Quality characterization of grape seed extract and dietary supplements based on multi-marker quantification by UPLC-UV

Grape seed extract (GSE) is a polyphenol-rich dietary ingredient, and its compositional complexity poses challenges for precise product quality assessment. At present, GSE quality control mainly relies on spectrophotometric assays that measure bulk indices, such as total polyphenols and total procyanidins. However, these methods cannot readily distinguish low-oligomeric procyanidins from highly polymerized components due to the lack of structural resolution. To enable more specific characterization of key constituents, this study developed and validated an ultra-performance liquid chromatography-ultraviolet detection (UPLC-UV) method for the simultaneous quantification of five phenolic markers: gallic acid (GA), (+)-catechin (C), procyanidin B2 (PCB2), (−)-epicatechin (EC), and procyanidin C1 (PCC1). Using this method, we systematically compared 14 industrial raw materials from different manufacturers and five commercial dietary supplements. The results showed that, among industrial raw materials with label-declared total procyanidins ≥ 95%, the measured concentrations of the target oligomeric markers differed markedly across samples. Analysis of commercial finished products further showed that substantial between-product differences in key components remained evident even after label-context normalization. These findings indicate that chromatographic quantification of structurally specific markers, used as a complement to conventional total-content indices, would provide more detailed compositional information and thereby serve as a valuable supplementary analytical approach for routine quality control of GSE and related products.