Michal Roth

Utilization of the solution-of-groups concept in gas—liquid chromatography

Abstract The possibilities of utilizing the UNIFAC group model of activity coefficients for the prediction of gas—liquid chromatographic retention data are discussed. Examples of the determination of the specific retention volumes of different types of solute compounds on both simple and mixed stationary phases, relative retentions, Kovats retention indices and differential sorption enthalpies are given. It appears that the original version of the UNIFAC model can be used merely to give a rough estimation of relative retentions, Kovats retention indices and sorption enthalpies.

Column pressure drop in capillary supercritical fluid chromatography

Abstract Some relationships have been derived describing the pressure profile along a capillary column in supercritical fluid chromatography (SFC) in terms of e

Enthalpy of transfer in supercritical fluid chromatography

Abstract The enthalphy of solute transfer from the mobile to the stationary phase in supercritical fluid chromatograhy has often been calculated from the plot of solute retention against the reciprocal absolute termperature at a constant density of the mobile phase fluid. In this treatment it has been assumed that enthalpy of solute tranfer is directly proportional to the slope of this plot. In the present report, the above procedure is proven to be incorrect. A thermodynamic analysis shows that the slope of a constant-density plot is related neither to enthalpy nor to integral energy of solute transfer between the two phases.

Correlation of gas chromatographic specific retention volumes of homologous compounds with temperatures and methylene number

Abstract The specific retention volumes of homologous compounds can be expressed by log V g = p + q / T + nr + s , where n is the number of methylene groups in the solute molecule, T is the absolute temperature and p, q, r and s are constants related to the thermodynamic properties of the chromatographic system. Generally, the specific retention volumes measured for four homologues at different temperatures are sufficient for determining the constants of the equation. The accuracy of the V g values calculated for given n,T data by the equation is commensurate with the precision of replicate determinations of the V g value of a given compound on a given instrument.

Solute retention in supercritical fluid chromatography as a function of pressure: A thermodynamic analysis

Abstract The pressure dependence of the solute retention in a hypothetical, isothermal, isobaric, chromatographic column is discussed in terms of the thermodynamic properties involved. In general, the logarithm of the solute capacity ratio decreases with increasing pressure along an S-shaped curve. To a first approximation, the point of inflection occurs at the pressure corresponding to the maximum compressibility of the mobile-phase fluid at the column temperature. The occurrence of a high-pressure minimum in retention at a constant temperature of the column is possible but unlikely to be observed within the practical range of operating pressure in supercritical fluid chromatography (SFC. The pressure course of the solute retention in SFC is compared with the pressure dependence of the solute solubility in pure solid-supercritical fluid phase equilibria. The sorption of the mobile phase into the stationary phase is shown to be a significant source of uncertainty in the thermodynamic interpretation of the solute retention in SFC.

Interference effects in trapping trace components from gases on chromatographic sorbents : Sorption of benzene in the presence of o-xylene

Abstract The sorption isotherms of benzene in the presence of o-xylene as a co-sorbate were measured in various chromatographic sorbent—nitrogen systems at 35°C. The benzene gas-phase concentration was kept at 0.5 ppm while that of o-xylene was varied within about 0.025–25 ppm. With Tenax GC, Porapak P and Porapak Q as sorbents the partition coefficients of benzene were substantially decreased due to the presence even of about 0.01 ppm o-xylene in the gaseous phase. With Apiezon K and QF-1 liquid sorbents the partition coefficients of benzene remained practically constant up to about 2.5 ppm of o-xylene in the gaseous phase.

Activity coefficients in binary liquid mixtures measured by reversed-flow gas chromatography

Abstract Three methods of reversed-flow gas chromatography are presented for the determination of activity coefficients in binary liquid mixtures. The systems used were n-hexane-n-hexadecane and benzene-n-octane. the average activity coefficients obtained for n-hexane in the first system were close to those given in the literature, when the n-hexane mole fraction was close to unity, whereas the values found for benzene in the second system were close to those calculated theoretically, using the Margules equation, when the benzene mole fraction approached zero or unity. On the other hand, the activity coefficients of n-octane in the same system were close to the theoretical values only when the n-octane mole fraction was close to unity. All these deviations are discussed and possible causes, such as liquid diffusion, fugacity correction and thermal non-equilibrium considered.

Sensitivity of gas chromatographic analysis on packed and capillary columns with different types of detector under resolution-normalized conditions

Abstract A comparison of the sensitivity of gas chromatographic analysis on packed and capillary columns coupled with a concentration-sensitive or a mass rate-sensitive detector under resolution-normalized conditions was carried out, the sensitivity being defined as the peak-maximum detector response per unit mass of analyte in the eluted zone. Capillary columns generally yield a substantially higher sensitivity as compared to packed columns under these conditions. With concentration-sensitive detectors the gain in sensitivity produced on replacing a packed column by a capillary one is about two orders of magnitude larger than that produced when a mass rate-sensitive detector is employed.

Effect of column inlet concentration profile on the sensitivity of analysis : A quantitative assessment

In column chromatography, especially when trace amounts of analytes are to be determined, it is instructive to describe quantitatively the effects of the individual experimental parameters on the sensitivity of analysis1-4. The sensitivity of chromatographic analysis depends, among other factors, on the manner in which the sample is introduced into the column. However, a quantitative evaluation of the actual significance of the column inlet concentration profile in this respect has been lacking. The purpose of this paper is to present a simple general model that shows the extent to which the sensitivity of analysis is affected by the shape of the column inlet concentration profile of the analyte. The sensitivity of analysis, i.e., the performance of the chromatographic column detector system, is expressed here through the sensitivity of analyte mass determination. The sensitivity of analyte mass determination is related to the zone-maximum analyte concentration per unit mass of analyte in the mobile phase at the column 0utleW.

Gaussian Concentration Profile Modified by the Column-Inlet Concentration Profile and by an Additional Exponential-Decay Process

The chromatographic elution profile is described by the convolution of the Gaussian curve with the column-inlet concentration profile and with an additional exponential distribution. Two different column-inlet profiles are considered, viz., the rectangular and exponential-decay concentration profiles. The relationships obtained are used to show the effect of an extracolumn exponential-decay process on the peak height, and on the sensitivity of quantitative chromatographic analysis. A previously proposed criterion of the effect of column-inlet concentration profile on the peak height is also extended to accommodate the extracolumn exponential-decay process.