title: The advantage of mixed-mode separation in the first dimension of comprehensive two-dimensional liquid-chromatography.
abstract: Comprehensive two-dimension liquid chromatography (LC×LC) has exhibited its powerful ability to separate complex samples. However, the use of a single chromatographic mode in 1st dimension has been limited to the separation of components by their individual characteristics, such as hydrophobicity, ionic properties etc. The use of mixed-mode stationary phases has revealed opportunities to combine different retention mechanisms. In this respect, stationary phases featuring both RP-like hydrophobic and ion-exchange interactive sites promise great versatility in retaining both polar and more apolar ionic and non-ionic compounds. We have therefore developed an LC×LC system based on mixed-mode (strong anion exchange and reversed phase) in the first dimension and a C18 phase in the second dimension. The system has been evaluated with standard compounds and applied for the separation of white wine and Chinese Herbal Medicine (CHM). The mixed-mode system SAX-PFP×C18 results in a better separation than a single mode system such as SAX×C18 or PFP×C18. Although little improvement in orthogonality (0.91 instead 0.86) is achieved with SAX×C18, the mixed-mode SAX-PFP×C18 gives a much larger effective peak distribution area in the analysis of e. g. white wine. But the analysis of aqueous extracts of CHM (Hdyotis diffusa and Scutellaria barbata) with SAX-PFP×RP leads to a very long analysis time because of strong hydrophobic interactions with the PFP column. Thus, the system was changed by using a cyano phase instead of a PFP phase. The improved SAX-CN×C18 system shows a better peak distribution and more importantly a reasonable analysis time.
authors.affiliation: Applied Analytical Chemistry, Faculty of Chemistry, University of Duisburg-Essen, Universitaetsstr. 5-7, 45141 Essen, Germany; Institute of Pure and Applied Mass Spectrometry, University of Wuppertal, Gaussstr. 20, 42119 Wuppertal, Germany.
authors.forename: Duxin
authors.initials: D
authors.lastname: Li
authors.affiliation: Institute of Pure and Applied Mass Spectrometry, University of Wuppertal, Gaussstr. 20, 42119 Wuppertal, Germany.
authors.forename: Roman
authors.initials: R
authors.lastname: Dück
authors.affiliation: Applied Analytical Chemistry, Faculty of Chemistry, University of Duisburg-Essen, Universitaetsstr. 5-7, 45141 Essen, Germany; Institute of Pure and Applied Mass Spectrometry, University of Wuppertal, Gaussstr. 20, 42119 Wuppertal, Germany. Electronic address: oliver.schmitz@uni-due.de.
authors.forename: Oliver J
authors.initials: OJ
authors.lastname: Schmitz
mesh_terms: D002852:Chromatography, Ion Exchange / Q000379:methods / Q000592:standards; D056148:Chromatography, Reverse-Phase / Q000379:methods / Q000592:standards; D004365:Drugs, Chinese Herbal / Q000032:analysis*; D057927:Hydrophobic and Hydrophilic Interactions; D012015:Reference Standards; D031363:Scutellaria / Q000737:chemistry; D014920:Wine / Q000032:analysis*
chemical_list: D004365:Drugs, Chinese Herbal
keywords: Chinese Herbal Medicine; Comprehensive two-dimensional liquid chromatography; Mixed mode; Peak-distribution area; White wine