Angel Miguel Nardillo

Retention index in temperature-programmed gas chromatography

A persistent feature in the development of temperature-programmed gas chromatography (TPGC) has been the search of a retention index IT, defined as a relationship between the retention of the analyte and two members of an homologous series enclosing it. The motivation is to find for TPGC a parameter IT resembling the properties of the isothermal Kovats index I. The latter is a relative thermodynamic parameter which provides basic information on solute–solvent interactions, independently from the chromatographic flow conditions, column’s dimensions and phase ratios. Several years ago, Rijks and coworkers pointed out that the isothermal Kovats index I and the TPGC index IT are incommensurable. The implications from their remark have not received sufficient attention. As a consequence, in many recent publications IT is still presented as a general useful parameter to be reported, specifically with reference to its correlation to solutes structural properties and their partition coefficients, analogous to the applicability of I. A renewed discussion on IT is proposed, with the aim of divulging the unsolvable difficulties for obtaining relevant information from this that could be shared between laboratories. The influence of the column inlet pressure pi and the ratio L/dc on the reproducibility of IT is utilized as an example to illustrate and discuss the basic concepts. Conditions were selected involving capillary gas chromatography of solutes presenting a retention reversal with the bracketing n-alkanes. Several important aspects of IT were illustrated under these conditions. Since basic information of general applicability cannot be derived from IT, it is concluded that only a compilation of thermodynamic parameters can reliably be shared by chromatographers.

Regression against temperature of gas chromatographic retention data

Abstract Specific retention volumes were measured in the range 25–75°C for benzene + squalane, benzene + triethylene glycol and n -hexane + squalane and in the range 15–55°C for benzene + tetraethylene glycol dimethyl ether, each in steps of 5°C. Values for the solution thermodynamic properties and their errors were obtained by fitting the experimental data to equations with two or more constant by using the method of Clarke and Glew. The best thermodynamic results were obtained by means of the three-constant equation; adding a fourth constant did not improve the adjustment. However, when the objective is interpolation, the results obtained with the classical, two-constant equation, are of adequate accuracy.

Thermodynamic study of the behaviour of two molten organic salts as stationary phases in gas chromatography

Abstract The gas chromatographic retention behaviour of hydrocarbons of different types and of a group of halogenated compounds was measured at four temperatures from 20 to 30°C in columns containing ethylammonium nitrate (EAN) or n-propylammonium nitrate (PAN) as stationary phases. Thermodynamic functions of solution calculated from these data are compared to those in non-ionic organic solvents and in water. The solvent behaviour of both molten salts fits well with those of other solvents of high cohensive energy. Alkanes and olefins are retained by mixed processes of solution and adsorption at the gas-liquid interface.

Theoretical and practical aspects of flow control in programmed-temperature gas chromatography

Abstract Behavior patterns for the temperature dependence of column head pressure and outlet volumeteric flow-rate are analyzed for some particular configurations of the chromatographs flow control system. Quantitative prediction of this behavior, and especially its effect on gas hold up time, is evaluated. Attention is centered on the application of programmed-temperature gas chromatography (PTGC) retention simulation.

Aspects of the elution order inversion by pressure changes in programmed-temperature gas chromatography

The phenomenon of elution order inversion in programmed-temperature gas chromatography, as a result of only changing the head pressure of the column while retaining the same temperature program, is known to be experimentally detectable. In this paper we analyze some theoretical aspects of this phenomenon. These aspects concern the dependence of the separation on the pressure program for a couple of analytes presenting a retention reversal in isothermal gas chromatography (GC). In the theoretical context, the reason why the pressure program will not have a uniform effect on the separation of all crossing-over couples from a mixture, and why it is not possible to obtain a simple correlation for the separation of a given couple under different pressure conditions is visualized. A complete simulation of the process is required to evaluate the effect of the chosen head pressure-temperature program on the separation of the solutes.

Considerations on the dependence of gas–liquid chromatographic retention of n-alkanes with the carbon number

The contributions to the free energy change involved in the chromatographic retention of n-alkanes, which are functions of the solute chain length, are discussed with the aid of recent advances in chain statistics. In the context of a simplified theoretical analysis, a discussion is promoted on the molecular nature of the observed slight deviations from linearity of ln tR′ vs. n. A non-linear function ln tR′(n) is derived from the application of the generalized Van der Waals fluid partition function to the liquid stationary phase. The excluded volume, particularly its dependence on n, is analyzed through the theory of Flory for dilute solutions of chain molecules.

Behavior of n-alkanes on poly(oxyethylene) capillary columns evaluation of interfacial effects

The solvation behavior of n-alkanes on poly(oxyethylene) was studied employing capillary gas chromatography. Interfacial effects were discriminated and evaluated through the analysis of retention data from six commercial fused-silica capillary columns, having film thicknesses of 0.15-5 microm. Expressions for the mixed retention mechanism in capillary columns were deduced from assumptions of a general character. Partition coefficients were determined for the n-alkanes up to 28 carbon atoms, at temperatures ranging from 40 to 240 degrees C. In agreement with other authors, it was observed that interfacial phenomena contribute poorly to the chromatographic retention, being negligible over 140 degrees C for homologues with less than 16 carbons.

Thermodynamics of hydrocarbon solutions using g.l.c. n-Hexane, n-heptane, benzene, and toluene as solutes each at infinite dilution in n-hexadecane, in n-octadecane, and in n-eicosane

Abstract Infinite-dilution activity coefficients were measured by g.l.c. over a range of 50 K for n-hexane, n-heptane, benzene, and toluene in n-hexadecane, n-octadecane, and n-eicosane; measurements for cyclohexane and tetrachloromethane in n-hexadecane are also discussed. The experimental results were fitted to an adaptation of the equation of Clarke and Glew, thus calculating partial molar excess enthalpies and heat capacities. The results are compared with those obtained by extrapolation from static results at finite mole fractions.

Solution and adsorption thermodynamics in propylene glycol by gas chromatography : A comparative study with other polyhydroxylated solvents

Abstract The retention volumes of eighteen hydrocarbons representative of different families and of acetone, diisopropyl ether and three chloromethanes were measured at four temperatures within the range 12–22°C on columns containing five different percentages of propylene glycol (PG) as the stationary phase. The relative magnitudes of the adsorption on PG, on formamide and on four glycols are compared and discussed in terms of the thermodynamic properties of the bulk mixtures with hydrocarbons, From this comparison, PG is found to possess the weakest adsorption effects, and some of the solutes studied exhibit no adsorption within experimental error. The thermodynamic properties of solution in PG are calculated and compared to those in formamide and in the glycols. Recent extensions of the regular solutions model are inefficient for calculating infinite dilution activity coefficients, but yield a very acceptable correlation between heats of solution and solute solubility parameters.

Revision of a theoretical expression for gas-liquid chromatographic retention.

In this communication, we revise some aspects of the [ideal gas/Van der Waals fluid] partition, derived in an earlier publication. The general character of the conclusions concerning the dependence of the partial molar free energy of solution, deltaGs(n), on the chain length, n, of linear solute molecules is shown through the relationship with more general partition formulations. Simultaneously, the correction of an error in the expression of the retention time dependence on the phase ratio of the chromatographic column, beta, is carried out. The misleading source of this error was redundant accounting in the solute translational contribution to deltaGs(n).