit of detection of the dual-wavelength superposition method was 3.23 μg/L. When applying this method to detect cysteine in hepatoprotective agent and soy sauce, the relative standard deviations (n = 6) were within 3%, which meets the requirements of quantitative analysis.

Key words: cysteine; malachite green; supramolecular; resonance light scattering spectrometry; hepatoprotective cysteine capsules; soy sauce