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.     

Comments on the paper “Characterization of stationary phases by a linear solvation energy relationship utilizing supercritical fluid chromatography” by C. R. Mitchell,N. J. Benz,S. Zhang

In a recent paper published by Mitchell et al. in this journal, some results obtained in supercritical fluid chromatography and interpreted with the solvation parameter model to characterize interactions for “novel stationary phases” were surprising to us. Indeed, we had already published results for most of the stationary phases reported, but, except for polar phases, our results were not in agreement with those, despite the use of identical mobile phases in both studies. These data were disturbing because they suggest that supercritical fluid chromatography is always a normal‐phase mode, while we have shown that it is a reversed‐phase mode when working with non‐polar stationary phases. In the process of establishing the reason for the differences between our works, we examined several different factors. This paper deals with practice of linear solvation energy relationships: choice of dead‐volume marker, choice of test‐solutes to adequately probe the possible interactions and appropriate column length for characterization of chromatographic systems with highly eluting mobile phases are discussed. The importance of control experiments to validate retention models and confirm their accordance with the chromatographer's experience is evidenced. Recommendations for good linear solvation energy relationship practice are suggested in order to avoid the publication of results leading to erroneous conclusions.     

Selectivity of amino-, cyano- and diol-bonded silica in reversed-phase liquid chromatography

Amino-, cyano- and diol-bonded silica stationary phases were characterized by estimating their characteristic interaction constants in reversed-phase liquid chromatography (RPLC) based on linear solvation energy relationships. Five characteristic interaction constants of the stationary phases, the hydrophobicity (v), polarizability (r), dipolarity (s), hydrogen bond (HB) acceptor basicity (a) and HB donor acidity strength (b) were determined by multiple regression analyses of logarithmic retention factors (k) for a set of test solutes measured on them in 10% (v/v) methanol-water vs. the solute properties represented by characteristic molecular volume (Vx), excess polarization (R2), dipolarity/polarizability (pi*), HB donor acidity (alpha) and HB acceptor basicity (beta). Magnitudes of the five constants for the phases in RPLC were compared with those in normal-phase LC to see the differences in chromatographic selectivity in the two LC modes.     

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.     

A unified classification of stationary phases for packed column supercritical fluid chromatography

The use of supercritical fluids as chromatographic mobile phases allows to obtain rapid separations with high efficiency on packed columns, which could favour the replacement of numerous HPLC methods by supercritical fluid chromatography (SFC) ones. Moreover, despite some unexpected chromatographic behaviours, general retention rules are now well understood, and mainly depend on the nature of the stationary phase. The use of polar stationary phases improves the retention of polar compounds, when C18-bonded silica favours the retention of hydrocarbonaceous compounds. In this sense, reversed-phase and normal-phase chromatography can be achieved in SFC, as in HPLC. However, these two domains are clearly separated in HPLC due to the opposite polarity of the mobile phases used for each method. In SFC, the same mobile phase can be used with both polar and non-polar stationary phases. Consequently, the need for a novel classification of stationary phases in SFC appears, allowing a unification of the classical reversed- and normal-phase domains. In this objective, the paper presents the development of a five-dimensional classification based on retention data for 94-111 solutes, using 28 commercially available columns representative of three major types of stationary phases. This classification diagram is based on a linear solvation energy relationship, on the use of solvation vectors and the calculation of similarity factors between the different chromatographic systems. This classification will be of great help in the choice of the well-suited stationary phase, either in regards of a particular separation or to improve the coupling of columns with complementary properties.     

Characterization of stationary phases in subcritical fluid chromatography by the solvation parameter model. I. Alkylsiloxane-bonded stationary phases

Varied types of alkylsiloxane-bonded and fluoroalkylsiloxane-bonded stationary phases, all commercially available, were investigated with subcritical fluid mobile phase. The effect of the alkyl chain length (from C4 to C18) and of the nature of the bonding (fluorodecylsiloxane, phenyl-C18 and polar-embedded-C18) on the chromatographic behaviour was investigated by the use of a linear solvation energy relationship (LSER), the solvation parameter model. A large set of test compounds provides precise and reliable information on the intermolecular interactions responsible for retention on these stationary phases used with a subcritical mobile phase. First of all, the results underline the close properties between subcritical fluid and organic liquid. The use of non aqueous mobile phases reduces the cavity energy and the mobile phase acidity generally encountered with aqueous liquid phases, allowing other interactions to take a part in retention. As expected, an increase in the alkyl chain length favours the dispersive interactions between the solutes and the stationary phases. Changes in basicity and acidity of the stationary phases are also related to the chain length, but, in this case, mobile phase adsorption onto the stationary phase is supposed to explain these behaviours. The addition of a phenyl group at the bottom of the C18 chain, near the silica, does not induce great modifications in the retentive properties. The fluorodecylsiloxane and the polar-embedded alkylsiloxane phases display very different properties, and can be complementary to the classical alkylsiloxane-bonded phases. In particular, the fluorinated phase does not favour the dispersive interactions, in comparison to hydrogenated stationary phases, when the basicity of the polar-embedded phase is obviously greater than the one of classical alkylsiloxane-bonded phases, due to the amide function. Finally, logk-logk curves plotted between the different phases illustrate the effect of the interaction properties on the retention of different classes of compounds.     

Retention characteristics of a new butylimidazolium-based stationary phase

A new HPLC stationary phase has been synthesized based on the ionic liquid n-butylimidazolium bromide. Imidazolium was covalently immobilized on a silica substrate through an n-alkyl tether and the retention characteristics of the resulting stationary phase were evaluated systematically. Using 28 small aromatic test solutes and reversed phase conditions, the linear solvation energy relationship approach was successfully used to characterize this new phase. The retention characteristics of the test solutes show remarkable similarity with phenyl stationary phases, despite the presence of a positive charge on the new imidazolium phase. Operated in the reversed phase mode, this new stationary phase shows considerable promise for the separation of neutral solutes and points to the potential for a truly multi-modal stationary phase.     

Characterization of surface-confined ionic liquid stationary phases: impact of cation and anion identity on retention

A series of surface-confined ionic liquid (SCIL) stationary phases for high-performance liquid chromatography were synthesized in-house. The synthesized phases were characterized by the linear solvation energy relationship (LSER) method to determine the effect of residual linking ligands and the role of the cation and the anion on retention. Statistical analysis was utilized to determine whether the system coefficients returned from multiple linear regression analysis of chromatographic retention data for a set of 28 neutral aromatic probe solutes were significantly different. Examination of the energetics of retention via κ−κ plots agrees with the results obtained from the LSER analysis. Residual linking ligands were determined to contribute reversed-phase-type retention character to the chromatographic system. Furthermore, retention on the SCIL phases was observed to be more profoundly affected by the identity of the anion than by that of the cation.     

Effect of water on the retention on diol and amide columns in hydrophilic interaction liquid chromatography

The amount of water adsorbed on polar columns plays important role in hydrophilic interaction liquid chromatography. It may strongly differ for the individual types of polar columns used in this separation mode. We measured adsorption isotherms of water on an amide and three diol‐bonded stationary phases that differ in the chemistry of the bonded ligands and properties of the silica gel support. We studied the effects of the adsorbed water on the retention of aromatic carboxylic acids, flavonoids, benzoic acid derivatives, nucleic bases, and nucleosides in aqueous‐acetonitrile mobile phases over the full composition range. The graphs of the retention factors versus the volume fraction of water in mobile phase show “U‐profile” characteristic of a dual hydrophilic interaction–reversed phase retention mechanism. The minimum on the graph that marks the changing retention mechanism depends on the amount of adsorbed water. The linear solvation energy relationship model suggests that the retention in the hydrophilic interaction liquid chromatography mode is controlled mainly by proton–donor interactions in the stationary phase, depending on the column type. Finally, the accuracy of hydrophilic interaction liquid chromatography gradient prediction improves for columns that show a high water adsorption.     

Development of a decaaza‐cyclophane stationary phase for high‐performance liquid chromatography

A new stationary phase for high‐performance liquid chromatography was prepared by covalently bonding a heteroatom‐bridged cyclophane onto silica gel using 3‐aminopropyltriethoxysilane as the coupling reagent. The structure of the new material was characterized by infrared spectroscopy, elemental analysis, and thermogravimetric analysis. The linear solvation energy relationship method was successfully employed to evaluate the new phase with a set of 25 solutes, and compared with octadecylsilyl and p‐tert‐butyl‐calix[4]arene bonded stationary phases. The retention characteristics of the new phase are similar to the octadecylsilyl and conventional calixarene phases, and it also has distinctive features. In addition, the chromatographic behavior of the phase was illustrated by eluting alkylbenzenes and inorganic anions in the reversed‐phase mode and anion‐exchange mode, respectively. Thus, multi‐interaction mechanisms and mixed‐mode separation of the new phase can very likely guarantee its promising application in the analysis of complex samples. The column has been successfully employed for the analysis of triazines in milk, and it is demonstrated to be a competitive alternative analytical method for the determination of triazine herbicide residues.     

Preparation and comparison of three zwitterionic stationary phases for hydrophilic interaction liquid chromatography

Surface‐bonded zwitterionic stationary phases have shown highlighted performances in separation of polar and hydrophilic compounds under hydrophilic interaction chromatography mode. So, it would be helpful to evaluate the characteristics of zwitterionic stationary phases with different arranged charged groups. The present work involved the preparation and comparison of three zwitterionic stationary phases. An imidazolium ionic liquid was designed and synthesized, and the cationic and anionic moieties respectively possessed positively charged imidazolium ring and negatively charged sulfonic groups. Then, the prepared ionic liquid, phosphorylcholine and an imidazolium‐based zwitterionic selector were bonded on the surface of silica to obtain three zwitterionic stationary phases. The selectivity properties were characterized and compared through the relative retention of selected solute pairs, and different kinds of hydrophilic solutes mixtures were used to evaluate the chromatographic performances. Moreover, the zwitterionic stationary phases were further characterized by the modified linear solvation energy relationship model to probe the multiple interactions. All the results indicated that the types and arrangement of charged groups in zwitterionic stationary phases mainly affect the retention and separation of ionic or ionizable compounds, and for interaction characteristics the contribution from n and π electrons and electrostatic interactions displayed certain differences.     

The Properties of the Nucleodur HILIC Stationary Phase in Supercritical Fluid Chromatography

Russian Journal of Physical Chemistry A - The Nucleodur HILIC zwitterionic stationary phase was characterized by linear solvation energy relationship (LSER) analysis in the supercritical fluid...     

Preparation and evaluation of a novel bonded imidazolium ionic liquid as stationary phase for gas chromatography

1‐Butyl‐3‐[(3‐trimethoxysilyl)propyl]imidazolium chloride ionic liquid was synthesized and chemically modified onto the inner wall of a fused capillary column as a stationary phase for gas chromatography. The 1‐butyl‐3‐[(3‐trimethoxysilyl)propyl]imidazolium chloride ionic liquid bonded capillary column was evaluated in detail. The results revealed that the ionic liquid bonded capillary column exhibited high column efficiency of 1.08 × 10⁴ plates/m, and good chromatographic separation selectivity (α ) for polar and non‐polar substances, and a good thermal stability between room temperature and 400°C. Moreover, the determination of thermodynamic parameters and the linear solvation energy relationship were further carried out. The results indicated that the chromatographic retention of each probe molecule on the ionic liquid bonded stationary phase was an enthalpy‐driven process, and the system constants of the linear solvation energy relationship signified that the dispersion interaction, the hydrogen bonding acidity and hydrogen bonding basicity were dominant interactions between probes and stationary phase among five interactions during the chromatographic separation. However, the contribution of each specific interaction for the stationary phase is ranked as the dispersion interaction > the hydrogen bonding basicity > the hydrogen bonding acidity.     

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.     

Liquid crystal stationary phases for gas chromatography and supercritical fluid chromatography

Novel monomeric and polymeric liquid crystalline compounds were synthesized as stationary phases for gas chromatography (GC) and supercritical fluid chromatography (SFC). Monomeric liquid crystalline compounds were used in packed column gas chromatography for the separation of isomeric aromatic compounds and insect sex pheromones. Liquid crystalline polymers possess long nematic ranges and a uniform coating was easily achieved in glass and fused silica capillaries, which could stand temperatures up to 250°C in GC and pressures of 200 MPa at 160°C in SFC. The columns provide excellent selectivity and resolution for fused ring aromatic compounds such as the isomers anthracene and phenanthrene or triphenylene and chrysene.     

Characterization of a novel pyridinium bromide surface confined ionic liquid stationary phase for high-performance liquid chromatography under normal phase conditions via linear solvation energy relationships

Utilizing linear solvation free energy relationship methodology, a novel pyridinium bromide surface confined ionic liquid (SCIL) stationary phase was characterized under normal phase high-performance liquid chromatographic conditions. A limited set of neutral aromatic probe solutes were utilized to rapidly assess the utility of the LSER model, using mobile phases of hexane modified with 2-propanol. The excellent correlation of the global fit across the mobile phase composition range used in this study for the experimental and calculated retention values (R(2)=0.994) indicates that the LSER model is an appropriate model of characterizing this polar bonded phase under normal phase conditions. For a limited subset of compounds, retention on the pyridinium bromide SCIL stationary phase is more highly correlated with that obtained on a cyano column than on a diol column under NP conditions.