Characterization of C18-bonded liquid chromatographic stationary phases by Raman spectroscopy: the effect of temperature

Surface-bubble-modulated liquid chromatography: an experimental strategy for identification of molecular processes of solute retention in reversed-phase separation systems

Molecular level understanding of the chemistry at the aqueous/hydrophobe interface is crucial to separation processes in aqueous media, such as reversed-phase liquid chromatography (RPLC) and solid-phase extraction (SPE). Despite significant advances in our knowledge of the solute retention mechanism in these reversed-phase systems, direct observation of the behavior of molecules and ions at the interface in reversed-phase systems still remains a major challenge and experimental probing techniques that provide the spatial information of the distribution of molecules and ions are required. This review addresses surface-bubble-modulated liquid chromatography (SBMLC), which has a stationary gas phase in a column packed with hydrophobic porous materials and enables one to observe the molecular distribution in the heterogeneous reversed-phase systems consisting of the bulk liquid phase, the interfacial liquid layer, and the hydrophobic materials. The distribution coefficients of organic compounds referring to their accumulations onto the interface of alkyl- and phenyl-hexyl-bonded silica particles exposed to water or acetonitrile-water and into the bonded layers from the bulk liquid phase are determined by SBMLC. The experimental data obtained by SBMLC show that the water/hydrophobe interface exhibits an accumulation selectivity for organic compounds, which is quite different from that of the interior of the bonded chain layer, and the overall separation selectivity of the reversed-phase systems is determined by the relative sizes of the aqueous/hydrophobe interface and the hydrophobe. The solvent composition and the thickness of the interfacial liquid layer formed on octadecyl-bonded (C₁₈) silica surfaces are also estimated from the bulk liquid phase volume determined by the ion partition method employing small inorganic ions as probes. It is clarified that various hydrophilic organic compounds as well as inorganic ions recognize the interfacial liquid layer formed on the C₁₈-bonded silica surfaces as being different from the bulk liquid phase. The behavior of some solute compounds exhibiting substantially weak retention in RPLC or the so-called negative adsorption, such as urea, sugars, and inorganic ions, can rationally be interpreted with a partition between the bulk liquid phase and the interfacial liquid layer. The spatial distribution of solute molecules and the structural properties of the solvent layer on the C₁₈-bonded layer determined by the liquid chromatographic methods are discussed in comparison to the results obtained by other research groups using molecular simulation methods.

Profiling of Unsaturated Lipids by Raman Spectroscopy Directly on Solid-Phase Microextraction Probes

Lipids play a critical role in cellular signaling, energy storage, and the construction of cellular membranes. In this paper, we propose a novel on-site approach for detecting and differentiating enriched unsaturated lipids based on the direct coupling of SPME probes with Raman spectroscopy. To this end, different SPME particles, namely, hydrophilic-lipophilic balanced (HLB), mixed-mode (C8-SCX), and C18, were embedded in polyacrylonitrile (PAN) and tested for their efficacy as biocompatible coatings. The C18/PAN coating showed less background interference compared to the other sorbent materials during the analysis of unsaturated lipids. In addition, different SPME parameters that influence extraction efficiency, such as extraction temperature, extraction time, and washing solvent, were also investigated. Our results indicate a clear dependence between the Raman band intensity related to the number of double bonds in fatty acids mixture and the number of double bonds in a fatty acid. Our findings further show that Raman spectroscopy is especially useful for the analysis of lipid unsaturation, which is calculated as the ratio of n(C═C)/n(CH₂) using the intensities of the Raman bands at 1655/1445 cm^-1. Furthermore, the developed protocol reveals great SPME activity and high detection ability for several unsaturated lipids in different complex matrixes, such as cod liver oil. Finally, the applicability of this technology was demonstrated via the characterization of cod liver oil and other vegetable oils. Thus, the proposed SPME-Raman spectroscopy approach has a great future potential in food, environmental, clinical, and biological applications.

Hybrid-Lipid Bilayers Induce n-Alkyl-Chain Order in Reversed-Phase Chromatographic Surfaces, Impacting their Shape Selectivity for Aromatic Hydrocarbon Partitioning

Shape selectivity is important in reversed-phase liquid chromatographic separations, where stationary phases are capable of separating geometric isomers, thereby resolving solutes based on their three-dimensional structure or shape rather than other chemical differences. Numerous chromatographic studies have been carried out using n-alkyl-chain-modified columns to understand how molecular shape affects retention. For polycyclic aromatic hydrocarbons (PAHs), it was found that planar compounds were selectively retained over nonplanar structures of comparable molecular weight on surfaces with longer n-alkyl chains, higher chain-density, or at lower temperatures, where selectivity likely arises with greater ordering of the n-alkyl chains. A limitation of these studies, however, is the small range of chain ordering that can be achieved and lack of a direct measure of the n-alkyl-chain order of the stationary phases. In this work, we employ a C₁₈ stationary phase modified with a monolayer of phospholipid as a means of significantly varying the n-alkyl chain order. These hybrid-supported lipid bilayers, which have previously been employed as membrane-like stationary phases for measuring lipophilicity, provide a unique approach to control n-alkyl chain ordering by varying the acyl chain length and degree of unsaturation of the phospholipid modifier. The degree of alkyl-chain order of the resulting modified surfaces is determined from the ratio of trans- versus gauche-conformers, measured in situ within individual porous particles by confocal Raman microscopy. This methodology was also used to assess the affinity of these surfaces for planar versus nonplanar PAH molecules. The retention selectivity for the planar versus nonplanar compounds, thus determined, was found to vary significantly and systematically with the degree of order of the acyl/alkyl chains in the hybrid-supported lipid bilayers. The investigation also demonstrates the utility of confocal Raman microscopy for interrogating the impact of solute partitioning on stationary-phase structure within porous chromatographic particles.

Teicoplanin-Modified HPLC Column as a Source of Experimental Parameters for Prediction of the Anticonvulsant Activity of 1,2,4-Triazole-3-Thiones by the Regression Models

The cell membrane is a complex system that consists of lipids, proteins, polysaccharides, and amphiphilic phospholipids. It plays an important role in ADME processes that are responsible for the final pharmaceutical effects of xenobiotics (bioavailability, activity). To study drug-membrane interaction at the molecular level, several high-performance liquid chromatography (HPLC) membrane model systems have been proposed which are mimicking mainly its lipid character. The aim of this work was to study interactions of new synthesized antiepileptic compounds of 4-alkyl-5-(3-chlorophenyl)-2,4-dihydro-3H-1,2,4-triazole-3-thione derivatives with Chirobiotic column containing glycoprotein ligand attached to the silica matrix. The affinity of the analytes to immobilized glycoprotein ligand was examined chromatographically in reversed-phase mode. The thermodynamics of interactions between bioactive compounds and teicoplanin was studied in terms of the van't Hoff linear relationship ln k vs. 1/T in the range of 5-45 °C. Change in enthalpy (ΔH°), change in entropy (ΔS°) and change in Gibbs free energy (ΔG°) were estimated utilizing graphical extrapolation and interpolation methods. The density functional theory (DFT) approach and docking simulations were used to get the molecular interpretation and prove the obtained experimental results. Cross-correlations of chromatographic and thermodynamic parameters with non-empirical topological and quantum chemical indices suggest that the polarizability of analytes appears to be responsible for the interactions of the tested molecules with teicoplanin and, ultimately, their retention on the column. Experimental and theoretical parameters were subjected to statistical analysis using regression models. Partial least squares (PLS) regression model showed the usefulness of the experimentally measured parameter φ0 (MeOH) to discriminate between anticonvulsant active and inactive 1,2,4-triazole-3-thione derivatives. Obtained results point out the usefulness of interaction of potential anticonvulsants with glycoprotein class of compounds to anticipate their activity.

Confocal Raman Microscopy Investigation of Self-Assembly of Hybrid Phospholipid Bilayers within Individual Porous Silica Chromatographic Particles

Hybrid-supported phospholipid bilayers are a model structure utilized for measurement of molecular interactions that typically occur at cell membranes. These membrane models are prepared by adsorption of a lipid monolayer onto a stable n-alkyl chain layer that is covalently bound to a support surface. Hybrid bilayers have been adapted to chromatographic retention measurements of lipophilicity through the assembly of a phospholipid monolayer onto n-alkane-modified silica surfaces in reversed-phase chromatographic particles. Recent Raman microscopy studies of these particles have shown that the acyl chains of the phospholipid interact with the C₁₈-alkyl chains immobilized on the silica surface, where both lipid and C₁₈ alkyl chains become ordered because of chain interdigitation. Confocal Raman microscopy has also been used to investigate the association of small molecules with hybrid-lipid bilayers in C₁₈ chromatographic silica particles; the partitioning of model solutes compares favorably to that in lipid vesicle membranes with similar changes in acyl-chain structure (disordering) with solute partitioning. The present study seeks information about how these membrane-mimetic bilayers assemble onto the C₁₈-derivatized silica surfaces of reversed-phase chromatographic silica particles. Confocal Raman microscopy is capable of interrogating the time-dependent internal composition and structure within individual silica particles. The Raman scattering data can be resolved into component Raman spectra and corresponding composition vectors that describe the time-dependent changes in intensity of the component spectra. This analysis provides insight into how the structures of both the lipid and C₁₈ alkyl chains of hybrid lipid bilayers evolve during deposition and organization on the internal surfaces of reversed-phase chromatographic silica particles.

Column Characterization and Selection Systems in Reversed-Phase High-Performance Liquid Chromatography

Reversed-phase high-performance liquid chromatography (RP-HPLC) is the most popular chromatographic mode, accounting for more than 90% of all separations. HPLC itself owes its immense popularity to it being relatively simple and inexpensive, with the equipment being reliable and easy to operate. Due to extensive automation, it can be run virtually unattended with multiple samples at various separation conditions, even by relatively low-skilled personnel. Currently, there are >600 RP-HPLC columns available to end users for purchase, some of which exhibit very large differences in selectivity and production quality. Often, two similar RP-HPLC columns are not equally suitable for the requisite separation, and to date, there is no universal RP-HPLC column covering a variety of analytes. This forces analytical laboratories to keep a multitude of diverse columns. Therefore, column selection is a crucial segment of RP-HPLC method development, especially since sample complexity is constantly increasing. Rationally choosing an appropriate column is complicated. In addition to the differences in the primary intermolecular interactions with analytes of the dispersive (London) type, individual columns can also exhibit a unique character owing to specific polar, hydrogen bond, and electron pair donor-acceptor interactions. They can also vary depending on the type of packing, amount and type of residual silanols, "end-capping", bonding density of ligands, and pore size, among others. Consequently, the chromatographic performance of RP-HPLC systems is often considerably altered depending on the selected column. Although a wide spectrum of knowledge is available on this important subject, there is still a lack of a comprehensive review for an objective comparison and/or selection of chromatographic columns. We aim for this review to be a comprehensive, authoritative, critical, and easily readable monograph of the most relevant publications regarding column selection and characterization in RP-HPLC covering the past four decades. Future perspectives, which involve the integration of state-of-the-art molecular simulations (molecular dynamics or Monte Carlo) with minimal experiments, aimed at nearly "experiment-free" column selection methodology, are proposed.

Confocal Raman Microscopy Investigation of Molecular Transport into Individual Chromatographic Silica Particles

Porous silica is used as a support in a variety of separation processes, including chromatographic separation and solid-phase extraction. The resolution and efficiency of these applications is significantly impacted by the kinetics of partitioning and molecular transport into the interior of the porous particles. Molecular transport in porous silica has been explored previously by measuring chromatographic elution profiles, but such measurements are limited to relatively low retention conditions, where within-particle molecular transport must be inferred from elution profiles of solutes emerging from a packed column. In this work, a measurement of within-particle molecular transport is carried out using confocal Raman microscopy to probe the time-dependent accumulation of pyrene from an aqueous mobile phase into the center of individual C₁₈-chromatographic particles. The measured time constants for pyrene accumulation were much slower than diffusion-limited transport of solute in solution to the particle surface. Furthermore, the accumulation into the center of the particle did not show a time-lag characteristic of slow-transport into the particle interior. The exponential rise of pyrene concentration is, however, consistent with first-order Langmuir adsorption kinetics at low surface coverages. The linear dependence of the time-constant on particle radius indicates an adsorption barrier near the outer boundary of the particle, where the accumulation rate depends on flux across the boundary (proportional to the particle area) to satisfy the within-particle capacity at equilibrium (proportional to the particle volume). The pyrene accumulation kinetics into the porous particle, expressed as a heterogeneous rate constant, were nearly 50-times faster than the pyrene adsorption rate at a planar C₁₈-functionalized silica surface, which demonstrates the impact of multiple surface encounters within the porous structure leading to much greater capture efficiency compared to a planar surface. Monte Carlo simulations of within-particle pyrene diffusion, with the adsorption efficiency estimated from the planar-surface adsorption rate, predict a diffusion-to-capture distance within the porous particle that is within 40% of that observed in the radial dependence of the pyrene within-particle accumulation results.

Molecular simulation studies of reversed-phase liquid chromatography

Over the past 20 years, molecular simulation methods have been applied to the modeling of reversed-phase liquid chromatography (RPLC). The purpose of these simulations was to provide a molecular-level understanding of: (i) the structure and dynamics of the bonded phase and its interface with the mobile phase, (ii) the interactions of analytes with the bonded phase, and (iii) the retention mechanism for different analytes. However, the investigation of chromatographic systems poses significant challenges for simulations with respect to the accuracy of the molecular mechanics force fields and the efficiency of the sampling algorithms. This review discusses a number of aspects concerning molecular simulation studies of RPLC systems including the historical development of the subject, the background needed to understand the two prevalent techniques, molecular dynamics (MD) and Monte Carlo (MC) methods, and the wealth of insight provided by these simulations. Examples from the literature employing MD approaches and from the authors' laboratory using MC methods are discussed. The former can provide information on chain dynamics and transport properties, whereas the latter techniques are uniquely suited for the investigation of phase and sorption equilibria that underly RPLC retention, and both can be used to elucidate the bonded-chain conformations and solvent distributions.

Nanoscale reversed-phase liquid chromatography-mass spectrometry of permethylated N-glycans

Reversed-phase liquid chromatography on the nanoscale coupled to electrospray tandem mass spectrometry was used to analyse a mixture of four commercial glycan standards, and the method was further adapted to N-glycans enzymatically released from alpha-1-acid glycoprotein and immunoglobulin gamma. Glycans were permethylated to enable their separation by reversed-phase chromatography and to facilitate interpretation of fragmentation data. Prior to derivatization of glycans by permethylation, they were reduced to cancel anomerism because, although feasible, it was not desired to separate α- and β-anomers. The effect of supplementing chromatographic solvent with sodium hydroxide to guide adduct formation was investigated. Raising the temperature in which the separation was performed improved chromatographic resolution and affected retention times as expected. It was shown by using the tetrasaccharides sialyl Lewis X and sialyl Lewis A that reversed-phase chromatography could achieve the separation of methylated isobaric glycan analytes. Isobaric glycans were detected among the N-glycans of immunoglobulin gamma and further analysed by tandem mass spectrometry.

Confocal Raman microscopy of protein adsorbed in chromatographic particles

Confocal Raman microscopy is a nondestructive analytical technique that combines the chemical information from vibrational spectroscopy with the spatial resolution of confocal microscopy. It was applied, for the first time, to measure conformation and distribution of protein adsorbed in wetted chromatographic particles. Monoclonal antibody was loaded into the Fractogel EMD SO(3) (M) cation exchanger at 2 mS/cm or 10 mS/cm. Amide I and III frequencies in the Raman spectrum of the adsorbed protein suggest that there are no detectable changes of the original β-sheet conformation in the chromatographic particles. Protein depth profile measurements indicate that, when the conductivity is increased from 2 mS/cm to 10 mS/cm, there is a change in mass transport mechanism for protein adsorption, from the shrinking-core model to the homogeneous-diffusion model. In this study, the use of confocal Raman microscopy to measure protein distribution in chromatographic particles fundamentally agrees with previous confocal laser scanning microscopic investigations, but confocal Raman spectroscopy enjoys additional advantages: use of unlabeled protein to eliminate fluorescent labeling, ability for characterization of protein secondary structure, and ability for spectral normalization to provide a nondestructive experimental approach to correct light attenuation effects caused by refractive index (RI) mismatching in semiopaque chromatographic particles.

Structural modifications and adsorption capability of C18-silica/binary solvent interphases studied by EPR and RP-HPLC

The structure of the octadecyl (C18) chain layer attached to a silica surface in the presence of binary solvents (acetonitrile/water; methanol/water) was investigated by electron paramagnetic resonance (EPR) and reverse-phase high-performance liquid chromatography (RP-HPLC), using 4-hydroxy-2,2,6,6 tetramethylpiperidine-N-oxyl (Tempol) to mimic the behavior of pollutants with medium-low polarity. The computer-aided analysis of the EPR spectra provided structural and dynamical information of the probe and its environments which clarified the modifications of the chain conformations that occur at different solvent compositions. Capacity factors, k', were calculated as a function of the percentage of water/organic solvent (mobile phase), and the retention behavior of the C18-functionalized silica surface (stationary phase) was compared with the results obtained with EPR analysis under static conditions. In particular, EPR analysis showed that, at percentages of ACN equal or higher than 50%, the chain layer assume a quite ordered structure, whereas at percentages lower than 50% the chains tend to collapse and fold on the silica surface. In this latter situation, the hydrophobic net of the C18 chains strongly limits Tempol mobility. In methanol/water mixtures, both EPR and RP-HPLC analysis showed that the probe was adsorbed into a poorly ordered interphase. If the residual silanols at the silica surface were bonded to a sililating agent (endcapping), both EPR and RP-HPLC analysis showed a decreased adsorption of the probe with respect to the non-endcapped silica at the same mobile phase composition.

Microscopic Origins of Band Broadening in Chromatography. Polarity Distribution in C18 Stationary Phase Probed by Confocal Ratiometric Imaging of Nile Red

Band broadening is a major factor that influences the efficiency and resolution of chromatographic separations. Studies of microscopic origins of band broadening, such as the micropolarity distribution of chromatographic stationary phase, can provide a better understanding of many chromatographic phenomena and retention behavior. In this work, we probe the chemical environments of C18 chromatographic stationary phase with quantitative confocal fluorescence microscopy under real reversed-phase liquid chromatography conditions. Ratiometric imaging of C18 interface is achieved by loading the stationary phase with a polarity-sensitive dye, Nile red, and optical sectioning with confocal microscopy. The results reveal that there are uniform micropolarity distributions inside individual chromatographic beads, but the polarity may differ between stationary-phase particles. The homogeneity of micropolarity of individual beads suggests that there are not any spatially large exposed silica sites beyond the optical resolution in C18 stationary phase. The strong adsorption sites are smaller in size than the optical resolution of a few hundred nanometers. The heterogeneity between chromatographic beads indicates that the interactions of Nile red with C18 bonded phase are different between beads. This contributes to the broad overall polarity distribution of the C18 stationary phase and can be one of the factors that cause band broadening in separations. With its high spatial resolution and optical sectioning capabilities, confocal fluorescence imaging is shown to be an ideal method to probe the chromatographic stationary phase. The distribution of micropolarity sheds light on the microscopic heterogeneity in chromatographic processes and its influence on chemical separations.

Effect of temperature on gradient reequilibration in reversed-phase liquid chromatography

The effect of mobile phase modifier and temperature on gradient reequilibration is examined using three different stationary phases. The stationary phases studied are a traditional C18 phase, a polar endcapped C18 phase, and an alkyl phase with a polar embedded group. It was observed that both temperature and choice of mobile phase organic modifier had an effect on gradient reequilibration volume on both the traditional C18 stationary phase and the polar endcapped phase. On both these phases, at any given temperature, the reequilibration volume was generally smaller when methanol was used as the mobile phase modifier as compared to acetonitrile. As the temperature is increased from 10 to 50 degrees C, significant reductions in reequilibration volume were observed with both mobile phase modifiers. In contrast, neither temperature nor choice of modifier appeared to have much effect on reequilibration volume when the polar embedded group stationary phase was considered.

Raman Spectral Conformational Order Indicators in Perdeuterated Alkyl Chain Systems

Conformational order indicators for perdeuterated alkyl chain systems are identified in the Raman spectra of nonadecane-d40, polyethylene-d4, and stearic acid-d35. Six spectral indicators are identified: I[nu(a)(CD2)2196]/I[nu(a)(CD2)2172], I[nu(C-C)G]/I[nu(C-C)T], I[nu(s)(CD3)]/I[nu(a)(CD2)2172], I[nu(s)(CD2)]/I[nu(a)(CD2)2172], and the full width at half-maximum (fwhm) and frequency of the nu(s)(CD2) mode. Among these indicators, the ratio of I[nu(a)(CD2)2196]/I[nu(a)(CD2)2172] is considered a primary indicator of conformational order, since it responds to alkyl chain conformational changes in a manner similar to the I[nu(a)(CH2)]/I[nu(s)(CH2)] primary indicator for hydrogenated systems. Other indicators are correlated to this primary indicator to derive a better understanding of the effect of structural attributes on conformational order in perdeuterated alkyl chain systems. These Raman spectral order indicators are applicable to any perdeuterated alkyl-containing system including lipids, biological membranes, alkylsilane-based chromatographic stationary phases, and self-assembled monolayers.

Effect of temperature on the adsorption behavior of tryptophan in reversed-phase liquid chromatography

Single-component adsorption isotherm data of l-tryptophan on a C(18)-bonded silica column were acquired by frontal analysis (FA), with aqueous mobile phases containing 5% of acetonitrile at five different temperatures between 23 and 62 degrees C. The non-linear fitting of these data provided the bi-Moreau model for all temperatures as the best isotherm model. The inverse method (IM) was used to derive the parameters at these temperatures from the parameters of the 25 degrees C isotherm. The adsorption constants and the saturation capacities of the low and high-energy sites decreases by increasing the temperature, while the adsorbate-adsorbate parameters of both sites increase. An excellent agreement was found between the experimental and calculated overloaded band profiles at all the temperatures used. The breakthrough curves obtained and the overloaded band profiles obtained were found to have different shapes according to the range of concentration studied and the temperatures. At low concentration 0.05-0.5 g/L the breakthrough curves and the overloaded band profiles have a front shock and diffuse rear, which indicates langmuirian behavior, but at intermediate 1-2 g/L and high concentration 8 g/L they start to have diffuse fronts and shocks at the rear or more than one shock at the rear which indicates non-langmuirian behavior. At 23 degrees C the isotherm has another langmuirian part, which appears at high concentration. The behavior of the breakthrough curves is explained by the shape of the isotherm in which all of the isotherms have a langmuirian part (the isotherm is concave upward) and an antilangmuirian part (the isotherm is concave downward). The temperature affected the breakthrough curves by decreasing the time of the appearance of the fronts for all concentration ranges studied, and by decreasing the time difference between the highest concentration and lowest concentration of the fronts, especially the low concentration range at 0.5 g/L. The fronts of the breakthrough curves at high concentration seems to be the most affected by temperature.

Chain conformation and solvent partitioning in reversed-phase liquid chromatography: Monte Carlo simulations for various water/methanol concentrations

Many structural models for the stationary phase in reversed-phase liquid chromatography (RPLC) systems have been suggested from thermodynamic and spectroscopic measurements and theoretical considerations. To provide a molecular picture of chain conformation and solvent partitioning in a typical RPLC system, a particle-based Monte Carlo simulation study is undertaken for a dimethyl octadecyl (C(18)) bonded stationary phase on a model siliceous substrate in contact with mobile phases having different methanol/water concentrations. Following upon previous simulations for gas-liquid chromatography and liquid-liquid phase equilibria, the simulations are conducted using the configurational-bias Monte Carlo method in the Gibbs ensemble and the transferable potentials for phase equilibria force field. The simulations are performed for a chain surface density of 2.9 micromol/m(2), which is a typical bonded-phase coverage for mono-functional alkyl silanes. The solvent concentrations used here are pure water, approximately 33 and 67% mole fraction of methanol and pure methanol. The simulations show that the chain conformation depends only weakly on the solvent composition. Most chains are conformationally disordered and tilt away from the substrate normal. The interfacial width increases with increasing methanol content and, for mixtures, the solvent shows an enhancement of the methanol concentration in a 10 Angstrom region outside the Gibbs dividing surface. Residual surface silanol groups are found to provide hydrogen bonding sites that lead to the formation of substrate bound water and methanol clusters, including bridging clusters that penetrate from the solvent/chain interfacial region all the way to the silica surface.

Structure−Function Relationships in High-Density Docosylsilane Bonded Stationary Phases by Raman Spectroscopy and Comparison to Octadecylsilane Bonded Stationary Phases

Raman spectroscopy is used to investigate the effects of temperature, surface coverage, polymerization method (surface or solution polymerized), and nature of the alkylsilane precursor on alkyl chain conformational order in a series of high-density docosylsilane (C22) stationary phases at surface coverages ranging from 3.61 to 6.97 mumol/m(2). The results of this study contribute to an enhanced understanding of the shape-selective retention characteristics of these phases at a molecular level. Conformational order is evaluated using the intensity ratio of the antisymmetric and symmetric nu(CH2) modes as well as the frequency at which these modes are observed. Alkyl chain order is shown to be dependent on surface coverage, alkyl chain length, and polymerization method. In general, alkyl chain order increases with surface coverage. Temperature-induced changes are observed between 250 and 350 K for the three phases with surface coverages between 3.61 and 4.89 mumol/m(2). These changes occur over a broad range of temperatures characteristic of two-dimensional systems, but in general adhere to the behavior predicted for a simple first-order transition. This change is not believed to be an abrupt cooperative disassociation characteristic of a phase transition in a bulk phase, but instead is thought to involve significant changes in conformational order in segments of the surface-tethered molecules, mostly those segments at the outer edge of the alkylsilane. In contrast to the changes observed in coverages below 5 mumol/m(2), a first-order change is not observed for the stationary phase with coverage of 6.97 mumol/m(2). A molecular picture of the temperature-induced disorder is proposed with disorder originating at the distal carbon and propagating only a short distance toward the proximal carbon. A comparison is made between these C22 stationary phases and similar high-density octadecylsilane (C18) bonded phases.

Adsorption of Pure and Mixed Solvent Solutions of Spin Probes onto Stationary Phases

Water, methanol (MeOH), acetonitrile (ACN), and binary MeOH-water and ACN-water solutions of different spin probes (nitroxides), selected to mimic the behavior of different pollutants, were adsorbed onto stationary phases usually used in reversed-phase high-performance liquid chromatography (RP-HPLC). These stationary phases are constituted by porous silica and differ from each other regarding the surface area, the pore size, the particle size, the surface functions (NH2, C8, and C18), and the percentage of functionalization. The electron paramagnetic resonance (EPR) spectra of the probe solutions adsorbed into the pores were analyzed by computer-aided computation of the spectral line shape, which provided structural and dynamical parameters of the probes and their environments. These parameters provided information on the surface properties of the stationary phases, such as alkyl chain density, solvent penetration, stationary-phase ordering, and residual silanol effects, which modify the retention times in HPLC. A different availability of polar surface groups in the pure and mixed solvents was found for the different stationary phases depending on (1) the different functionalization degree, (2) the surface-chain length, (3) the particle size, and (4) the polarity of both the probe and the solvent. The C8 functionalization rendered the surface more hydrophobic with respect to C18. The endcapping process of the residual silanols strongly enhanced the surface hydrophobicity tested by the probes. At the highest water content, the adsorption of the polar or charged probes onto the hydrophobic surface is the lowest and self-aggregation occurs. When the probes bear both hydrophilic and hydrophobic moieties, the adsorption is enhanced by a synergy between hydrophilic and hydrophobic bonds with the surface. A balance between the hydrophilic and hydrophobic forces leads to high adsorption and partial insertion of the surfactant probes in an ordered C18 chain layer at the solid surface which forms in the binary mixtures; this layer is ascertained between 40% and 70% of the less hydrophilic solvent, depending on the type of both the solid and the probe. This insertion and the response on the formation of the ordered layer were favored in ACN-water with respect to MeOH-water.

Influence of synthetic routes on the conformational order and mobility of C18 and C30 stationary phases

Silica gels modified with n-alkyl chains (n = 18, 30) are prepared by two different synthetic routes and are examined by variable temperature FTIR and solid-state NMR spectroscopy. HPLC measurements of SRM 869, cis/trans ss-carotene isomers and xanthophylls isomers confirm the dependence of the separation mechanism on the alkyl chain length and the synthetic routes. The determination of the silane functionality and degree of cross-linking of silane ligands on the silica surface is achieved by 29Si CP/MAS NMR measurements. The structural order and mobility of the alkyl chains are investigated by means of variable temperature 13C CP/MAS NMR measurements. Variable temperature FTIR studies are performed where conformational order and flexibility of the alkyl chains in C18 and C30 phases are monitored through conformational sensitive CH2 symmetric, anti-symmetric stretching and wagging modes. In addition, the chromatographic properties of the C18 and C30 phases are determined. The results derived from the FTIR, NMR and HPLC measurements are discussed in the context of the applied synthetic routes and alkyl chain lengths.

Influence of stationary phase solvation on shape selectivity and retention in reversed-phase liquid chromatography

In the past few decades, shape selectivity has drawn a great deal of attention from chromatographers. The chemistry and characteristics of bonded stationary phases such as phase type, length of bonded phase, surface coverage, and silica surface material have an effect on the shape selectivity of the columns. Although the effects of bonded phase shape selectivity are relatively well understood, one remaining question is the effect of intercalated solvent on shape selectivity. The intercalation of organic modifier and water molecules into the stationary phase is believed to introduce more rigidity into bonded alkyl chains in RPLC. The use of gas chromatography (GC) opens a new dimension to approach this question. C18 columns 4 cm in length were prepared in our laboratory and used in both LC and GC experiments. Shape selectivity and thermodynamic constants for the transfer of a solute from the mobile phase to the stationary phase have been determined as a function of monomeric octadecyl stationary phase bonding densities over the range of 1.44-3.43 micromol/m2 and a polymeric phase (nominal surface coverage 4.77 micromol/m2). Comparing LC and GC experiments, we observed: (a) similar relationships between shape and phenyl selectivities with monomerically bonded C18 phase densities; (b) different correlation of thermodynamic quantities (DeltaH degrees , DeltaS degrees , and DeltaG degrees ) versus bonded phase densities. The effects of high temperature and residual silanol groups are sources of difficulty in elucidation of the intercalated mobile phase role in selectivity and retention for GC measurements.

Molecular Dynamics Simulations of Alkylsilane Stationary-Phase Order and Disorder. 1. Effects of Surface Coverage and Bonding Chemistry

"Shape-selective" polymeric alkylsilane stationary phases are routinely employed over the more common monomeric phases in reversed-phase liquid chromatography (RPLC) to improve the separation of geometric isomers of shape-constrained solutes. We have investigated the molecular dynamics of chromatographic models that represent both monomeric and polymeric stationary phases with alkylsilane surface coverages and bonding chemistries typical of actual materials in an effort to elucidate the molecular-level structural features that control shape-selective separations. The structural characterization of these models is consistent with previous experimental observations of alkyl chain order and disorder: (1) alkyl chain order increases with increased surface coverage; and (2) monomeric and polymeric phases with similar surface coverages yield similar alkyl chain order (although subtle differences exist). In addition, a significant portion of the alkyl chain proximal to the silica surface is disordered (primarily gauche conformations) and the distal end is most ordered. Models that represent shape-selective RPLC phases possess a significant region of distal end chain order with primarily trans dihedral angle conformations. This is consistent with the view that the alkyl chains comprising polymeric stationary phases contain a series of well-defined and rigid voids in which shape-constrained solutes can penetrate and hence be selectively retained.

Molecular Dynamics Simulations of Alkylsilane Stationary-Phase Order and Disorder. 2. Effects of Temperature and Chain Length

In an effort to elucidate the molecular-level structural features that control shape-selective separations, we have investigated the molecular dynamics of chromatographic models that represent both monomeric and polymeric stationary phases with alkylsilane length and temperature conditions analogous to actual materials of low to high shape selectivity. The structural characterization of these models is consistent with previous experimental observations of alkyl chain order and disorder: alkyl chain order increases both with alkyl chain length and with reduced temperature. Models that represent shape-selective reversed-phase liquid chromatography (RPLC) phases possess a significant region of distal end chain order with primarily trans dihedral angle conformations; the extension of these ordered regions into the phase increases with an increase in chain length. Models with extended chain length (C30) possess a higher degree of conformational order and are relatively insensitive to changes in surface coverage, bonding chemistry, and temperature. Chromatography models of various chain lengths and over a temperature range that represents highly shape-selective RPLC stationary phases all contain a series of well-defined and rigid cavities; the size and depth of these "slots" increase for the C30 models, which may promote the enhanced separations of larger size shape-constrained solutes, such as carotenoids.

Investigation on conformational order and mobility of DiamondBond-C18 and C18-alkyl modified silica gels by Fourier transform infrared and solid-state NMR spectroscopy

The effect of surface coverage and solid supports on the conformational order of alkyl chains of commercially available carbon clad zirconia based supports and synthesised C18-alkyl modified silica based supports are probed in the dry state for the first time using variable temperature Fourier transform infrared (FT-IR) and solid-state 13C NMR spectroscopy. From FT-IR spectroscopy, the conformational order of alkyl chains tethered to the substrates is examined by the analysis of CH2 symmetric and anti-symmetric stretching bands. Through solid-state 13C NMR spectroscopy, the order is inferred from the relative intensity of the main methylene carbon resonance assigned to trans and trans-gauche conformations. It is found that molecules tethered to the graphite layer experience a strongly diamagnetic component of the highly anisotropic magnetic susceptibility of the graphite lattice, which reflects upfield shift in the 13C NMR spectra of commercially available octadecyl-modified carbon clad zirconia based column materials. The present results prove that temperature, surface coverage and solid supports have an influence on the conformational order and mobility of alkyl chains tethered to the carbon clad inorganic metal oxides.

Conformational order of n-alkyl modified silica gels as evaluated by Fourier transform infrared spectroscopy

The conformational behaviour of non-deuterated and selectively deuterated alkyl modified silica gels in the dry state is examined by variable temperature FT IR spectroscopy. In the present study, three systems are considered, which are distinguished by the length of the tethered alkyl chains (C9Hl9-, C18H37-, C22H45-). The desired information is obtained by the analysis of various conformational-sensitive IR bands, including CH2 wagging, CD2 stretching and CD2 rocking bands. The analysis of the CH2 wagging bands provides the relative amounts (i.e., integral numbers over the whole chain) of the kink/gauche-trans-gauche, double-gauche and end-gauche conformers in the tethered alkyl chains. From the analysis of the CD2 stretching and CD2 rocking bands information about the conformational order at a specific deuterated methylene segment is available. Here, the CD2 rocking band data are used to determine the amount of gauche conformers at the deuterated carbon positions C-4 and C-6, and C-12. It is found that the conformational order critically depends on the actual alkyl chain length, chain position and sample temperature. Particular emphasis is given to the impact of the external pressure during sample preparation on the alkyl chain conformations, about which so far no information is available. It is observed that the samples prepared as KBr pellets, which experienced a pressure of about 10 kbar, are characterised by a lower amount of gauche conformers. This substantial increase of conformational order is attributed to better alkyl chain packing along with a gain of intermolecular chain interactions.

Order and disorder in alkyl stationary phases

Covalently modified surfaces represent a unique state of matter that is not well described by liquid or solid phase models. The chemical bond in tethered alkanes imparts order to the surface in the form of anisotropic properties that are evident in chromatographic and spectroscopic studies. An understanding of the structure, conformation, and organization of alkyl-modified surfaces is requisite to the design of improved materials and the optimal utilization of existing materials. In recent years, the study of alkyl-modified surfaces has benefited from advances in modern analytical instrumentation. Aspects of alkyl chain conformation and motion have been investigated through the use of nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, fluorescence spectroscopy, and neutron scattering studies. Chromatography provides complementary evidence of alkyl chain organization through interactions with solute probes. Computational simulations offer insights into the structure of covalently modified surfaces that may not be apparent through empirical observation. This manuscript reviews progress achieved in the study of the architecture of alkyl-modified surfaces.

Probing Strong Adsorption of Solute onto C18-Silica Gel by Fluorescence Correlation Imaging and Single-Molecule Spectroscopy under RPLC Conditions

Understanding molecular adsorption at a chromatographic interface is of great interest for addressing the tailing problem in chemical separations. Single-molecule spectroscopy and confocal fluorescence correlation imaging are used to study the adsorption sites of C(18) silica beads under RPLC chromatographic conditions. The experiments show that cationic molecule rhodamine 6G laterally diffuses through the chromatographic interface of a C(18) hydrocarbon monolayer and acetonitrile with occasional reversible strong adsorptions. Fluorescence correlation imaging extracts the rare strong adsorption events from large data sets, revealing that the strong adsorption sites are randomly distributed throughout the silica beads. Virtually every imaging pixel of silica beads adsorbs molecules. Single-molecule spectroscopy of the 584 strong adsorption events observed indicates that the strong adsorptions persist on the time scales from several milliseconds to seconds, having an average desorption time of 61 ms. The strong adsorption events are rare, comprising 0.3% of the total observation time. The sizes of strong adsorption sites are within the optical resolution of confocal imaging.

Effect of phase ratio on van't Hoff analysis in reversed-phase liquid chromatography, and phase-ratio-independent estimation of transfer enthalpy

In analysis of the thermodynamics of the transfer of a solute from the mobile phase to the stationary phase in reversed-phase liquid chromatography, it is nearly always assumed that the phase ratio is constant. This type of analysis is typically performed by applying a form of the van't Hoff equation, which relates the retention factor to temperature via the enthalpy and entropy of transfer. When non-linear van't Hoff plots are observed, it is often assumed that the enthalpy and entropy of transfer change with temperature. However, when the possibility of a change in the phase ratio is considered, it becomes apparent that non-linear van't Hoff behavior may or may not be due to changes in enthalpy or entropy. In this work, we present mathematical evidence that phase ratio changes, if they occur, can cause deviations from linearity in a van't Hoff plot. We also show that the phase ratio influence can be eliminated by considering the molecular difference between two solutes instead of the solutes themselves. The resulting selectivity van't Hoff plots may be linear, even when the van't Hoff plots of the two solutes are non-linear. In such cases, temperature-dependent phase ratio changes, and not necessarily changes in the transfer enthalpy, may be responsible for the curved van't Hoff plots of the individual solutes. In addition, we present chromatographic evidence that different solutes may "see" different thermodynamic phase ratios. It is clear that the concept of a phase ratio in reversed-phase chromatography is not nearly as well defined as a phase ratio in a bulk system like a liquid-liquid extraction.

Structure−Function Relationships in High-Density Octadecylsilane Stationary Phases by Raman Spectroscopy. 3. Effects of Self-Associating Solvents

Raman spectroscopy is used to examine the subtle effects of polar, hydrogen-bonding solvents; temperature; and the surface grafting method (surface- or solution-polymerized) on alkyl chain rotational and conformational order in a series of high-density octadecylsilane stationary phases ranging in surface coverage from 3.09 to 6.45 micromol/m2. Rotational and conformational order is assessed using the intensity ratio of the antisymmetric to symmetric v(CH2) modes as well as the frequencies at which these Raman bands are observed. Alkyl rotational and conformational order decreases with decreasing surface coverage in these polar solvents, consistent with the behavior of these materials in air. For homogeneously distributed, high surface coverage materials, these polar solvents induce rotational ordering that is proposed to be due to the self-association of these solvents through hydrogen bonding or other dipole interactions at the alkylsilane-solvent interface. From these observations, molecular pictures of these solvent-stationary-phase interfaces are proposed in which solvent interaction with the stationary phase occurs primarily at the distal methyl group of the alkyl chains.

Determination of minor conformational changes of a doxorubicin-peptide conjugate under chromatographic conditions

Thermodynamic analysis of the reversed-phase retention behavior of a doxorubicin-peptide conjugate demonstrated that the degree of non-linearity observed in Van't Hoff plots was impacted by mobile phase acetonitrile content over the 25-38% acetonitrile (v/v) range tested. Small decreases in the non-polar surface area of the doxorubicin-peptide conjugate as a function of temperature were estimated from these data using linear solvent strength relationships, suggesting that the retention behavior may be the result of minor analyte conformational changes during the chromatographic experiment. This hypothesis was supported via circular dichroism (CD), Raman and 1H NMR spectroscopic studies of the doxorubicin-peptide conjugate in selected chromatographic mobile phase compositions. The CD and Raman data indicated small changes to the apparent analyte microenvironment as a function of temperature and bulk solvent environment, while 1H NMR studies specifically demonstrated the environmental sensitivity of protons on three non-polar peptide residues and the proximal aromatic region of the analyte. Together, these data suggest that minor changes to the conformational order of the essentially random structure of the doxorubicin-peptide conjugate are sufficient to impact chromatographic performance.

Probing transport and microheterogeneous solvent structure in acetonitrile-water mixtures and reversed-phase chromatographic media by NMR quadrupole relaxation

Mixtures of CH(3)CN and H(2)O are the predominant solvent systems used in reversed-phase liquid chromatographic (RPLC) separations, as well as in a multitude of other applications. In addition, acetonitrile is the simplest model for an amphiphilic molecule possessing both organic and polar functional groups. Although many studies have focused on this solvent system, the general nature of the intermolecular interactions are not fully understood, and a microscopic description of the proposed microheterogeneity that exists is still not clearly established. In the present study, we measure the spin-lattice relaxation times (T(1)) of (14)N to determine reorientational correlation times (tau(c)) of CH(3)CN-H(2)O solvent mixtures over the entire binary composition range and at temperatures ranging from 25.0 to 80.0 degrees C. At all compositions, the microscopic observable, tau(c), is found to be directly proportional to the macroscopic solution viscosity when scaled for temperature (eta/T). This indicates that for a constant composition, this system's dynamics are well described by hydrodynamic theory on a microscopic level. These results suggest that under appropriate conditions, the measurement of changes in quadrupolar relaxation times is a reliable means of determining changes in solution viscosity. We stress the importance of this approach in systems not amenable to traditional viscosity measurements, such as those having species in interfacial regions. This approach is used to examine the changes in the interfacial solution viscosity of CH(3)CN-H(2)O mixtures in contact with a commercially available C(18)-bonded stationary phase. The measurements indicate that CH(3)CN is motionally hindered at the stationary phase surface. The surface affected CH(3)CN has a larger dependence of tau(c) on temperature than the bulk CH(3)CN, indicating greater changes in the interfacial viscosity as a function of temperature. Additionally, the bulk relaxation data show direct correlations to existing models of proposed regions of structure for CH(3)CN-H(2)O mixtures. Using a microscopic hydrodynamic approach, we show that, quite unexpectedly, each of the experimentally determined parameters in the viscosity correlation plots change simultaneously, and we propose that these are indicative of changes in the distribution of species for this microheterogeneous liquid system. Although distinct regions for the onset of microheterogeneity have previously been proposed, within the framework of a microscopic hydrodynamic model and the recently proposed model of Reimers and Hall,(1) the present data support the existence of a microheterogeneous solvent structure that varies continuously over the full range of temperatures and compositions examined.

Structure-function relationships in high-density octadecylsilane stationary phases by Raman spectroscopy. 1. Effects of temperature, surface coverage, and preparation procedure

Raman spectroscopy is used to examine the effects of temperature, surface coverage, nature of the alkylsilane precursor (octadecyltrichlorosilane, methyloctadecyldichlorosilane, or dimethyloctadecylchlorosilane), and surface grafting method (surface or solution polymerized) on alkyl chain conformational order in a series of high-density octadecylsilane stationary phases ranging in surface coverage from 3.09 to 6.45 micromol/m2. Conformational order is assessed using the intensity ratio of the antisymmetric and symmetric v(CH2) modes as well as the frequency at which these Raman bands are observed. Conformational order increases with surface coverage. Temperature-induced surface phase changes are observed between 258 and 343 K for this homologous series of stationary phases that are demonstrated to adhere to the Clapeyron equation for a simple first-order transition. Phase changes are discussed in terms of variation of the molar enthalpy, molar entropy, and molar volume of the stationary phase, all of which depend on surface coverage. For the limited range of systems investigated, a correlation between stationary-phase preparation (surface versus solution polymerized and nature of the silane precursor) and extent of alkyl chain order is not clearly observed.Instead, akyl chain order is largely dependent on bonding density. A molecular picture of temperature-induced disorder in octadecylsilane stationary phases is proposed, with disorder originating at the distal carbon and propagating toward the proximal carbon.

Structure-function relationships in high-density octadecylsilane stationary phases by Raman spectroscopy. 2. Effect of common mobile-phase solvents

Raman spectroscopy is used to examine the effects of solvent, temperature, and surface grafting method (surface or solution polymerized) on alkyl chain rotational and conformational order in a series of high-density octadecylsilane stationary phases ranging in surface coverage from 3.09 to 6.45 micromol/m2. Rotational and conformational order is assessed using the intensity ratio of the antisymmetric to symmetric v(CH2) modes as well as the frequency at which these Raman bands are observed. Solvents studied include perdeuterated hexane, toluene, chloroform, tetrahydrofuran, benzene, methanol, acetone, acetonitrile, and water. Stationary-phase order was investigated at temperatures between 258 and 323 K. Alkyl chain rotational and conformational order, and hence, solvation of the stationary phase, is dependent on solvent parameters (polarity, size, etc.), temperature, and stationary-phase properties (polymerization method and surface coverage). Information on stationary-phase conformational order allows solvent-stationary-phase interactions to be described in terms of a combination of adsorption and partitioning models for reversed-phase liquid chromatography. Finally, a distinct interplay between solvent- and temperature-induced ordering of these stationary phases is documented that is also a function of solvent and stationary-phase properties.