Retention in Gas-Liquid Chromatography

Taking the above considerations into account, a general model for retention in gas-liquid chromatography is given by Eq (2.2) [29,184-188] 

Relative contribution (%) of different retention mechanisms for different solutes on Apiezon M and Carbowax 4000 based on Eq. (2.2) at 70° C. 

VsKs = contribution due to partition with the structured liquid phase layer 

By working at intermediate to high phase loadings the contributions of the structured liquid phase to retention can be neglected allowing Eq. (2.2) to be simplified and rearranged to 

OV-275. For solutes retained solely by gas-liquid partitioning (e.g. nitromethane, dioxane and ethanol) on Carbowax 20M the plots have a zero slope. The n-alkanes are retained by a mixed retention mechanism on Carbowax 20M as indicated by the slope. The relatively large intercept is an indication that gas-liquid partitioning makes a significant contribution to the retention mechanism. Interfacial adsorption is important for all compounds on OV-275 and is dominant for the n-alkanes, which have a near zero intercept, indicating that gas-liquid partitioning is of minor importance to their retention. The lack of a reliable method to estimate the surface area of the liquid phase prevents Eq. (2.3) from being used to determine the gas-liquid adsorption coefficient. 

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Taking the above considerations into account Nikolov proposed a general model to describe retention in gas-liquid chromatography given by equation (2.1)... 

L. Sun, J. I. Siepmann, W. L. Klotz and M. R. Schure, Retention in gas-liquid chromatography with a polyethylene oxide stationary phase Molecular simulation and experiment, /. Chromatogr., A, 2006, 1126, 373-380. 

D., Balahan, A. T. Comparison of weighting schemes for molecular graph descriptors application in quantitative structure-retention relationship models for alkylphenols in gas-liquid chromatography. J. Chem. Inf. Comput. Sci. 2000, 40, ITl-lM,. 

It is standard practice in gas-liquid chromatography to compare the retention time of each component to that of an arbitrary standard. The actual retention-time for any compound may differ greatly from column to column. Many authors, unfortunately, only report relative retention-times, and give no indication of the absolute time involved. Admittedly, this retention time will differ slightly according to the... 

VR has been used in gas-solid chromatography to represent corrected retention volume (49), whereas corrected retention volume in gas-liquid chromatography is given the symbol VR- Specific retention volume in gas-solid chromatography is V, but in... 

Veening, H., Huber, J. F. K. Phenomena which influence the retention of metal fluoroacetylacetonates in gas-liquid chromatography. J. Gas Chomatog. 6, 326 (1968). 

M.V. Budahegyi, E.R. Lombosi, T.S. Lombosi, G. Tarjan, I. Timar and J.M. Takacs, Twenty-fifth anniversary of the retention index system in gas-liquid chromatography, J. Chromatogr., 271, 213-307 (1983). 

Retention time The time a vaporized compound takes to pass through the column in gas-liquid chromatography (i.e. the time it is retained in the column). 

It has been shown that gas-Hquid chromatographic methods are particularly suitable for a quantitative characterization of the polarity of solvents. In gas-liquid chromatography it is possible to determine the solvent power of the stationary liquid phase very accurately for a large number of substances [98, 99, 259, 260]. Many groups of substances exhibit a certain dependence of their relative retention parameters on the solvation characteristics of the stationary phase or of the separable components. In determining universal gas-chromatographic characteristics, the so-called retention index, I, introduced by Kovats [100], is frequently used. The elution maxima of individual members of the homologous series of n-alkanes (C H2 +2) form the fixed points of the system of retention indices. The retention index is defined by means of Eq. (7-41),... 

Both of these approaches used in the characterization of stationary liquid-phase polarities by means of retention indices have been further explored and expanded [104, 259-261]. For a review on the characterization of solvent properties of phases used in gas-liquid chromatography by means of the retention index system, see reference [344]. Similar methods for the characterization of solvent polarity in liquid-liquid and liquid-solid chromatography can be found in references [105-107] cf also Section A-7 and Tables A-10 and A-11 in the Appendix. 

Retention volume Change in temperature can affect the distribution of a sample between the mobile and stationary phases and thereby affect retention behavior. In ion-exchange chromatography the effect of temperature is slight, but when the value of the partition ratio depends on adsorption, temperature variation is more significant. In gas-liquid chromatography the effects of temperature have been thoroughly studied, and the fundamental retention relation is... 

J. Mraz, P. Jheeta, A. Gescher, M.D. ThreadgUl (1993) Unusual deuterium isotope effect on the retention of formamides in gas-liquid chromatography. J. Chromatogr. 641. 194-198... 

In gas-liquid chromatography, the stationary phase is a liquid that is immobilized on a solid. Retention of sample constituents involves equilibria between a gaseous and a liquid phase. In ga.s-solid chromatography, the stationary phase is a solid surface that retains analytes by physical adsorption. Here separation involves adsorption equilibria. 

Fig. 1.—Quantities in Gas-Liquid Chromatography. (A, peak representing component A B, peak representing component B I, point of injection of sample VrA, retention volume of peak A, also equals d in expression (2) VrB, retention volume of peak B. All horizontal measurements are made along the base line.)...

So far we have discussed solvation properties at a reference temperature of 120°C. The choice of reference temperature arises from historical considerations. McReynolds chose this temperature to compile his extensive database of retention measurements for volatile solutes on a large number of stationary phases. His database has been widely used for exploring new approaches to stationary phase classification and has influenced others into using the same temperature to collect additional reference data to maintain compatibility with the original database. The choice of a standard reference temperature is of less concern than whether a single reference temperature is sufficient to classify solvent properties for use at temperatures distant from the reference temperature. There is only a limited amount of data for the influence of temperature on selectivity in gas-liquid chromatography [53,81,103,121,122]. In general polar interactions are... 

The appropriate sample size depends on whether the investigations are carried out in the range of infinite dilution or of finite-concentration . At infinite dilution, the concentrations of the components in the carrier gas may be neglected and thus the gaseous phase may be considered ideal. At finite concentration, a correction for retention calculations should be introduced, which makes them more complicated. Certain limitations of GC methods for physicochemical determinations should also be mentioned. These are mainly restricted to the study of interactions that occur on solids, in liquids, in the mobile gas phase, and at their interfaces. While measurements can be made simultaneously, it is possible that interference from other physicochemical properties of the materials may introduce inaccuracy. Another important limitation is the volatility of the stationary phase in gas-liquid chromatography. 

The system of McReynolds constants inspired many researchers to carry out more extensive explorations. Based on this system, Tacacs constructed a unified system for the prediction of retention data in gas—liquid chromatography. 

Gawdzik, B. Studies on selectivity of porous polymers based on polyarfxnatic esters. J. Chromatogr. 1990,503,41-49. Tacacs, J.M. Unified system for the prediction of retention data in gas-liquid chromatography. J. Chromatogr. A. 1998, 799,185 205. 

Fig. 2 Predicted Kovats retention indices for various analytes in gas-liquid chromatography with squalane and polyethylene oxide stationary phases. See text for description of the analyte molecules...

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