title: Surface charge fine tuning of reversed-phase/weak anion-exchange type mixed-mode stationary phases for milder elution conditions.
abstract: A series of new mixed-mode reversed-phase/weak anion-exchange (RP/WAX) phases have been synthesized by immobilization of N-undecenyl-3-α-aminotropane onto thiol-modified silica gel by thiol-ene click chemistry and subsequent introduction of acidic thiol-endcapping functionalities of different type and surface densities. Click chemistry allowed to adjust a controlled surface concentration of the RP/WAX ligand in such a way that a sufficient quantity of residual thiols remained unmodified which have been capped by thiol click with either 3-butenoic acid or allylsulfonic acid as co-ligands. In another embodiment, performic acid oxidation of N-undecenyl-3-α-aminotropane-derivatized thiol-modified silica gave a RP/WAX phase with high density of sulfonic acid end-capping groups. ζ-Potential determinations confirmed the fine-tuned pI of these mixed-mode stationary phases which was shifted from 9.5 to 8.2, 7.8, and 6.5 with 3-butenoic acid and allylsulfonic acid end-capping as well as performic acid oxidation. For acidic solutes, the co-ionic endcapping leads to strongly reduced retention times and clearly allowed elution of these analytes under lower ionic strength thus milder elution conditions. In spite of the acidic endcapping, the new mixed-mode phases maintained their hydrophobic and anion-exchange selectivity as well as their multimodal nature featuring RP and HILIC elution domains at acetonitrile percentages below and above 50%, respectively. Column classification by principal component analysis of an extended retention map in comparison to a set of polar commercial and in-house synthesized stationary phases confirmed complementarity of the new mixed-mode phases with respect to HILIC, polar RP, amino and commercial mixed-mode phases.
authors.affiliation: Institute of Pharmaceutical Sciences, Pharmaceutical (Bio)Analysis, University of Tübingen, Auf der Morgenstelle 8, 72076 Tübingen, Germany.
authors.forename: Aleksandra
authors.initials: A
authors.lastname: Zimmermann
authors.affiliation: Institute of Pharmaceutical Sciences, Pharmaceutical (Bio)Analysis, University of Tübingen, Auf der Morgenstelle 8, 72076 Tübingen, Germany.
authors.forename: Jeannie
authors.initials: J
authors.lastname: Horak
authors.affiliation: Biomembranes Group, Institute of Molecular Science, Universitat de València, C/ Catedrático José Beltrán 2, 46980 Paterna, Valencia, Spain.
authors.forename: Orlando L
authors.initials: OL
authors.lastname: Sánchez-Muñoz
authors.affiliation: Institute of Pharmaceutical Sciences, Pharmaceutical (Bio)Analysis, University of Tübingen, Auf der Morgenstelle 8, 72076 Tübingen, Germany. Electronic address: michael.laemmerhofer@uni-tuebingen.de.
authors.forename: Michael
authors.initials: M
authors.lastname: Lämmerhofer
mesh_terms: D000097:Acetonitriles; D000838:Anions; D002852:Chromatography, Ion Exchange / Q000295:instrumentation / Q000379:methods*; D056148:Chromatography, Reverse-Phase / Q000295:instrumentation / Q000379:methods*; D057930:Click Chemistry; D057927:Hydrophobic and Hydrophilic Interactions; D012822:Silicon Dioxide / Q000737:chemistry; D012997:Solvents; D013438:Sulfhydryl Compounds / Q000737:chemistry; D014326:Tropanes / Q000737:chemistry*
chemical_list: D000097:Acetonitriles; D000838:Anions; C000603190:N-undecenyl-3-alpha-aminotropane; D012997:Solvents; D013438:Sulfhydryl Compounds; D014326:Tropanes; D012822:Silicon Dioxide; C032159:acetonitrile
keywords: Ionic endcapping; Mixed-mode chromatography; Multi-modal stationary phases; Thiol-ene click reaction; Zwitterionic media; ζ-Potential of modified silica particles