Retention parameters of aromatic hydrocarbons with mono-substituted polar groups in binary RP-HPLC systems

Summary Capacity factor (k′) values of aromatic hydrocarbons with mono-substituted polar-groups are correlated for reversed-phase systems involving stationary phases with C₁₈ or C₄ ligands chemically bonded to silica and a binary aqueous eluent containing modifiers: methanol, acetonitrile, tetrahydrofuran, isopropanol, dioxane or dimethoxyethane. The relative retention variations of the solutes are interpreted with special consideration of their interactions with non-polar stationary phases and the molecular structure of the modifiers and solutes. Rules for retention and selectivity optimisation in RP-HPLC systems are given.     

Column-induced selectivity in separation of polynuclear aromatic hydrocarbons by reversed-phase,high-performance liquid chromatography

Summary Differences in retention time and relative elution order were observed when a standard mixture of 11 PAH was injected on three C-18 columns from different manufacturers under equivalent conditions.Working on a grant from the Swedish Natural Science Research Council.     

Comparison of different retention models in normal- and reversed-phase liquid chromatography with binary mobile phases

The dependence on mobile phase composition of the retention of selected test analytes in different normal- and reversed-phase chromatographic systems is studied. The aim of this study is to compare the performance of six valuable retention models reported in the literature with a new empirical equation, which is first introduced in this study. All of these models are compared for different thin-layer chromatographic and high-performance liquid chromatographic systems by use of three criteria: the sum of the squared differences between the experimental and theoretical data, approximation of the standard deviation, and the Fisher test.     

Regularities in the retention of nonsubstituted polynuclear aromatic hydrocarbons in liquid chromatography on polar adsorbents

Summary The retention of polynuclear aromatic hydrocarbons (PAH) is studied in liquid chromatography, using columns packed with silica gel with a hydroxylated surface and the same silica gel with surface-bonded amine and nitrile groups. A linear relationship was found between retention and the number of π-electrons in the molecule, the size of the molecule and the different types of carbon atoms in the molecule. The different carbon atoms in a PAH molecule contribute differently to the retention and this contribution depends on the nature of the polar functional groups on the adsorbent's surface and on the nature and concentration of a polar modifier present in a non-polar eluent.     

Models of retention of adamantylamidrazones in reversed-phase high-performance liquid chromatography

Rules governing the chromatographic behavior of some amidrazones of the adamantane series were studied under the conditions of reversed-phase high-performance liquid chromatography. The characteristics of the retention of sorbates in elution by aqueous-acetonitrile phases with various compositions were calculated. Correlations between the structure and physicochemical characteristics of sorbate molecules and their retention were studied.     

Compatibility and universality of the retention prediction system for polycyclic aromatic hydrocarbons in reversed-phase liquid chromatography

Summary System-compatibility and universality of the retention prediction concept has been investigated for polycyclic aromatic hydrocarbons in reversed-phase liquid chromatography. The results clearly indicate that the retention prediction approach has a very high potential for optimization of separation conditions in almost all reversed-phase systems, and allows more precise and rapid analysis. This approach may be one of the best optimization techniques, because the system does not require any standard materials.     

Prediction of peptide retention time in reversed-phase high-performance liquid chromatography.

Peptide retention in reversed-phase chromatography depends mainly on the amino acid composition of peptides and can therefore be predicted by summing the relative hydrophobic contributions of each constitutive amino acid residue. The prediction is correct for small peptides but overestimates the retention times of peptides larger than 10-15 residues. A new prediction model is proposed in which the contribution to peptide retention of each amino acid residue is not a constant but a decreasing function of peptide length. From the retention times of 104 peptides, the parameters of decreasing functions were estimated by a non-linear multiple regression analysis. The contribution to peptide retention of charged, polar and non-polar residues appears to be differently affected by peptide length. The secondary structure of most peptides during reversed-phase high-performance liquid chromatography could be responsible for this. The high correlation between the predicted and observed retention times of peptides which were not used to establish the model indicates a good predictive accuracy of the new model.     

Comparison of selected retention models in reversed-phase liquid chromatography

Retention models are usually compared by how well the model equation fits retention data for one solute taken over a range of mobile phase compositions. Even when retention data for multiple solutes are used, the quality of the fit is often judged by the statistical goodness-of-fit alone. This study compared four different RPLC retention models, encompassing three distinct mathematical forms. Each model was fit to the retention data of multiple solutes and the sets of best-fit parameters were examined in terms of the underlying physico-chemical assumptions of the models. Next, for the linear and quadratic models, some of the model parameters were calculated a priori and the rest of the model parameters were then obtained in subsequent fittings. The sets of best-fit parameters obtained in this manner were more consistent with the underlying assumptions of these models than were the sets of parameters obtained entirely through regressions to the experimental data. Thus, the extraction of parameters by fitting a model to the retention data of a single solute may result in unreliable values for those parameters, even in the case of a fit that would be considered good when judged by conventional statistical criteria. That is, although parameters extracted in such a fashion may be suitable for optimization or similar uses, they may not be suitable for determining the appropriateness of the underlying assumptions of retention models.     

Differences between retentions of various classes of aromatic hydrocarbons in reversed-phase high-performance liquid chromatography. Implications of using retention data for characterizing hydrophobicity

Capacity factors of a series of alkylbenzenes (C1-C10), 12 chlorobenzenes, 9 chlorotoluenes, 17 chloronaphthalenes and 65 chlorobiphenyls have been measured on an octadecylsilica column. Aqueous methanol of four different compositions (80-95% methanol) was used as eluent. Logarithms of capacity factors of all eluites are linearly related to the amount of organic modifier in the eluent. In addition, linear relationships between the solvent strength and the logarithms of capacity factors extrapolated to zero methanol have been revealed. The proportionality factors are dependent on the structures of the eluites. Thermodynamic consideration of the retention processes shows that, within each type of eluite, enthalpy-entropy compensation is found. The compensation temperatures are not significantly different for the various types of eluite. Furthermore, it is shown that the compensation temperatures increase with increasing water content of the eluent. Since the intercepts of the delta G0-delta H0 plots are not equal for the various types of eluite, it was concluded that the distribution processes causing retention of benzene, naphthalene and biphenyl are different. When only the free energies of retention (i.e. the capacity factors) of different types of eluite are compared, no accurate information on the hydrophobicity of the eluites can be obtained if aqueous methanol is used as eluent. Therefore the possibilities for relating or predicting other physico-chemical parameters of the test compounds, such as octan-1-ol-water partition coefficients with isocratic retention data, will be limited.     

Retention behavior of aromatic hydrocarbons in reversed-phase liquid chromatography with ion-exchange-induced stationary phases

Chromatographia - The retention behavior of aromatic hydrocarbons on anion-exchangers modified with alkylsulfate or alkanesulfonate has been examined by column liquid chromatography. Several...     

Application of reversed-phase high-performance liquid chromatography for the separation of deuterium and hydrogen analogs of aromatic hydrocarbons

Substitution of hydrogen by deuterium in aromatic hydrocarbons can alter various properties significantly. Benzene, toluene, naphthalene, anthracene, phenanthrene, biphenyl and durene are separated from their deuterium analogs by reversed-phase (C₁₈ liquid chromatography with acetonitrile/water mobile phases and ultraviolet detection. Deuterium compounds always elute before the hydrogen analog; possible explanations are given. The elution pattern and retention times of anthracene and phenanthrene were unchanged when D₂O replaced H₂O as the mobile phase component.     

Ion-pair reversed-phase high-performance liquid chromatography analysis of oligonucleotides: retention prediction

An ion-pair reversed-phase HPLC method was evaluated for the separation of synthetic oligonucleotides. Mass transfer in the stationary phase was found to be a major factor contributing to peak broadening on porous C18 stationary phases. A small sorbent particle size (2.5 microm), elevated temperature and a relatively slow flow-rate were utilized to enhance mass transfer. A short 50 mm column allows for an efficient separation up to 30mer oligonucleotides. The separation strategy consists of a shallow linear gradient of organic modifier, optimal initial gradient strength, and the use of an ion-pairing buffer. The triethylammonium acetate ion-pairing mobile phases have been traditionally used for oligonucleotide separations with good result. However, the oligonucleotide retention is affected by its nucleotide composition. We developed a mathematical model for the prediction of oligonucleotide retention from sequence and length. We used the model successfully to select the optimal initial gradient strength for fast HPLC purification of synthetic oligonucleotides. We also utilized ion-pairing mobile phases comprised of triethylamine (TEA) buffered by hexafluoroisopropanol (HFIP). The TEA-HFIP aqueous buffers are useful for a highly efficient and less sequence-dependent separation of heterooligonucleotides.     

Modern state of high-performance liquid chromatography of polycyclic aromatic hydrocarbons

The review is devoted to the recent studies in the field of high-performance liquid chromatography (HPLC) of polycyclic aromatic hydrocarbons (PAHs). The existing certified techniques, developments of synthesis of new sorbents for separating PAH isomers, and study of the retention mechanism are discussed. It also considers the modern methods of extraction and concentration of PAHs, mainly from environmental objects, which are compatible with subsequent identification by HPLC, as well as the necessity of analytical control of contamination of the environment, food sources, and food supplies with PAHs.     

The influence of the structure of some aromatic heterocyclic derivatives on their retention in reversed-phase high-performance liquid chromatography

The chromatographic behavior of aromatic heterocyclic derivatives in reversed-phase high-performance liquid chromatography was investigated. The retention characteristics of the substances under the conditions of chromatography with water-acetonitrile mobile phases (retention factors, relative retentions, distribution coefficients, Henry adsorption constants, differences in the differential molar energy of sorption, and Gibbs energies of sorption) were determined. It was shown that the chromatographic retention of the sorbates depended on their molecular structure. The influence of the nature of heteroatoms and their number on the sorption of heterocyclic compounds was discussed. Original Russian Text ? S.V. Kurbatova, B.R. Saifutdinov, O.G. Larionov, V.V. Meshkovaya, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 3, pp. 557–564.     

Thermodynamic and kinetic characterization of nitrogen-containing polycyclic aromatic hydrocarbons in reversed-phase liquid chromatography

A series of five nitrogen-containing polycyclic aromatic hydrocarbons (NPAHs) was studied on polymeric octadecylsilica using methanol and acetonitrile as the mobile phase. The thermodynamic and kinetic behavior was examined as a function of ring number, annelation structure, and position of the nitrogen atom. The retention factors for the NPAHs are smaller than those for the parent PAHs in methanol, while the converse is true in acetonitrile. The changes in molar enthalpy are relatively comparable in both mobile phases with 1-aminopyrene having values of -5.0 +/- 0.2 kcal/mol in methanol and -6.3 +/- 0.7 kcal/mol in acetonitrile (1 cal = 4.184 J). However, the rate constants from mobile to stationary phase (k(sm)) and from stationary to mobile phase (k(ms)) demonstrate large differences as a function of mobile phase. For example, the rate constants k(ms) for 1-aminopyrene and 4-azapyrene are 675 and 62 s(-1), respectively, in methanol at 303 K. In contrast, the same solutes demonstrate rate constants of 3.47 and 3.9 x 10(-3) s(-1), respectively, in acetonitrile. The activation energies for transfer from mobile phase to transition state (deltaE(double dagger(m)) and from stationary phase to transition state (deltaE(double dagger(s)) also differ as a function of mobile phase. For example, the activation energies deltaE(double dagger(s)), for 1-aminopyrene are 21 and approximately 0 kcal/mol, whereas those for 4-azapyrene are 19 and 23 kcal/mol, in methanol and acetonitrile, respectively. Based on these thermodynamic and kinetic results, the relative contributions from the partition and adsorption mechanisms are discussed.     

Determination of polycyclic aromatic hydrocarbons in Diesel soot by high-performance liquid chromatography

Summary The described identification and determination of polycyclic aromatic hydrocarbons (PAH) in Diesel soot is based on a class-fractionation using an open Al₂O₃-column and a following separation on a reversed-phase HPLC-column with post-chromatographic derivatization. The optimized analysis has been applied to the determination of PAH in soots from different engines such as locomotives and motor vehicles by variation of the fuel additives and different Diesel/water emulsions. The results obtained by locomotives show that a special Diesel/water emulsion emits a minor amount of mutagenic and cancerogenic polycyclic aromatic hydrocarbons, whereas the emission of the comparatively harmless compounds like phenanthrene and benzo(h)quinoline increases.     

Retention and selectivity of aromatic hydrocarbons with mono-substituted polar groups in ternary RP-HPLC systems

Summary The variation in selectivity for aromatic hydrocarbons with mono-substituted polar groups is investigated in ternary mobile-phase systems on C₁₈ stationary phases. The dependence of log k′ on the proportions of two modifiers was determined for the solutes within the concentration range of ternary systems obtained by mixing two binary eluents: methanol + water and tetrahydrofuran + water or acetonitrile + water and tetrahydrofuran + water. The nature of the relationships is explained in terms of molecular shape, molecular interactions between solute and extracted modifier and the ordering of solvated ligands on the stationary phase.     

New practical algorithm for modelling retention times in gradient reversed-phase high-performance liquid chromatography

Computer models have been widely used to predict the chromatographic behaviour of liquid chromatography systems. With the introduction of mass spectrometric detection and the use of lower mobile phase flow rates with conventional LC equipment, the influence of the dwell volume on the shape of the gradient curve becomes an issue in predicting the retention times. A new straight forward algorithm is proposed for the modelling of retention times in reversed-phase LC, taking the effect of the dwell volume on the gradient shape into account. The results show that the dwell volume has a large effect at lower flow rates and on the retention times of the analytes eluting at the end of fast gradient curves. The proposed model is able to make reliable predictions and can be helpful in LC-MS method development.