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The methodology of the amino acid lysine (Lys) analysis using ninhydrin (Nin) reagent by spectrophotometric method was in-depth studied and improved. The optimization of time, Lys to Nin volume ratio and temperature were explicitly investigated. A specific wavelength of 479 nm was set according to the peak of the absorbance spectrum. The reaction time between Lys and Nin, in aqueous solution, occurs during 50 min. The Lys/Nin ratio 1.67 appeared most effective for this reaction. Moreover, the absorbance increased with temperature until a stable value was achieved nearly at 85°C. It was also found that the reaction is more efficient at initial pH 6 corresponding to formation of high amount of coloured product. Furthermore, the pH of post-reaction mixture was around 4 for different concentrations of lysine and initial pH range 3–10. For lysine solution contaminated with potassium (a representative interfering species) and after variation of the reaction conditions, the maximum wavelength (479 nm) was not affected. Mastering the use of these parameters leads to a good usage of this analytical technique which is simple, fast, accurate, less expensive and especially less harmful towards the environment and human health.

A New Approach on the Amino Acid Lysine Quantification by UV-Visible Spectrophotometry