Separation and characterization of phenolic compounds and triterpenoid saponins in licorice (Glycyrrhiza uralensis) using mobile phase-dependent reversed-phase × reversed-phase comprehensive two-dimensional liquid chromatography coupled with mass spectrom

• Mobile phase-dependent comprehensive 2DLC was used to separate licorice extracts.
• Phenolic compounds were separated from saponins and flavonoid glycosides.
• Different selectivity of MeOH and MeCN remarkably improved the orthogonality.
• 311 peaks (175 peaks at 250 nm) were separated, and the orthogonality reached 79.8%.
• 8 phenolic compounds were discovered in G. uralensis for the first time by 2DLC/MS.

Licorice is one of the most popular herbal medicines worldwide. It contains a big array of phenolic compounds (flavonoids, coumarins, and diphenylethanones). Due to high structural diversity, low abundance, and co-elution with licorice saponins, these phenolic compounds are difficult to be separated by conventional chromatography. In this study, a mobile phase-dependent reversed-phase × reversed phase comprehensive two-dimensional liquid chromatography (RP × RP 2DLC) method was established to separate phenolic compounds in licorice (the roots of Glycyrrhiza uralensis). Organic solvents in the mobile phase were optimized to improve orthogonality of the first and second dimensions, and a synchronized gradient mode was used to improve chromatographic resolution. Finally, licorice extracts were eluted with methanol/water/formic acid in the first dimension (Acquity CSH C18 column), and acetonitrile/water/formic acid in the second dimension (Poroshell Phenyl-Hexyl column). By using this 2DLC system, a total of 311 compounds were detected within 40 min. The practical and effective peak capacity was 1329 and 524, respectively, and the orthogonality was 79.8%. The structures of 21 selected unknown compounds were tentatively characterized by mass spectrometry, and 8 of them were discovered from G. uralensis for the first time. The mobile phase-dependent 2DLC/MS system could benefit the separation and characterization of natural products in complicated herbal extracts.