Waruna Kiridena

Chromatographic methods for the determination of the logL16 solute descriptor

A squalane packed column and two open-tubular columns coated with an immobilized stationary phase film of poly(dimethylsiloxane) or poly(methyloctylsiloxane) are evaluated as surrogate chromatographic systems for the determination of the logL16 solute descriptor by gas chromatography. Retention on the squalane column is dominated by gas-liquid partitioning with a significant temperature-dependent contribution from interfacial adsorption at the liquid-solid interface. Using the gas-liquid partition coefficient as the dependent model variable allows logL16 to be estimated to +/- 0.026 log units over the temperature range 60-120 degrees C. Without correction for interfacial adsorption a single column estimation of logL16 with phase loadings of 8 to 20% (m/m) over the temperature range 80-120 degrees C is possible for compounds with moderate hydrogen-bond basicity. The poly(dimethylsiloxane) stationary phase is both dipolar and hydrogen-bond basic and less suitable than the poly(methyloctylsiloxane) stationary phase, which is less cohesive and has similar dipolarity but no hydrogen-bond basicity. The determination of logL16 on the poly(methyloctylsiloxane) column requires prior knowledge of the solute dipolarity/polarizability descriptor to avoid significant errors in the measurement of logL16 for polar compounds. In such circumstances a single column estimation of logL16 over the temperature range 60-140 degrees C with an error of +/- 0.05-0.09 log units is possible.

Selectivity equivalence of poly(ethylene glycol) stationary phases for gas chromatography

The solvation parameter model is used to study differences in selectivity for poly(ethylene glycol) stationary phases for packed column (Carbowax 20M) and fused-silica, open-tubular column (HP-20M, AT-Wax, HP-INNOWax and DB-FFAP) gas chromatography. All phases are dipolar, strongly hydrogen-bond basic with no hydrogen-bond acidity and of moderate cohesion. No two phases are exactly alike, however, and selectivity differences identified with cavity formation and dispersion interactions, n- and pi-electron pair interactions, dipole-type interactions and hydrogen-bond interactions are quantified by differences in the system constants at a fixed temperature where retention occurs solely by gas-liquid partitioning. The system constants vary linearly with temperature over the range 60-140 degrees C (except for n- and pi-electron pair interactions which are temperature invariant) facilitating a general comparison of the importance of temperature on selectivity differences for compared phases. From a mechanistic point of view it is demonstrated that selectivity differences can result from chemical differences between the poly(ethylene glycol) stationary phases and from differences in the relative contribution of interfacial adsorption to the retention mechanism. The latter depends on both system properties and solute characteristics.

Selectivity assessment of popular stationary phases for open-tubular column gas chromatography.

The solvation parameter model is used to study the influence of temperature and composition on the selectivity of nine poly(siloxane) and two poly(ethylene glycol) stationary phase chemistries for open-tubular column gas chromatography. A database of system constants for the temperature range 60-140 degrees C was constructed from literature values with additional results determined for HP-50+, DB-210, DB-1701, DB-225 and SP-2340 columns. The general contribution of monomer composition (methyl, phenyl, cyanopropyl, and trifluoropropyl substituents) on the capacity of poly(siloxane) stationary phases for dispersion, electron lone pair, dipole-type and hydrogen-bond interactions is described. The selectivity coverage of the open-tubular column stationary phases is compared with a larger database for packed column stationary phases at a reference temperature of 120 degrees C. The open-tubular column stationary phases provide reasonable coverage of the range of dipole-type and hydrogen-bond base interactions for non-ionic packed column stationary phases. Deficiencies are noted in the coverage of electron lone pair interactions. None of the open-tubular column stationary phases are hydrogen-bond acids. The system constants are shown to change approximately linearly with temperature over the range 60-140 degrees C. The intercepts and slopes of these plots are used to discuss the influence of temperature on stationary phase selectivity.

Influence of solute size and site-specific surface interactions on the prediction of retention in liquid chromatography using the solvation parameter model

The solvation parameter model was used to characterize the retention properties of silica and a cyanopropylsiloxane-bonded silica sorbent in liquid–solid chromatography using hexane and various volume fractions of methyl tert-butyl ether as a mobile phase. The relative capacity of the solvated sorbent for dipole-type interactions and hydrogen-bond interactions, solute size and differences in the apparent phase ratio have to be considered to explain retention and selectivity differences for the two sorbents. Dipole-type and hydrogen-bond interactions favor retention whereas increasing solute size reduces retention for both sorbents, although the sorbent capacity and solvent dependence for these interactions are different. Solvent composition (range 10–50% v/v methyl tert-butyl ether) produces a similar trend for changes in cohesion and the solvated sorbent’s capacity for dipole-type interactions and capacity as a hydrogen-bond base, but different results for sorbent lone-pair electron and hydrogen-bond acid interactions. The quality of the model fits is excellent for the cyanopropylsiloxane-bonded sorbent but only approximate for silica (excluding nitrogen-containing bases in both cases). The poor fit for silica is probably due to the inadequacy of the characteristic volume to represent the projection of the cross-sectional area of the solute at the solvated sorbent surface and site-specific interactions of polar compounds with the heterogeneous surface sorption sites of the solvated sorbent. The anomalous behavior of the nitrogen-containing bases is possibly due to electrostatic interactions resulting from ion-exchange behavior that is not included in the model.

Structure-driven retention model for solvent selection and optimization in reversed-phase thin-layer chromatography

Abstract A new approach to method development in reversed-phase thin-layer chromatography is proposed based on the use of calculations employing the solvation parameter model. System constants are provided for aqueous binary mobile phase mixtures containing the organic solvents methanol, 2-propanol, 2,2,2-trifluoroethanol, acetone, N,N-dimethylformamide and acetonitrile on a cyanopropylsiloxane-bonded layer. Good agreement between experimental and predicted RF values (±0.03 RF units) for steroids, phenols and naphthalene derivatives is demonstrated for mobile phase optimization.

Extension of the system constants database for open-tubular columns: system maps at low and intermediate temperatures for four new columns.

The solvation parameter model is used to characterize the separation properties of four open-tubular columns for gas chromatography at low and intermediate column temperatures covering the range 60-240 degrees C. Solute descriptors for compounds suitable for characterizing columns over the intermediate temperature range are optimized using an iterative procedure. These compounds, and those previously recommended for the lower temperature range, are used to provide system constant maps for Rxi-5Sil MS, Rxi-17, Rtx-TNT and Rtx-TNT2 columns suitable for merging with a system constants database with entries for more than 50 columns. The Rxi-5Sil MS column is shown to have separation properties similar to the silphenylene-dimethylsiloxane copolymer stationary phase (DB-5ms) but these two columns are not selectivity equivalent. The Rxi-17 column has similar separation properties to the Rxi-50 column but is not selectivity equivalent to it. Rxi-17 is a poly(dimethyldiphenylsiloxane) stationary phase containing 50% diphenylsiloxane monomer and Rxi-50 a poly(methylphenylsiloxane) stationary phase with the same nominal composition but a different monomer structure. The difference in monomer structure results in only small changes in selectivity, and for all but the most demanding separations, the columns are interchangeable. The application-specific column (energetic materials) Rtx-TNT is shown to be selectivity equivalent to columns coated with the poly(dimethyldiphenylsiloxane) stationary phases containing 5% diphenylsiloxane monomer. The Rtx-TNT2 column is selectivity equivalent to the proprietary Rtx-OPPesticides column. Rtx-OPPesticides is a low bleed stationary phase, possibly based on silarylene-siloxane chemistry, with a composition designed to mimic the separation properties of the poly(dimethylmethyltrifluoropropylsiloxane) stationary phases containing 35% methyltrifluoropropylsiloxane monomer. Selectivity equivalence of columns is determined by the statistical agreement in system constants at 20 degrees C intervals over the full temperature range from 60 to 240 degrees C, and by the construction of correlation plots for the retention factors of varied compounds for the same temperature intervals.

Nanometer-scale elasticity measurements on organic monolayers using scanning force microscopy

Local elastic compliance of organic monolayers (octadecyltriethoxysilane/mica and alkanethiol/gold) has been investigated with nanometer resolution by applying a force modulation technique to an atomic force microscope. Systematic measurements were taken as a function of modulation frequency and amplitude, as well as the local environment surrounding the surface. The topography and local elasticity of the monolayers are contrasted to the bare substrate created by the tip of the atomic force microscope at high imaging force. Under ambient laboratory conditions, the Youngs modulus of mica calculated from the elasticity images is lower than the organic monolayer. Soch an observation is not intuitive and can be explained by the thin film of water adsorbed on mica. Water adsorption can change the microscope tip surface interaction. As a result, mica appears as a softer surface than the organic layers. In addition, the elasticity is dramatically enhanced if the modulation frequency coincides with or is close to the natural resonance frequency of the tips of the atomic force microscope. Measurements taken under liquid provide more reproducible and accurate results because the resonance frequency is damped out and capillary interactions are avoided. The measured Youngs modulus is also found to increase slightly with increasing modulation amplitude.

Influence of composition and temperature on the selectivity of stationary phases containing either mixtures of poly(ethylene glycol) and poly(dimethylsiloxane) or copolymers of cyanopropylphenylsiloxane and dimethylsiloxane for open-tubular column gas chromatography

The solvation parameter model is used to determine the system constants for three columns containing mixtures of poly(dimethylsiloxane) and poly(ethylene glycol) and a poly(cyanopropylphenyldimethylsiloxane) containing 6% of cyanopropylphenylsiloxane monomer at five equally spaced temperatures in the range 60-140°C. Together with literature data for a poly(dimethylsiloxane) and a poly(ethylene glycol) stationary phase the influence of temperature and composition on selectivity is studied for mixing ratios of 0 to 1 poly(ethylene glycol) for the temperature range 60-140°C. Using literature data for two poly(cyanopropylphenyldimethylsiloxane) stationary phases containing 14% and 50% of cyanopropylphenylsiloxane monomer groups the influence of temperature and replacing dimethylsiloxane monomer groups by cyanopropylphenylsiloxane groups on selectivity is studied for incorporation of 0 to 0.5 cyanopropylphenylsiloxane groups over the temperature range 60-140°C. Addition of poly(ethylene glycol) or introduction of cyanopropylphenylsiloxane monomer groups into a poly(dimethylsiloxane) influences selectivity through an increase in dipolarity/ polarizability, hydrogen-bond basicity, electron lone pair interactions, and changes in cohesion. The changes in system constants as a function of temperature and composition are simply modeled as smooth quadratic response surfaces. Curvature in the response surfaces along the composition axis is significant while changes along the temperature axis are modest for both stationary phase types. Cluster analysis is used to demonstrate that the mixed poly(dimethylsiloxane)/poly(ethylene glycol) stationary phases containing 0.5 and 0.85 weight fraction of poly(ethylene glycol) have different selectivity to a database of common open-tubular column stationary phases. The mixed poly(dimethylsiloxane)/poly(ethylene glycol) stationary phase containing 0.10 weight fraction of poly(ethylene glycol) has similar selectivity to the poly(cyano-propylphenyldimethylsiloxane) containing 6% cyanopropylphenyl monomer groups, and could replace the mixed phase for all but the most critical of separations.

Reversed-Phase Chromatography on a Polar Endcapped Octadecylsiloxane-Bonded Stationary Phase with Water as the Mobile Phase

SummarySynergiTM Hydro-RP is a polar endcapped octadecylsiloxane-bonded silica packing for reversed-phase chromatography compatible with water as a mobile phase. The effect of temperature on retention and selectivity for a varied group of compounds is evaluated from plots of the retention factors against temperature and by application of the solvation parameter model. Retention is shown to increase with the size and capability of the solute for electron lone pair interactions and is reduced by hydrogen-bonding and dipole-type interactions. Increasing temperature over the range 25 to 65°C reduces retention with only a small affect on selectivity. Increasing temperature effectively increases the phase ratio for the column and diminishes the relative contribution of electron lone pair interactions with the stationary phase. The SynergiTM Hydro-RP column in suitable for the separation of small polar molecules with good peak shape and reasonable efficiency in an acceptable separation time.

Selectivity assessment of DB-200 and DB-VRX open-tubular capillary columns

The solvation parameter model is used to study the influence of composition and temperature on the selectivity of two poly(siloxane) stationary phases used for open-tubular capillary column gas chromatography. The poly(methyltrifluoropropyldimethylsiloxane) stationary phase, DB-200, has low cohesion, intermediate dipolarity/polarizability, low hydrogen-bond basicity, no hydrogen-bond acidity, and repulsive electron lone pair interactions. The DB-VRX stationary phase has low cohesion, low dipolarity/polarizability, low hydrogen-bond basicity and no hydrogen-bond acidity and no capacity for electron lone pair interactions. The selectivity of the two stationary phases is complementary to those in a database of 11 stationary phase chemistries determined under the same experimental conditions.