6 aminoquinolyl n hydroxysuccinimidyl carbamate aqc

The pH was measured by using a Sartorius pH indicator. Lactic acid, acetic acid, and free amino acids were determined by using ACQUITY UPLC M-Class System with BEH C18 Column (2.1 × 50 mm × 1.7 μm) and PDA detector (Waters Corporation, Milford, MA, USA). The supernatants of Suanzhou samples were subjected to filtration by the syringe filter (0.2 μm pore size). The filtrate was directly used for the lactic acid and acetic acid analysis. The mobile phase used was prepared by mixing 0.01 mol/l KH₂PO₄ (pH 3.0) and CH₃CN in a ratio of 98:2. Free amino acids in the supernatant samples were determined according to the manual from Waters and the method described previously (Fiechter et al., 2011 (link)). The reagent 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC, Waters) was used to derivatize amino acids. Amino acids AAS18 and A9906 (Sigma-Aldrich) were used as analytical standards. All experiments were repeated three times. The values for pH, organic acids, and free amino acids were subjected to one-way analysis of variance (ANOVA) by Tukey's method of the statistical software Statistica 7.0.

All chemicals and reagents used in the study were of analytical or HPLC grade with the highest purity. Ortho-phthalaldehyde (OPA) was obtained from Millipore Sigma, St Louis, MO, USA. Amino acid derivatization reagent, 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC), part of the AccQ•Tag^TM Ultra Derivatization Kit (P/N 186003836), was obtained from Waters Corp., Milford, MA, USA.