Characterization of various reversed-phase columns using the linear free energy relationship

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.

Design of peptide standards with the same composition and minimal sequence variation to monitor performance/selectivity of reversed-phase matrices

The present manuscript extends our de novo peptide design approach to the synthesis and evaluation of a new generation of reversed-phase HPLC peptide standards with the same composition and minimal sequence variation (SCMSV). Thus, we have designed and synthesized four series of peptide standards with the sequences Gly-X-Leu-Gly-Leu-Ala-Leu-Gly-Gly-Leu-Lys-Lys-amide, where the N-terminal is either N(α)-acetylated (Series 1) or contains a free α-amino group (Series 3); and Gly-Gly-Leu-Gly-Gly-Ala-Leu-Gly-X-Leu-Lys-Lys-amide, where the N-terminal is either N(α)-acetylated (Series 2) or contains a free α-amino group (Series 4). In this initial study, the single substitution position, X, was substituted with alkyl side-chains (Ala Collapse

Key Words Collapse

MESH Headings

• Amino Acid Sequence
• Chromatography, High Pressure Liquid/standards
• Chromatography, Reverse-Phase/standards
• Peptides/chemistry
• Reference Standards

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Grants

• R01 GM061855 NIGMS NIH HHS
• R01 GM061855-10 NIGMS NIH HHS
• R01 GM61855 NIGMS NIH HHS

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Affiliation(s)

Colin T. Mant Department of Biochemistry and Molecular Genetics, University of Colorado Denver, School of Medicine, Aurora, CO 80045, USA
Robert S. Hodges Department of Biochemistry and Molecular Genetics, University of Colorado Denver, School of Medicine, Aurora, CO 80045, USA

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A visual approach to stationary phase selectivity classification based on the Snyder-Dolan Hydrophobic-Subtraction Model

A novel type of stationary phase selectivity classification "triangle" has been developed based on the Snyder-Dolan (S-D) Hydrophobic-Subtraction Model, wherein the apices of a set of four triangles represent the relative contributions of steric hindrance (chi(S)), hydrogen-bonding acidity (chi(A)), hydrogen-bonding basicity (chi(B)), cation-exchange capacity (chi(C)) to selectivity. We found that "effective selectivity" of a stationary phase is mathematically given by the ratio of system dependent interaction coefficients but not their absolute values. Thus by normalizing the S*, A, B and C terms of the S-D model by H, we were able to obtain four parameters which fully define the chromatographic selectivity of the stationary phases. By examining the parameters in groups of three, we can represent all the result in a set of four "selectivity triangles". The distinctive feature of this approach compared to the S-D phase classification scheme is that it allows the visualization of column selectivity by plotting three-dimensional data in a two-dimensional space. Moreover, it very clearly shows that the RPLC columns thus far characterized cover only a small fraction of separation selectivity space leaving a great deal of room for researchers to develop novel RPC materials. Various applications of these "selectivity triangles" will be discussed in this paper.

Lipophilicity measurement of drugs by reversed phase HPLC over Wide pH range using an alkaline-resistant silica-based stationary phase, XBridge Shield RP(18)

We propose a reversed phase HPLC (RP-HPLC) with an alkaline-resistant silica-based stationary phase, XBridge Shield RP(18), for the determination of the lipophilicity of drugs with diverse chemical nature ranging from acidic to basic. A set of 40 model compounds with well-defined solvatochromic parameters was selected to allow a broad distribution of structural properties. The chromatographic results showed that the lipophilicity index log k(w) obtained with XBridge Shield RP(18) was well correlated with experimental log P(oct) values (r(2)=0.96). Linear solvation free-energy relationship (LSER) analyses revealed that the retention mechanism of the stationary phase and 1-octanol/water partitioning were controlled by almost the same balance of intermolecular forces (hydrophobicity as expressed by the van der Waals volume V(w), H-bond acceptor basicity beta, and dipolarity/polarizability pi*). The results showed that XBridge Shield RP(18) phase overcomes the shortcomings of the silica-based stationary phases, the application of which to lipophilicity measurements had been limited to neutral and acidic compounds.

Stationary phases for hydrophilic interaction chromatography, their characterization and implementation into multidimensional chromatography concepts

Hydrophilic interaction chromatography (HILIC) is becoming increasingly popular for separation of polar samples on polar columns in aqueous-organic mobile phases rich in organic solvents (usually ACN). Silica gel with decreased surface concentration of silanol groups, or with chemically bonded amino-, amido-, cyano-, carbamate-, diol-, polyol-, or zwitterionic sulfobetaine ligands are used as the stationary phases for HILIC separations, in addition to the original poly(2-sulphoethyl aspartamide) strong cation-exchange HILIC material. The type of the stationary and the composition of the mobile phase play important roles in the mixed-mode HILIC retention mechanism and can be flexibly tuned to suit specific separation problems. Because of excellent mobile phase compatibility and complementary selectivity to RP chromatography, HILIC is ideally suited for highly orthogonal 2-D LC-LC separations of complex samples containing polar compounds, such as peptides, proteins, oligosaccharides, drugs, metabolites and natural compounds. This review attempts to present an overview of the HILIC separation systems, possibilities for their characterization and emerging HILIC applications in 2-D off-line and on-line LC-LC separations of various samples, in combination with RP and other separation modes.

Characterization and applications of reversed-phase column selectivity based on the hydrophobic-subtraction model

A total of 371 reversed-phase columns have now been characterized in terms of selectivity, based on five solute-column interactions (the hydrophobic-subtraction model). The present study illustrates the use of these data for interpreting peak-tailing and column stability. New insights are also provided concerning column selectivity as a function of ligand and silica type, and the selection of columns for orthogonal separations is re-examined. Some suggestions for the quality control of reversed-phase columns during manufacture are offered.

Characterisation of stationary phases in supercritical fluid chromatography with the solvation parameter model

Characterisation of chromatographic systems with the solvation parameter model provides satisfactory information on the main non-ionic interactions developed in a chromatographic system. The procedure requires the analysis of a large number of compounds to warrant the relevance and the accuracy of the calculated models, and even if retention time is lower in supercritical fluid chromatography (SFC) than in HPLC (3-5 times lower), a decrease in the time required for that procedure would favour the use of this model in method development. Consequently, in order to establish a rapid testing procedure that would provide equivalent information, nine key solutes were carefully selected among the hundred we classically use. The separation factors calculated between these key solutes, taken two by two to establish new equations, allow the calculation of the model coefficients. The normal testing procedure is thus reduced from one or two days down to 2h. Precision and accuracy of the models provided are assessed through back-calculation of the coefficients that served for the establishment of the procedure, then through calculation of the coefficients of 13 new SFC systems. The applicability of the rapid testing procedure in SFC is evidenced with three examples: the elaboration of a system map, by varying the modifier concentration in the mobile phase, and the comparison of six ODS phases bonded on the same silica base. The simplified procedure presented here does not pretend to characterize the chromatographic systems as precisely as the complete testing procedure does, but is only aimed at rapidly evaluating the chromatographic retention characteristics when operating parameters are varied.

Stationary phase characterization and method development

The properties of stationary phases and their characterization methods are reviewed. New and significant developments have occurred in the last few years, and new methods for stationary phase characterization have become available. The characterization methods are discussed, and the differences between the different methods are pointed out. In addition, method development approaches are reviewed, with special emphasis on recent developments that employ multiple parameters in parallel. Also, the renewed interest of temperature as a tool in method development is surveyed.

Combined supercritical fluid chromatographic methods for the characterization of octadecylsiloxane-bonded stationary phases

In this paper, we present a combination of a key-solute test based on retention and separation factors of large probe solutes (carotenoid pigments) and a quantitative structure-retention relationship analysis based on the retention factors of small probe solutes (aromatic compounds) to investigate the different chromatographic behavior of octadecylsiloxane-bonded stationary phases of all sorts: classical, protected against silanophilic interactions or not, containing polar groups (endcapping groups or embedded groups). Varied chemometric methods are used to enlighten the differences between the 27 phases tested. The results indicate that the two approaches chosen (carotenoid test and solvation parameter model) are complementary and provide precise information on the chromatographic behavior of ODS phases.

Selectivity in reversed-phase separations

The selectivity difference between 15 different stationary phases was measured using a large number of analytes at 2 or 3 different pH values (3, 7 and 10) with acetonitrile and methanol as the mobile phase modifiers. The packings discussed include standard C(8) and C(18) packings, packings with embedded polar groups, a phenyl packing, a pentafluoro-phenyl packing, an adamantylethyl packing and others. The major selectivity differences observed are discussed in detail. Specific effects such as pi-pi interactions on phenyl packings or hydrogen-bond interactions on phases with embedded polar groups are confirmed.

Column selectivity for two-dimensional liquid chromatography

Selectivity of phase system is of primary concern when designing a 2-D separation, as it affects the 2-D system orthogonality and consequently the peak capacity controlling the number of peaks that can be separated in the available 2-D retention space limited by the time of analysis. Possibilities for characterization of LC phase system selectivity with respect to different polar and nonpolar structural units are compared, with special attention to multidimensional samples with various types of repeat groups, such as homopolymers, (co)polymers, fatty acid esters with various acyl lengths and number and position of double bonds, etc. Possibilities of the 2-D LC separations of these and other sample types, including pharmaceuticals, natural phenolic compounds, biopolymers, etc., using various combinations of separation modes are reviewed. Rules for design of comprehensive 2-D LC x LC systems are discussed, with respect to mobile phase compatibility in the two systems and modulation techniques suppressing band broadening connected with the sample fraction transfer from the first to the second dimension. Pitfalls connected with online connection of normal-phase and RP LC systems and their possible practical solutions are addressed and illustrated by practical examples.

Linear free energy relationship as a tool for characterization of three teicoplanin-based chiral stationary phases under various mobile phase compositions

Teicoplanin, teicoplanin aglycon, and methylated teicoplanin aglycon chiral stationary phases (CSPs) have been compared on the basis of the regression coefficients calculated from the linear free energy relationship (LFER) equation. The parameters have been obtained from the measurements of a set of 34 structurally diverse solutes. Influence of mobile phase composition - variation of methanol (MeOH) content - on the participation of different interactions types in the retention mechanism has been evaluated. Retention of the various interaction forces in analytes differs with both the CSP and the mobile phase composition. Hydrophobic interactions play a major role in mobile phases for high buffer contents. The more hydrophobic the CSP, the more important are they in the retention mechanism. With increase of MeOH contents in the mobile phase the major role in the interaction mechanism is shifted to more polar forces in which basicity and dipolarity/polarizability dominate. Although the LFER model does not address chiral aspects, we have attempted to explore the importance of the individual interactions in chiral discrimination of amino acids and their N-tert-butyloxycarbonyl derivatives.

Characterization of the surface properties of Mg/Al oxides by the solvation parameter model

The oxides of different Mg/Al ratios (Mg/Al = 0, 0.1, 5, 10 and infinity) were prepared, and the characterization of these oxides was attempted by estimating characteristic interaction parameters based on the solvation parameter model. The magnitudes of the regression coefficients varied with the increase of Mg/Al ratio. For the oxide of Mg/Al = 0.1 and Al2O3, the contribution of these characteristic interactions for solutes' retention was similar to that of the common silica, and the dipolarity/polarizability (pi*), the solute hydrogen bond donating (alpha2(H)) and accepting (beta2(H)) abilities played an important role. Yet, the retention behavior at a higher Mg/Al ratio (Mg/Al = 5, 10 and MgO) drastically changes, and V(i)/100 (the intrinsic molar volume), pi*, alpha2(H) and beta2(H) all favored the solutes retention. By comparison of the regression coefficients on various normal-phase (NP) and reversed-phase (RP) columns, a new model was developed to correlate the solute retention factors on Mg/Al = 5, 10 and MgO columns with the data of NP and RP columns.

Intermolecular interactions on multiwalled carbon nanotubes in reversed-phase liquid chromatography

Retention on multiwalled carbon nanotubes (MWCNTs) in RPLC has been correlated with solute descriptors of dispersion, polarizability, dipolarity, hydrogen bond donor acidity, and hydrogen bond acceptor basicity through the use of the linear solvation energy relationship. Intermolecular interactions influencing solute retention on MWCNTs were compared with those on a graphitic carbon-deposited zirconia and a common RPLC stationary phase, octylsilane-bonded silica.

Characterisation of stationary phases in subcritical fluid chromatography with the solvation parameter model

In this third paper, varied types of polar stationary phases, namely silica gel (SI), cyano (CN)- and amino-propyl (NH2)-bonded silica, propanediol-bonded silica (DIOL), poly(ethylene glycol) (PEG) and poly(vinyl alcohol) (PVA), were investigated in subcritical fluid mobile phase. This study was performed to provide a greater knowledge of the properties of these phases in SFC, and to allow a more rapid and efficient choice of polar stationary phase in regard of the chemical nature of the solutes to be separated. The effect of the nature of the stationary phase on interactions between solute and stationary phases and between solute and carbon dioxide-modifier mobile phases was studied by the use of a linear solvation energy relationship (LSER), the solvation parameter model. The retention behaviour observed with sub/supercritical fluid with carbon dioxide-methanol is close to the one reported in normal-phase liquid chromatography with hexane. The hydrogen bond acidity and basicity, and the polarity/polarizability favour the solute retention when the molar volume of the solute reduces it. As with non-polar phases, the absence of water in the subcritical fluid allows the solute/stationary phase interactions to play a greater part in the retention behaviour. As expected, the DIOL phase and the bare silica display a similar behaviour towards acidic and basic solutes, when interactions with basic compounds are lower with the NH2 phase. On the CN phase, all interactions (hydrogen bonding, dipole-dipole and charge transfer) have a nearly equivalent weight on the retention. The polymeric phases, PEG and PVA, provide the most accurate models, possibly due to their better surface homogeneity.

Characterization of stationary phases in subcritical fluid chromatography by the solvation parameter model

Varied types of alkylsiloxane-bonded and fluoroalkylsiloxane-bonded stationary phases, all commercially available, were investigated with subcritical fluid mobile phase. The effect of the alkyl chain length (from C4 to C18) and of the nature of the bonding (fluorodecylsiloxane, phenyl-C18 and polar-embedded-C18) on the chromatographic behaviour was investigated by the use of a linear solvation energy relationship (LSER), the solvation parameter model. A large set of test compounds provides precise and reliable information on the intermolecular interactions responsible for retention on these stationary phases used with a subcritical mobile phase. First of all, the results underline the close properties between subcritical fluid and organic liquid. The use of non aqueous mobile phases reduces the cavity energy and the mobile phase acidity generally encountered with aqueous liquid phases, allowing other interactions to take a part in retention. As expected, an increase in the alkyl chain length favours the dispersive interactions between the solutes and the stationary phases. Changes in basicity and acidity of the stationary phases are also related to the chain length, but, in this case, mobile phase adsorption onto the stationary phase is supposed to explain these behaviours. The addition of a phenyl group at the bottom of the C18 chain, near the silica, does not induce great modifications in the retentive properties. The fluorodecylsiloxane and the polar-embedded alkylsiloxane phases display very different properties, and can be complementary to the classical alkylsiloxane-bonded phases. In particular, the fluorinated phase does not favour the dispersive interactions, in comparison to hydrogenated stationary phases, when the basicity of the polar-embedded phase is obviously greater than the one of classical alkylsiloxane-bonded phases, due to the amide function. Finally, logk-logk curves plotted between the different phases illustrate the effect of the interaction properties on the retention of different classes of compounds.

Novel RPLC stationary phases for lipophilicity measurement: Solvatochromic analysis of retention mechanisms for neutral and basic compounds

An RPLC was developed to rapidly determine lipophilicity of neutral and basic compounds using three base deactivated RPLC stationary phases particularly designed for the analysis of basic compounds, namely, Supelcosil ABZ(+)Plus, Discovery RP Amide C16, and Zorbax Extend C18. The work consisted of three sets of experiments. In the first log kw values of neutral compounds were extrapolated using hydroorganic mobile phases at different compositions. Good correlation between log kw and log Poct indicated that the method was appropriate for these supports, without adding a silanol masking agent. In the second set of experiments, isocratic log k values of neutral and basic compounds were measured with three different mobile phases. The best estimation of lipophilicity was obtained for neutral and basic compounds when the secondary interactions were strongly reduced (i. e., when basic compounds were under their neutral form). In the third set of experiments, isocratic retention factors of basic compounds (in their neutral form) were measured with a high-pH mobile phase, on a chemically stable support (Zorbax Extend C18). Under these chromatographic conditions, correlation between the isocratic retention factors and log Poct (log D10.5) for basic compounds was similar to that for neutral compounds.

Chromatographic classification and comparison of commercially available reversed-phase liquid chromatographic columns containing polar embedded groups/amino endcappings using principal component analysis

Polar embedded phases have become increasingly popular in liquid chromatography (LC) analysis. These phases can produce diverse chromatographic selectivities as a result of their differing base silica, the type of polar embedded group (i.e. amide, urea, carbamate, ether or sulphonamide moieties) and the length of the alkyl ligand. Four column characterization protocols, using differing test probes, have been used to characterize 18 of these phases together with 17 alkyl phases (some of which contained novel polar endcapping, i.e. amino), which have been evaluated using principal component analysis (PCA). PCA provided graphical comparisons of the differences/similarities between these phases and between their corresponding C-alkyl, amino endcapped and enhanced polar selectivity phases.

Comparative study of three teicoplanin-based chiral stationary phases using the linear free energy relationship model

Teicoplanin (T) is a macrocyclic glycopeptide that is highly effective as a chiral selector for enantiomeric separations. In this study, we used three teicoplanin-based chiral stationary phases (CSPs) - native teicoplanin, teicoplanin aglycon (TAG) and recently synthesized methylated teicoplanin aglycon (MTAG). In order to examine the importance of various interaction types in the chiral recognition mechanism the three related CSPs were evaluated and compared using a linear free energy relationship (LFER). The capacity factors of 19 widely different solutes, with known solvation parameters, were determined on each of the columns under the same mobile phase conditions used for the chiral separations. The regression coefficients obtained revealed the magnitude of the contribution of individual interaction types to the retention on the compared columns under those specific experimental conditions. Statistically derived standardized regression coefficients were used to evaluate the contribution of individual molecular interactions within one stationary phase. It has been concluded that intermolecular interactions of the hydrophobic type significantly contribute to retention on all the CSPs studied here. Other retention increasing factors are n- and pi-electron interactions and dipole-dipole or dipole-induced dipole ones, while hydrogen donating or accepting interactions are more predominant with the mobile phase than with the stationary phases. However, these types of interactions are not equally significant for all the CSPs studied.

Micellar proportion: a parameter to compare the hydrophobicity of the pseudostationary phases or that of the analytes in micellar electrokinetic chromatography

Micellar proportion, t(prop,mic) = t(mic)/t(m), a quantity expressing how much time is spent by the analyte in the micellar phase related to its whole migration time (t(m)) has been introduced by utilizing the micellar phase residence time (t(mic)). The t(prop,mic) values have been determined for analytes of different chemical structures (alkyl benzene and alkyl phenone homologous series, alcohols, strongly hydrophobic peptides) studied by micellar elektrokinetic chromatography (MEKC) using various cationic and anionic pseudostationary phases. A good linear correlation was obtained between t(prop,mic) and the calculated hydrophobicity (CLOGP) of the analytes for all pseudostationary phases (CLOGP = A.logt(prop,mic) + B). Considering a given pseudostationary phase, t(prop,mic) as a relative quantity is a suitable parameter to characterize and compare experimentally the behavior of the various analytes in MEKC. Applying a set of probe molecules with known hydrophobicity, the CLOGP(50) value (showing the value of hydrophobicity of a virtual molecule spending exactly 50% of its migration time in the pseudostationary phase) has been calculated for each pseudostationary phase applied here. This experimentally determinable numerical value (characterizing the pseudostationary phase) can be utilized to compare the hydrophobicity and hence retention ability of the pseudostationary phases. The t(prop,mic) value was found to be applicable to compare the methylene selectivity of the different pseudostationary phases as well: logt(prop,mic) = A.Z + B, where Z is the number of carbon atoms of the alkyl chain in the alkyl benzene homologous series.

Column selectivity in reversed-phase liquid chromatography. VI. Columns with embedded or end-capping polar groups

A previous model of column selectivity for reversed-phase liquid chromatography (RP-LC) has been applied to an additional 21 columns with embedded or end-capping polar groups (EPGs). Embedded-polar-group columns exhibit a significantly different selectivity vs. non-EPG, type-B columns, generally showing preferential retention of hydrogen-bond donors, as well as decreased retention for hydrogen-bond acceptors or ionized bases. EPG-columns are also generally less hydrophobic (more polar) than are non-EPG-columns. Interestingly, columns with polar end-capping tend to more closely resemble non-EPG columns, suggesting that the polar group has less effect on column selectivity when used to end-cap the column versus the case of an embedded polar group. Column selectivity data reported here for EPG-columns can be combined with previously reported values for non-EPG columns to provide a database of 154 different columns. This enables a comparison of any two of these columns in terms of selectivity. However, comparisons that involve EPG columns are more approximate.

Insights into the retention mechanism of neutral organic compounds on polar chemically bonded stationary phases in reversed-phase liquid chromatography

The solvation parameter model is used to characterize the retention properties of a 3-aminopropylsiloxane-bonded (Alltima amino), three 3-cyanopropylsiloxane-bonded (Ultrasphere CN, Ultremex-CN and Zorbax SB-CN), a spacer bonded propanediol (LiChrospher DIOL) and a multifunctional macrocyclic glycopeptide (Chirobiotic T) silica-based stationary phases with mobile phases containing 10 and 20% (v/v) methanol-water. The low retention on the polar chemically bonded stationary phases compared with alkylsiloxane-bonded silica stationary phases arises from the higher cohesion of the polar chemically bonded phases and an unfavorable phase ratio. The solvated polar chemically bonded stationary phases are considerably more hydrogen-bond acidic and dipolar/polarizable than solvated alkylsiloxane-bonded silica stationary phases. Selectivity differences are not as great among the polar chemically bonded stationary phases as they are between the polar chemically bonded phases and alkylsiloxane-bonded silica stationary phases.

System maps for retention of neutral organic compounds under isocratic conditions on a reversed-phase monolithic column

The solvation parameter model is used to create systems maps for the separation of neutral organic compounds on a Chromolith Performance RP-18e octadecylsiloxane-bonded silica-based monolithic column for water-acetonitrile and water-methanol mobile phase compositions from 10 to 70% (v/v) organic solvent. These results demonstrate that the retention properties of the monolithic column are similar to those of conventional octadecylsiloxane-bonded silica particle-packed columns. It is further shown that the selectivity for the monolithic column falls within the selectivity range for typical particle-packed columns at two mobile phase compositions for which a direct comparison is possible.

Column selectivity from the perspective of the solvation parameter model

The solvation parameter model is a useful tool for delineating the contribution of defined intermolecular interactions to retention of neutral molecules in separation systems based on a solute equilibrium between a gas, liquid or fluid mobile phase and a liquid or solid stationary phase. The free energy for this process is decomposed into contributions for cavity formation and the set up of intermolecular interactions identified as dispersion, electron lone pair, dipole-type and hydrogen bonding. The relative contribution of these interactions is indicated by a series of system constants determined by the difference of the defined interaction in the two phases. The interpretation of these system constants as a function of experimental factors that affect retention in the chromatographic system provides the connection between relative retention (selectivity) and the control variables for the separation system. To aid in the understanding of these processes we perform an analysis of system constants for gas chromatography, liquid chromatography, supercritical fluid chromatography and micellar electrokinetic chromatography as a function of different experimental variables as a step towards gaining a theoretical understanding of selectivity optimization for method development.

Column selectivity in reversed-phase liquid chromatography I. A general quantitative relationship

Retention factors k have been measured for 67 neutral, acidic and basic solutes of highly diverse molecular structure (size, shape, polarity, hydrogen bonding, pKa, etc.) on 10 different C18 columns (other conditions constant). These data have been combined with k values from a previous study (86 solutes, five different C8 and C18 columns) to develop a six-term equation for the correlation of retention as a function of solute and column. Values of k can be correlated with an accuracy of +/- 1-2% (1 standard deviation). This suggests that all significant contributions to column selectivity have been identified (and can be measured) for individual alkyl-silica columns which do not have an embedded polar group. That is, columns of the latter kind can be quantitatively characterized in terms of selectivity for use in the separation of any sample.

Effect of molecular interactions on retention and selectivity in reversed-phase liquid chromatography

The linear solvation energy relationships (LSERs) have been applied in the last years for description and prediction of retention and selectivity in reversed-phase liquid chromatography with good results. Widely different stationary phases have been compared and characterized by LSERs. In recent publications the influence of the type of the organic moderator and the composition of the mobile phase have also been described. However, the influence of the molecular properties of the solutes to be separated has never been discussed. According to the LSER model variation in retention factors (log k) with solute structure can be related to their potential for various intermolecular interactions. The retention factor is given as the sum of the terms of the LSER equation representing various types of molecular interactions. For this reason the influence of the structure and molecular properties of the solutes to be separated can also be investigated using the LSER equation. In this study we shall demonstrate how the specific molecular interactions influence chromatographic retention and selectivity. We intend to show that retention and selectivity depend on all participants of the system. In addition to the structure and properties of the stationary phase and the type and composition of the mobile phase the molecular properties of the solutes, characterized by the solvation parameters, will also influence the type and extent of the various molecular interactions governing retention and selectivity.