An equation for the calculation of retention indices in temperature-programmed gas chromatography with allowance for the nonlinear variation of the retention parameters ofn-alkanes

In view of the nonlinear variation of the temperature increments ofn-alkanes found previously, the accuracy of the calculations of the retention indices (I _pr) of substances in temperature-programmed capillary gas chromatography carried out in terms of six known equations was verified. A new four-parameter equation was proposed, and a general method for the calculation of its coefficients, suitable for all stationary phases, based on the adjusted retention times ofn-alkanes was suggested. The coefficients of the equation for 12 temperature variation programs were determined. Using the homologous series of methyl esters of fatty acids as an example, it was shown that the proposed equation ensures the minimum error of determination ofI _pr under various conditions. The equation also makes it possible to carry out interpolation and extrapolation calculations. The coefficients of the equation are found using the least-squares method based on data for any 4–5 referencen-alkanes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 323–327, February, 1997.     

Nonlinear variation of sorption parameters of n-alkane homologsl in temperature-programmed gas chromatography (tpgc) and new equation for calculation of retention indixces

It has been demonstated that the considaerable difference in temperature increments of sorption parameters of n-alkanes under isothermal conditions is the main reason for nonlinear dependence of sorption parameters on molecular mass of homolog in temperaturre programmed gas chromatography (TPGC). A new nonlinear 4th parameter equation has been given for calculation of the retention indices. Coefficients of the equation are calculated from n-alkanes. The equation allows extrapolation and interpolation calculations of retention indices under TPGC conditions with experimental precision. The results obtained; for fatty acidkl methyl esters demonstrate the advantage of ovr equation in comparison with van den Dool and Kratz's equation.     

Correlation of unified retention indices for OV-101 and squalane

Summary Gas chromatographic unified retention indices for 43 hydrocarbons (alkanes and cycloalkanes) are given for squalane and OV-101. Comparison of these values and unified retention index increments are presented as linear regression equations with high correlation coefficients and acceptable standard deviations.     

Cubic splines compared with other methods for the calculation of programmed temperature retention indices

Summary Program temperature retention indices for fifteen nonalkane solutes have been determined by cubic splines, by other procedures found in the literature and by interpolation of the n-alkanes retention time logarithm for eleven temperature programs. A comparison in terms of variance of the differences between PTRI calculated by CS and each of the remaining methods is made for each of the eleven program runs, for each of the three stationary phases used and for many of the programs. The smallest variances obtained result when the Zenkevich, van den Dool & Kratz and Chen et al. methods are tested. The stationary phase polarity is of no relevance since it has no effect on the specific PTRI found by the different methods employed in this work.     

Prediction of the retention indices of methyl pyridines and pyrazines in capillary gas chromatography based on the non-linear additivity of the sorption energy

Summary Methylbenzenes, pyridines and pyrazines were investigated on fused-silica and glass capillary columns coated with SE-30 and PEG-40M/KF liquid phases, at two temperatures, 80° and 110°C. The contribution of the methylene groups to the partial molar free sorption energy was determined for methylpyridines and pyrazines. Equations are proposed for the calculation of the retention indices of methyl pyridines and pyrazines. These equations are based on the ortho- and α-effects of the methyl groups. The predicted indices have been experimentally tested for six dimethyl- and trimethylpyridines, and four methylpyrazines. Good accuracy of the calculation permits to use this method for the identification of methylpyridines and pyrazines in complex mixtures. Enlarged text of the paper presented at the Eighth International Symposium on Capillary Chromatography, Riva del Garda, Italy, May 19–21, 1987.     

Theoretical interpretation of the retention indices of alkylbenzenes

Summary The correlation between retention data of alkylbenzenes in GLC (measured on the stationary phase OV-101) and contribution of the Gibbs solvation energy are investigated in order to determine the dominant factor controlling retention. The retention in GLC can be predicted with help of our thermodynamic retention model.     

Unified retention indices of hydrocarbons on BP-1 dimethylsiloxane stationary phase

Summary Gas chromatographic retention indices for 23 hydrocarbons were determined on BP-1 dimethylsiloxane between 45 and 65°C. From the data measured unified retention indices were calculated. These values agree well with the corresponding experimental values. The values of unified retention indices obtained on BP-1 are compared with those determined on OV-101 and on squalane. The differences between the unified and the experimental retention indices on BP-1, OV-101 and squalane columns are shown.     

The system of universal retention indices and aspects of its application in gas chromatography

Summary The universal retention index has been developed on the basis of a critical analysis of the forms of chromatographic retention data presentation. Methods are presented for the determination of the universal retention index. The advantages of the universal retention index system in the evaluation of the selectivity and classification of stationary phases and in the determination of the composition of binary and polynary stationary phases are discussed. The thermodynamic aspect of the suggested system of chromatographic retention data presentation is also considered.     

Unified retention indices of alkylbenzenes on OV-101 and SE-30

Summary Gas chromatographic, unified retention-indices of 56 alkylbenzenes up to C₁₅ on dimethylsiloxane OV-101 and SE-30 stationary phases are given. These values agree well with the corresponding experimental values used in the statistical treatment of the experimental data.     

Calculation of the thermondynamic values of alkylthiazoles from the Kovats retention indices on the basis of the non-linear additivity principle

Summary Retention indices and their energy equivalents were determined for ten alkylthiazoles analysed on the stationary phases Apiezon L, Triton X-305 and Carbowax 1000. It was found that the energy contributions of identical alkyl groups to the partial molar free energy of sorption of a molecule are not equivalent. Smaller energy contribututions are due to the α-position of alkyl groups with respect to the heteroatoms in the thiazole ring (α-effect). This effect was most pronounced for 2-alkylthiazoles. Higher sorption energies were found for dimethylthiazoles with two methylgroups in a vicinal position. The retention indices were predicted for ten other di- and tri-substituted methyl, ethyl and propyl thiazoles on the basis of the so-called α- and ortho-effects. The predicted values are in good agreement with the experimentally determined values. Dedicated to Prof. Dr. E. sz. Kováts (Ecole Polytechnique Fédérale de Lausanne) on the occasion of his sixtieth birthday.     

Comparison of mathemical methods for the calculation of retention indices at high temperature in gas chromatography

The retention indices of three homologous series (2-alkanones, 1-alkanols, cycloalksanones) have been determined at high temperature by the application of two new adaptation methods: A multiparametric least-squares regressions iterative method based on the dertermination of the adjusted retention times and a cubic interpolation directly using the uncorrected retention times without dead time correction. The two methods were applied to two types of columns. The first group includes eight packed columns (seven OV polymethylphenylsiloxane and Apolane-87 stationary phases), while the second includes five glass capillary columns (four methyl-silicons with different film thicknesses and Apolane-87 stationary phases). The retention indices obtained with a multiparametric and a cubic interpolation methods were compared with each other and with those calculated by Grobler's, Guardino's, Kaiser's and Kovàts' methods. The influence of coating, film thickness, and temperature on them was investigated.     

A method of calculating the specific retention volumes of substances from their retention indices and specific retention volumes of n-alkanes

The choice of stationary phase for gas chromatography is dictated by the nature of the analytes. Polarity and selectivity are known to play an important role. This paper suggests equations for calculation of specific retention volumes of any compounds from their retention indices.     

On the influence of the solute sample size on temperature-programmed retention indices

The effect of the solute sample size on the measurement of temperature-programmed retention indices (I_TP) has been studied on glass capillary columns with different stationary phases, film thicknesses, and temperature programs. The sample capacity of a solute is little affected by the parameters of temperature program, i. e. initial temperature and heating rate. However, it has been found that in a temperature programmed run, allowable sample sizes for high I_TP values are lower than for low I_TP values. About 10 ng of each compound is recommended as the sample size for accurate measurement of I_TP values on commonly used capillary columns.     

超高效液相色谱塔板高度方程的推导

近年来,分析工作者采用超高效液相色谱(UPLC)完成了许多过去不能完成的分离分析工作。但是在阐述UPLC原理时不少人却采用了van Deemter方程。这是不对的。本文研究了UPLC色谱过程动力学,从热传导方程出发运用色谱动力学原理推导了包括考虑流动相摩擦生热影响的UPLC塔板高度方程H=2γD_m/u+((2λd_pu^1/3)/(u^1/3+ω(D_m/d_p)^1/3))+((2ku)/((1+k)²(1+κ₀)κ_d))+ ((θ(κ₀+κ₀k+k)²d_p²u)/(D_mκ₀(1+κ₀)²(1+k)²)) +(κ_i(κ₀+κ₀k+k)²d_p^5/2u^2/3)/(3κ₀Ω D_m^2/3(1+κ₀)²(1+k)²+(r₀²(κ₀+κκ₀k+k)u)/(4(1+k)D_r)·exp(-Kr₀²α)。上述方程右端最后一项代表了流动相摩擦生热对塔板高度的贡献。当流动相线速度较低时,流动相摩擦生热对塔板高度的贡献趋近于零,塔板高度方程还原成Horvath和Lin的方程;当流动相线速度较高时,由于流动相摩擦生热,柱轴心与边缘温差增加,流动相线速度径向分布差异导致柱效率降低,而柱轴心与边缘的温差与流动相线速度平方成正比。作者明确指出:UPLC的柱效率与柱内径密切相关,采用细内径柱有利于实现高效率;过高的流动相线速度将导致柱效率崩溃。     

The use of a nonlinear equation for calculation of the retention indices of polar substances in gas chromatography with linear temperature programming

New possibilities for using the equation that takes into account the nonlinear variation of parameters of the reference n-alkane scale for the calculation of retention indices of polar substances at different modes of temperature programming were considered. The advantages of this equation over the linear scale used traditionally were demonstrated in relation to C₃—C₁₁ alkan-1-ols. The equation appears to have considerable promise regarding the search for the equivalent isothermal index.     

The influence of the carrier gas on retention in capillary gas-solid chromatography

In capillary gas-solid chromatography where interactions between solute and carrier gas and adsorption of the solute on the surface of the adsorbent are considered to be imperfect, it has been shown that chromatographic retention is determined largely by adsorption processes. It has been established that correlation relationships k(P₂)=A k(P₁) + B, where k is the retention factor, and A and B are equation constants, was valid for use of different carrier gases P₁ and P₂. Column efficiency could be improved by use of carbon dioxide. The advantages of using carbon dioxide as the carrier gas were investigated.     

Data for the gas-liquid chromatographic analysis of essential oils. Determination of the composition of the essential oil of Marjoram

Summary By identification of the essential oil component ‘cis-sabinene hydrate acetate’ of marjoram we demonstrate that the mass spectrum—furnish to the chemical composition and molecular configuration of compounds—can lead to incorrect qualitative identification if retention data are not taken into consideration. We studied also the temperature-dependence of the retention data of different compounds, and our measurements showed that because of the different temperature-dependence, a change in the temperature of the analysis results in a change in order of elution. The use of this phenomenon, originating from the change of the retention data, both increases the safety of qualitative identification and aids determination of the optimum measurement conditions in both isothermal and temperature-programmed modes of operation. Presented at Balaton Symposium '01 on High-Performance Separation Methods, Siófok, Hungary, September 2–4, 2001     

Live retention database for identification of compounds in capillary gas chromatographic analysis --Principle and structure

A live retention database for compound identification in isothermal and any step temperature programmed capillary gas chromatography has been developed. The database utilizes the Kovats retention indices of compounds on a given stationary phase and the retention time of n-alkanes measured at isothermal conditions on the column to be used, together with the programming parameters. Identification is performed by search operation that compares the calculated results with the retention values of unknown peaks. Cross-reference of the search results of different operating conditions is performed automatically by the database in order to increase the reliability of the identification. The error of the database conversion is ≤± 0.5 index unit, or ≤± 1% on retention time. This paper describes the principle and the structure of the database in detail. The experimental results for different calsses of compounds tested at divers operating conditions will be presented in Part Ⅱ.