Essential oil-removed fennel extract, which is used as an antioxidant agent derived from natural source, was analyzed by using HPLC-PDA/MS. The main constituents of the commercial products were sinapyl alcohol 4-O-β-D-glucoside (syringin), quercetin 3-O-β-D-glucuronide (Q3GA), and acyl-quinic acids, such as 5-O-caffeoylquinic acid (chlorogenic acid), 3-O-caffeoylquinic acid, 4-O-caffeoylquinic acid, and 3,5-di-O-caffeoylquinic acid. The reagents of these main constituents were subjected to external calibration quantitative 1H-NMR. There were approximately 10% differences in the absolute purities between the reagents. By reflecting the absolute purities to each calibration curve, a reliable HPLC quantification showed that the contents of main constituents in the products were 0.05%–0.2%. Furthermore, the antioxidant activities of the products and the main constituents were evaluated by antioxidant assay: 1,1-diphenyl-2-picrylhydrazyl (DPPH) and thiocyanate methods. Among the main constituents, Q3GA and chlorogenic acids were responsible to the antioxidative potency of the products.
