Nicole Lenca

Estimation of the environmental properties of compounds from chromatographic measurements and the solvation parameter model

This article provides an overview of chromatographic methods as surrogate models for environmental processes and for the determination of descriptors for compounds of environmental interest. The solvation parameter model is the link to the identification of suitable chromatographic models for the estimation of environmental properties using a set of tools that allow screening of chromatographic databases for the selection of candidate systems. As an alternative approach, many transport and distribution properties of environmental interest can be described directly by the solvation parameter model. Environmental properties for compounds with known descriptors can then be predicted through these models. The central role chromatographic methods, together with liquid-liquid partition coefficients, occupy in the determination of the six descriptors used in the solvation parameter model is detailed. There is a current need to accelerate efforts to expand the coverage of environmental process models by incorporating more complex molecules of contemporary environmental interest. For many of these molecules descriptor values are unavailable and their determination should be prioritized.

Gas chromatography on wall-coated open-tubular columns with ionic liquid stationary phases.

Ionic liquids have moved from novel to practical stationary phases for gas chromatography with an increasing portfolio of applications. Ionic liquids complement conventional stationary phases because of a combination of thermophysical and solvation properties that only exist for ionic solvents. Their high thermal stability and low vapor pressure makes them suitable as polar stationary phases for separations requiring high temperatures. Ionic liquids are good solvents and can be used to expand the chemical space for separations. They are the only stationary phases with significant hydrogen-bond acidity in common use; they extend the hydrogen-bond basicity of conventional stationary phases; they are as dipolar/polarizable as the most polar conventional stationary phases; and some ionic liquids are significantly less cohesive than conventional polar stationary phases. Problems in column coating techniques and related low column performance, column activity, and stationary phase reactivity require further exploration as the reasons for these features are poorly understood at present.

Applications of the solvation parameter model in reversed-phase liquid chromatography

The solvation parameter model is widely used to provide insight into the retention mechanism in reversed-phase liquid chromatography, for column characterization, and in the development of surrogate chromatographic models for biopartitioning processes. The properties of the separation system are described by five system constants representing all possible intermolecular interactions for neutral molecules. The general model can be extended to include ions and enantiomers by adding new descriptors to encode the specific properties of these compounds. System maps provide a comprehensive overview of the separation system as a function of mobile phase composition and/or temperature for method development. The solvation parameter model has been applied to gradient elution separations but here theory and practice suggest a cautious approach since the interpretation of system and compound properties derived from its use are approximate. A growing application of the solvation parameter model in reversed-phase liquid chromatography is the screening of surrogate chromatographic systems for estimating biopartitioning properties. Throughout the discussion of the above topics success as well as known and likely deficiencies of the solvation parameter model are described with an emphasis on the role of the heterogeneous properties of the interphase region on the interpretation and understanding of the general retention mechanism in reversed-phase liquid chromatography for porous chemically bonded sorbents.

Liquid chromatography with room temperature ionic liquids

Room temperature ionic liquids are a new class of solvents of potential interest for liquid chromatography. Ionic liquids possess a combination of physical and solvation properties that are complementary to conventional organic solvents. Applications in liquid chromatography are currently limited by their unfavorable viscosity and low-wavelength absorption in the ultraviolet (UV) region. In addition, for planar chromatography, the absence of a vapor pressure does not allow evaporation of ionic liquid solvents after development. The room temperature ionic liquids are good solvents for nonionic compounds with a different blend of intermolecular interactions compared with conventional organic solvents as indicated by solvatochromic measurements and the system constants of the solvation parameter model. Current applications in column and planar chromatography are reviewed to demonstrate the potential of room temperature ionic liquids as mobile phases or mobile phase additives in separation science. A real breakthrough in their use, however, requires the identification of new room temperature ionic liquids with viscosity closer to those of conventional organic solvents as well as addressing other minor issues described in the text.

A system map for the ionic liquid stationary phase 1,12-di(tripropylphosphonium)dodecane bis(trifluoromethylsulfonyl)imide for gas chromatography

The solvation parameter model is used to prepare a system map for the retention of volatile organic compounds on the ionic liquid stationary phase 1,12-di(tripropylphosphonium)dodecane bis(trifluoromethylsulfonyl)imide (SLB-IL60) by gas chromatography over the temperature range 80-280°C. Retention is governed by dispersion, dipole-type and hydrogen-bonding interactions with a different temperature dependence. The hydrogen-bond acidity of the SLB-IL60 column is unexpected since the stationary phase contains no hydrogen-bond acid groups and is not obviously connected to contributions from the deactivated column wall. The polarity number is shown to be a poor indicator of column retention properties. Principal component analysis with the system constants of the solvation parameter model as variables indicates that the properties of SLB-IL60 are not duplicated by any of the common poly(siloxane) and poly(ethylene glycol) stationary phase chemistries in common use for column preparation. The SLB-IL60 column has similar selectivity to a poly(cyanopropylphenyldimethylsiloxane) stationary phase containing 50% cyanopropylphenyl siloxane monomer but the two columns are not selectivity equivalent. Poly(ethylene glycol) stationary phases indicated as most similar to SLB-IL60 based on their polarity numbers are shown to have quite different selectivity.

A system map for the ionic liquid stationary phase 1,12-di(tripropylphosphonium)dodecane bis(trifluoromethylsulfonyl)imide trifluoromethanesulfonate for gas chromatography

The solvation parameter model is used to prepare a system map for the retention of volatile organic compounds on the ionic liquid stationary phase 1,12-di(tripropylphosphonium)dodecane bis(trifluoromethylsulfonyl)imide trifluoromethanesulfonate (SLB-IL61) over the temperature range 80-260°C. Retention is governed by dispersion, dipole-type and hydrogen-bonding interactions each with its own temperature dependence. The exchange of a bis(trifluoromethylsulfonyl)imide anion in SLB-IL60 for a trifluromethanesulfonate anion (SLB-IL61) results in a change in selectivity indicated by an increase in the hydrogen-bond basicity and a decrease in hydrogen-bond acidity of the stationary phase without change in either the cohesion or dipolarity/polarizability of the stationary phases. At high temperatures there are small differences in electron lone pair interactions but these are relatively unimportant in terms of selectivity differences. Since the disclosed chemical structures for SLB-IL60 and SLB-IL61 does not contain obvious hydrogen-bond acid functional groups the modest hydrogen-bond acidity of these stationary phases was unexpected but does not appear to be obviously connected to adsorption sites at the column wall. The polarity number is shown to be a poor indicator of column retention properties for SLB-IL61. Principal component analysis with the system constants as variables indicates that the retention properties of SLB-IL61 are not duplicated by any of the common poly(siloxane) and poly(ethylene glycol) stationary phase chemistries in current use for column preparation. The SLB-IL61 column is closest in separation properties to poly(cyanopropylphenyldimethylsiloxane) and poly(cyanopropylmethyldimethysiloxane) stationary phases with a high percentage of cyanopropyl-containing monomer but the two stationary phase types are not selectivity equivalent.

Estimation of descriptors for hydrogen-bonding compounds from chromatographic and liquid-liquid partition measurements

Retention factors obtained by gas chromatography and reversed-phase liquid chromatography on varied columns and partition constants in different liquid-liquid partition systems are used to estimate WSU descriptor values for 36 anilines and N-heterocyclic compounds, 13 amides and related compounds, and 45 phenols and alcohols. These compounds are suitable for use as calibration compounds to characterize separation systems covering the descriptor space E=0.2-3, S=0.4-2.1, A=0-1.5, B=0.1-1.5, L=2.5-10.0 and V=0.5-2.2. Hydrogen-bonding properties are discussed in terms of structure, the possibility of induction effects, intramolecular hydrogen bonding and steric factors for anilines, amides, phenols and alcohols. The relationship between these parameters and observed descriptor values are difficult to predict from structure but facilitate improving the general occupancy of the descriptor space by creating incremental changes in hydrogen-bonding properties. It is verified that the compounds included in this study can be merged with an existing database of compounds recommended for characterizing separation systems.

Chapter 4 - Reversed-phase liquid chromatography

Abstract Reversed-phase liquid chromatography encompasses a family of separation techniques characterized by the distribution of compounds between a water-containing mobile phase and a relatively nonselective stationary phase. As a consequence, for method development, variation of mobile-phase properties are more influential on retention and selectivity than the variation of stationary-phase properties. Empirical models provide useful relationships to predict changes in retention for variation in solvent strength and temperature. Quantitative structure-retention relationships facilitate the selection of separation conditions for mixtures of known composition. The heterogeneous nature of the solvated stationary phase has so far, however, frustrated the development of fundamental thermodynamic retention models. In their absence, linear free energy relationship models have been developed to facilitate a global understanding of chromatographic selectivity and its variation with experimental parameters and for the classification of stationary-phase selectivity for the separation of neutral low-mass compounds. These tools are invaluable for obtaining insight into the retention mechanism in reversed-phase liquid chromatography.