Analysis was performed using a Shimadzu Nexera X2 LC-30AD HPLC system (Shimadzu, Tokyo, Japan) composed of a quaternary pump, an on-line degasser, a column temperature controller, the SIL-30AC autosampler (Shimadzu, Tokyo, Japan); the CTO-20AC thermostat (Shimadzu, Tokyo, Japan) as well as the SPD-M20A diode array detector (DAD) and a chromatographic column ACE 5 C18 with particle size of 5 µm (250 × 4.6 mm) with a pre-column. Acetonitrile (HPLC grade, Sigma-Aldrich GmbH, Switzerland), HPLC grade water that was obtained from a water purifying system (Millipore, Bedford, MA, USA), sodium phosphate dibasic and phosphoric acid (Sigma-Aldrich GmbH, Switzerland) were used in the analysis work.
The chromatographic conditions: a mobile phase A—acetonitrile, a mobile phase B—3.0 g/L sodium phosphate dibasic solution adjusted to pH 3.0. The chromatography was carried out by the following gradient program: 0–3 min 25% A, 75% B; 3–13 min 25→50% A, 75→50% B; 13–20 min 50% A, 50% B; 20–21 min 50→25% A, 50→75% B; 21–25 min 25% A, 75% B. The flow rate—1 mL/min, the injection volume—10 µL, detection at the wavelength of 260 nm, the column temperature −30 °C.
The chromatographic conditions: a mobile phase A—acetonitrile, a mobile phase B—3.0 g/L sodium phosphate dibasic solution adjusted to pH 3.0. The chromatography was carried out by the following gradient program: 0–3 min 25% A, 75% B; 3–13 min 25→50% A, 75→50% B; 13–20 min 50% A, 50% B; 20–21 min 50→25% A, 50→75% B; 21–25 min 25% A, 75% B. The flow rate—1 mL/min, the injection volume—10 µL, detection at the wavelength of 260 nm, the column temperature −30 °C.