J.A. García-Domínguez

The hold-up time in gas chromatography II. Validation of the estimation based on the concept of a zero carbon atoms alkane

Abstract The procedure of estimating the hold-up time in gas chromatography recently presented by Lebron-Aguilar et al. [J. Chromatogr. A, 760 (1997) 219] as the retention of an alkane of zero carbon atoms has been exhaustively tested with packed and capillary columns of various stationary phase types, between 30 and 150°C. Several permanent gases injected have shown that there is not a general rule to decide which is the least retained gas under all circumstances. Results of hold-up times obtained with the new procedure have been compared with those of other well established methods. The conclusion arrived at is that the retention of an n-alkane of n carbon atoms is best described by the expression: tR (n)=A+exp(B+ C nD), and the best estimation of the hold-up time is obtained from that expression for n=0.

Solubility parameters of gas chromatographic mixed stationary phases

Gas chromatography was used to measure activity coefficients, ϒ∞, partial molar free energies of mixing, ΔG−∞1, Flory-Huggins interaction parameters, χ∞, and its enthalpic and entropic contributions, of a number of solutes on five single and fourteen mixed stationary phases at 120°C. Solubility parameters of the stationary phases, δ2, were deduced by the method of DiPaola-Baranyi and Guillet for the different solute-stationary phase combinations. The polymer-polymer interaction parameter, χ′2,3, was also calculated for the various mixed systems.

Inverse gas chromatography in characterization of surfactants : determination of binary parameter

Abstract Binary parameters corresponding to hydrogen bonding (I12 h) and polar (I12 p) interactions were determined by inverse gas chromatography. The influence of the structures of broad-range and narrow-range distributed oxyethylene derivatives of 1-hexadecanol on the binary parameters was studied. The relationship between the binary parameters (I12 h and I12 p) and the polarity of the compounds used as stationary phases is presented and discussed, in addition to the effect of the solubility parameter and its increments on the values of the two binary parameters.

Evaluation of the effect of the cyanopropyl radical on the interaction of the methylene group with silicone stationary phases

Values of the solute-polymer interaction parameter of n-alkanes and polar substances between 90C and 200°C were obtained on two packed columns (OV-101 and OV-105). Experiments carried out with a capillary column confirmed that with non-polar solute-polymer systems the values of both the specific retention volumes and the thermodynamic parameters derived from them are equivalent to those obtained with packed columns. The results showed that n-alkanes mix better with OV-101 and polar substances are better solvents of OV- 105. The differences are small, however. Partial molar enthalpies and free energies of solution and mixing were calculated. The contribution of the methylene group of the n-alkanes to the thermodynamic parameters of solution was evaluated. The effect of cyanopropyl group substitution on the contribution of the methylene group to the partial molar free energy of solution of n-alkanes was found to be roughly linear with the cyanoalkyl group content, decreasing as the polarity of the polymer increases. The solubility parameter of the stationary phase increases only slightly when a 5% substitution of cyanopropyl groups is introduced in the polymer.

Programmed-temperature retention indices : A survey of calculation methods

Abstract Retention temperatures of various solutes were measured on an OV-105 column under different heating rates. The experimental results agree with those evaluated by means of the equation of Curvers et al. Several methods of assigning programmed temperature retention indices to each solute were tested. Comparison of the results does not allow a clear choice of any of the methods, although a procedure based on an interpolation through the use of cubic splines polynomials is preferred. The sensitivity of the various procedures towards small changes in the input data is reported.

Effects of solvent density on retention in gas–liquid chromatography: I. Alkanes solutes in polyethylene glycol stationary phases

Gas-liquid chromatographic columns were prepared coating silica capillaries with poly(oxyethylene) polymers of different molecular mass distributions, in the range of low number-average molar masses, where the density still varies significantly. A novel, high-temperature, rapid evaporation method was developed and applied to the static coating of the low-molecular-mass stationary phases. The analysis of alkanes retention data from these columns reveals that the dependence of the partition coefficient with the solvent macroscopic density is mainly due to a variation of entropy. Enthalpies of solute transfer contribute poorly to the observed variations of retention. Since the alkanes solubility diminishes with the increasing solvent density, and this variation is weakly dependent with temperature, it is concluded that the decrease of free-volume in the liquid is responsible for this behavior.

Gas chromatographic comparative study of superox 20m immobilized in different ways

Abstract Three capillary columns of Superox 20M, two of them immobilized in different ways, were prepared. All columns showed a transition zone from adsorption of the solutes at low temperatures to absorption at higher temperatures. The minimum allowed operating temperature of the column was found to change from 64°C to 50°C when the polymer was immobilized. Partial molar enthalpies of mixing showed that alkanes are poor solvents of the stationary phase, while aromatics are good solvents, with little dependence on the degree of immobilization. It might be expected that Superox 20M will mix well with polymers with phenyl components, and will not mix with polymers of long hydrocarbon-type chains.

Mixed stationary phases in gas—liquid chromatography : Partition coefficients and retention indices in OV-101-OV-25, OV-101-carbowax 20M and OV-225-SP-2340 mixtures

Abstract Partition coefficients of n-alkanes and other substances were measured at 393 K on mixed stationary phase columns prepared by mixing OV-101 with OV-25, OV-101 with Carbowax 20M and OV-225 with SP-2340. The results are examined with respect to the Purnell—Vargas de Andrade and Perry—Tiley relationships. The solvent—solvent interaction parameter was calculated in the three systems. When the value of this parameter is higher than 2 a clear departure from the linear approximation was observed. Retention indices were also calculated and compared with those predicted by the above relationships.

Thermodynamic characterization of Superox 20M by inverse gas chromatography

Abstract Inverse gas chromatographic experiments carried out between 80 and 140°C were used to characterize Superox 20M deposited on glass capillary columns, using 28 substances belonging to five different chemical types. No absorption in the gas—liquid interphase was observed except in three cases corresponding to the highest molecular masses. The thermodynamic parameters of solution and mixing and the Flory—Huggins solute—stationary phase interaction parameter were measured. The dependences of all magnitudes on temperature and chain length were also calculated, and conclusions on the relationship between their values and the chromatographic behaviour of the solutes in the stationary phase were drawn. The solubility parameter of Superox 20M was measured between 80 and 140°C and was found to have a linear dependence on temperature. Values at 50, 64 and 280°C were deduced.

Gibbs energy of solution and molecular structural coefficients of 85 solutes on 20 gas chromatographic stationary phases

Abstract Molecular structural coefficients of Takacs ( S ci ), relative Gibbs solution energies ( y i ), and their respective increments, [ ΔS ci (RX)] and [ Δy i (RX)], with respect to parent RH n -alkanes have been determined using specific retention volumes of 85 solutes on 20 stationary phases at two common temperatures. Fair correlations between all these parameters and the cavity/dispersion, ΔG (C/D), and the polar, ΔG (P), Gibbs solution energy contributions, calculated with Abrahams solvation model, were obtained. It is proved that the polar solute-solvent interactions calculated for several RX chemical functions, according to the model proposed in this work, agree satisfactorily with both the polar and the cavity formation and dispersion contributions to the Gibbs solvation energy for the said RX chemical functions, deduced from Abrahams solvation model. The relationship between y i and S ci was also investigated.