Geometry of chemically modified silica

Effect of Hydrophobic Moieties on the Assembly of Silica Particles into Colloidal Crystals

To boost the implementation of colloidal crystals (CCs) in separation science, the effects of the most common chromatographic reversed phases, that is, butyl and octadecyl, on the assembly of silica particles into CCs and on the optical properties of CCs are investigated. Interestingly, particle surface modification can cause phase separation during sedimentation because the assembly is highly sensitive to minute changes in surface characteristics. Solvent-induced surface charge generation through acid-base interactions of acidic residual silanol groups with the solvent is enough to promote colloidal crystallization of modified silica particles. In addition, solvation forces at small interparticle distances are also involved in colloidal assembly. The characterization of CCs formed during sedimentation or via evaporative assembly revealed that C4 particles can form CCs more easily than C18 particles because of their low hydrophobicity; the latter can only form CCs in tetrahydrofuran when C18 chains with a high bonding density have extra hydroxyl side groups. These groups can only be hydrolyzed from trifunctional octadecyl silane but not from a monofunctional one. Moreover, after evaporative assembly, CCs formed from particles with different surface moieties exhibit different lattice spacings because their surface hydrophobicity and chemical heterogeneity can modulate interparticle interactions during the two main stages of assembly: the wet stage of crystal growth and the late stage of nano dewetting (evaporation of interparticle solvent bridges). Finally, short, alkyl-modified CCs were effectively assembled inside silica capillaries with a 100 μm inner diameter, laying the foundation for future chromatographic separation using capillary columns.

The use of zeolite 13X as a stationary phase for direct determination of water in organic solvents by high-performance liquid chromatography

High-performance liquid chromatography (HPLC) method for direct determination of water impurities in various organic solvents using a column (50 × 4.6 mm, ID) packed with 5 μm faujasite and methanol as an eluent was developed. Detection of the water peaks was performed by using either refractometric or indirect spectrophotometric method at 204 nm. In the first case the limit of detection (LOD) and limit of quantitation (LOQ) were 0.001 wt% and 0.0033 wt%, respectively for a sample volume of 20 μL. A linear calibration plot (R² = 0.9998) for the water content between 0.01 and 9.55 wt% was obtained. The developed method was applied for the direct analysis of various polar organic solvents including acetonitrile, nitromethane, 1,4-dioxane, 2-ethoxyethanol, methanol, dimethyl sulfoxide (DMSO), ethanol and 2-propanol.

Stationary Phases for Green Liquid Chromatography

Industrial research, including pharmaceutical research, is increasingly using liquid chromatography techniques. This involves the production of large quantities of hazardous and toxic organic waste. Therefore, it is essential at this point to focus interest on solutions proposed by so-called “green chemistry”. One such solution is the search for new methods or the use of new materials that will reduce waste. One of the most promising ideas is to perform chromatographic separation using pure water, without organic solvents, as a mobile phase. Such an approach requires novel stationary phases or specific chromatographic conditions, such as an elevated separation temperature. The following review paper aims to gather information on stationary phases used for separation under purely aqueous conditions at various temperatures.

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.

Experimental determination of phase ratio of C8 columns employing retention factors and octane-mobile phase partition coefficients of homologous series of linear alkylbenzenes

This work proposes an experimental method for the estimation of the phase ratio of reversed-phase C8 columns by employing the equation log(k)=alog(K_om)+log(Φ), where k is the retention factor, K_omis the octane-mobile phase partition coefficient, a is a proportionality constant and Φ is the phase ratio (defined as volume ratio of the stationary phase to the mobile phase). The immiscible liquid octane and mobile phase are chosen as the surrogate model for the C8 stationary phase and mobile phase of the chromatographic system. The octane-mobile phase is used for measuring the partition coefficient K_om of six compounds of the homologous series of linear alkylbenzenes, viz. benzene, toluene, ethylbenzene, propylbenzene, butylbenzene and pentylbenzene. The distribution of a compound between the octane and mobile phase is proposed to simulate the partitioning process in the chromatography. The retention factor k of each compound is measured using the same mobile phase for two C8 columns (Zorbax Eclipse XDB-C8 and Symmetry C8). The set of data of k and K_om is fitted to the above linear equation to give the best-fit values of a and log(Φ) for each column and various mobile phase compositions (methanol-water or acetonitrile-water). The regression analyses have coefficients of determination r² > 0.992. This observed linear relationship can therefore be expressed as k=K_om^aΦ. The experimental values of Φ for the C8 columns are in the range of 0.206 to 0.842, with a from 0.544 to 0.811, respectively.

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.

The combination of partition, size exclusion, and hydrodynamic models in chromatography, and application to bonded phases on porous supports

Liquid-liquid partition chromatography has been used for many years as a model and teaching introduction to column chromatography. However, the partition model does not describe separations on bonded phases with porous supports particularly well, especially regarding the thermodynamics controlling solute distribution. Further difficulties arise when more than one mechanism is involved in solute retention. Nomenclature is not perfectly aligned with the underlying thermodynamic descriptors and is inconsistently applied over various chromatographic techniques. Presented here is a general description of retention that spans partition, size exclusion, and hydrodynamic separation processes, and is then applied to bonded-phase separations on porous supports. The model provides a general description applicable to adsorption, reversed-phase, hydrophilic interaction, size-exclusion, hydrodynamic chromatography, and any combination of these techniques including liquid chromatography at the critical condition. Further expansion to include retention by ion-exchange and field-flow fractionation appears to be possible. Recommendations on retention factor definition and evaluation are given.

A Concentration-Dependent Insulin Immobilization Behavior of Alkyl-Modified Silica Vesicles: The Impact of Alkyl Chain Length

The insulin immobilization behaviors of silica vesicles (SV) before and after modification with hydrophobic alkyl -C₈ and -C₁₈ groups have been studied and correlated to the grafted alkyl chain length. In order to minimize the influence from the other structural parameters, monolayered -C₈ or -C₁₈ groups are grafted onto SV with controlled density. The insulin immobilization capacity of SV is dependent on the initial insulin concentrations (IIC). At high IIC (2.6-3.0 mg/mL), the trend of insulin immobilization capacity of SV is SV-OH > SV-C₈ > SV-C₁₈, which is determined mainly by the surface area of SV. At medium IIC (0.6-1.9 mg/mL), the trend changes to SV-C₈ ≥ SV-C₁₈ > SV-OH as both the surface area and alkyl chain length contribute to the insulin immobilization. At an extremely low IIC, the hydrophobic-hydrophobic interaction between the alkyl group and insulin molecules plays the most significant role. Consequently, SV-C₁₈ with longer alkyl groups and the highest hydrophobicity show the best insulin enrichment performance compared to SV-C₈ and SV-OH, as evidenced by an insulin detection limit of 0.001 ng/mL in phosphate buffered saline (PBS) and 0.05 ng/mL in artficial urine determined by mass spectrometry (MS).

Organic solvent modifier and temperature effects in non-aqueous size-exclusion chromatography on reversed-phase columns

Common reversed-phase columns (C₁₈, C₄, phenyl, and cyano) offer inert surfaces suitable for the analysis of polymers by size-exclusion chromatography (SEC). The effect of tetrahydrofuran (THF) solvent and the mixtures of THF with a variety of common solvents used in high performance liquid chromatography (acetonitrile, methanol, dimethylformamide, 2-propanol, ethanol, acetone and chloroform) on reversed-phase stationary phase characteristics relevant to size exclusion were studied. The effect of solvent on the elution of polystyrene (PS) and poly(methyl methacrylate) (PMMA) and the effect of column temperature (within a relatively narrow range corresponding to typical chromatographic conditions, i.e., 10°C-60°C) on the SEC partition coefficients K_SEC of PS and PMMA polymers, were also investigated. The bonded phases show remarkable differences in size separations when binary mixtures of THF with other solvents are used as the mobile phase. The solvent impact can be two-fold: (i) change of the polymeric coil size, and possible shape, and (ii) change of the stationary phase pore volume. If the effect of this impact is properly moderated, then the greatest benefit of optimized solute resolution can be achieved. Additionally, this work provides an insight on solvent-stationary phase interactions and their effects on column pore volume. The only effect of temperature observed in our studies was a decreased elution volume of the polymers with increasing temperature. SEC partition coefficients were temperature-independent in the range of 10°C-60°C and therefore, over this temperature range elution of PS and PMMA polymers is by near-ideal SEC on reversed-phase columns. Non-ideal SEC appears to occur for high molar mass PMMA polymers on a cyano column when alcohols are used as mobile phase modifiers.

Evaluation of the phase ratio for three C18 high performance liquid chromatographic columns

For a chromatographic column, phase ratio Φ is defined as the ratio between the volume of the stationary phase Vst and the void volume of the column V0, and it is an important parameter characterizing the HPLC process. Although apparently simple, the evaluation of Φ presents difficulties because there is no sharp boundary between the mobile phase and the stationary phase. In addition, the boundary depends not only on the nature of the stationary phase, but also on the composition of the mobile phase. In spite of its importance, phase ratio is seldom reported for commercially available HPLC columns and the data typically provided by the vendors about the columns do not provide key information that would allow the calculation of Φ based on Vst and V0 values. A different procedure for the evaluation of Φ is based on the following formula: log k'j=a log Kow,j+log Φ, where k'j is the retention factor for a compound j that must be a hydrocarbon, Kow,j is the octanol/water partition coefficient, and a is a proportionality constant. Present study describes the experimental evaluation of Φ based on the measurement of k'j for the compounds in the homologous series between benzene and butylbenzene for three C18 columns: Gemini C18, Luna C18 both with 5 μm particles, and a Chromolith Performance RP-18. The evaluation was performed for two mobile phase systems at different proportions of methanol/water and acetonitrile/water. The octanol/water partition coefficients were obtained from the literature. The results obtained in the study provide further support for the new procedure for the evaluation of phase ratio.

All-in-one assay for β-d-galactoside sialyltransferases: Quantification of productive turnover, error hydrolysis, and site selectivity

Sialyltransferases are important enzymes of glycobiology and the related biotechnologies. The development of sialyltransferases calls for access to quick, inexpensive, and robust analytical tools. We have established an assay for simultaneous characterization of sialyltransferase activity, error hydrolysis, and site selectivity. The described assay does not require expensive substrates, is very sensitive (limit of detection=0.3 μU), and is easy to perform. It is based on sialylation of nitrophenyl galactosides; the products thereof are separated and quantified by ion pair reversed phase high-performance liquid chromatography with ultraviolet detection.

Molecular simulations of retention in chromatographic systems: use of biased Monte Carlo techniques to access multiple time and length scales

The use of configurational-bias Monte Carlo simulations in the Gibbs ensemble allows for the sampling of phenomena that occur on vastly different time and length scales. In this review, applications of this simulation approach to probe retention in gas and reversed-phase liquid chromatographic systems are discussed. These simulations provide an unprecedented view of the retention processes at the molecular-level and show excellent agreement with experimental retention data.