Partition and displacement models in reversed-phase liquid chromatography with mixed eluents

Volume and composition of semi-adsorbed stationary phases in hydrophilic interaction liquid chromatography. Comparison of water adsorption in common stationary phases and eluents

Pycnometric and homologous series retention methods are used to determine the volume and mean composition of the water-rich layers partially adsorbed on the surface of several hydrophilic interaction liquid chromatography (HILIC) column fillings with acetonitrile-water and methanol-water as eluents. The findings obtained in this work confirm earlier studies using direct methods for measuring the stationary phase water content performed by Jandera's and Irgum's research groups. Water is preferentially adsorbed on the surface of the HILIC bonded phase in hydroorganic eluents containing more than 40% acetonitrile or 70% methanol, and a gradient of several water-rich transition layers between the polar bonded phase and the poorly polar bulk mobile phase is formed. These layers of reduced mobility act as HILIC stationary phases, retaining polar solutes. The volume of these layers and concentration of adsorbed water is much larger for acetonitrile-water than for methanol-water mobile phases. In hydroorganic eluents with less than 20-30% acetonitrile or 40% methanol the amount of preferentially adsorbed water is very small, and the observed retention behavior is close to the one in reversed-phase liquid chromatography (RPLC). In eluents with intermediate acetonitrile-water or methanol-water compositions a mixed HILIC-RPLC behavior is presented. Comparison of several HILIC columns shows that the highest water enrichment in the HILIC retention region for acetonitrile-water mobile phases is observed for zwitterionic and aminopropyl bonded phases, followed in minor grade for diol and polyvinyl alcohol functionalizations. Pentafluorophenyl bonded phase, usually considered a HILIC column, does not show significant water adsorption, nor HILIC retention.

Comparison of the Retention and Separation Selectivity of Aromatic Hydrocarbons with Polar Groups in RP-HPLC Systems with Different Stationary Phases and Eluents

In this manuscript, the retention of aromatic hydrocarbons with polar groups has been compared for systems with various nonpolar columns of the types from C3 to C18 and different mobile phases composed of methanol, acetonitrile, or tetrahydrofuran as modifiers. The selectivity separation of the solutes in systems with different adsorbents, when one eluent modifier is swapped by another, has been explained, taking into account molecular interactions of the solutes with components of the stationary phase region (i.e., extracted modifier depending on the chain length of the stationary phase).

The combination of partition, size exclusion, and hydrodynamic models in chromatography, and application to bonded phases on porous supports

Liquid-liquid partition chromatography has been used for many years as a model and teaching introduction to column chromatography. However, the partition model does not describe separations on bonded phases with porous supports particularly well, especially regarding the thermodynamics controlling solute distribution. Further difficulties arise when more than one mechanism is involved in solute retention. Nomenclature is not perfectly aligned with the underlying thermodynamic descriptors and is inconsistently applied over various chromatographic techniques. Presented here is a general description of retention that spans partition, size exclusion, and hydrodynamic separation processes, and is then applied to bonded-phase separations on porous supports. The model provides a general description applicable to adsorption, reversed-phase, hydrophilic interaction, size-exclusion, hydrodynamic chromatography, and any combination of these techniques including liquid chromatography at the critical condition. Further expansion to include retention by ion-exchange and field-flow fractionation appears to be possible. Recommendations on retention factor definition and evaluation are given.

Thermodynamics Study of Solvent Adsorption on Octadecyl-Modified Silica

Elution and solvation processes in liquid chromatography may be controlled by temperature changes. In the case of solvent adsorption, the temperature influences the amount of adsorbed solvent as well as the enthalpy and entropy of the solvation process. In this work, the thermodynamic parameters of organic solvents used as organic modifiers in the reversed-phase high-performance liquid chromatography elution process were determined. The changes of enthalpy and entropy in a series of chemically bonded stationary phases were measured to determine the effects of the temperature and surface coverage density of octadecyl ligands on the thermodynamic parameters of the solvation. For both the enthalpy and entropy a parabolic trend was observed with the minimum for medium surface coverage. The correlation of solvent adsorption values with the enthalpy of solvation was also investigated. The highest influence of the temperature on solvation process was observed for stationary phases with high surface coverage.

Functionalized anion exchange stationary phase for separation of anionic compounds

Synthesis of the multilayered stationary phases containing quaternary ammonium functional groups on the silica support was described. Bonded phases were characterized by elemental analysis, solid state (13)C NMR spectroscopy and chromatographic methods. The surface of silica support was coated with different number of polymeric layers formed by condensation polymerization of primary amine (methylamine) with diepoxide (1,4-butanedioldiglycidyl ether). A series of stationary phases with different number of polymerized layers were tested. Separation of an inorganic anions sample (F(-), Cl(-), NO2(-), Br(-), NO3(-)) and nucleotides was performed.

Retention and bandwidths prediction in fast gradient liquid chromatography. Part 2-Core-shell columns

Recently, we confirmed that the well-established theory of gradient elution can be employed for prediction of retention in gradient elution from the isocratic data, method development and optimization in fast gradient chromatography employing short packed fully porous and monolithic columns and gradient times in between 1 and 2min, or even less. In the present work, we extended this study to short core-shell reversed-phase columns. We investigated the effects of the specification of the stationary phase in the core-shell structure on the prediction of gradient retention data. Two simple retention models describing the effects of the mobile phase on the retention by two-parameter equations yield comparable accuracy and can be used for prediction of elution times. The log-log model provides improved prediction of gradient bandwidths, especially for less retained compounds. A more sophisticated three-parameter model did not offer significant improvement of prediction. We compared the efficiency, selectivity and peak capacity of fast gradient separations of alkylbenzenes, phenolic acids and flavones on seven core shell columns with different lengths and chemistry of bonded shell stationary phase. Within the limits dictated by a fixed short separation time, appropriate adjustment of the range of the composition of mobile phase during gradient elution is the most efficient means to optimize the gradient separation. The gradient range affects sample bandwidths equally or even more significantly than the column length. Both 5-cm and 3-cm core-shell columns may provide comparable peak capacity in a fixed short gradient time.

Design of peptide standards with the same composition and minimal sequence variation to monitor performance/selectivity of reversed-phase matrices

The present manuscript extends our de novo peptide design approach to the synthesis and evaluation of a new generation of reversed-phase HPLC peptide standards with the same composition and minimal sequence variation (SCMSV). Thus, we have designed and synthesized four series of peptide standards with the sequences Gly-X-Leu-Gly-Leu-Ala-Leu-Gly-Gly-Leu-Lys-Lys-amide, where the N-terminal is either N(α)-acetylated (Series 1) or contains a free α-amino group (Series 3); and Gly-Gly-Leu-Gly-Gly-Ala-Leu-Gly-X-Leu-Lys-Lys-amide, where the N-terminal is either N(α)-acetylated (Series 2) or contains a free α-amino group (Series 4). In this initial study, the single substitution position, X, was substituted with alkyl side-chains (Ala Collapse

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MESH Headings

• Amino Acid Sequence
• Chromatography, High Pressure Liquid/standards
• Chromatography, Reverse-Phase/standards
• Peptides/chemistry
• Reference Standards

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Grants

• R01 GM061855 NIGMS NIH HHS
• R01 GM061855-10 NIGMS NIH HHS
• R01 GM61855 NIGMS NIH HHS

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Affiliation(s)

Colin T. Mant Department of Biochemistry and Molecular Genetics, University of Colorado Denver, School of Medicine, Aurora, CO 80045, USA
Robert S. Hodges Department of Biochemistry and Molecular Genetics, University of Colorado Denver, School of Medicine, Aurora, CO 80045, USA

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Hydrophilic interaction liquid chromatography (HILIC)--a powerful separation technique

Hydrophilic interaction liquid chromatography (HILIC) provides an alternative approach to effectively separate small polar compounds on polar stationary phases. The purpose of this work was to review the options for the characterization of HILIC stationary phases and their applications for separations of polar compounds in complex matrices. The characteristics of the hydrophilic stationary phase may affect and in some cases limit the choices of mobile phase composition, ion strength or buffer pH value available, since mechanisms other than hydrophilic partitioning could potentially occur. Enhancing our understanding of retention behavior in HILIC increases the scope of possible applications of liquid chromatography. One interesting option may also be to use HILIC in orthogonal and/or two-dimensional separations. Bioapplications of HILIC systems are also presented.

Simple models for the effect of aliphatic alcohol additives on the retention in reversed-phase liquid chromatography

Four retention models for the effect of aliphatic alcohol additives on the retention of analytes in reversed-phase liquid chromatography have been developed following either a semi-thermodynamic treatment or an empirical approach. Their performance was tested using the experimental retention times of six non-polar analytes (alkylbenzenes) and ten o-phthalaldehyde derivatives of amino acids under different isocratic chromatographic runs when a small amount of ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol or 1-heptanol was added to methanol/water mixtures containing a constant amount of methanol. It was shown that for the structurally simple alkylbenzenes all the models can be adopted for retention prediction with good results. In contrast, just one out of four models, that with the fewest approximations, predicts satisfactorily the retention properties of amino acids derivatives. However, the most interesting feature is that this model can predict the effect of an alcohol-additive on the retention properties of solutes, even if this additive was not used in chromatographic runs done for the fitting procedure, provided that it belongs to the same homologous series of alkanols. This feature is also observed in all models described the retention of alkylbenzenes.

Study of solvent adsorption on chemically bonded stationary phases by microcalorimetry and liquid chromatography

A detailed, molecular-level description of the sorption mechanism in reversed-phase liquid chromatography is of great interest to analytical chemists. For this purpose, solvent adsorption in the octadecyl stationary bonded phase was investigated. Preferential adsorption of solvents from an acetonitrile-water and methanol-water mobile phase was measured on a series of non-end-capped octadecyl bonded phases with different coverage densities of bonded ligands using the minor disturbance method. For a comparison, a microcalorimetric study of organic solvent adsorption on the stationary phase was executed. The results from the excess isotherm measurement agree well with the experimental measurement of the heat of immersion of the bonded stationary phases by the test solvents. The microcalorimetric measurement is another method for determination of solvation processes of the stationary phases. Changes of the heat of immersion provide information about the surface accessibility for interaction with solvent molecules. The increase of the stationary phase coverage density reduces the free space between bonded chains and penetration of solvent between organic chains.