Classification of stationary phases and other materials by gas chromatography

Experimental determination and prediction of the gas-liquid n-hexadecane partition coefficients

Experimental methods based on gas-phase chromatography were tested with a view to determine the gas-liquid n-hexadecane partition coefficients, log L16 of non-volatile compounds at 298.2 K. It was demonstrated that reliable values of log L16 of compounds more volatile than n-docosane can be obtained using either capillary, or packed columns. The main limitation of both methods is the column stability at high temperatures. Here we propose a new method based on the temperature gradient mode, to obtain log L16 of high-boiling compounds. A group contribution model is also presented in view to predicting log L16 values of non-volatile compounds.

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.

Linear free energy relationships used to evaluate equilibrium partitioning of organic compounds

In environmental chemistry, one often wants to interpret or predict equilibrium partitioning of organic compounds between any two phases. Hence, one needs to understand the partition variability that stems from using different types of compounds and the variability that arises from looking at different natural phases, e.g. different soil organic matter. It is current practice in environmental chemistry to evaluate equilibrium partitioning with the help of double logarithmic correlations between the unknown partition constant and a well-known partition constant of the compounds, e.g., partitioning between natural organic matter and water or air is correlated with the octanol/water or octanol/air partition constant, respectively. However, these relationships (in the following called one-parameter LFERs) can only predict the compound variability within a single substance class. They supply no means to understand the variability between substance classes or the variability between different natural organic phases. The reasons for these limitations are that (a) the complete compound variability cannot be described by a single parameter because partitioning results from different kinds of interactions that vary independently from each other and (b) the specific properties of the studied phase are represented in the slope and intercept of the double logarithmic correlation and not in a variable parameter. In contrastto one-parameter LFERs, polyparameter LFERs are based on a concept that considers all interactions involved in partitioning by separate parameters. They allow for predicting the complete compound variability by a single equation, and they also provide the possibility to evaluate and predict the variability in the sorption characteristics of different natural phases. Thus future research in the field of environmental partition processes should focus on adapting and improving the more comprehensive polyparameter LFERs rather than trying to refine existing one-parameter LFERs.

Prediction of retention indices. V. Influence of electronic effects and column polarity on retention index

The retention index increment for addition of a methylene group to an analyte molecule is shown for 1-halo-n-alkanes to be different from 100 i.u., a value that is customarily assigned according to the current convention in retention index prediction. In temperature-programmed gas chromatography using linearly interpolated retention index I, a linear regression equation, I=AZ+(GRF), with the number of atoms (Z) in the molecule as variable can describe the retention of 16 homologous series of organic compounds on non-polar and polar columns with characteristic A (linear regression coefficient) and (GRF) (group retention factor) values. A molecular model of retention on the basis of electron density and electron density distribution relative to that of n-alkane is proposed. This model brings out the inter- and intramolecular electronic effects in the analyte molecule and its dipole-dipole interaction with the stationary liquid phases, as variations in the A value. The (GRF) value varies with the connectivity ability of a functional group for extended conjugation, substitution, etc., but is most influenced by hydrogen bonding (H-bonding) with the stationary liquid phase. One can estimate the sequence of elution of a mixture of organic compounds from any two of the three parameters on the right-hand side of the above equation or retrieve the retention indexes of an entire homologous series from its A and (GRF) values. The fact that each analyte molecule has its own A value on different columns makes column difference (deltaI) compound-specific rather than column-specific, a departure from previous assumptions.

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.

Practitioner's guide to method development in thin-layer chromatography

The authors provide a personal perspective of method development in thin-layer chromatography for the novice and more experienced chromatographer alike. No attempt has been made at a comprehensive survey of the literature. Instead we provide an overview with insights into a smaller number of approaches that the authors have found useful in their own work and indicate the factors responsible for the variation in retention and their control. The main topics covered are the relationship between sorbent chemistry and retention, the selection of primary solvents for mobile phase optimization and mobile phase optimization using the PRISMA and solvation parameter models.

Rapid method for estimating the octanol--water partition coefficient (log P ow) by microemulsion electrokinetic chromatography

Several surfactant systems were evaluated based on their system constants determined by the solvation parameter model for the design of a surrogate chromatographic model for the rapid estimation of octanol-water partition coefficient (log Pow) by microemulsion electrokinetic chromatography. The system constant ratios responsible for the log Pow partition system are (nearly) the same as those for the microemulsion system containing sodium dodecyl sulfate (1.4% w/v), butan-1-ol (8% v/v) and heptane (1.2% v/v). Neutral and basic compounds are analyzed using a fused-silica capillary column with a 50 mM sodium phosphate-sodium borate (3:2) buffer at pH 10. Weakly acid compounds require the use of sulfonated silica capillary column and a 50 mM sodium phosphate buffer at pH 3. For 29 varied neutral and weakly basic compounds the average error between log Pow estimated using MEEKC and literature values was +/-0.12 over a log Pow range from 0.3 to 5.8.

Contributions of theory to method development in solid-phase extraction

The kinetic and retention properties of solid-phase extraction devices are reviewed from the perspective of method development strategies. Models based on frontal analysis are used to correct retention properties of solid-phase extraction devices to account for the fact that too few theoretical plates are provided for retention to be independent of kinetic factors. The available pressure drop for the sampling device largely dictates the choice of useful particle sizes and maximum bed length. The use of octanol--water partition coefficients and extrapolated values of the retention factor obtained by liquid chromatography are poor empirical models for the estimation of breakthrough volumes with water as the sample solvent. The solvation parameter model provides an adequate description of sorbent retention for the estimation of breakthrough volumes, rinse solvent volume and composition, and elution solvent volume and composition. Combining the frontal analysis and solvation parameter models offers a comprehensive approach to computer-aided method development in solid-phase extraction. This is the first step in the development of a structure-driven approach to method development in solid-phase extraction that should be more reliable and less tedious than traditional trial and error approaches.

Revised linear solvation energy relationship coefficients for the 77-phase McReynolds data set based on an updated set of solute descriptors

Linear solvation energy relationship (LSER) coefficients for the 77-phase McReynolds data set have been recalculated using updated solute descriptors in the revised solvation equation: [equation: see text] These revised LSER coefficients are presented and classified by cluster analysis into groupings of stationary phases which have comparable solubility properties. It was found that the groupings were similar to those proposed by Abraham using the original solvation equation and that any dissimilarities were readily explainable by the grouping methods that were applied. Comparison of the original coefficients with the revised set also shows that several stationary phases which had a statistically insignificant b1 value with the original equation now have significant b1 values when utilizing the revised solvation equation.