title: How ternary mobile phases allow tuning of analyte retention in hydrophilic interaction liquid chromatography.
abstract: An attractive yet hardly explored feature of hydrophilic interaction liquid chromatography (HILIC) is the tuning of analyte retention through the addition of an alcohol to the water (W)-acetonitrile (ACN) mobile phase (MP). When retention times increase sharply between 10/90 and 5/95 (v/v) W/ACN, intermediate retention values are stepwise accessible with a ternary MP of 5/90/5 (v/v/v) W/ACN/alcohol by switching from methanol to ethanol to isopropyl alcohol. We investigate the physicochemical basis of this retention tuning by molecular dynamics simulations using a model of a 9 nm silica pore between two solvent reservoirs. Our simulations show that alcohol molecules insert themselves neatly into the retentive W-rich layer at the silica surface, without disrupting the layer's structure or altering its essential properties. With the decreasing tendency of an alcohol (methanol > ethanol > isopropyl alcohol) to move toward the silica surface, the contrast between the W-rich layer and the bulk MP sharpens as the latter becomes more organic, while the W density near the silica surface remains high. Analyte retention increases with the ratio between the W mole fraction in the diffuse part of the W-rich layer and that in the bulk MP. We predict that tuning of HILIC retention is possible over a wide range through the choice of the third solvent in a W/ACN-based ternary MP, whereby the largest retention values can be expected from W-immiscible solvents that fully remain in the bulk MP.
authors: Department of Chemistry, Philipps-Universität Marburg , Hans-Meerwein-Strasse, 35032 Marburg, Germany.
authors: Sergey M
authors: SM
authors: Melnikov
authors: Alexandra
authors: A
authors: Höltzel
authors: Andreas
authors: A
authors: Seidel-Morgenstern
authors: Ulrich
authors: U
authors: Tallarek