Revisionist look at solvophobic driving forces in reversed-phase liquid chromatography

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.

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.

Tris(hydroxymethyl)aminomethane-functionalized agarose particles: parameters affecting the binding of bovine serum albumin

A new protein adsorbent is introduced based on the coupling of the common buffer ion, tris(hydroxymethyl)aminomethane, to the agarose gel Sepharose HP from GE Healthcare Bio-Sciences AB, Uppsala, Sweden. The article describes the synthesis of the new adsorbent and the use of BSA as a model in a binding study. By optimization of the coupling procedure, a maximum ligand density of 63.5 μmol/mL gel could be obtained. Adsorption equilibria were investigated in the pH range 5.0-8.0 and at salt concentrations of 0-0.4 mol/L. Binding of BSA under different conditions indicated that both electrostatic interaction and hydrogen bonding were involved in the adsorption process where the former played a major role.

Simple models for the effect of aliphatic alcohol additives on the retention in reversed-phase liquid chromatography

Four retention models for the effect of aliphatic alcohol additives on the retention of analytes in reversed-phase liquid chromatography have been developed following either a semi-thermodynamic treatment or an empirical approach. Their performance was tested using the experimental retention times of six non-polar analytes (alkylbenzenes) and ten o-phthalaldehyde derivatives of amino acids under different isocratic chromatographic runs when a small amount of ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol or 1-heptanol was added to methanol/water mixtures containing a constant amount of methanol. It was shown that for the structurally simple alkylbenzenes all the models can be adopted for retention prediction with good results. In contrast, just one out of four models, that with the fewest approximations, predicts satisfactorily the retention properties of amino acids derivatives. However, the most interesting feature is that this model can predict the effect of an alcohol-additive on the retention properties of solutes, even if this additive was not used in chromatographic runs done for the fitting procedure, provided that it belongs to the same homologous series of alkanols. This feature is also observed in all models described the retention of alkylbenzenes.

Fast, comprehensive two-dimensional liquid chromatography

The absolute need to improve the separating power of liquid chromatography, especially for multi-constituent biological samples, is becoming increasingly evident. In response, over the past few years, there has been a great deal of interest in the development of two-dimensional liquid chromatography (2DLC). Just as 1DLC is preferred to 1DGC based on its compatibility with biological materials we believe that ultimately 2DLC will be preferred to the much more highly developed 2DGC for such samples. The huge advantage of 2D chromatographic techniques over 1D methods is inherent in the tremendous potential increase in peak capacity (resolving power). This is especially true of comprehensive 2D chromatography wherein it is possible, under ideal conditions, to obtain a total peak capacity equal to the product of the peak capacities of the first and second dimension separations. However, the very long timescale (typically several hours to tens of hours) of comprehensive 2DLC is clearly its chief drawback. Recent advances in the use of higher temperatures to speed up isocratic and gradient elution liquid chromatography have been used to decrease the time needed to do the second dimension LC separation of 2DLC to about 20s for a full gradient elution run. Thus, fast, high temperature LC is becoming a very promising technique. Peak capacities of over 2000 and rates of peak capacity production of nearly 1 peak/s have been achieved. In consequence, many real samples showing more than 200 peaks with signal to noise ratios of better than 10:1 have been run in total times of under 30 min. This report is not intended to be a comprehensive review of 2DLC, but is deliberately focused on the issues involved in doing fast 2DLC by means of elevating the column temperature; however, many issues of broader applicability will be discussed.

Theory of Retention in Reversed Phase Liquid Chromatography with Ternary Mobile Phases

The partition model of retention is developed for reversed phase liquid chromatography with multicomponent mobile phases. Simple equations for the retention and selectivity in ternary mobile phases are derived. For the systems in which the ratio of volume fractions of organic modifiers remains fixed, new linear dependences for retention factor and selectivity are proposed. These equations are successfully used to describe experimental data found in the literature. An influence of the nature of organic solvents and proportion in which they are mixed on retention and selectivity is discussed.

Reversed-phase liquid chromatography testing

Column testing is a primary concerns for analysts. It is of use not only for the choice of set of development columns with different behaviors, but also for a substitution column in a validated method or as a quality control of new batches of stationary phase. A validated chromatographic procedure for column testing was applied to 42 commercially available columns, including alkyl, polar embedded and Aqua type stationary phases. This procedure was based on the use of two different solvents: MeOH and MeCN; and two different solvent/aqueous buffer fractions. Principal component analysis has been combined to hierarchical cluster analysis to provide both rational and objective classifications. The solvent effects were then studied on the obtained representations, revealing the necessity for considering both the solvent nature and its fraction in RPLC column testing. Differences observed depending on the solvent nature and fractions revealed quite different chromatographic behaviors according to these parameters.

Retention process in reversed phase TLC systems with polar bonded stationary phases

The retention of a solute in RP chromatography is a very complex process which depends on many factors. Therefore, the study of the influence of a mobile phase modifier concentration on the retention in different reversed phase chromatographic systems is very important for understanding the rules governing retention and mechanisms of substance separation in a chromatographic process. Composition changes and the nature of mobile phases enable tuning of the separated analytes' retention over a wide range of retention parameters and optimization of the chromatographic process as well. Optimization of the chromatographic process can be achieved by several different methods; one of them is the so-called interpretative strategy. The key approach adopted in this strategy is the implementation of adequate retention models that couple the retention of solute with the composition of a mixed mobile phase. The use of chemically bonded stationary phases composed of partially non-bonded silica matrix and organic ligands bonded to its surface in everyday chromatography practice leads to questions of the correct definition of the retention model and the dominant retention mechanism in such chromatographic systems. The retention model for an accurate prediction of retention factor as a function of modifier concentration and the heterogeneity of the adsorbent surface should be taken into consideration. In this work the influence of mobile-phase composition on the retention of sixteen model substances such as phenols, quinolines, and anilines used as test analytes in different RP-TLC systems with CN-, NH2-, and Diol-silica polar bonded stationary phases has been studied. The aim of this study is to compare the performance of three valuable retention models assumed as the partition, adsorption/partition, and adsorption mechanism of retention. All the models were verified for different RP-TLC systems by three statistical criteria. The results of investigations presented in this work demonstrate that the best agreement between the experimental and calculated Rf values was obtained by the use of new-generation retention models, which assume heterogeneity of adsorbent surface. The results reported here show that heterogeneity of the adsorbent surface may be important in analysis of the elution process in liquid chromatography. Consideration of the goodness of fit for the experimental data to the examined retention models is in conformity with the adsorption mechanism of retention on all polar bonded stationary phases in most eluent systems for most investigated compounds.

New insights on the retention mechanism of non-polar solutes in reversed-phase liquid chromatographic columns

A clarification of the retention mechanism of non-polar solutes in octadecyl reversed-phase chromatographic columns is attempted based on a systematic comparison of the retention in C18 and C2 columns under the assumption that the retention in C2 columns is due to adsorption. The comparison involves curve fitting procedures and tests based on the properties of special functions suggested in the present paper. For the application of this approach the retention behaviour of six non-polar solutes, benzene, toluene, ethylbenzene, propylbenzene, isopropylbenzene and tert-butylbenzene, is studied from aqueous mobile phases modified with methanol, isopropanol, acetonitrile and tetrahydrofuran using C18 and C2 reversed-phase columns. It was found that the retention mechanism in C18 columns is not the same in the four modifiers. In particular, our results show that the adsorption mechanism has a significant contribution in mobile phases modified by acetonitrile and tetrahydrofuran, the partition mechanism is likely to predominate in isopropanol-water mobile phases provided that the mole fraction of isopropanol is higher than 0.2, whereas the case of MeOH is rather obscure, since the various tests did not give a clear picture about the retention mechanism in methanol-water mobile phases.

Part II. Chromatography using ultra-stable metal oxide-based stationary phases for HPLC

In this part of the review authors discuss methods used for modification of metal oxide surfaces. On the basis of literature data it is shown, that silanization of the surfaces do not form stable supports for chromatography. On the other hand, the success of polymer modified surfaces such as polybutadiene (PBD) and polystyrene (PS) is emphasized. Permanent modification of metal oxide surfaces with Lewis bases is also widely discussed. Chromatographic properties of polymer modified surfaces of zirconia are discussed in details. The perspectives of carbon-coated metal oxide surfaces in HPLC and high temperature separations are described.

Thermodynamics and kinetics of solute transfer in reversed-phase liquid chromatography

In this study, the thermodynamic and kinetic behavior of a homologous series of fatty acids is examined using a polymeric octadecylsilica stationary phase and a methanol mobile phase. The zone profiles are evaluated as the temperature is varied from 20 to 60 degrees C and the average pressure from 400 to 4570 p.s.i. (1 p.s.i.=6894.76 Pa). The rate constant for solute transfer from mobile to stationary phase (k(ms)) appears to be relatively constant with carbon number. In contrast, the rate constant from stationary to mobile phase (k(sm)) decreases logarithmically with increasing carbon number. This suggests that the mass transport processes become progressively slower, owing to the smaller diffusion coefficients of the larger solutes in the stationary phase. The activation energy decreases slightly in the mobile phase and increases slightly in the stationary phase with increasing carbon number. The activation energy in the stationary phase ranges from 41.6 to 55.9 kcal/mol, while the thermodynamic change in internal energy ranges from -9.8 to -29.0 kcal/mol for C10 to C22, respectively (1 cal=4.184 J). The activation volume increases with increasing carbon number in both the mobile and stationary phase. The activation volume in the stationary phase ranges from 31.7 to 211 cm3/mol, while the thermodynamic change in molar volume ranges from -27.1 to -104 cm3/mol for C10 to C22, respectively. These large changes in activation energy and volume suggest that the solutes do not enter and leave the stationary phase in a single step, but in a stepwise or progressive manner.

Interaction mechanisms on octadecyl packed columns in subcritical fluid chromatography with CO2-modifier mobile phases

An experimental design was carried out for describing the interaction mechanisms between solutes and octadecyl bonded silicas in subcritical fluid chromatography (SubFC), with CO2-methanol and CO2-acetonitrile mobile phases. The effects of modifier amount, temperature and outlet pressure were studied. The homologous series of alkylbenzenes was mainly used as probe, and results were in part assessed with other series. Curves between the methylene selectivity (alphaCH2) and the alkyl chain carbon number (Cn) were plotted, because changes of slope or discontinuity in these curves are yielded by interaction mechanism modifications. Moreover, the linearity of the Van 't Hoff curves with CO2-acetonitrile mobile phases has enabled one to calculate the transfer enthalpy (deltaH) for each homologue. The curves log k = f(-deltaH) allow a discrimination of the retention behaviors between the short and the long homologues for CO2-acetonitrile mobile phases. Depending on the analytical conditions, different oriented partition mechanisms occur for the long homologues, when the short ones seem to be fully embedded into the grafted chains near the silica surface. With methanol-CO2 mobile phases the discrimination between the homologues disappears and the methylene selectivity curves correspond to a bulk partition mechanism. The differences in the interaction mechanisms following the modifier nature are related to the adsorption the mobile phase onto the stationary phase, because the amount of adsorbed mobile phase modifies the bonded chain mobility. With methanol, an important adsorption of the mobile phase occurs, when this adsorption is reduced with acetonitrile. In this latter case, an anisotropy in the stationary phase mobility can explain the observed difference in the interaction mechanisms of homologues. Finally, effects of stationary phase chain length (from C18 to C22) and bonding density (from 2.5 to 3.4 micromol m(-2)) were also reported.

Repeatability and reproducibility of retention data and band profiles on six batches of monolithic columns

Chromatographic data were acquired for eight different mixtures, under five different sets of experimental conditions, for a total of 30 neutral, acidic and basic test compounds, on a series of six Chromolith Performance columns from Merck. These columns are made of a C18 chemically bonded silica monolith. Each column belonged to a different production batch, so the data reported here characterize their batch-to-batch reproducibility. The parameters studied in this work were the retention times, the retention and separation factors, the hydrophobic and the steric selectivities, the column efficiencies, and the tailing factors for all 30 compounds.

Enhancement of selectivity in reversed-phase liquid chromatography

In an effort to gain insight into the relationship between stationary phase solvation and selectivity, the use of short- and medium-chained-length alcohols (methanol, n-propanol, n-butanol, and n-pentanol) as mobile phase modifiers in reversed-phase liquid chromatography (RPLC) was investigated to determine their impact on chromatographic selectivity. A wide range of mobile phase compositions was evaluated because of the large effect exerted by solvent strength on selectivity. Employing a set of six vanillin compounds as retention probes, evidence is presented to support the view that an increase in the hydrophobicity of the organic modifier used in RPLC can increase the selectivity of the C18 alkyl bonded phase while simultaneously decreasing the retention time of the eluting solutes. Thus, we are presented with an interesting paradox: higher selectivity and shorter retention times, which can be attributed to changes in either solvent selectivity and/or stationary phase solvation by the organic modifier.

Effect of the organic modifier concentration on the retention in reversed-phase liquid chromatography I. General semi-thermodynamic treatment for adsorption and partition mechanisms

A semi-thermodynamic treatment is adopted to account for adsorption or partition of solute molecules from aqueous mobile phases on/in reversed-phase liquid chromatography stationary phases. The theoretical expressions of ln k' versus organic modifier content are tested against 10 data sets covering a variety of solute molecules. It is shown that the mean field approximation, adopted widely in ptevious studies, is marginally valid in aqueous mobile phases, especially in the presence of solute molecules, and the lattice model approximation, which is also used in relevant studies, is a poor approximation. Clear conclusions about the validity of either the adsorption or the partition model for the retention mechanism could not be drawn. The equations of the adsorption model describe all data sets absolutely satisfactorily and yield a physically reasonable picture about the behavior of modifier and solvent at the adsorbed layer. However, the high applicability of the adsorption model may not safely entail the validity of the adsorption mechanism at a molecular level, especially in the case of solutes with small and non-polar molecules, where our analysis gives strong indications about the validity of the partition mechanism. The next steps needed for the final elucidation of the retention mechanism in reversed-phase chromatographic columns are indicated.

Modeling the effect of solvation on solute retention in reversed-phase liquid chromatography

The retention of a homologous series of alkylbenzenes was determined on octyl and octadecyl reversed-phase columns in several polar organic liquids. Free energies of transfer were calculated by the SM5.0R classical solvation model for each organic liquid tested and for several alkanes. The relationships between the measured retention factors and the calculated free energies of transfer were then investigated. Although the natural logarithms of the retention factor and the calculated free energies of transfer were linearly correlated, the obtained free energies of transfer of the solutes did not completely explain the retention behavior of the solutes. Nonetheless, even in these pure organic liquids, the energetics of RPLC retention behaved very similarly to those of partitioning.

Low-energy interactions in high-performance liquid chromatography

Liquid chromatographic systems with very weak excessive analyte-adsorbent interactions have been studied. These systems consisted of a homologous series of n-alkanes as both analytes and mobile phases with a C18 reversed-phase adsorbent. A linear decrease of the analyte retention volume with an increase of the number of analyte carbon atoms was found. Corresponding increases of analyte retention with an increase in the number of eluent carbon atoms was also discovered. An explanation of these two effects on the basis of adsorption theory is proposed. A good correlation of column hold-up volume calculated by interpolation of the retention dependencies for above mentioned systems with that measured by the minor disturbance method has been shown. A study of the temperature dependencies of these alkane systems has shown entropy-governed retention dependencies.

Studies of retention and stability of a horizontally polymerized bonded phase for reversed-phase liquid chromatography

We have studied the novel horizontally polymerized mixed trichloropropyl-trichlorooctadecyl silane bonded phase described by Wirth. These materials can be reproducibly prepared and give very high bonded phase density (>7.5 micromol m(-2)). They show significantly improved alkaline stability and chromatographic selectivity towards PAHs similar to conventional monomeric phases. Study of retention by the Linear Solvation Energy Relationship approach as well as measurement of dead volume and retention of methanol indicate that less mobile phase is sorbed by a horizontally polymerized phase than by conventional phases. Silanophilic interaction of amines are decidedly weaker on a silica modified by horizontal polymerization compared to a conventionally modified phase. In addition, this work provides additional support for the "partition-like" retention mechanism of bonded phase RPLC.

Repeatability and reproducibility of retention data and band profiles on reversed-phase liquid chromatography columns. III. Results obtained with Kromasil C18 columns

The reproducibility of the retention data and the band profiles was investigated with Kromasil C18 columns (silica-based monomeric type reversed-phase packing material). High precision data were obtained and statistically compared among five columns from the same batch (column-to-column reproducibility) and six columns, one from each of six different batches (batch-to-batch reproducibility). These data were acquired under five different sets of chromatographic conditions, for a group of 30 neutral, acidic and basic compounds selected as probes following an experimental protocol previously described. Data characterizing the retention time, the retention factor, the separation factor, the column efficiency and the peak asymmetry for the different probe compounds are reported. Factors describing the silica surface interaction with the selected probe compounds, such as the hydrophobic interaction selectivity, the steric selectivity, and the separation factors of basic compounds at different pH values were also determined. The influence of the underlying silica on these data and correlations between the chromatographic and physico-chemical properties of the different batches are discussed.

Comparative study of hydrocarbon, fluorocarbon, and aromatic bonded RP-HPLC stationary phases by linear solvation energy relationships

The retention properties of eight alkyl, aromatic, and fluorinated reversed-phase high-performance liquid chromatography bonded phases were characterized through the use of linear solvation energy relationships (LSERs). The stationary phases were investigated in a series of methanol/water mobile phases. LSER results show that solute molecular size and hydrogen bond acceptor basicity under all conditions are the two dominant retention controlling factors and that these two factors are linearly correlated when either different stationary phases at a fixed mobile-phase composition or different mobile-phase compositions at a fixed stationary phase are considered. The large variation in the dependence of retention on solute molecular volume as only the stationary phase is changed indicates that the dispersive interactions between nonpolar solutes and the stationary phase are quite significant relative to the energy of the mobile-phase cavity formation process. PCA results indicate that one PCA factor is required to explain the data when stationary phases of the same chemical nature (alkyl, aromatic, and fluoroalkyl phases) are individually considered. However, three PCA factors are not quite sufficient to explain the whole data set for the three classes of stationary phases. Despite this, the average standard deviation obtained by the use of these principal component factors are significantly smaller than the average standard deviation obtained by the LSER approach. In addition, selectivities predicted through the LSER equation are not in complete agreement with experimental results. These results show that the LSER model does not properly account for all molecular interactions involved in RP-HPLC. The failure could reside in the V2 solute parameter used to account for both dispersive and cohesive interactions since "shape selectivity" predictions for a pair of structural isomers are very bad.

Non-flammable preparative reversed-phase liquid chromatography of recombinant human insulin-like growth factor-I

Acetonitrile is used as an eluent for reversed-phase chromatography. However, because it is a flammable solvent, using acetonitrile on a large scale requires expensive equipment and facilities specially designed for flammable solvents. Using a non-flammable solvent as an eluent eliminates this expense. A method was developed to purify recombinant human insulin-like growth factor I by reversed-phase high-performance liquid chromatography using gradient elution with hexylene glycol, a non-flammable replacement for acetonitrile. The separation produced equivalent yield, purity and throughput as reversed-phase chromatography using elution with acetonitrile.

Retention in reversed-phase chromatography: partition or adsorption?

A unified framework within the hermeneutics of the solvophobic theory is employed for the treatment of experimental data with nonpolar and weakly polar substances in reversed-phase chromatography (RPC), oil-water partitioning and adsorption on activated charcoal from dilute aqueous solution. This approach sheds light on the energetic similarities between such processes driven by the hydrophobic effect. Among several stationary phase models that have been proposed in the literature for the physical representation of alkyl-silica bonded phases, the isolated solvated hydrocarbon chains model is adopted for the retention in RPC since it represents most closely the stationary phase configuration and is not based a priori on a partition or adsorption mechanism as some other models are for the retention in RPC. Using the fundamental framework of the solvophobic theory, the free energy change per unit nonpolar surface area for octanol-water and hexadecane-water partitioning, retention in RPC as well as adsorption on activated charcoal from dilute aqueous solution at 25 degrees C are evaluated and they are found to be in good agreement with the corresponding experimental data. Furthermore, such quantities are very similar for all the above mentioned processes involving aqueous solution, in contradistinction to the predictions by the lattice theory. From the results it follows that these apparently disparate processes are subject to the same physicochemical principle. The present study demonstrates the capability of the solvophobic theory in describing the energetics of processes involving hydrophobic interactions, and exposes the difficulties in distinguishing between partition and adsorption mechanisms in RPC by using partition models based on the lattice approach. It is concluded that a clear distinction between partition and adsorption in RPC of nonpolar elites is not apparent from thermodynamic analysis.