Hydrocarbons In Reversed-phase Liquid Chromatography Research Articles

Retention mechanism for polycyclic aromatic hydrocarbons in reversed-phase liquid chromatography with monomeric stationary phases

Reversed-phase liquid chromatography (RPLC) is the foremost technique for the separation of analytes that have very similar chemical functionalities, but differ only in their molecular shape. This ability is crucial in the analysis of various mixtures with environmental and biological importance including polycyclic aromatic hydrocarbons (PAHs) and steroids. A large amount of effort has been devoted to studying this phenomenon experimentally, but a detailed molecular-level description remains lacking. To provide some insight on the mechanism of shape selectivity in RPLC, particle-based simulations were carried out for stationary phases and chromatographic parameters that closely mimic those in an experimental study by Sentell and Dorsey [J. Chromatogr. 461 (1989) 193]. The retention of aromatic hydrocarbons ranging in size from benzene to the isomeric PAHs of the formula C(18)H(12) was examined for model RPLC systems consisting of monomeric dimethyl octadecylsilane (ODS) stationary phases with surface coverages ranging from 1.6 to 4.2 μmol/m(2) (i.e., stationary phases yielding low to intermediate shape selectivity) in contact with a 67/33 mol% acetonitrile/water mobile phase. The simulations show that the stationary phase acts as a very heterogeneous environment where analytes with different shapes prefer different spatial regions with specific local bonding environments of the ODS chains. However, these favorable retentive regions cannot be described as pre-existing cavities because the chain conformation in these local stationary phase regions adapts to accommodate the analytes.

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 × 10 −3 s −1, respectively, in acetonitrile. The activation energies for transfer from mobile phase to transition state (Δ E ‡m) and from stationary phase to transition state (Δ E ‡s) also differ as a function of mobile phase. For example, the activation energies Δ E ‡s for 1-aminopyrene are 21 and ∼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.

Retention Behaviour of Higher Fluorinated Polycyclic Aromatic Hydrocarbons in Reversed-Phase Liquid Chromatography

The retention behaviour of 18 di-, tri- and tetra-fluorinated phenanthrenes, chrysenes and benzo[g]chrysenes in column liquid chromatography was studied for a monomeric and a polymeric octadecyl-bonded silica phase. The retention times were much longer than those of the parent compounds and strongly dependent on the F-substitution pattern. The influence of geometric factors like calculated length-to-breadth ratios (LB ratios) and planarity, and electronic factors such as dipole moment and London forces was investigated. A linear correlation was found between the dipole moment and/or the degree of fluorination and the retention factor for the monomeric phase, and additionally between the maximum LB ratio and the retention factor for the polymeric phase. These findings are interpreted using the models of partition and the slot-model, respectively. Polyfluorinated polycyclic aromatic hydrocarbons are not very similar to parent PAHs and are, therefore, less suitable as internal standards in PAH analysis than the monofluorinated analogues especially in cases when the similarity of physico-chemical properties is critical.

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

The retention of six polycyclic aromatic hydrocarbons (PAHs) was characterized by reversed-phase liquid chromatography. The PAHs were detected by laser-induced fluorescence at four points along an optically transparent capillary column. The profiles were characterized in space and time using an exponentially modified Gaussian equation. The resulting parameters were used to calculate the retention factors, as well as the concomitant changes in molar enthalpy and molar volume, for each PAH on monomeric (2.7 micromol/m2) and polymeric (5.4 micromol/m2) octadecylsilica. The changes in molar enthalpy become more exothermic as ring number increases and as annelation structure becomes less condensed. The changes in molar volume become more negative as ring number increases for the planar PAHs, but are positive for the nonplanar solutes. In addition, the rate constants, as well as the concomitant activation enthalpy and activation volume, are calculated for the first time. The kinetic data demonstrate that many of the PAHs exhibit very fast transitions between the mobile and stationary phases. The transition state is very high in energy, and the activation enthalpies and volumes become greater as ring number increases and as annelation structure becomes less condensed. The changes in thermodynamic and kinetic behavior are much more pronounced for the polymeric phase than for the monomeric phase.

Quadrupolar Effects on the Retention of Aromatic Hydrocarbons in Reversed-Phase Liquid Chromatography

Because of their large molecular quadrupole moments, aromatic hydrocarbons can give rise to strong electrostatic interactions with polar molecules such as acetonitrile and water. Such interactions may have some significant effects on the retention and separation of the aromatic compounds in reversed-phase liquid chromatography. We report here an experimental investigation of these effects in which we studied the retention behavior of a series of aromatic hydrocarbons in comparison with that of aliphatic hydrocarbons which have no appreciable quadrupole moments. The elution order of benzene, cyclohexane, and n-hexane was determined and shown to follow inversely the increasing magnitude of the molecular quadrupole moments. A linear dependence of retention on molecular polarizability was observed for both the aliphatics and aromatics, the latter being consistently less retained than the former under the same chromatographic conditions. The molecular polarizability and quadrupole moment were regressed as independent variables against the logarithmic retention factors of both the aliphatic and aromatic solutes, and an excellent correlation was found with all five different chromatographic systems studied. These results clearly show that, while the retention of aromatic hydrocarbons is primarily governed by the dispersion interactions with the nonpolar stationary phase, the electrostatic interactions of the aromatic solutes with the polar mobile phase act to reduce the retention and thus contribute favorably to the control of the relative retention and selectivity in reversed-phase liquid chromatography.

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

The retention behavior of aromatic hydrocarbons on anion-exchangers modified with alkylsulfate or alkanesulfonate has been examined by column liquid chromatography. Several parameters affected the retention of analytes, involving concentration of the modifier in the mobile phase, its alkyl chain length and mobile phase composition. Stationary phases modified with a reagent having longer alkyl chains achieved better column efficiency. The separation factor achieved on ion-exchange-induced stationary phases foro-terphenyl and triphenylene was larger than that on alkyl-bonded silica gel.

Anomalous behavior of selected methyl-substituted polycyclic aromatic hydrocarbons in reversed-phase liquid chromatography

In the reversed-phase liquid chromatographic separation of polycyclic aromatic hydrocarbons (PAHs) on C 18 phases, methyl-substituted PAHs are expected to elute after the unsubstituted parent PAH based on the increase in hydrocarbonaceous contact area of the methyl-substituted PAHs. However, we have observed that several methyl-substituted PAHs elute prior to the parent compound, e.g., 1-methylperylene, 1-methylpicene and 13-methylpicene. To investigate this anomalous retention behavior, the retention characteristics of all methyl-substituted isomers of chrysene, picene and perylene were compared on a series of sixteen commercially prepared C 18 phases. The anomalous retention behavior was observed only on polymeric C 18 phases ( i.e., those prepared using trifunctional silanes) whereas the methyl-substituted PAHs elute after the parent PAH, as would be expected, on monomeric C 18 phases ( i.e., those prepared using monofunctional silanes). The early elution of some of these methyl-substituted isomers is related to the non-planarity of these PAHs due to the presence of the methyl group in the so-called “bay-region” of the PAH structure. The non-planarity of these methyl-PAHs can be characterized by the dihedral angle of distortion between the aromatic rings.

Retention behaviour of large polycyclic aromatic hydrocarbons in reversed-phase liquid chromatography on a polymeric octadecylsilica stationary phase

Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy and differential scanning calorimetry experiments were performed in order to study the retention mechanism in reversed-phase liquid chromatography for large polycyclic aromatic hydrocarbons. The effects of changing the mobile phase composition and column temperature were evaluated. The results suggest that a change in mobile phase composition from pure methanol to pure dichloromethane induces further non-planarity in non-planar solutes and also slightly changes the conformation of a polymeric octadecyl stationary phase. Conversely, a change to higher column temperatures drastically changes the structure of the stationary phase from solid-like to liquid-like, with only a small change of non-planar solutes to more non-planar conformations

Application of the electrostatic interaction concept for the study of the retention behaviour of peropyrene-type polycyclic aromatic hydrocarbons in reversed-phase liquid chromatography

Capacity factors of peropyrene-type polycyclic aromatic hydrocarbons were measured using 15 different liquid chromatographic reversed phase systems. On the basis of the retention data the electric interaction indices have been calculated. Application of these indices for structure-retention studies on peropyrene-type polycyclic aromatic hydrocarbons is demonstrated.

Structure-retention relationship analysis for some mono- and polycyclic aromatic hydrocarbons in reversed-phase liquid chromatography using molecular connectivity

A multivariate regression model involving molecular connectivity is described which successfully predicts the reversed-phase liquid chromatographic retention for 58 hydroaromatic compounds (compounds containing only carbon and hydrogen and an aromatic site). Molecular connectivity refers to a collection of indexes that provide quantitative information about molecular structure. The multiparameter approach produced highly correlated relationships between molecular connectivity and retention data. Attempts at single-variable regression were not successful due to the insensitivity of an individual index to the more subtle molecular differences such as position isomerism (o-, m- p-xylene). The /sup 0/chi/sup v/, /sup 0/chi, /sup 4/chi/sup v//sub pc/, and /sup 4/chi/sub pc/ combination of indexes were accurate descriptors of the individual compounds, and this unique set of variables successfully predicted the retention characteristics of the hydroaromatics on various C/sub 18/ stationary phases in aqueous methanol and acetonitrile. The complementary nature of these two sets of nonvalence/valence pairs is discussed.

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

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.

The influence of the pore size of bonded phases containing phenyl groups on the retention of polycyclic aromatic hydrocarbons in reversedphase liquid chromatography

Differences in bonded phase properties were studied for triphenyl, diphenyl, phenyl and benzyl phases prepared on a variety of silicas. A total of six silicas with pore diameters ranging from 100 to 200 A were used in the synthesis. The effect of the pore diameter of the silicas on the retention of polycyclic aromatic hydrocarbons are discussed. Differences in retention were observed as a function of pore size, but not much difference was observed with substrates having pore sizes larger than about 170A. The observed phenomena are explained by two steric exclusion mechanisms, one during the bonding reaction, and the other in the chromatographic analysis. Statistical calculations were performed using the descriptors showing molecular size and/or shape. The results indicated that the difference between bulky phases and less bulky phases was only observed in the degree of shape-recognition ability of the solute while no differences exist in the retention mechanisms on the different phases.