Laterally attached liquid-crystalline polymers as stationary phases. Effect of temperature in RP HPLC and comparison with a commercial C18 stationary phase

Summary Specific side-on-fixed liquid-crystalline polymers (SOLCP) have been synthesized for use in silica-modified stationary phases in high-performance liquid chromatography (HPLC). The mesogenic side group of the SOLCP is composed of three benzoate-type phenyl rings with terminal alkoxy chains and is laterally linked to a polysiloxane backbone via an alkyl ester spacer arm. The dependence of the logarithm of the retention factor on the reciprocal temperature showed that the liquid-crystalline anisotropic order was conserved in the small pores (200 ? diameter) of the silica gel. The first-order nematic-isotropic transition is lost and probably becomes second-order. Adsorption enthalpies for the liquid-crystalline stationary phases have been measurement for three polycyclic aromatic hydrocarbon isomers (ortho-terphenyl, triphenylene, and chrysene) and compared with those for a commercial C₁₈ phase. The adsorption enthalpies never exceeded 30 kJ mol⁻¹, i.e. ten times the thermal agitation energy,RT. They were always less on the SOLCP stationary phase than on the C₁₈ column, emphasizing the more rigid structure of the liquid crystalline phase and its mechanism based upon adsorption. Better separation of steroids, pesticides and amino acids were obtained with the LCP-coated silica than the commercial bonded C₁₈ column. Four small peptides were successfully separated by using pure water as mobile phase.     

Laterally attached liquid crystalline polymers as stationary phases in reversed-phase high-performance liquid chromatography. V. Study of retention mechanism using linear solvation energy relationships

A linear solvation energy relationship model was used to characterize the retention behavior of a stationary phase based upon a nematic side-on liquid crystalline polymer (SOLCP) in reversed-phase liquid chromatography. The set of solutes was constituted of a high variety of compounds whose molecular sizes were considerably smaller than the mesogenic unit size. The results showed good statistical fits for these retention data in 65:35, 75:25 and 85:15 (v/v) methanol-water mobile phases. Both the cavity term and excess molar refraction are the most important favorable retention-governing parameters, whereas the solute hydrogen bond acceptor basicity is the most unfavorable retention parameter. Hydrophobicity and pi-pi interactions decrease strongly when the percentage of methanol increases, leading to an important retention decrease despite the fact that the hydrogen bond interaction weakens as the organic solvent is added. The shape recognition ability of this side-on liquid crystalline stationary phase on polycyclic aromatic hydrocarbon solutes is partly explained by the solutes' high polarizability due to the presence of pi-electrons. However, the solute polarizability is not sufficient and a stationary phase's "structure effect" must to be taken into account for the shape discrimination observed. The strong interaction between liquid crystal molecules caused likely a adsorption retention mechanism rather than a partition mechanism.     

Thermotropic laterally attached liquid crystalline polymers: I. New stationary phases for high-performance liquid chromatography

Up to now thermotropic liquid crystalline side chain polymers have been seldom used as stationary phases in high-performance liquid chromatography (HPLC). The preparation of a new class of surface modified silica gels is reported. They are obtained by coating on the silica support liquid crystalline polysiloxanes with mesogenic side groups laterally attached to the polymer backbone through a flexible spacer. Their chromatographic behavior in reversed-phase HPLC is described for the separation of polycyclic aromatic hydrocarbons. The results show excellent planarity and rod shape recognition capabilities. Comparisons with low-molecular-mass liquid crystalline-bonded silica and longitudinally attached liquid crystalline polymer-coated stationary phase are also reported. Finally, comparisons to commercially available C₁₈ phases are described for the separation of complex mixtures.     

Study of Anion Effects on Separation Phenomenon for the Vinyloctylimidazolium Based Ionic Liquid Polymer Stationary Phases in GC

Poly-vinyloctylimidazolium ionic liquid polymers with different counter ions (bromide, hexafluorophosphate and bis-trifluoromethanesulfonylimide) were directly coated in capillary fused silica tubing as the stationary phases for gas chromatography. The anion effects on the polymers were examined and discussed. The results suggest that the poly-vinyloctylimidazolium, bis-trifluoromethanesulfonylimide capillary column has the highest thermal stability and separation efficiency. Column-to-column reproducibility was also studied. The results indicate that anions significantly influence ionic liquid polymers which give them potential to extend the range of options for the stationary phases in gas chromatography.     

Thermotropic Liquid Crystalline Side Group Polymers as Stationary Phases in High Performance Liquid Chromatography. II. the Mechanism of Separation

Abstract

Liquid crystalline side group polymers support coated on silica gels have been applied as stationary phases in high performance liquid chromatography. It has been possible to show that also in liquid chromatography, separations based on the mesophase structure can be observed in analogy to gas chromatography. From results of separations in which temperature, flow rate, sample concentration and the solvent strength of the mobile phase were varied, this work derives views on the fundamental mechanisms involved. In addition, it will be shown that different mechanisms are probably involved in the separation of steroids and dinitrobenzene isomers on these stationary phases.     

HPLC of synthetic polymers characterization of polystyrenes by high performance precipitation liquid chromatography (HPPLC)

Summary The advantages and disadvantages of high performance precipitation liquid chromatography have been demonstrated for polystyrene homopolymers. Depending on the mobile phase composition at the dissolution point of the polymeric sample and surface properties of the stationary phase, elution is governed either by a solution process or by adsorption. A contribution by adsorption was noticed on silica as well as on reversed phases based on silica with a normal phase gradient of increasing polarity (heptane to dichloromethane). Elution was solely governed by solubility of the polymers on both types of stationary phase for polystyrenes with a molecular weight above 35 000 and reversed phase gradient of decreasing polarity (methanol to dichloromethane). Under these conditions an identical dependence of elution solvent composition on sample size was found as for turbidity titrations. Due to differences in the velocity of the eluent front and the polymeric sample with porous stationary phases the polymers can be eluted as colloidal solutions Non-porous stationary phases are superior in this respect because the velocities of eluent and solutes are identical.     

Laterally attached liquid crystalline polymers as stationary phases in reversed-phase high-performance liquid chromatography. II. Optimization of the molecular parameter of the polymer

Stationary phases obtained by coating side-chain liquid crystalline polymers (LCPs) with the mesogenic rod like units laterally attached to a polysiloxane backbone via a flexible spacer have been already reported. These phases show excellent planarity and shape recognition for polynuclear aromatic hydrocarbon (PAH) solutes in reversed-phase liquid chromatography. Optimization of these stationary phases in terms of molecular parameters of the polymer is here described. Fifteen stationary phases have been prepared varying different parameters such as the spacer length, the aliphatic tail length, and the proportion of laterally attached mesogenic units along the polymer chain. The results show that the combination of a long spacer and long terminal chains, which generates a smectic phase in the polymer bulk, leads to the best chromatographic performances towards planarity and shape recognition for PAH solutes.     

Evaluation of the Effect of Citronellol Group on Functionalized Mesogenic Materials by Capillary GC

In this paper, the effects of functionalization with terpenes on two new liquid-crystalline stationary phases for gas chromatography (GC) are described. Citronellol was used as the terminal group in the first material, and tetrahydrogeraniol was used with a second material. Inverse GC showed that the new materials have wide liquid-crystalline ranges (mesophases), 371–500 and 395–501 K, respectively. Moreover, they show good thermal stability up to 523 K and good potential as stationary phases for capillary GC. To clarify the effects of the liquid crystal structures and functional groups on retention and separation, the chromatographic behaviors of the two stationary phases were compared by eluting alkylbenzenes, polycyclic aromatic hydrocarbons, aromatic compounds, and terpenoids. The selectivities for a wide range of analytes achieved using the citronellol column were significantly better than those obtained using the tetrahydrogeraniol column. The columns showed different retention behaviors and fine resolutions for some of the main constituents of essential oils. Introduction of the double bond of citronellol greatly improved the polarization interactions involved in the shape recognition of the liquid-crystalline state for isomers. The new citronellol liquid-crystalline stationary phase, therefore, has a high affinity for natural compounds.

     

环果糖色谱柱分离4组结构类似物

考察了聚苯乙烯键合天然环果糖色谱柱MCI Gel CRS100、硅胶键合天然环果糖色谱柱Frulic N、硅胶键合异丙基氨基甲酸酯衍生环果糖色谱柱Larihc P、硅胶键合R-萘乙基氨基甲酸酯衍生环果糖色谱柱Larihc RN在正相模式下对免疫抑制剂、维生素E、姜黄类化合物、辣椒碱4组结构类似物的分离能力。考察了固定相支撑物、固定相功能基团、流动相组成等条件对色谱分离效率的影响。结果显示MCI Gel CRS100由于其固定相较强的疏水性适用于正相色谱而不适用于亲水作用色谱。衍生化的环果糖色谱柱Larihc P和Larihc RN比天然环果糖色谱柱Frulic N具有更高的选择性。三氟乙酸的加入对环果糖色谱柱在正相色谱中分离能力的影响较小。     

Additional studies on shape selectivity by using the carotenoid test to classify C18 bonded silica

Numerous chromatographic tests are applied to study the C18 bonded phases, either to classify these phases, or to better determine their properties or their chromatographic behaviours. Because the carotenoid test is developed in supercritical fluid chromatography (SFC), many correlations with high performance liquid chromatography (HPLC) results have been necessary to ensure the trueness of the classification reached in these analytical conditions. Consequently, the analytical conditions of the carotenoid test were chosen to fit with the TbN/BaP values from the NIST 869a test, which describe the shape selectivity of the alkyl bonded phases. Additional studies performed in this paper by using well standardized silica (J'Sphere, YMC Pack ODS A, Wakosil II), which varied from their surface coverage (J'Sphere), their pore shape (YMC), or their bonding type and carbon content (Wakosil II). The use of these C18 stationary phases allows to reach more accurate conclusion on the comparison of the shape selectivity values provided either by the cis/trans β-carotene selectivity or by the TRI/oTER or TbN/BaP ones. The extension of the studies to many other C18 stationary phases allows clarifying the relationships between the carotenoid test and the tests based on the use of PAH, as well as the usage limit of TRI/oTER and TBN/BaP selectivity in regards of the bonding density of the stationary phases for both monomeric and polymeric phases. By checking other aromatic compounds, another selectivity (anthracene/oTER) display greater correlation with the carotenoid test, that suggest an improvement of the relevance of this new compound couple for the steric selectivity study.     

An attempt directed toward enhanced shape selectivity in reversed-phase liquid chromatography: preparation of the dodecylaminated beta-cyclodextrin-bonded phase.

With the aim of preparing a stationary phase with a high shape-recognition ability for liquid chromatography, a new bonded phase was synthesized by coupling multiply dodecylamino-substituted beta-cyclodextrin (beta-CD) to 3-glycidoxypropyl-derivatized silica gel. The stationary phase prepared in this way was expected to have increased shape selectivity compared with that of conventional reversed-phase materials, due to solute interactions with the alkyl chain piles built up on the beta-CDs bonded to silica. The separation characteristics of the bonded phase were investigated using polycyclic aromatic hydrocarbons (PAHs) with different molecular shapes and compared with those of monomeric ODS and native beta-cyclodextrin-bonded phases. The newly developed stationary phase was found to be highly selective for PAHs.     

Recent progress in carbohydrate separation by high-performance liquid chromatography based on size exclusion

Since the introduction of stationary phases based on microparticulate porous silica and polymeric sorbents, rigid and semi-rigid, size-exclusion chromatography (SEC) has become established as a form of high-performance liquid chromatography. In recent years, there have beeen revolutionary developments in detection systems for high-performance SEC, which have placed the use of the method for the determination of molecular-size and molecular-weight distributions of polymers on a sound theoretical basis andincreased the range of information on molecular characteristics that can be retrieved from SEC data. This review surveys these changes in SEC systems and their application to the separation and molecular-weight distribution analysis of carbohydrates.     

Review on the chemical and thermal stability of stationary phases for reversed-phase liquid chromatography

At present, in high-performance liquid chromatography (HPLC) for the majority of analyses, reversed-phase liquid chromatography (RPLC) is the separation mode of choice. Faster method development procedures using aggressive eluents under elevated temperature conditions, the need for improved selectivities, efficiencies and resolution, the reduction of solvent consumption and also the decrease of analysis times require reversed-phase (RP) columns of high chemical and thermal stability. Until now, the majority of columns for RPLC separations are manufactured from silica substrates. Silica has many favorable properties making this material nearly ideal as a support for RP columns. However, its solubility, that increases considerably in eluents of pH above +/-7, is a drawback preventing its widespread use over the entire pH range. In addition, also the thermal stability of silica is limited. Recently, however, substantial progress has been made in the synthesis of RPLC silica-based stationary phases showing satisfactory thermal and chemical stability under many different experimental conditions. Also, new substrates mainly based on other inorganic substrates like, e.g. alumina and zirconia have been developed now as a starting material for the preparation of RPLC stationary phases of improved chemical and thermal stability. In addition, for the same reasons, many efforts have also been made to synthesize polymer and also polymer-coated phases. These latter phases, more particularly those based on zirconia, but also polymer phases show a high degree of chemical and thermal stability compared to silica counterparts. In this paper, an overview will be given of the state-of-the-art of the thermal and chemical stability of the different available stationary phases for RPLC.     

Chromatographic behavior of new deazapurine ribonucleosides in hydrophilic interaction liquid chromatography

The chromatographic behavior of new biogenic purine nucleosides in hydrophilic interaction liquid chromatography was examined on three different stationary phases, namely bare silica, and amide‐ and cyclofructan‐based stationary phases. The effects of buffer concentration, pH and acetonitrile‐to‐aqueous‐part ratio in the mobile phase on retention and peak shape were assessed. The retention coefficients and peak symmetry values substantially differed with respect to analytes´ structures, stationary phase properties and mobile phase composition. The bare silica column was unsuitable for these compounds under the chromatographic conditions tested due to very broad and asymmetrical peaks. Furthermore, the cyclofructan‐based stationary phase provided almost Gaussian peak shapes of all deazapurine nucleosides under most conditions tested. Therefore, the cyclofructan‐based stationary phase is the most suitable choice for the chromatographic analysis of nucleosides.     

Molecularly imprinted polymers: towards highly selective stationary phases in liquid chromatography and capillary electrophoresis

Molecularly imprinted polymers (MIPs) are synthetic polymers with a predetermined selectivity for a given analyte, or group of structurally related compounds, that make them ideal materials to be used in separation processes. In this sense, it is not surprising that the first applications of MIPs were as tailor-made chiral stationary phases in liquid chromatography. However, peak broadening and tailing, especially of the more retained enantiomers, were observed. Accordingly, this paper gives an overview of the attempts carried out during the recent years to improve the chromatographic performance of MIPs in liquid chromatography and capillary electrophoresis as well as the more recent applications. We conclude that MIPs are very promising materials to be used as selective stationary phases in chromatography although further developments are necessary in order to fully exploit their potential.     

Comparison of Different Silica-Based Imidazolium Stationary Phases for LC in Separation of Alkaloids

Three stationary phases based on silica modified with imidazolium ionic liquids were prepared and characterized for their use in high-performance liquid chromatography. Multiple interactions of separation mechanism of the three stationary phases were studied by using caffeine, theophylline and theobromine. The comparison of differential retentions was established and the hydrophobic interaction of the alkylimidazolium groups showed stronger impact than other interactions in separation of alkaloids. The effects of temperatures and different concentrations of additives in eluent were also investigated. Furthermore, the study of retention mechanism was used to predict the order of elution of cytosine, thymine and guanine successfully.     

Development of silica-based stationary phases for high-performance liquid chromatography

Stationary phases are the basis of the development and application of high-performance liquid chromatography (HPLC). In this review we focused on the development of silica-based stationary phases, including the synthesis of silica gel and the application of silica in hydrophilic interaction chromatography (HILIC), reversed-phase liquid chromatography (RPLC), chiral separation chromatography, and ion chromatography. New stationary phases, advances in ionic liquid-modified silica, silica-based core-shell materials, and silica-based monolithic columns for HPLC are introduced separately.     

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.     

Comparative study of chromatography on thin layers impregnated with organic stationary phases. Chromatographic separation of nitrophenols.

The chromatographic behaviour of nitrophenols on thin layers of silica gel and cellulose was compared, both without impregnation and impregnated with non-aqueous polar stationary phases (formamide, dimethylformamide) and less polar stationary phases (liquid paraffin, octan-1-ol, 1-bromonaphthalene). Cellulose is preferred when using formamide or dimethylformamide if a pure partition process is required. For each particular analyte a certain amount of the stationary phase is always required to suppress the adsorption activity of silica gel. Separation by reversed-phase thin-layer chromatography is strongly affected by the type of stationary phase (the possibility of forming charge-transfer complexes with 1-bromonaphthalene) and its support (the acidic properties of silica gel and its adsorption activity), the mobile phase (content of organic modifier, pH, presence of salts) and the properties of the solutes (polarity, ionizability).     

Synthesis and evaluation of ginsenosides imprinted polymer-based chromatographic stationary phase

The molecular imprinting technique has aroused great interest in preparing novel stationary phases, and the resulting materials named molecularly imprinted polymers coated silica packing materials exhibit good performance in separating diverse analytes based on their good characteristics (including high selectivity, simple synthesis, and good chemical stability). To date, mono-template is commonly used in synthesizing molecularly imprinted polymers-based stationary phases. The resulting materials always own the disadvantages of low column efficiency and restricted analytes, and the price of ginsenosides with high purity was very high. In this study, to overcome the weaknesses of molecularly imprinted polymers-based stationary phases mentioned above, the multi-templates (total saponins of folium ginseng) strategy was used to prepare ginsenosides imprinted polymer-based stationary phase. The resulting ginsenosides imprinted polymer-coated silica stationary phase has a good spherical shape and suitable pore structures. Additionally, the total saponins of folium ginseng were cheaper than other kinds of ginsenosides. Moreover, the ginsenosides imprinted polymer-coated silica stationary phase-packed column performed well in the separation of ginsenosides, nucleosides, and sulfonamides. The ginsenosides imprinted polymer-coated silica stationary phase possesses good reproducibility, repeatability, and stability for seven days. Therefore, a multi-templates strategy for synthesizing the ginsenosides imprinted polymer-coated silica stationary phase is considered in the future.