Comparison of retention of aromatic hydrocarbons with polar groups in binary reversed-phase high-performance liquid chromatography systems

The retention of aromatic hydrocarbons with polar groups has been correlated as log k1 versus log k2 for reversed-phase high-performance liquid chromatography systems with different binary aqueous mobile phases containing methanol, acetonitrile or tetrahydrofuran as modifiers. Distinct changes in separation selectivity have been observed between tetrahydrofuran and acetonitrile or methanol systems. Methanol and acetonitrile systems show lower diversity of separation selectivity. The changes in retention and selectivity of aromatic hydrocarbons with various polar groups between any two chromatographic systems with binary aqueous eluents (tetrahydrofuran vs. acetonitrile, tetrahydrofuran vs. methanol and methanol vs. acetonitrile) have been interpreted in terms of molecular interactions of the solute with especially one component of the stationary phase region, i.e. extracted modifier, and stationary phase ordering. The ordering of the stationary phase region caused by modifier type influences the chromatographic selectivity of solutes with different molecular shape.     

Mobile phase effects in reversed-phase liquid chromatography: a comparison of acetonitrile/water and methanol/water solvents as studied by molecular simulation

Molecular simulations of water/acetonitrile and water/methanol mobile phases in contact with a C(18) stationary phase were carried out to examine the molecular-level effects of mobile phase composition on structure and retention in reversed-phase liquid chromatography. The simulations indicate that increases in the fraction of organic modifier increase the amount of solvent penetration into the stationary phase and that this intercalated solvent increases chain alignment. This effect is slightly more apparent for acetonitrile containing solvents. The retention mechanism of alkane solutes showed contributions from both partitioning and adsorption. Despite changes in chain structure and solvation, the molecular mechanism of retention for alkane solutes was not affected by solvent composition. The mechanism of retention for alcohol solutes was primarily adsorption at the interface between the mobile and stationary phase, but there were also contributions from interactions with surface silanols. The interaction between the solute and surface silanols become very important at high concentrations of acetonitrile.     

Characterisation of stationary phases in subcritical fluid chromatography with the solvation parameter model IV. Aromatic stationary phases

The purpose of the present work was to systematically study the chromatographic behaviour of different aromatic stationary phases in a subcritical fluid mobile phase. We attempted to assess the chemical origin of the differences in retention characteristics between the different columns. Various types of aromatic stationary phases, all commercially available, were investigated. The effect of the nature of the aromatic bonding on interactions between solute and stationary phases and between solute and carbon dioxide-methanol mobile phase was studied by the use of a linear solvation energy relationship (LSER): the solvation parameter model. This study was performed to provide a greater knowledge of the properties of these phases in subcritical fluid chromatography, and to allow a more rapid and efficient choice of aromatic stationary phase in regard of the chemical nature of the solutes to be separated. Charge transfer interactions naturally contribute to the retention on all these stationary phases but are completed by various other types of interactions, depending on the nature of the aromatic group. The solvation vectors were used to compare the different phase properties. In particular, the similarities in the chromatographic behaviour of porous graphitic carbon (PGC), polystyrene-divinylbenzene (PS-DVB) and aromatic-bonded silica stationary phases are evidenced.     

Characterisation of stationary phases in subcritical fluid chromatography with the solvation parameter model. III. Polar stationary phases

In this third paper, varied types of polar stationary phases, namely silica gel (SI), cyano (CN)- and amino-propyl (NH2)-bonded silica, propanediol-bonded silica (DIOL), poly(ethylene glycol) (PEG) and poly(vinyl alcohol) (PVA), were investigated in subcritical fluid mobile phase. This study was performed to provide a greater knowledge of the properties of these phases in SFC, and to allow a more rapid and efficient choice of polar stationary phase in regard of the chemical nature of the solutes to be separated. The effect of the nature of the stationary phase on interactions between solute and stationary phases and between solute and carbon dioxide-modifier mobile phases was studied by the use of a linear solvation energy relationship (LSER), the solvation parameter model. The retention behaviour observed with sub/supercritical fluid with carbon dioxide-methanol is close to the one reported in normal-phase liquid chromatography with hexane. The hydrogen bond acidity and basicity, and the polarity/polarizability favour the solute retention when the molar volume of the solute reduces it. As with non-polar phases, the absence of water in the subcritical fluid allows the solute/stationary phase interactions to play a greater part in the retention behaviour. As expected, the DIOL phase and the bare silica display a similar behaviour towards acidic and basic solutes, when interactions with basic compounds are lower with the NH2 phase. On the CN phase, all interactions (hydrogen bonding, dipole-dipole and charge transfer) have a nearly equivalent weight on the retention. The polymeric phases, PEG and PVA, provide the most accurate models, possibly due to their better surface homogeneity.     

Retention mechanisms in subcritical water reversed-phase chromatography

Differences in the properties of subcritical water and conventional water/acetonitrile and water/methanol mobile phases for reversed phase separations are explored. Using van’t Hoff plots enthalpies and entropies of transfer are compared among the mobile phases while linear solvation energy relationships are used to quantify contributions to retention based on a solute's polarizability, dipolarity, hydrogen bond donating ability, hydrogen bond accepting ability, and molecular size. Results suggest the presence of acetonitrile or methanol in the mobile phase may decrease dispersive interactions of the solute with the stationary phase compared to subcritical water, thereby lowering enthalpic contributions to retention. Enthalpic contributions are found to drive the retention of a methylene group in all systems studied.     

Enhancement of selectivity in reversed-phase liquid chromatography

In an effort to gain insight into the relationship between stationary phase solvation and selectivity, the use of short- and medium-chained-length alcohols (methanol, n-propanol, n-butanol, and n-pentanol) as mobile phase modifiers in reversed-phase liquid chromatography (RPLC) was investigated to determine their impact on chromatographic selectivity. A wide range of mobile phase compositions was evaluated because of the large effect exerted by solvent strength on selectivity. Employing a set of six vanillin compounds as retention probes, evidence is presented to support the view that an increase in the hydrophobicity of the organic modifier used in RPLC can increase the selectivity of the C18 alkyl bonded phase while simultaneously decreasing the retention time of the eluting solutes. Thus, we are presented with an interesting paradox: higher selectivity and shorter retention times, which can be attributed to changes in either solvent selectivity and/or stationary phase solvation by the organic modifier.     

Retention characteristics of porous graphitic carbon in subcritical fluid chromatography with carbon dioxide-methanol mobile phases

Numerous relationships usually used in high-performance liquid chromatography (HPLC) for describing the retention on porous graphitic carbon (PGC) have been applied in subcritical fluid chromatography, with CO2-methanol mobile phases. As reported in HPLC, octanol-water partition coefficient failed to fit the retention, whereas satisfactory results were obtained with the sum of partial negative charges. A better fit was reached by using the solvation parameter model, allowing a better understanding of the interactions developed between the solute, the stationary and the mobile phases. Results show that the dominant contribution to retention was given by the polarizability (E) and the volume (V), while the hydrogen-bond basicity (B) was not selected in the retention model, whatever the methanol content. The increase in methanol percentage favours the retention decrease, mainly through the volume for hydrophobic compounds, and through the hydrogen-bond acidity for polar compounds.     

Insights into the Retention Mechanism of Small Neutral Compounds on Octylsiloxane-Bonded and Diisobutyloctadecylsiloxane-Bonded Silica Stationary Phases in Reversed-Phase Liquid Chromatography

The system constants of the solvation parameter model are used to prepare system maps for the retention of small neutral compounds on an octylsiloxane-bonded (Kinetex C8) and diisobutyloctadecylsiloxane-bonded (Kinetex XB-C18) superficially porous silica stationary phases for aqueous mobile phases containing 10–70% (v/v) methanol or acetonitrile. Electrostatic interactions (cation-exchange) are important for the retention of weak bases with acetonitrile–water but not for methanol–water mobile phases. Compared with an octadecylsiloxane-bonded silica stationary phase (Kinetex C18) retention is reduced due to a less favorable phase ratio for both the octylsiloxane-bonded and diisobutyloctadecylsiloxane-bonded silica stationary phases while selectivity differences are small and solvent dependent. Selectivity differences for neutral compounds are larger for methanol–water but significantly suppressed for acetonitrile–water mobile phases. The selectivity differences arise from small changes in all system constants with solute size and hydrogen-bond basicity being the most important due to their dominant contribution to the retention mechanism. Exchanging the octadecylsiloxane-bonded silica column for either the octylsiloxane-bonded or diisobutyloctadecylsiloxane-bonded silica column affords little scope for extending the selectivity space and is restricted to fine tuning of separations, and in some cases, to obtain faster separations due to a more favorable phase ratio. For weak bases larger differences in relative retention are expected with acetonitrile–water mobile phases on account of the additional cation exchange interactions possible that are absent for the octadecylsiloxane-bonded silica stationary phase.     

Characterization of temperature dependent modifier effects in SFC using linear solvation energy relationships

Summary Linear solvation energy relationships (LSERs) are used to probe the changes in mobile and stationary phase properties of a carbon dioxide-based mobile phase and a polymeric stationary phase under near-critical conditions. Four mobile phase modifiers are compared with respect to dipolarity/polarizability, hydrogen bond donating and accepting ability, and other intermolecular interactions as a function of temperature. As temperature nears the mixture critical point, the differences in these properties between the mobile and stationary phases change to reflect the growing heterogeneity in mobile phase component distribution at the chromatographic interface. The stationary phase loses many of its original characteristics and takes on characteristics typical of the mobile phase modifier due to preferential adsorption of the modifier at the surface of the stationary phase.     

Effects of organic modifiers on retention mechanism and selectivity in micellar electrokinetic capillary chromatography studied by linear solvation energy relationships

The effects of six organic modifiers (urea, methanol, dioxane, tetrahydrofuran, acetonitrile and 2-propanol) on the retention mechanism and separation selectivity of the bulk buffer in micellar electrokinetic capillary chromatography (MECC) with sodium dodecyl sulfate (SDS) micelles as pseudo-stationary phase have been investigated through linear solvation energy relationships (LSERs). It is found that the retention value in MECC systems with or without organic modifier is primarily dependent on the solvophobic interaction and the hydrogen bonding interaction with the solute as proton acceptor, while the dipolar interaction and the hydrogen bonding interaction with the solute as proton donor play minor roles. The effects of the organic modifiers on the solvophobic, dipolar and hydrogen bonding interactions are evaluated in terms of the relationship between regression coefficient of the LSER equations and the modifier concentration. The variations of the solvophobic interaction and the dipolar interaction with change of the modifier concentration can be approximately explained using the solubility parameter and the dipolarity/polarizability parameter of the organic modifier, respectively. However, the relationships between the hydrogen bond acidity and basicity of the bulk buffer and the organic modifiers are rather complicated. Those results may be caused from the displacement of organic modifiers to the water adsorbed on the micellar surface as well as changes in the acidity and basicity of the bulk buffer with the addition of organic modifiers. In addition, it is found that the phase ratio is influenced significantly by the use of organic modifier.     

Insights into the Retention Mechanism on a Pentafluorophenylpropylsiloxane-Bonded Silica Stationary Phase (Discovery HS F5) in RP-LC

The solvation parameter model is used to elucidate the retention mechanism of neutral compounds on the pentafluorophenylpropylsiloxane-bonded silica stationary phase (Discovery HS F5) with methanol-water and acetonitrile-water mobile phases containing from 10 to 70% (v/v) organic solvent. The dominant factors that increase retention are solute size and electron lone pair interactions while polar interactions reduce retention. A comparison of the retention mechanism with an octadecylsiloxane-bonded silica stationary phase based on the same silica substrate and with a similar bonding density (Discovery HS C18) provides additional insights into selectivity differences for the two types of stationary phase. The methanol-water solvated pentafluorophenylpropylsiloxane-bonded silica stationary phase is more cohesive and/or has weaker dispersion interactions and is more dipolar/polarizable than the octadecylsiloxane-bonded silica stationary phase. Differences in hydrogen-bonding interactions contribute little to relative retention differences. For mobile phases containing more than 30% (v/v) acetonitrile selectivity differences for the pentafluorophenylpropylsiloxane-bonded and octadecylsiloxane-bonded silica stationary phases are no more than modest with differences in hydrogen-bond acidity of greater importance than observed for methanol-water. Below 30% (v/v) acetonitrile selectivity differences are more marked owing to incomplete wetting of the octadecylsiloxane-bonded silica stationary phase at low volume fractions of acetonitrile that are not apparent for the pentafluorophenylpropylsiloxane-bonded silica stationary phase. Steric repulsion affects a wider range of compounds on the octadecylsiloxane-bonded than pentafluorophenylpropylsiloxane-bonded silica stationary phase with methanol mobile phases resulting in additional selectivity differences than predicted by the solvation parameter model. Electrostatic interactions with weak bases were unimportant for methanol-water mobile phase compositions in contrast to acetonitrile-water where ion-exchange behavior is enhanced, especially for the pentafluorophenylpropylsiloxane-bonded silica stationary phase. The above results are compatible with a phenomenological interpretation of stationary phase conformations using the haystack, surface accessibility, and hydro-linked proton conduit models.     

Selectivity comparison of acetonitrile-methanol-water ternary mobile phases on an octadecylsiloxane-bonded silica stationary phase

The solvation parameter model was used in this study to investigate various intermolecular interactions that influence retention on the standard C18 stationary phase for the solvent system acetonitrile:methanol (ACN:MeOH, 1:1). In comparison to the organic mobile phase modifiers acetonitrile, acetone, methanol, 2-propanol, and tetrahydrofuran, the solvent strength for the ACN:MeOH (1:1) solvent system was evaluated. To facilitate the interpretation of various intermolecular interactions that contribute to retention on a standard C18 stationary phase for the solvent system ACN:MeOH (1:1), system maps were constructed and compared with those of acetone, tetrahydrofuran, acetonitrile, 2-propanol, and methanol. The solvation parameter models were constructed for the ternary solvent system ACN:MeOH (1:1)-water, and in the models constructed, the coefficient of determination values were from 0.998 to 0.999, the Fisher statistic values for the models were from 1687 to 4015, and the standard error of the estimate values ranged from 0.022 to 0.029. The solvent system ACN:MeOH (1:1) has retention properties more similar to methanol than acetonitrile, indicating methanol's influence is more dominant.     

Linear Solvation Energy Relationships in the Determination of Specificity and Selectivity of Stationary Phases

The retention of fifty structurally different compounds has been studied using linear solvation energy relationships. Investigations were performed with the use of six various stationary phases with two mobile phases (50/50 % v/v methanol/water and 50/50 % v/v acetonitrile/water). Packing materials were home-made and functionalized with octadecyl, alkylamide, cholesterol, alkyl-phosphate and phenyl molecules. This is the first attempt to compare all of these stationary phases synthesized on the same silica gel batch. Therefore, all of them may be compared in more complex and believable way, than it was performed earlier in former investigations. The phase properties (based on Abraham model) were used to the classification of stationary phases according to their interaction properties. The hydrophilic system properties s, a, b indicate stronger interactions between solute and mobile phase for most of the columns. Both e and v cause greater retention as a consequence of preferable interactions with stationary phase by electron pairs and cavity formation as well as hydrophobic bonds. However, alkyl-phosphate phase has different retention properties, as it was expressed by positive sign of s coefficient. It may be concluded that most important parameters influencing the retention of compounds are volume and hydrogen bond acceptor basicity. The LSER coefficients showed also the dependency on the type of organic modifier used as a mobile phase component.

     

Linear Solvation Energy Relationships in the Determination of Specificity and Selectivity of Stationary Phases

The retention of fifty structurally different compounds has been studied using linear solvation energy relationships. Investigations were performed with the use of six various stationary phases with two mobile phases (50/50?% v/v methanol/water and 50/50?% v/v acetonitrile/water). Packing materials were home-made and functionalized with octadecyl, alkylamide, cholesterol, alkyl-phosphate and phenyl molecules. This is the first attempt to compare all of these stationary phases synthesized on the same silica gel batch. Therefore, all of them may be compared in more complex and believable way, than it was performed earlier in former investigations. The phase properties (based on Abraham model) were used to the classification of stationary phases according to their interaction properties. The hydrophilic system properties s, a, b indicate stronger interactions between solute and mobile phase for most of the columns. Both e and v cause greater retention as a consequence of preferable interactions with stationary phase by electron pairs and cavity formation as well as hydrophobic bonds. However, alkyl-phosphate phase has different retention properties, as it was expressed by positive sign of s coefficient. It may be concluded that most important parameters influencing the retention of compounds are volume and hydrogen bond acceptor basicity. The LSER coefficients showed also the dependency on the type of organic modifier used as a mobile phase component.     

Liquid chromatography analysis of monosubstituted sulfobutyl ether-beta-cyclodextrin isomers on porous graphitic carbon

The retention behaviour of the three positional isomers of monosubstituted sulfobutyl ether-beta-cyclodextrin was investigated on a porous graphitic carbon (PGC) column. The influence of the mobile phase composition (nature and concentration of organic and electronic modifiers) was studied as well as the effect of column temperature. These hydrophilic and anionic analytes were highly retained on the PGC stationary phase compared to octadecyl bonded phases. The retention is mainly governed by a reversed-phase mechanism with electronic interaction playing a secondary role. An increase in solute retention and efficiency with temperature was observed. Successful isocratic separation with satisfactory baseline resolution of the three isomers of monosubstituted sulfobutyl ether-beta-cyclodextrin was achieved at 75 degrees C on a Hypercarb column by using ammonium acetate as electronic modifier in water-acetonitrile (83:17). The chromatographic methodology developed can be easily used for relative quantification of each isomer within a mixture and can be applied for semi-preparative purification of each one. The evaporative light scattering detector allows the detection of these non UV-visible absorbing molecules.     

High-performance liquid chromatography of substituted p-benzoquinones and p-hydroquinones. II. Retention behavior, quantitative structure-retention relationships and octanol-water partition coefficients

The retention behavior of methoxy-substituted p-benzoquinones and the corresponding hydroquinones in reversed-phase chromatography was examined on octylsilica and two octadecylsilica stationary phases and with five hydroorganic mobile phases containing acetonitrile, methanol or tetrahydrofuran and additionally in most cases (NH3OH)3PO4 used as a reducing and buffering agent. The retention order of benzoquinones and hydroquinones was the same on each stationary phase with either methanol or acetonitrile as the organic modifier. On the other hand, minor differences in the retention order were observed with the various stationary phases. In all cases, satisfactory quantitative structure-retention relationships (QSRRs) were found and the data suggest that the differences in the retention behaviour of octadecylsilicas used in this study are silanophilic interactions which, together with solvophobic interaction contribute to the retention of these eluites. Further analysis showed that QSRRs of sterically crowded molecules must take into account reduced surface area available for binding. The retention data obtained with use of aqueous tetrahydrofuran as mobile phase failed to give rise to satisfactory QSRRs. This was attributed to selective solvation of eluite by tetrahydrofuran and/or nearly equipotent binding of eluite and tetrahydrofuran to stationary phase.     

Influence of solvent effects on retention in reversed-phase liquid chromatography and solid-phase extraction using a cyanopropylsiloxane-bonded,silica-based sorbent

Summary The solvation parameter model is used to characterize the retention properties of a cyanopropylsiloxanebonded, silica-based sorbent with methanol, acetonitrile, tetrahydrofuran, and isopropanol in water as mobile phases. The system constants over the composition range 1 to 50 % (v/v) organic solvent indicate that retention occurs because of the relative ease of cavity formation in the solvated stationary phase compared to the same process in the predominantly aqueous mobile phase as well as from more favorable stationary phase interactions with solutes containing π- and n-electrons. The capacity of the solute for dipole-type interactions is not important whereas all hydrogen-bond-type interactions result in reduced retention. Graphing the system constants as a function of mobile phase composition provides a simple mechanism for interpreting the change in capacity of the chromatographic system for retention in terms of changes in the relative weighting of fundamental intermolecular interactions. A comparison is also made with the retention properties of an octadecylsiloxane-bonded, silica-based sorbent with 30 % (v/v) methanol in water as the mobile phase and the extraction characteristics of a porous polymer sorbent with 1 % (v/v) methanol, acetonitrile, tetrahydrofuran, and isopropanol in water as the sample processing solvent. Changes in sorbent selectivity due to selective uptake of the processing solvent are much smaller for the cyanopropylsiloxane-bonded sorbent than the results found for a porous polymer sorbent.     

Retention properties of acetone-water mobile phases on a biphenylsiloxane-bonded silica stationary phase in reversed-phase liquid chromatography

The solvation parameter model system constants and retention factors were used to interpret retention properties of 39 calibration compounds on a biphenylsiloxane-bonded stationary phase (Kinetex biphenyl) for acetone-water binary mobile phase systems containing 30–70% v/v. Variation in system constants, phase ratios, and retention factors of acetone-water binary mobile phases systems were compared with more commonly used acetonitrile and methanol mobile phase systems. Retention properties of acetone mobile phases on a Kinetex biphenyl column were more similar to that of acetonitrile than methanol mobile phases except with respect to selectivity equivalency. Importantly, selectivity differences arising between acetone and acetonitrile systems (the lower hydrogen-bond basicity of acetone-water mobile phases and differences in hydrogen-bond acidity, cavity formation and dispersion interactions) could be exploited in reversed-phase liquid chromatography method development on a Kinetex biphenyl stationary phase.     

Solvent effects on the retention of oligosaccharides in porous graphitic carbon liquid chromatography

Porous graphitic carbon (PGC) is known as well suited adsorbent for liquid chromatography of carbohydrates. In this work we report on systematic investigations of solvent effects on the retention mechanism of fluorescence labeled malto-oligosaccharides on PGC. The adsorption mechanism was found to depend on the type of organic modifier used in the mobile phase. Positive adsorption enthalpies and entropies, which have already been reported in the literature, were solely produced using acetonitrile. Both alternative solvents (tetrahydrofuran, 2-propanol) yielded in contrast negative enthalpies. As plausible retention mechanism for oligosaccharides on PGC applying acetonitrile as mobile phase component we propose the formation of a dense and highly ordered solvation layer of the PGC surface with the linear acetonitrile molecules. Adsorption of analyte molecules requires a displacement of numerous acetonitrile molecules, which explains the positive enthalpy and entropy values measured. The interplay of enthalpic and entropic contributions to the overall adsorption phenomena results in strongly temperature dependent chromatographic selectivity values.     

Mobile phase effects on retention on a new butylimidazolium-based high-performance liquid chromatographic stationary phase

A new HPLC stationary phase based on n-butylimidazolium bromide has been characterized by a linear solvation energy relationship (LSER) approach in the binary acetonitrile/water mobile phases. The retention properties of the stationary phase were systematically evaluated in terms of intermolecular interactions between 28 test solutes and the stationary phase. The results and further comparisons with conventional reversed phase system confirm that retention properties are similar to phenyl phases in acetonitrile/water mixtures. The results obtained with acetonitrile/water mixtures are also compared with results obtained using methanol/water mixtures.