Net Retention Volumes Research Articles

Description of substituent effects in gas-liquid chromatography

Substituent effects in gas-liquid chromatography are defined as the deviations, ΔΔlog V N , of the additivity of group contributions to log V N . To determine ΔΔlog V N values, corrected net retention volumes, V N were measured for o-, m- and p-substituted anilines, phenols and pyridines, and for monosubstituted benzenes, cyclohexanes and hexanes on polyethylene glycol 20,000 (at several temperatures) and on Apiezon M (at 206°), the substituents being H, CH 3, C 2H 5, F, Cl, Br, I, OCH 3, CN and NO 2. The ΔΔlog V N values were correlated with Hammett-type substituent constants using a three-parameter regression equation. The precision of the regressions was generally comparable to the experimental precision. The correlations provide information on the mechanism of the interaction between solutes and stationary phases. An interpretation of the effect of sample size on V N is given.

Thermodynamics of adsorption based on gas–solid chromatography

The net retention volume in gas–solid chromatography is expressed in terms of changes in the thermodynamic functions of adsorption and then is used for the experimental determination of these functions. The differential entropy of adsorption is a linear function of the differential enthalpy of adsorption, accounted for by assuming a heterogeneous surface and an exponential distribution of adsorption sites with respect to the energy. A criterion for predicting the above “thermodynamic compensation effect” is derived.The net retention volume is also expressed in terms of molecular properties and its dependence on the number of carbon atoms in the molecule is explained.Finally, from the results of adsorption of alkanes and cycloalkanes on aluminium oxide, the frequency of the vibrations of the adsorbate molecules normal to the surface is calculated.

Peak asymmetry in gas-liquid chromatography caused by adsorption exchange between the molecules of the sorbate and the stationary liquid phase

The influence of the process of competitive adsorption or adsorption exchange between the molecules of the eluted substance and the liquid phase specifically interacting with the active centers of the solid support particles on the peak shape and the (net) retention volume is discussed. It is derived theoretically and shown experimentally that the process of adsorption exchange on the sites of the solid support's surface covered with a monomolecular adsorption layer of the liquid phase molecules leads to peak asymmetry in the region of small samples and to a linear relationship between the (net) retention volumes and the reciprocal values of the square roots of the peak heights.

The properties of column packings containing small amounts of stationary phase in gas-liquid chromatography

Graphs are presented of the relations between the net retention volume per gram of column packing, V Ng, and the percentage of loading of Polsorb C with the stationary phases n-octadecanol and n-octadecane. From these relations and the value of the contact angle between the surface of diatomite and solutions of the stationary phases, it is shown that mono- and bimolecular layers of the stationary phase are formed on the support surface. The distribution of the stationary phases on the surface of the support is discussed in terms of the energetics of the support surface.

Quantitative separation of nucleotides on mercurated dextran

Dextran gels of varying porosities (Sephadex G series) were chemically modified so as to contain covalently bound monofunctional mercury. Mercurated Sephadex of the porosity G-25 was then used to fractionate mixtures of mono- and dinucleotides into the constituent components. Separation is based on the affinity of the nitrogen binding sites of the purine and pyrimidine derivatives for organomercurial Hg +. Thus, a mixture composed of the four mononucleotides Cyd-5′- P, Ado-3′- P, Guo-2′(3′)- P, dThd-5′- P and of the four dinucleotides Cyd- P-Cyd, Ado- P-Ado, Guo- P-Urd, and Urd- P-Urd could be separated into eight well-resolved fractions by using a combination Tris-bicarbonate/carbonate buffer system of increasing pH as an eluent. Complete separation was also achieved when a mixture of Ado-3′:5′- P, Ado-5′- P, Ado-5′- PP, and Ado-5′- PPP was fractionated on mercurated Sephadex G-25. Again, Tris-bicarbonate/carbonate buffer served as an eluent. Lastly, fractionation can also be performed at a constant pH by offering other suitable ligands, for instance Cl −, that will compete with nucleotides for monofunctional Hg +. The fractionation behavior of mercurated Sephadex G-25 can be fully understood on the basis of the complexing properties of monofunctional Hg +. This has been shown by calculating the net retention volume ratios of several nucleotides with the help of the known interaction parameters of corresponding nucleosides with CH 3 HgOH and by comparing the predicted ratios with the experimentally measured ones. Finally, the acid-base properties of mercurated Sephadex G-25 as well as its affinity for chloride and iodide ions have been determined. The data agree quite well with those known for CH 3 HgOH.

Evaluation of errors in the determination of gas chromatographic retention parameters and derivative quantities

The problem of the evaluation of errors in the determination of gas chromatographic retention parameters was discussed with reference to the types of errors that can be found practically, with reference to the type of quantities measured, to the retention parameters thereby obtained and finally to the type of derivative quantities (Δ H of evaporation). The influence of random and systematic errors and the variation in the experimental conditions on the precision and accuracy of the net retention volume can be obtained by using the error transmission. The determination of the specific retention volume also requires the correct determination of the quantities and types of phases that contribute to the retention, and the handling of correct expressions. The precision and accuracy of the determination of Δ H obtained from the relation ln V 0 = b/T e + a (where T c is the temperature of the column in °K) were discussed with reference to the presence of the following types of errors in the retention volume; 1. (1) random errors; 2. (2) systematic errors, constant in their relative values; 3. (3) systematic errors, constant in their absolute values; 4. (4) the retention volumes determined at different temperatures are affected by errors that depend stochastically on the time and on the temperature of operation of the gas chromatographic column; 5. (5) the partition system is not homogeneous (many phases contribute to the retention). In the first instance there is a no accurate value of Δ H, while Δ H is accurate in the second instance. In the other instances the determination of Δ H is incorrect or not reliable from the physical point of view. When there are only systematic errors, which are constant in their absolute values, it is then possible to apply the following expression: ▪ where x = 1/ T c and ( V N + Δ) is the measured retention value, which is affected by a systematic constant error. We can obtain from such an expression, by successive approximation, the value of Δ and thence also of b and δ H.

A gas‐chromatographic investigation of the thermodynamics of solutions of some normal and branched alkanes, cyclohexane, benzene and tetrachloromethane in polyisobutylene

AbstractFrom net retention volumes, measured over a wide range of temperatures, solute molar volumes and solute fugacities, standard partial molar free enthalpies, entropies and enthalpies of mixing at infinite dilution in polyisobutylene solution were calculated over a range of temperatures.Gas‐chromatography is an unique method in the sense that it gives thermodynamic data at polymer mole fractions near unity whereas conventional methods usually are not capable of giving accurate data for very concentrated polymer solutions.The residual standard partial molar free enthalpies and entropies and the standard partial molar enthalpies of mixing of the normal and branched alkanes were analysed by the (simplified) Prigogine solution theory. Those of the n‐alkanes, cyclohexane, benzene and tetrachloromethane were discussed in terms of the solution theory of Flory, Orwoll and Vrij.

The effect of carrier gas nonideality and adsorption on the net retention volume in gas—solid chromatography

The effect of carrier gas nonideality and adsorption on the net retention volume has been considered and certain approximate relationships are developed. These in turn are applied to the net retention volume of methane obtained on a 200-ft. (I/8 in. O.D.) Porapak S column at o°C where He, Ar and CO 2 were employed as carrier gases. In all cases it was found that the effect of carrier gas nonideality on the net retention volume was much smaller than that arising from carrier gas adsorption. The relative degree of adsorption was found to be in the expected order, that is, He < Ar<CO 2. The adsorption of all three gases was found to be governed by a single pseudo adsorption isotherm of the form θ = k P n (I + k P n . All indications are that due to the uncertainty in the nature of the carrier gas adsorption isotherms, gas—solid chromatography is not a suitable method in the determination of the B 12 terms.

Relationship between net retention volume and column temperature in gas chromatography

Relationship between net retention volume and column temperature in gas chromatography

Determination of the relationship between retention index and column temperature in gas chromatograpy through the temperature-dependence of the net retention volume

By working with one of the most characteristic fundamentals of gas chromatography, viz. the net retention volume, we have shown that the temperature-dependence of the retention index may be generally determined by an equation of Antoine-type: ▪ The three constants of the equation depend only on the material quality of the substance and of the stationary phase and can easily be determined from experimental results. In this work we also describe the relationships necessary for the definition in question. Furthermore, we were able to show that in spite of the fact that the graphic image of the Antoine-type equation is a hyperbola, in the temperature-interval used in gas chromatography this plot has a linear section. Finally we present some practical examples for the application of the equation.

Gas-liquid chromatographic determination of cross-term second virial coefficients using glycerol. Benzene + nitrogen and benzene + carbon dioxide at 50°C

This paper extends the theory of gas-chromatographic elution of a highly dilute solute to the second order in gas-phase imperfection terms, yielding a precise expression for the ideal retention volume in terms of the thermodynamic parameters of the system. The effect of carrier gas dissolution in the solvent is included explicitly; the cross-term second virial coefficient B12 for the system solute + carrier gas cannot be evaluated unambiguously for an appreciably soluble carrier gas. The use in such cases of polar solvents in which non-polar carrier gases are effectively insoluble is discussed with reference to surface adsorption and chromatographic non-ideality, which are important in polar solvents. Extrapolation of observed retention volume to zero flowrate for each of a series of columns covering a range of solvent loadings should give, corresponding to the limit of infinite loading, unambiguous values of bulk-phase solute-in-solvent activity coefficient and of B12. The theory has been applied to measurements on the systems benzene + nitrogen + glycerol, and benzene + carbon dioxide + glycerol, using four columns loaded at 15.7, 25.3, 33.6 and 23.3 % by weight glycerol on Celite, and the system benzene + hydrogen + glycerol using a column loaded at 44.0 %. The flowrate-dependence of the net retention volume is approximately linear on all columns, the gradient correlating well with empirical plate height and with solvent loading, in accordance with theory and with the known facts about the distribution of a polar solvent on Celite. The zero-flowrate B12 values are effectively the same with all columns. These values of B12 at 50°C are, for benzene + nitrogen, –98 ± 9 cm3 mole–1, and for benzene + carbon dioxide, –250 ± 15 cm3 mole–1, both being in fair agreement with theoretical predictions for systems of non-spherical molecules of different sizes. The activity coefficient for benzene at infinite dilution in glycerol at 50°C is logγ∞13= 2.084 ± 0.005.

Temperature independent retention in gas chromatography.

THE infinite dilution net retention volume (solubility) of solutes in GLC is related to the corresponding partition coefficient K∞ and solvent volume VL via Although VL usually increases slightly with increasing temperature, K diminishes so rapidly that dVR/dT is inevitably negative. The possibility that means may be found to cause VL to increase with increasing temperature faster than K decreases seems never to have been explored although it obviously presents an approach to practical attainment of a novel GLC situation in which, at least for some range of temperature, d VR/dT is positive or, more interesting, can be made zero.