A rapid and efficient method of microwave-assisted extraction and hydrolysis and automatic amino acid analyzer determination of 17 amino acids from mulberry leaves

The microwave-assisted extraction and hydrolysis (MAEHM) and automatic amino acid analyzer (AAA) determination method (MAEHM-AAA) were developed and applied to extract and determine amino acids from mulberry leaves. The optimal extraction conditions were an extraction time of 10 min, an extraction temperature of 110 °C, a solid-to-liquid ratio of 1/50 and a microwave power of 400 W. The preponderance of MAEHM was demonstrated by comparison with the conventional heating acid hydrolysis method (CHAHM) using a first-order kinetics equation. The equilibrium concentrations (C∞) of the target compounds obtained by MAEHM were similar with those obtained by CHAHM. In addition, MAEHM had higher extraction reaction rate constants (k) than CHAHM, which suggests that the target compounds extracted by MAEHM could be much more effectively transferred from the plant matrix into the solvent than those by CHAHM. Compared with the preprocessing time of CHAHM (22 h), the preprocessing time of MAEHM was reduced to 15 min, which substantially reduced detection time. MAEHM-AAA provides a linear response over the concentration range of 0.1–500 nmol/mL with correlation coefficients above 0.99 in all cases. The low limits of detection (LODs) of this method were 0.025–3.75 nmol/mL, which shows its great sensitivity. In addition, the MAEHM was validated in terms of accuracy (the intra- and inter-day relative standard deviations were less than 6.62%), precision (80–120%) and stability (recovery was 90.38–121.66%). The MAEMH-AAA method was also successfully applied to quantify 17 target compounds in mulberry leaves.