Wrong gas/liquid partition data by gas chromatography

In a paper published in 1992 [K.S. Reddy, J.-Cl. Dutoit, E.sz. Kováts, Pair-wise interactions by gas chromatography. I. Interaction free enthalpies of solutes with non-associated primary alcohol groups, J. Chromatogr. 609 (1992) 229] retention indices and standard chemical potential differences (SPOT-s) are given for about 160 solutes on a liquid branched paraffin hydrocarbon of the carbon number, z = 78. For the temperature dependence of the SPOT-s the proposal of Kirchhoff was accepted i.e. that at higher temperatures the molar heat capacity difference of the solute between the gas phase and the solution is nearly constant in a broad temperature range. The data were determined under conditions where the effect of adsorption could be neglected. By comparing the published data with those determined on another branched paraffin of the carbon number, z = 87 [F. Riedo, D. Fritz, G. Tarján, E.sz. Kováts, A tailor-made C87 hydrocarbon as a possible non-polar standard stationary phase for gas chromatography, J. Chromatogr. 126 (1976) 63] it is shown that the reported thermodynamic constants of solutes determined experimentally at higher temperatures (the group "H") are wrong on C78. In the present paper, the method of correction and the resulting corrected data are given. In later papers retention data are given on eight polar derivatives of the hydrocarbon C78 relative to those measured on the nonpolar standard [R. Cloux, G. Défayes, K. Fóti, J.-Cl. Dutoit, E.sz. Kováts, Pair-wise interactions by gas chromatography. III. Synthesis of isosteric stationary phases for gas chromatography, Synthesis (1993) 909; G. Défayes, K.S. Reddy, A. Dallos, E.sz. Kováts, Pair-wise interactions by gas chromatography. V. Interaction free enthalpies of solutes with primary chloro- and bromo-alkanes, J. Chromatogr. A 699 (1995) 131; K.S. Reddy, E.sz. Kováts, Pair-wise interactions by gas chromatography. II. Sociation free enthalpies of some hydrogen bonding solutes with non-associated primary alcohol groups, Chromatographia 34 (1992) 249; K.S. Reddy, R. Cloux, E.sz. Kováts, Pair-wise interactions by gas chromatography. IV. Interaction free enthalpies of solutes with trifluoromethyl-substituted alkanes, J. Chromatogr. A 673 (1994) 181; K.S. Reddy, R. Cloux, E.sz. Kováts, Pair-wise interactions by gas chromatography. VI. Interaction free enthalpies of solutes with primary methoxyalkane, cyanoalkane and alkanethiol groups, J. Chromatogr. A 704 (1995) 387; A. Dallos, A. Sisak, Z. Kulcsár, E.sz. Kováts, Pair-wise interactions by gas chromatography. VII. Interaction free enthalpies of solutes with secondary alcohol groups, J. Chromatogr. A 904 (2000) 211]. Obviously, the same data which are wrong in the nonpolar standard are also wrong in these solvents. The corrected data in the polar solvents are given in supplementary tables.     

Factors affecting the reproducibility and reliability of retention simulation in any form of temperature programmed capillary GC

Conversion of Kováts retention indices on a given stationary phase into the thermodynamic parameters of compounds on a given column leads to a simplified method for retention simulation in isothermal, linear, and multi-ramp temperature programmed capillary gas chromatography. The influence of numerical methods used in the computation, the temperature coefficient of Kováts indices, and the experimental factors such as isothermal temperatures selected in the measurement of n-alkanes, column characterization and sample overloading, on the reproducibility and accuracy of simulation were discussed and examined. When the column used is properly characterized, the error between the simulated values and the experimental data is within ± 0.5 index unit or less than ± 1% of retention time.     

Gas chromatographic retention data of wax esters

Higher wax esters within the range of C24 to C44 (205 standards) were analyzed by means of gas chromatography and Kováts indexes (I) and reduced Kováts indexes (RKI) were calculated. The dependences of these retention data on number of carbon atoms and on number and position of double bonds in acid and in alcohol moieties of esters were plotted.     

Gas chromatography study of silica modified with polyfluoroalkyl groups

Silicas with attached polyfluoroalkyl groups were studied by gas chromatography (GC) at zero surface coverage. Organic molecules (n-alkanes, ethers, esters, ketones, etc.) were used as test compounds. The contributions of molecule groups to the Gibbs energy of adsorption deltaGi, and the contributions of specific energy interactions deltaG(sp) as well as Kováts retention indices were calculated. The strong and weak points of the two approaches (deltaGi, deltaG(sp)) were discussed. It was shown on the basis of both approaches that adsorption process on polyfluoroalkyl silicas is to be described by the low contribution of hydrocarbon groups (dispersion interactions) and sufficiently high contribution of groups reinforcing specific interactions. The significant role of entropy in thermodynamic of retention was shown by example of adamantane adsorption.     

Gas chromatographic separation of substituted pyridines

Capillary gas chromatographic methodology for separation of complex mixtures of substituted pyridines has been demonstrated on polar (CAM) and non-polar (DB-5) columns. Separations are characterized by high resolution, high sensitivity, a wide dynamic detector range, and good reproducibility. For the first time, Kováts retention indices have been calculated for pyridine and substituted pyridines. Correlations of retention indices with physico-chemical properties, such as hydrogen bonding, pyridine pKa and Hammett substituent constants are discussed.     

Retention indices as identification tool in pyrolysis-capillary gas chromatography

Pyrolysis-capillary gas chromatography (Py-cGC) represents important method to identify the analytes in the mixture after thermal degradation. This combines high effective analyte separation on-line coupled with thermal degradation process that depends on analyte structure. System of retention indices has been used for identification of the analytes after on-line pyrolysis and chromatographic separation. The pyrolysate composition has been studied during thermal degradation of polymethylmethacrylate (PMMA) at different pyrolysis temperatures and chromatographic column conditions. Homologues series of n-alkanes have been used for calculation of pyrolysate Kováts retention indices (I) and compared with mass spectrometric (MS) data of pyrolysate model mixture. To identify PMMA thermal degradation products the high density polyethylene (HDPE) as additive standard producing triplets of the olefin homologous series during co-pyrolysis has been used. These homologous series enable to calculate programmed temperature retention indices (ITPGC) to identify the analytes present in the pyrolysate. Calculated I values were compared with published I values databases to identify analytes yielded at different pyrolysis temperatures.     

Development of a database of gas chromatographic retention properties of organic compounds

A comprehensive database of gas chromatographic retention properties of chemical compounds has been developed using multiple literature sources. The National Institute of Standards and Technology (NIST) database of retention data for non-polar and polar stationary phases currently contains 292,924 data records for 42,888 compounds. The database includes data for Kováts indices, linear indices, Lee indices, retention times and retention volumes. The first release of this database for non-polar stationary phases is available with NIST/US Environmental Protection Agency (EPA)/National Institutes of Health (NIH) Mass Spectral Database (June 2005) and through the internet (NIST Chemistry WebBook). The paper describes the database and the process by which it has been compiled. The format of data presentation and the quality control procedures are described. Data sources of gas chromatographic retention data are also discussed.     

On the Relationship Between Kováts and Lee Retention Indices

A relationship between the Kováts and Lee retention indices was obtained for dimethylsilicone stationary phase and isothermal conditions. The temperature dependence of the retention indices was taken into account. Based on numerous experimental data for dimethylsilicone stationary phase, parameters of the linear temperature dependence of the Kováts indices were determined for the reference compounds of the Lee scale.     

Use of the Tóth and Unilan Equations for Chromatographic Testing of the Adsorption Properties of Active Carbon

The adsorption and chromatographic properties of active carbon samples obtained by adsorptive/desorptive modification have been studied by inverse gas chromatography to assess their suitability for adsorption of analytes from gas and liquid phases. This paper may help in the choice of an appropriate method for characterizing the surface of an active carbon with regard to particular analytes. Therefore, the specific, V _{g (T )}, and net, V _{S ( T )}, retention volumes were determined from the retention times of the peak maxima and centres of gravity of the peaks. A mathematical link between retention volume data and the monolayer capacity, a _m , was derived by use of a hyperbolic-type equation which enables the values of the terms of the Tóth and Unilan equations to be found. Adsorption second virial coefficients, B _{2 S} , calculated from the values of the terms of the Tóth and Unilan equations were used as a sensitive test of the accuracy of the calculated data. There were no significant differences between the isosteric enthalpies obtained by use of virial coefficients or retention volumes. It was shown experimentally there is substantial parallelism between the magnitudes of the differential isosteric enthalpy and the differential entropy for propane chromatographed on the adsorption systems tested. The cause of non-linear behaviour of plots of ln V _{g ( T )} against T ^?1 for different dynamic adsorption systems is discussed.     

Comparison of Retention Indices of Some Monosubstituted Benzenes Calculated by Different Mathematical Methods in RP-LC

In reversed phase liquid chromatography, the retention indices of benzene and nine mono substituted benzenes with different functionality based on the alkan-2-ones and alkyl aryl ketones retention index standards have been determined by the application of two new mathematical adaptation methods, viz. a multiparametric least-squares regression iterative method based on the determination of the adjusted retention times and a local cubic interpolation method directly using the total retention times. The two methods were applied to two types of columns. The first group includes four octadecyl-C₁₈ columns with different packing materials obtained from different manufacturers, while the second comprises an octyl-C₈ column. The retention indices have been extensively studied using either methanol–water or acetonitrile–water mobile phase systems. The influences of the concentration of the organic modifier in the mobile phase (methanol or acetonitrile), the column temperature, and the column packing material on retention indices of the set of the ten monoaromatics studied were also investigated. The calculated multiparametric retention indices values, those obtained by the local cubic interpolation and Kováts’ methods are compared. Good agreement was observed between the retention indices calculated by the three methods.     

Saturated vapor pressures and the enthalpies of vaporization of dicyclohexyl esters of dicarboxylic acids

Saturated vapor pressures at 320–370 K and the enthalpies of vaporization of dicyclohexyl esters of linear dicarboxylic acids containing two to six carbon atoms were determined by the transpiration method. Linear correlations of the enthalpies of vaporization of the esters with the number of carbon atoms in molecules and Kováts retention indices were obtained.     

The correlation between the physical properties and structure of alcohols and their gas chromatographic behavior on polar and non-polar stationary phases : II. Branched-chain alcohols

Branched-chain alcohols with from one to eight carbon atoms have been analysed by gas chromatography using thermal conductivity and flame-ionization detectors. The specific retention volumes, V_g, and the retention indices relative to normal alcohols were determined for all the compounds, and the effects of the branching and of the position of the hydroxyl groups have been investigated. Boiling points, vapour pressures, molecular volumes were correlated with the measured retention values. The activity coefficients at infinite dilution and the normalized activity coefficients were calculated, and their influence on the separation is discussed. The results are compared with the behaviour of branched-chain halogenoalkanes on polar and non-polar stationary phases.     

Prediction of gas chromatographic retention indices of linear,branched, and cyclic alkanes from their physicochemical properties

Kováts retention indices for a series of linear, branched, and cyclic alkanes on squalane at any temperature, and on other stationary phases of different polarity at a given temperature, are related to physicochemical properties of the solutes, such as boiling point and molar refraction, by multiple regression analysis. The equations found permit calculation of the Kováts retention index for all alkanes, with standard deviations close to experimental error. The same equations can also be used for calculating the physicochemical parameters they contain.     

Capillary gas chromatography-mass spectrometry of all 93 acyclic octenes and their identification in fluid catalytic cracked gasoline

The Kováts retention indices of all 93 acyclic octenes on polydimethylsiloxane and squalane as stationary phases as well as their mass spectra were measured. The means of gas chromatography-mass spectrometry (GC-MS) were used for confirmation of GC identification as well as for mass spectrometric deconvolution of the majority of gas chromatographic unseparated isomeric octene peaks. The distinction between corresponding E and Z acyclic octenes, that is either difficult or even impossible by means of GC-MS, was obtained on the basis of larger temperature coefficients of retention indices for Z isomeric octenes than for corresponding E isomers. The retention data expressed as homomorphy factors were correlated with the degree of branching, position of double bond, and position of alkyl group with respect to the double bond of acyclic octenes, and the structure-retention relationships were formulated. The 81 acyclic octenes were identified in FCC gasoline.     

The analysis of C4–C11 hydrocarbons in naphtha and reformate with a new apolar fused silica column

This paper deals with the separation of alkanes, naphthenes, and aromatic compounds in naphtha and reformate, on a newly developed apolar high resolution GC column. The selectivity of this apolar phase has been compared with those of squalane, DB-1, and SE-30. A total of 95 hydrocarbons were reliably identified, mostly by GC-MS. Repeated measurements of Kováts retention indices are presented as evidence for the reproducible manufacture of fused silica columns coated with this phase.     

Supercooled liquid vapour pressures and related thermodynamic properties of polycyclic aromatic hydrocarbons determined by gas chromatography

A gas chromatographic method using Kováts retention indices has been applied to determine the liquid vapour pressure (P(i)), enthalpy of vaporization (DeltaH(i)) and difference in heat capacity between gas and liquid phase (DeltaC(i)) for a group of polycyclic aromatic hydrocarbons (PAHs). This group consists of 19 unsubstituted, methylated and sulphur containing PAHs. Differences in log P(i) of -0.04 to +0.99 log units at 298.15K were observed between experimental values and data from effusion and gas saturation studies. These differences in log P(i) have been fitted with multilinear regression resulting in a compound and temperature dependent correction. Over a temperature range from 273.15 to 423.15K, differences in corrected log P(i) of a training set (-0.07 to +0.03 log units) and a validation set (-0.17 to 0.19 log units) were within calculated error ranges. The corrected vapour pressures also showed a good agreement with other GC determined vapour pressures (average -0.09 log units).     

Structure-retention correlations of hydrocarbons in gas-liquid and gas-solid chromatography. Cycloalkenes and cycloalkadienes

Summary The retention indices of cycloalkenes and cycloalkadienes with C₆–C₁₃ rings are determined by gas-liquid chromatography (GLC) on glass capillary columns coated with OV-1 and Ucon LB 550X and by gas-solid chromatography (GSC) on a microcolumn packed with uncoated graphitized thermal carbon black (GTCB). Structure — retention correlations are derived on using index differences such as H^OV, H^GTCB and ΔI values, considering the differences in the stereochemistry of these compounds. It is shown that the combined application of index increments obtained in GLC and GSC provides more detailed structure informations. The value of the retention index units agree with the most stable conformations of the alicyclic compounds. The high value for the energy equivalent to an index unit (ΔG_I.U.=4.18kJ/mol) confirmes that graphitized thermal carbon black causes much stronger dispersive interactions than any nonpolar liquid phase. Dedicated to Prof. Dr. E. sz. Kováts (Ecola Polytechnique Fédérale de Lausanne) on the occasion of his sixtieth birthday.     

Identification of FAMEs using ECL values and a three-dimensional Kováts retention index system

n-Alkanes and fatty acid methyl esters have been chromatographed on four capillary columns of different polarity (LM-1 (100% polydimethylsiloxane), CB-225 (25% cyanopropyl, 25% phenyl polysiloxane), LM-100 (immobilized polyethylene glycol), and BPX-70 (70% cyanopropyl polysiloxane)) with the goal of identifying the major fatty acid methyl esters obtained by derivatization of vegetable oils. Chromatographic retention data were employed for the construction of a three-dimensional Kováts retention index system and for the elaboration of a table of equivalent chain length values.