F.R. Gonzalez

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.

Considerations on the temperature dependence of the gas–liquid chromatographic retention

A discussion on the temperature dependence of the partition coefficient K is developed. This discussion embraces topics such as the limitations of conventional thermodynamic approaches followed in the chromatographic literature, qualitative theoretical notions arising from molecular thermodynamics and the experimental information that is accessible through modern capillary gas chromatography. It is shown that the heat capacity difference of solute transfer for flexible molecules has at least one maximum in the chromatographic range of temperature. As a consequence, a great amount of experimental data is required for a correct thermodynamic interpretation of the chromatographic retention.

Consistency of gas hold-up determinations

Abstract The retention of methane on poly(dimethylsiloxane) capillary columns was studied focusing its relation with the gas hold-up time t M , which was derived from the retention of n -alkanes. The [ideal gas/Van der Waals fluid] partition equation was applied in order to provide the mathematical expression for the retention factor of the n -alkanes as a function of the carbon number n . In this context the partial molar free energy of solution is a non-linear function of the chain length Δ G s ( n ) ( n ≥5). The resultant retention factor k of methane, determined from its retention time and the calculated t M , approaches an almost constant value when the temperature is increased beyond 100°C. This value of k has the order of the reciprocal column phase ratio 1/ β . Precisely, this is the theoretically expected limit of a non-interacting solute or inert marker.

Permethylated β-cyclodextrin in liquid poly(oxyethylene) as a stationary phase for capillary gas chromatography

This paper reports the study of poly(oxyethylene) as a solvent for heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (PM-beta-CD) and the potential of these mixtures for constructing enantioselective gas chromatography columns. Enantioseparations of volatile racemic mixtures using capillaries coated with 10-50% PM-beta-CD diluted in Carbowax 20M were evaluated. The influence of the polymer type on retention, separation and reproducibility over a period of time is critically discussed. The poly(oxyethylene)-based columns were also compared to columns prepared by dilution of PM-beta-CD in two polysiloxanes, SE-30 and SE-54, at several temperatures. Finally, we evaluated a new stationary phase prepared by crosslinking the chiral selector to the poly(oxyethylene) matrix.

Distribution coefficients of n-alkanes measured on wall-coated capillary columns

Distribution coefficients K of n-alkanes were determined in wide ranges of temperature and carbon numbers from gas chromatographic retention data measured on wall-coated poly(dimethylsiloxane) commercial capillary columns. A discussion is centered on how to mitigate the difficulties for an accurate determination of K when using weakly retentive columns, as those bearing very high phase ratios or short lengths. Particularly, the errors associated with the estimation of the gas hold-up and the phase ratio of the column are considered. The chromatographic importance for determining K of n-alkanes relies on the fact that these are the most commonly applied references for reporting relative thermodynamic parameters such as the Kovats Index and the relative retention. A great amount of information has been compiled in this form. If K of the reference is known, absolute values of distribution coefficients for a myriad of substances are readily obtainable. The knowledge of K(T) functions of solutes in wide ranges of temperature is a primary necessity in temperature-programmed gas chromatography. This knowledge is needed for the prediction of absolute retention times and for computing separation optimizations of mixtures containing several critical pairs of analytes.

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.

Interpreting the gas chromatographic retention of n-alkanes

Non-linear regressions were applied to n-alkanes retention data for the determination of gas hold-up in a preceding paper. It was found that at temperatures over 100 degrees C the reduced partial molar free energy of solution, deltaG/RT, tends to be negligible for the solute methane in poly(dimethylsiloxane) stationary phases. A consequence of interest can be inferred from this fact. The C-H bonds from terminal methyl groups of n-alkane solute molecules should not contribute significantly to deltaG/RT in these conditions. The analysis of data confirms that, within the chromatographic experimental error, the contributions of n-alkane end C-H bonds are also negligible in this temperature range. Consequently, the regression parameter that contains the phase ratio of the column only includes the gas hold-up as the accompanying factor.