Integrated HPLC-DAD and multivariate analysis for exploratory prioritisation of candidate marker compounds in <i>Lactuca indica</i>

Lactuca indica L. is a medicinal plant widely used in traditional Vietnamese medicine. A validated HPLC-DAD method was developed for the simultaneous quantification of four selected phenolic compounds, chlorogenic acid, caffeic acid, luteolin-7-O-glucuronide, and luteolin, in 12 batches collected from northern Vietnam. The method showed good linearity (R2 ≥ 0.9999), recoveries of 97.8–102.7%, repeatability of 1.15–1.72% RSD, intermediate precision of 1.73–3.37% RSD, and LOD/LOQ values of 0.02–0.84/0.07–2.79 µg mL−1. DPPH radical scavenging activity showed substantial between-batch variation, with IC50, extract values ranging from 30.40 to 97.91 µg mL−1. Principal component analysis revealed notable variation in phenolic compound profiles, while partial least squares regression explored the relationship between phenolic composition and antioxidant activity. An integrated HPLC–DAD/PLSR analysis identified chlorogenic acid and luteolin-7-O-glucuronide as the variables most closely associated with DPPH radical-scavenging activity (VIP > 1), providing a data-driven basis for their prioritisation as candidate markers for DPPH-oriented quality evaluation of L. indica.