title: Restriction capillaries as an innovative mixing unit for intermediate mobile phase exchange in multidimensional analysis.
abstract: A novel mixing unit is proposed for the serial coupling of orthogonal columns to analyze polar and non-polar compounds in a single run. The principle relies on the isolation of unretained peaks eluting from a first dimension column in a sample loop, before directing them to a second column for separation. Since the mobile phases employed in highly orthogonal separations are not directly compatible, a mixing unit is required to alter the mobile phase composition before executing the second dimension separation. The mixing unit proposed in this work is based on the use of two restriction capillaries with different flow resistances to dilute the mobile phase eluting from the first dimension with a solvent appropriate for the second dimension separation. The restriction capillaries are implemented in an ultra-high performance liquid chromatography set-up using three high-pressure switching valves and two T-pieces. It is demonstrated that the dilution ratio can be adequately predicted using the law of Hagen-Poiseuille and can be adjusted easily by changing the dimensions of the restriction capillaries. The dilution volume required to obtain acceptable recoveries is investigated and the use of different column diameters in the first and second dimension is proposed to increase the sensitivity of the analysis. Under optimum dilution conditions, recoveries ranging between 82% and 99% are always obtained, while repeatability values are excellent. The proof-of-concept of the different set-ups is demonstrated for the separation of 20 pharmaceuticals with log D-values ranging between -5.75 and 4.22.
authors.affiliation: KU Leuven - University of Leuven, Department for Pharmaceutical and Pharmacological Sciences, Pharmaceutical Analysis, Herestraat 49 bus 923, B-3000 Leuven, Belgium.
authors.forename: Glenn
authors.initials: G
authors.lastname: Loos
authors.affiliation: Agilent Technologies R&D, Hewlett-Packard-Strasse 8, Waldbronn, Germany.
authors.forename: Konstantin
authors.initials: K
authors.lastname: Shoykhet
authors.affiliation: Agilent Technologies R&D, Hewlett-Packard-Strasse 8, Waldbronn, Germany.
authors.forename: Monika
authors.initials: M
authors.lastname: Dittmann
authors.affiliation: KU Leuven - University of Leuven, Department for Pharmaceutical and Pharmacological Sciences, Pharmaceutical Analysis, Herestraat 49 bus 923, B-3000 Leuven, Belgium. Electronic address: deirdre.cabooter@kuleuven.be.
authors.forename: Deirdre
authors.initials: D
authors.lastname: Cabooter
mesh_terms: D002851:Chromatography, High Pressure Liquid / Q000295:instrumentation* / Q000379:methods*; D057927:Hydrophobic and Hydrophilic Interactions; D012997:Solvents
chemical_list: D012997:Solvents
keywords: HILIC; Method development; Mixing unit; RPLC; Restriction capillaries; Serial coupling