Onion Peel Extract Phenolic Profiling

The OPE was characterized using liquid chromatography–mass spectrometry (LC/MS, 1260 Infinity LC, and 6410 Triple Quadrupole MS, Agilent Technologies, USA) to detect the phenolic compounds in the onion peel. One g of dried extract powder was dissolved in 10 mL of Milli-Q water (resistivity: 18.2 MΩ cm), and kept in the ultrasonic bath for 15 min. Then, the solution was filtered using a nylon filter (0.2 μm) and diluted. The sample was passed through the ZORBAX C18 column (2.1 × 50 mm², 3.5 μm) column (maintained at 35 °C) using mobile phase A (water + 0.1% formic acid) and mobile phase B (100% water) at a flow rate of 200 µL min⁻¹. The elution gradient profile was used as following: 2% B (0–3 min), 2–13% B (3–5 min), 13% B (5–9 min), 13–18% B (9–12 min), 18% B (12–13 min), 18–30% B (13–17 min), 30–50% B, (17–21 min), 50–98% B (21–22 min), 98% B (22–27 min). After elution, the mass spectra of the analytes were obtained by scanning m/z ratio from 50 to 950 in negative ionization mode at 300 °C of capillary temperature (Bedrníček et al. 2020 (link); Celano et al. 2021 (link)).