Critical contribution of nonlinear chromatography to the understanding of retention mechanism in reversed-phase liquid chromatography

Modelling the pH dependent retention and competitive adsorption of charged and ionizable solutes in mixed-mode and reversed-phase liquid chromatography

This study investigated the influence of pH on the retention of solutes using a mixed-mode column with carboxyl (-COOH) groups acting as weak cation exchanger bonded to the terminal of C18 ligands (C18-WCX column) and a traditional reversed-phase C18 column. First, a model based on electrostatic theory was derived and successfully used to predict the retention of charged solutes (charged, and ionizable) as a function of mobile phase pH on a C18-WCX column. While the Horváth model predicts the pH-dependent retention of ionizable solutes in reversed-phase liquid chromatography (RPLC) solely based on solute ionization, the developed model incorporates the concept of surface potential generated on the surface of the stationary phase and its variation with pH. To comprehensively understand the adsorption process, adsorption isotherms for these solutes were individually acquired on the C18-WCX and reversed-phase C18 columns. The adsorption isotherms followed the Langmuir model for the uncharged solute and the electrostatically modified Langmuir model for charged solutes. The elution profiles for the single components were calculated from these isotherms using the equilibrium dispersion column model and were found to be in close agreement with the experimental elution profiles. To enable modelling of two-component cases involving charged solute(s), a competitive adsorption isotherm model based on electrostatic theory was derived. This model was later successfully used to calculate the elution profiles of two components for scenarios involving (a) a C18 Column: two charged solutes, (b) a C18 Column: one charged and one uncharged solute, and (c) a C18-WCX Column: two charged solutes. The strong alignment between the experimental and calculated elution profiles in all three scenarios validated the developed competitive adsorption model.

Organic-solvent ditch overlap in reversed-phase liquid chromatography: A molecular dynamics simulation study in cylindrical 6-12 nm-diameter pores

Mass transport through the mesopore space of a reversed-phase liquid chromatography (RPLC) column depends on the properties of the chromatographic interface, particularly on the extent of the organic-solvent ditch that favors the analyte surface diffusivity. Through molecular dynamics simulations in cylindrical RPLC mesopore models with pore diameters between 6 and 12 nm we systematically trace the evolution of organic-solvent ditch overlap due to spatial confinement in the mesopore space of RPLC columns for small-molecule separations. Each pore model of a silica-based, endcapped, C18-stationary phase is equilibrated with two mobile phases of comparable elution strength, namely 70/30 (v/v) water/acetonitrile and 60/40 (v/v) water/methanol, to consider the influence of the mobile-phase composition on the onset of organic-solvent ditch overlap. The simulations show that, as the pore diameter decreases from 9 to 6 nm, the bonded-phase density extends and compacts towards the pore center, which leads to increased accumulation of organic-solvent excess and thus enhanced organic-solvent diffusivity in the ditch. Because the acetonitrile ditch is more pronounced than the methanol ditch, acetonitrile ditch overlap sets in at less severe spatial confinement than methanol ditch overlap. The pore-averaged methanol and acetonitrile diffusivities are considerably raised by ditch overlap in the 6 nm-diameter pore, but also benefit from the ditch (without overlap) in the 7 to 12 nm-diameter pores, whereby local and pore-averaged effects are generally larger for acetonitrile than methanol.

Preparation and investigation of two-component silica-modified hydrophobic layer for minimizing retention loss of reversed-phase chromatographic columns using fully aqueous mobile phases

Chromatographers often employ fully aqueous mobile phases to retain highly polar compounds in reversed-phase liquid chromatography (RPLC). However, when the flow rate is interrupted, either accidentally or intentionally, a substantial loss in retention occurs due to the spontaneous dewetting of water from the hydrophobic surface of conventional RPLC-C18 particles. Previous studies have shown that maintaining a low C18 surface coverage (approximately 1.5 μmol/m2) can mitigate water dewetting by increasing chain disorder, facilitating the intercalation of water clusters between the C18-bonded chains, and keeping the mesopores wetted. In this research, we explore the potential and additional benefits of using two-component surface bonding materials (C8/C18 and PhenylHexyl (PhHx)/C18) at a constant and low total surface coverage of 1.51 ± 0.15 μmol/m2. We synthesized seven one- and two-component modified silica particles with a volume average particle size of 5.22 μm and an average mesopore size of 104 Å. The surface coverage was increased from 0 to 0.54, 1.00, and to 1.66 μmol2 for C8 chains and from 0 to 0.52, 0.70, and to 1.65 μmol2 for PhHx ligands. To prevent interactions between water and any unreacted silanols, all seven derivatized particles were heavily endcapped with trimethylsilane (TMS) reagent. The fraction of the surface area remaining in contact with water was determined by measuring the retention times of weakly (thiourea) and strongly (thymine) retained compounds at intervals of 1, 2, 4, 8, 16, 32, and 64 minutes following the cessation of flow. Two distinct column temperatures, 24°C and 60°C, were employed in the experiments. Retention losses were found to be minimized in the presence of a small quantity of C8 chains (less than 40% of the total surface coverage). Additionally, it is essential to consider substantial fractions of PhHx chains, as long as the presence of the PhHx ligand does not significantly impact retention and selectivity. Combining mixed RPLC bondings with a low total surface coverage of approximately 1.5 μmol/m2 emerges as a viable solution for further minimizing retention loss in standard C18-bonded RPLC columns, particularly within the surface coverage range of 2.5-3.0 μmol/m2.

Hydrophilic Partitioning or Surface Adsorption? A Quantitative Assessment of Retention Mechanisms for Hydrophilic Interaction Chromatography (HILIC)

Retention mechanisms in HILIC have been investigated and reported in literature. However, the current understanding of retention mechanisms is qualitative and lacks quantitative details. Previously, mechanism elucidation was based on indirect evidence, and unambiguous assignment of retention mechanisms has not been reported based on direct data. This study aims to quantitatively determine the contributions of two major retention mechanisms in HILIC, hydrophilic partitioning and surface adsorption to the overall retention of neutral compounds. Using the methodologies we developed previously, the phase ratio for adsorbed water layer and distribution coefficients were measured and used to calculate the retention factors contributed by hydrophilic partitioning. The methodology allows the determination of the contribution of surface adsorption simultaneously. The evaluation of five test compounds demonstrates that the retention may be controlled by hydrophilic partitioning, surface adsorption or both depending on compound characteristics. Quantitative assessment of retention mechanisms also makes it possible to better understand the effect of acetonitrile on retention in HILIC.

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.

Separation of phosphorothioate oligonucleotide impurities by WAX HPLC under high organic content elution conditions

The separation of impurities in phosphorothioate diester (PS) oligonucleotides is complicated by (1) the presence of a very large number of diastereoisomers, e.g., 2¹⁹ for a 20-mer oligonucleotide, (2) peak broadening due to the hydrophobic character of the sulfur atom, and (3) the chemical similarity of the impurities to the parent oligonucleotide and each other. Further difficulties arise due to the chemical nature of oligonucleotides, which display a complex mixture of ionic, hydrophobic, H-bonding, and other functionalities. To minimize hydrophobic interactions and peak broadening due to the PS modification, we have developed a novel method that combines a weak anion exchange (WAX) column with a mobile phase elution system designed to maximize separation by a single ionic/electrostatic interaction. We found that although chaotropes are helpful, the most significant beneficial effect of the hydrophilic WAX column is that high-organic, low-salt mobile phase is required for product elution. Separations are also benefitted by pH gradient effects on stationary phase electrostatic potential and analyte ionization. An extraordinary degree of separation is achieved by the new WAX method in comparison to SAX (strong anion exchange) chromatography. For the first time, the extent of deamination of PS oligonucleotides is directly determined by a chromatography-only method. The approach, representative results, and the mechanisms of separation are discussed.

Confinement Effects on Distribution and Transport of Neutral Solutes in a Small Hydrophobic Nanopore

Molecular dynamics simulations are used to study confinement effects in small cylindrical silica pores with extended hydrophobic surface functionalization as realized, for example, in reversed-phase liquid chromatography (RPLC) columns. In particular, we use a 6 nm cylindrical and a 10 nm slit pore bearing the same C₁₈ stationary phase to compare the conditions inside the smaller-than-average pores within an RPLC column to column-averaged properties. Two small, neutral, apolar to moderately polar solutes are used to assess the consequences of spatial confinement for typical RPLC analytes with water (W)-acetonitrile (ACN) mobile phases at W/ACN ratios between 70/30 and 10/90 (v/v). The simulated data show that true bulk liquid behavior, as observed over an extended center region in the 10 nm slit pore, is not recovered within the 6 nm cylindrical pore. Instead, the ACN-enriched solvent layer around the C₁₈ chain ends (the ACN ditch), a general feature of hydrophobic interfaces equilibrated with aqueous-organic liquids, extends over the entire pore lumen of the small cylindrical pore. This renders the entire pore a highly hydrophobic environment, where, contrary to column-averaged behavior, neither the local nor the pore-averaged sorption and diffusion of analytes scales directly with the W/ACN ratio of the mobile phase. Additionally, the solute polarity-related discrimination between analytes is enhanced. The consequences of local ACN ditch overlap in RPLC columns are reminiscent of ion transport in porous media with charged surfaces, where electrical double-layer overlap occurring locally in smaller pores leads to discrimination between co- and counterionic species.

Review of the Use of Liquid Chromatography-Tandem Mass Spectrometry in Clinical Laboratories: Part I-Development

The process of method development for a diagnostic assay based on liquid chromatography-tandem mass spectrometry (LC-MS/MS) involves several disparate technologies and specialties. Additionally, method development details are typically not disclosed in journal publications. Method developers may need to search widely for pertinent information on their assay(s). This review summarizes the current practices and procedures in method development. Additionally, it probes aspects of method development that are generally not discussed, such as how exactly to calibrate an assay or where to place quality controls, using examples from the literature. This review intends to provide a comprehensive resource and induce critical thinking around the experiments for and execution of developing a clinically meaningful LC-MS/MS assay.

Retention modeling and adsorption mechanisms in reversed-phase liquid chromatography

To interpret the dependence of solute retention behavior on modifier content in reversed-phase liquid chromatography, a theoretical framework, based on the concentration dependence of solvophobic forces imposed on solutes and the competitive adsorptions of solutes and solvent modifiers, was proposed. The generality of the developed model was demonstrated by comparing the model with conventional retention models. The linear dependence of the Gibbs energy change of solute adsorption with respect to the modifier concentration was assumed, and the model was fitted to the experimental results, with good agreement demonstrated between the experimental data and the model. Retention behaviors were inferred to be determined by two key dimensionless groups that represented the reductions in the retention factors resulting from a weakened solvophobic interaction and modifier competitive adsorption. The retention behaviors were successfully deconvoluted for each contribution as a function of the modifier concentration by using the fitted parameters. The effects of both contributions on the retention behaviors were enhanced for the solutes with aromatic groups. The standard Gibbs energy change _SL^o of benzene adsorption was found to depend linearly on the number of modifier molecules present but independent of modifier identity. For the solutes associated with hydrogen-bonding groups, the degree of reduction in the solvophobic interactions was considerably reduced. Hence, the relative contributions of both mechanisms to solute retention depend greatly on the solute structure. Perturbation method was performed to investigate the modifier adsorption mechanisms. The results show that the standard Gibbs energy change _SL^o for the first-layer adsorption of modifiers changed linearly with the carbon number of modifier molecule. These results demonstrated that the proposed model can offer a physically consistent quantitative description of retention when solvent composition is varied.

Consequences of Cylindrical Pore Geometry for Interfacial Phenomena in Reversed-Phase Liquid Chromatography

The interfacial phenomena behind analyte separation in a reversed-phase liquid chromatography column take place nearly exclusively inside the silica mesopores. Their cylindrical geometry can be expected to shape the properties of the chromatographic interface with consequences for the analyte density distribution and diffusivity. To investigate this topic through molecular dynamics simulations, we introduce a cylindrical pore inside a slit pore configuration, where the inner curved and outer planar silica surface bear the same bonded phase. The present model replicates an average-sized (9 nm) mesopore in an endcapped C₁₈ column equilibrated with a mobile phase of 70/30 (v/v) water/acetonitrile. Simulations performed for ethylbenzene and acetophenone show that the surface curvature shifts the bonded phase and analyte density toward the pore center, decreases the solvent density in the bonded-phase region, increases the acetonitrile excess in the interfacial region, and considerably enhances the surface diffusivity of both analytes. Overall, the cylindrical pore provides a more hydrophobic environment than the slit pore. Ethylbenzene density is decidedly increased in the cylindrical pore, whereas acetophenone density is nearly equally distributed between the cylindrical and slit pore. The cylindrical pore geometry thus sharpens the discrimination between the apolar and moderately polar analytes while enhancing the mass transport of both.

Perspective on the Future Approaches to Predict Retention in Liquid Chromatography

The demand for rapid column screening, computer-assisted method development and method transfer, and unambiguous compound identification by LC/MS analyses has pushed analysts to adopt experimental protocols and software for the accurate prediction of the retention time in liquid chromatography (LC). This Perspective discusses the classical approaches used to predict retention times in LC over the last three decades and proposes future requirements to increase their accuracy. First, inverse methods for retention prediction are essentially applied during screening and gradient method optimization: a minimum number of experiments or design of experiments (DoE) is run to train and calibrate a model (either purely statistical or based on the principles and fundamentals of liquid chromatography) by a mere fitting process. They do not require the accurate knowledge of the true column hold-up volume V₀, system dwell volume V_dwell (in gradient elution), and the retention behavior (k versus the content of strong solvent φ, temperature T, pH, and ionic strength I) of the analytes. Their relative accuracy is often excellent below a few percent. Statistical methods are expected to be the most attractive to handle very complex retention behavior such as in mixed-mode chromatography (MMC). Fundamentally correct retention models accounting for the simultaneous impact of φ, I, pH, and T in MMC are needed for method development based on chromatography principles. Second, direct methods for retention prediction are ideally suited for accurate method transfer from one column/system configuration to another: these quality by design (QbD) methods are based on the fundamentals and principles of solid-liquid adsorption and gradient chromatography. No model calibration is necessary; however, they require universal conventions for the accurate determination of true retention factors (for 1 < k < 30) as a function of the experimental variables (φ, T, pH, and I) and of the true column/system parameters (V₀, V_dwell, dispersion volume, σ, and relaxation volume, τ, of the programmed gradient profile at the column inlet and gradient distortion at the column outlet). Finally, when the molecular structure of the analytes is either known or assumed, retention prediction has essentially been made on the basis of statistical approaches such as the linear solvation energy relationships (LSERs) and the quantitative structure retention relationships (QSRRs): their ability to accurately predict the retention remains limited within 10-30%. They have been combined with molecular similarity approaches (where the retention model is calibrated with compounds having structures similar to that of the targeted analytes) and artificial intelligence algorithms to further improve their accuracy below 10%. In this Perspective, it is proposed to adopt a more rigorous and fundamental approach by considering the very details of the solid-liquid adsorption process: Monte Carlo (MC) or molecular dynamics (MD) simulations are promising tools to explain and interpret retention data that are too complex to be described by either empirical or statistical retention models.

Thermodynamic interpretation of the drift and noise of gradient baselines in reversed-phase liquid chromatography using mobile phase additives

The drift and noise of acetonitrile-water gradient baselines (5-95%, v/v, 5 min linear gradient) in reversed phase liquid chromatography (RPLC) are recorded at a wavelength of 215 nm using 0.1% trifluoroacetic acid (TFA) as the mobile phase additive, a 4.6 mm  ×  150 mm 5 μm Symmetry-C₁₈ RPLC column, and an Arc system (low-pressure gradient proportioning valve or GPV, pump with a stroke volume of either 66 or 132 μL, no mixer) as the LC instrument. These observations are predicted from solid-liquid adsorption thermodynamics which requires the measurement of the excess adsorption isotherm of acetonitrile from water onto the RPLC column and of the variation of the Henry's constant of TFA as a function of the volume fraction of acetonitrile in the bulk mobile phase. The incomplete mixing of the acetonitrile and water packets delivered by the low-pressure GPV is represented by a sinusoidal perturbation of the programmed volume fraction of acetonitrile during the entire gradient. The variation of the TFA absorbance at 215 nm with increasing acetonitrile concentration is measured in order to transform TFA concentration into the observable absorbance unit. The drift and noise of the gradient baseline are calculated by solving numerically (Rouchon method) the equilibrium-dispersive (ED) mass balance equations of acetonitrile and TFA. The agreement between the calculated and observed gradient baselines is very good as the proposed model of chromatography accurately accounts for the displacement of TFA between stationary and mobile phases (early excess and late deficit of TFA concentration relative to 0.1%) and for the frequency (equal to the ratio of the applied flow rate to the stroke volume) and the amplitude of the periodic noise recorded during the gradient. From a practical viewpoint, the drift of the gradient baseline can be minimized by maximizing the ratio of the gradient volume to the hold-up volume ( > 10) and/or by minimizing the retention factor of the mobile phase additive in the water-rich eluent (k < 0.2). The reduction of the noise amplitude below 0.1 mAU as requested by the pharmaceutical industry imposes the ratio of the flow rate to the stroke volume of the pump to be larger than 1 Hz. This opens avenues towards the development of new GPV, pump, and mixers in order to mix efficiently the solvent packets delivered by conventional LC instrument.

Adsorption mechanism of triterpenoid saponins in reversed-phase liquid chromatography and hydrophilic interaction liquid chromatography: Mogroside V as test substance

In this paper, adsorption mechanism of triterpenoid saponins in reversed-phase liquid chromatography (RPLC) and hydrophilic interaction liquid chromatography (HILIC) was proposed based on the study of the retention behavior of mogroside V as test substance. The change of peak shape of mogroside V and its influencing factors was first investigated. As the increase of sample loading, a tailing peak of mogroside V was observed in MeOHH₂O of both two modes. It was the fronting peak in ACNH₂O of HILIC while there was a transition from fronting peak to tailing peak in ACNH₂O of RPLC that was largely affected by column temperature and ACN concentration. The adsorption isotherm of mogroside V in ACNH₂O of RPLC was fitted by Moreau model, where a monolayer adsorption with large inter-molecular interaction was formed on the C18 surface. While in ACNH₂O of HILIC, the adsorption of mogroside V was in accordance with BET model, showing multilayer adsorption behavior. In MeOHH₂O of both HILIC and RPLC, there was always monolayer adsorption, which was fitted by Langmuir model. At last, by choosing the suitable chromatographic mode, controlling the key factors such as the solvent concentration and column temperature, and predicting the broadening trend of peak, three methods were screened out, namely, C18 column with 22% ACN (30 °C), Click XIon column with 90% MeOH or 70% ACN, to get mogroside V of purity greater than 98% from Siraitia grosvenorii extract. Among them, the RPLC method of 22% ACN that showed the highest loading sample per hour (1.92%) and the lowest solvent consumption emerged as the best approach.

Ultra-high performance separation of basic compounds on reversed-phase columns packed with fully/superficially porous silica and hybrid particles by using ultraviolet transparent hydrophobic cationic additives

The use of the tetrabutylammonium additive was investigated in the ultra-high performance reversed-phase liquid chromatographic elution of basic molecules of pharmaceutical interest. When added to the mobile phase at low pH, the hydrophobic tetrabutylammonium cation interacts with the octadecyl chains and with the residual silanols, thus imparting a positive charge to the stationary phase, modulating retention and improving peak shape of protonated basic solutes. Two sources of additive were tested: a mixture of tetrabutylammonium hydroxide/trifluoroacetic acid and tetrabutylammonium hydrogen sulfate. Retention and peak shape of 11 basic pharmaceutical compounds were evaluated on commercially available ultra-fast columns packed with octadecyl stationary phases (Ascentis Express C18 2.0 µm, Acquity BEH C18 1.7 µm, Titan C18 1.9 µm). All columns benefit from the use of additive, especially tetrabutylammonium hydrogen sulfate, providing very symmetric peaks with reasonable retention times. Focusing on the probe compounds amitriptyline and sertraline, efficiency and asymmetry values were investigated at increasing retention factor. The trend is very different to that obtained in reversed-phase conditions and the effect lies in the complex molecular interaction mechanisms based on hydrophobic and ion exchange interactions as well as electrostatic repulsion.

Retention loss of reversed-phase chromatographic columns using 100% aqueous mobile phases from fundamental insights to best practice

This work deals with experimental investigations pertaining to the impact of chemical (electrolyte concentration from 0 to 100 mM, dissolved nitrogen gas from 0 to 6.7  ×  10^-4 M in water; surface chemistry including hexylphenyl, polyphenyl, C₃₀, C₁₈, and C₈; surface coverage in C₁₈-bonded chains from 1.5 to 3.5 µmol/m²; presence of surface dopant), physical (hydrostatic pressure of water from 50 to 500 bar; temperature from 27 ^∘C to 75 ^∘C), and structural parameters (average pore size from 50 Å to 400 Å; pore connectivity) on the dewetting kinetics of water from the hydrophobic mesopores of particles packed in RPLC columns. The results are explained from physico-chemical viewpoints involving intrusion and extrusion Laplace pressures, advancing and receding contact angles, surface tension of water, vapor pressure of water, 3D reconstruction of the actual mesoporous structure, pore connectivity, and the hysteresis in nitrogen adsorption and desorption isotherm onto reversed-phase chromatographic materials. A model of water dewetting consistent with the observations and the physical interpretations is then proposed. Finally, the most relevant practical solutions (pressurizing the column in absence of flow, pore size enlargement, using phenyl-bonded phase, polar embedded or surface doped C₁₈-bonded phases, reducing the C₁₈ surface coverage, doping the silica surface, lengthening of the alkyl-bonded chains, applying low temperatures, purging and degassing the mobile phase with helium gas) are suggested in order to eliminate or at least minimize the retention loss of RPLC columns when using fully aqueous mobile phases.

One-Step Purification of Microbially Produced Hydrophobic Terpenes via Process Chromatography

Novel and existing terpenes are already being produced by genetically modified microorganisms, leading to new process challenges for the isolation and purification of these terpenes. Here, eight different chromatographic resins were characterized for the packed bed adsorption of the model terpene β-caryophyllene, showing their applicability on an Escherichia coli fermentation mixture. The polystyrenic Rensa® RP (Ø 50 μm) shows the highest affinity, with a maximum capacity of >100 g L-1 and the best efficiency, with a height equivalent of a theoretical plate (HETP) of 0.022 cm. With this material, an optimized adsorption-based purification of β-caryophyllene from a fermentation mixture was developed, with the green solvent ethanol for desorption. A final yield of >80% and a purity of >99% were reached after only one process step with a total productivity of 0.83 g h-1 L-1. The product solution was loaded with a volume ratio (feed to column) of >500 and the adapted gradient elution yielded a 40 times higher concentration of β-caryophyllene. The adsorption-based chromatography represents therefore a serious alternative to the liquid-liquid extraction and achieves desired purities without the utilization of hazardous solvents.

Kinetic mechanism of water dewetting from hydrophobic stationary phases utilized in liquid chromatography

An experimental protocol was designed to accurately measure the dewetting kinetics of aqueous mobile phases from reversed-phase liquid chromatography (RPLC) columns. The protocol enables the determination of the losses in the wetted surface area and internal pore volume (leading to undesirable retention losses) of RPLC columns as a function of the dewetting time. It is used to evaluate the impact of the buffer/salt concentration in water (0-100 mM), nitrogen concentration dissolved in water (0-6.7 × 10^-4 M), column temperature (300-358 K), and of the internal structure (pore connectivity) of the stationary phase on the dewetting kinetics of various RPLC packing materials. From a fundamental viewpoint, the experimental facts demonstrate that dewetting kinetics are not solely driven by the pore size of the stationary phase and the contact angle with water. Temperature has a major influence on dewetting kinetics as it controls the nucleation rate of isolated water vapor bubbles over the entire mesoporous network. Additionally, the internal microstructure of the stationary phase (characterized by its internal porosity, pore size distribution, and pore connectivity) influences the rate at which the water vapor bubbles grow and coalesce in the entire particle volume. From a more practical viewpoint, the retention loss of RPLC columns due to water dewetting can be eliminated or at least minimized by (1) adjusting the surface and bonding chemistries to reduce the receding contact angle, (2) elevating the column outlet pressure, (3) operating at the lowest possible temperature, (4) minimizing the pore connectivity of the stationary phase (e.g., by increasing the degree of surface functionalization from C₈ to C₁₈-bonded phases), and (5) by degassing the aqueous mobile phase from any dissolved gases.

The pearl necklace model in protein A chromatography: Molecular mechanisms at the resin interface

Staphylococcal protein A chromatography is an established core technology for monoclonal antibody purification and capture in the downstream processing. MabSelect SuRe involves a tetrameric chain of a recombinant form of the B domain of staphylococcal protein A, called the Z-domain. Little is known about the stoichiometry, binding orientation, or preferred binding. We analyzed small-angle X-ray scattering data of the antibody-protein A complex immobilized in an industrial highly relevant chromatographic resin at different antibody concentrations. From scattering data, we computed the normalized radial density distributions. We designed three-dimensional (3D) models with protein data bank crystallographic structures of an IgG1 (the isoform of trastuzumab, used here; Protein Data Bank: 1HZH) and the staphylococcal protein A B domain (the native form of the recombinant structure contained in MabSelect SuRe resin; Protein Data Bank: 1BDD). We computed different binding conformations for different antibody to protein A stoichiometries (1:1, 2:1, and 3:1) and compared the normalized radial density distributions computed from 3D models with those obtained from the experimental data. In the linear range of the isotherm we favor a 1:1 ratio, with the antibody binding to the outer domains in the protein A chain at very low and high concentrations. In the saturation region, a 2:1 ratio is more likely to occur. A 3:1 stoichiometry is excluded because of steric effects.

HPLC-CUPRAC post-column derivatization method for the determination of antioxidants: a performance comparison between porous silica and core-shell column packing

BACKGROUND

An HPLC method employing a post-column derivatization strategy using the cupric reducing antioxidant capacity reagent (CUPRAC reagent) for the determining antioxidants in plant-based materials leverages the separation capability of regular HPLC approaches while allowing for detection specificity for antioxidants.

METHODS

Three different column types, namely core-shell and porous silica including two chemically different core-shell materials (namely phenyl-hexyl and C18), were evaluated to assess potential improvements that could be attained by changing from a porous silica matrix to a core-shell matrix. Tea extracts were used as sample matrices for the evaluation specifically looking at catechin and epigallocatechin gallate (EGCG).

RESULTS

Both the C18 and phenyl-hexyl core-shell columns showed better performance compared to the C18 porous silica one in terms of separation, peak shape, and retention time. Among the two core-shell materials, the phenyl-hexyl column showed better resolving power compared to the C18 column.

CONCLUSIONS

The CUPRAC post-column derivatization method can be improved using core-shell columns and suitable for quantifying antioxidants, exemplified by catechin and EGCG, in tea samples.

The importance of ion-pairing in peptide purification by reversed-phase liquid chromatography

The adsorption mechanism for three peptides was studied under overloaded conditions through adsorption isotherm measurements in the presence of an ion-pairing reagent, trifluoroacetic acid (TFA), on an end-capped C₁₈-bonded stationary phase. The overall aim of the study was to obtain a better understanding of how the acetonitrile and the TFA fractions in the eluent affected the overloaded elution profiles and the selectivity between peptides using mechanistic modelling and multivariate design of experiments. When studying the effect of TFA, direct evidence for ion pair formation between a peptide and TFA in acetonitrile-water solutions was provided by fluorine-proton nuclear Overhauser NMR enhancement experiments and the adsorption of TFA on the stationary phase was measured by frontal analysis. The adsorption isotherms for each peptide were then determined by the inverse method at eight TFA concentrations ranging from 2.6mM to 37.3mM (0.02-0.29vol-%) in isocratic elution. The equilibrium between the peptide ion and the peptide-TFA complex was modelled by coupling the mass-balance to reaction kinetics and determining separate adsorption isotherms for the two species. We found that a Langmuir isotherm described the elution profile of peptide-TFA complex well while the peptide ion was described by a bi-Langmuir adsorption isotherm since it exhibited strong secondary interactions. The elution profiles had an unfavorable shape at low TFA concentrations consisting of a spike in their front and a long tailing rear due to the secondary interactions for the peptide ion having very low saturation capacity. The acetonitrile dependence on the adsorption isotherms was studied by determination of adsorption isotherms directly from elution profiles obtained in gradient elution which enabled a broad acetonitrile interval to be studied. Here, it was found that the column saturation capacity was quickly reached at very low acetonitrile fractions and that there were significant variations in adsorption with the molecular weight. Finally, practical implications for method development are discussed based on an experimental design where gradient slope and TFA concentrations are used as factors.

Separation of peptides and intact proteins by electrostatic repulsion reversed phase liquid chromatography

A new brand of BEH-C18 hybrid particles chemically bonded to a leash carrying an amine group permits the implementation of electrostatic repulsive interactions chromatography. Using columns packed with this material, the influence of the concentration of positive charges bonded to the BEH-C18 surface on the overloaded band profiles of a few positively charged peptides and proteins was investigated in the gradient elution mode. Three columns packed with endcapped BEH-C18 particles bonded with three different surface-charge densities (LOW, MEDIUM and HIGH) were used and compared with those provided by a column packed with non-doped, endcapped BEH-C18 particles. The surface concentrations of fixed charges in the LOW, MEDIUM and HIGH columns were estimated at 0.029, 0.050, and 0.064μmol/m(2), for example, about two orders of magnitude smaller than the surface density of bonded C18 chains (2.1μmol/m(2)). Three different mobile phase additives (0.1% v/v of trifluoro-acetic, phosphoric, and formic acid) were used to optimize the purification levels of proteins under different loading conditions. The weak ion-pairing ions (formate and phosphate) generate smaller retention but broader, more fronting band profiles than those eluted with a stronger ion-pairing ion (trifluoroactate). This effect is worse in the presence of fixed charges at the surface of the BEH-C18 particles. This was explained by an enhanced anti-Langmuirian adsorption behavior of the charged proteins in the presence of fixed surface charges. As the protein concentration increases in the bulk, so does the internal ionic strength, the electrostatic repulsive interactions weaken, and retention increases. Band fronting is mostly eliminated by replacing weak ion-pairing acids with TFA with which the adsorption isotherm remains weakly langmuirian. Faster but still complete gradient separation of insulin and myoglobin were achieved with the HIGH column than with the reference neutral column, despite a measurable loss in selectivity.

The rationale for the optimum efficiency of columns packed with new 1.9μm fully porous Titan-C18 particles-a detailed investigation of the intra-particle diffusivity

In a previous report, it was reported that columns packed with fully porous 1.9μm Titan-C18 particles provided a minimum reduced plate height as small as 1.7 for the most retained compound (n-octanophenone) under RPLC conditions. These particles are characterized by a relatively narrow size distribution with a relative standard deviation (RSD) of only 10%. A column packed with classical 5μm Symmetry-C18 particles, used as a reference RPLC column, generated a minimum reduced plate height of 2.1 for the same retained compound. This work demonstrates that this was due to an unusually low intra-particle diffusivity across these particles, which leads to a small longitudinal diffusion coefficient along the column. The demonstration is based on the combination of accurate measurements of the height equivalent to a theoretical plate (HETP), inverse size exclusion chromatography (ISEC), peak parking (PP), and minor disturbance method (MDM) experiments. The experimental results show that the reduced eddy dispersion HETP term (A=0.8 for a reduced velocity of 5), the internal particle porosity (ϵp=0.35), and the enrichment of acetonitrile in the pore volume (75% acetonitrile in the bulk, 85% inside the mesoporous volume) are identical on both the Titan-C18 and Symmetry-C18 columns. The difference between the internal structures of these two brands of RPLC-C18 fully porous particles lies in the values of the internal obstruction factor γp, which is 0.42 for the Symmetry-C18 but only 0.26 for the Titan-C18 particles. This is in part related to the diffusion hindrance due to the small average pore size of the Titan-C18 particles, around 59Å versus 77Å for Symmetry-C18 particles. A simple model of constriction along diffusion paths having the shape of a truncated cone suggests that the width of the pore size distribution (RSD of 30% and 20% for Titan-C18 and Symmetry-C18 particles) is mostly responsible for the difference in their obstruction factors.