Determination of 21 Free Amino Acids in Fruit Juices by HPLC Using a Modification of the 6-Aminoquinolyl-N-hydroxysuccinimidyl Carbamate (AQC) Method

The present paper reports the application of an optimized pre-column derivatization procedure with aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) to the determination of 21 free amino acids in fruit juices. The method permitted the optimal separation of 21 amino acids and ammonium in 30 min. Excellent response linearity was obtained from 2 to 1,000 μM for all amino acids, except tryptophan, for which the linearity was 5–1,000 μM. The fluorescence detection limits ranged from 0.08 pmol (phenylalanine) to 0.69 pmol (cysteine), except for tryptophan (7.89 pmol), for a signal-to-noise ratio of 3. The relative standard deviations of peak areas ranged from 0.1 to 0.4 % for intraday analysis and from 2.1 to 5.0 % for interday analysis. The method was applied to analyze the free amino acids in six fruit juices, and the recoveries of the proposed method were 90.4 to 103.3 %. The total free amino acid content ranged from 56.97 mg L−1 (apple) to 469.45 mg L−1 (longan) in six fruit juices. Aspartic acid (Asp), glutamic acid (Glu), asparagine (Asn), serine (Ser), and glutamine (Gln) were present in higher levels in most samples, whereas cysteine (Cys) was below the detection limit in the selected samples. Notable advantages of this method include its speed, ease, low cost, good repeatability, high sensitivity, excellent precision and accuracy, and anti-interference in high-sugar matrices.