Solute Retention In Liquid Chromatography Research Articles

Application of solvation model to prediction of the solute retention in liquid chromatography over a wide range of mobile-phase compositions

Possibilities have been studied of using a solvation model to predict the retention behavior of solutes in liquid chromatography mobile phases of water-acetonitrile and water-methanol with the organic solvent content varying from 1 to 100 vol%. Twenty-one test solutes, both aliphatic and aromatic compounds, have been selected on the basis of two-level factorial designs. Using the multiple linear regression analysis, regression coefficients, which are characteristics of the stationary and mobile-phase system, were calculated for different mobile phases in the solvation model. Regression coefficients have been used for the prediction of the retention behavior. Unbiased results have been obtained by using two sets, one training and the other the testing set. The predicted retention has been compared with the experimental data. The methanol-water system provided good results at low and medium methanol concentrations; the retention prediction was unsatisfactory for mobile phases containing more than 90% of methanol. The acetonitrile-water system yielded similar results, but the retention prediction ceased to be at acetonitrile concentrations greater than 80%. The retention has primarily been determined by cohesive and acid-base interactions. The dependences of the regression coefficients on the mobile-phase composition were similar for the acetonitrile-water and methanol-water systems.

The thermodynamic properties of methanol–water association and its effect on solute retention in liquid chromatography

Employing density data reported for a range of methanol–water mixtures measured at different temperatures, the methanol–water association constant was calculated at each temperature. The standard free enthalpy and entropy of association were then determined from the linear relationship that exists between the logarithm of the association constant and the reciprocal of the absolute temperature. It is shown that the standard free enthalpy is very high for simple association, viz., 1.916 kcal mol−1, and the standard free entropy of distribution was 2.05 cal K−1 mol−1. These values indicate strong association and the relatively low standard free entropy shows a comparatively small change in molecular freedom after association. This indicates that the solvents are probably strongly associated with themselves when not associating with each other. Using the thermodynamic parameters and molar volumes derived from the calculation, the volume fractions of unassociated methanol, unassociated water and methanol–water associate were calculated for a range of initial methanol–water mixtures at different temperatures. The association decreased fairly rapidly with increased temperature as would be expected. From the calculated composition of the ternary mixtures, together with published retention data for the distribution coefficients of a series of aromatic hydrocarbons and aliphatic acids between methanol–water mixtures and hexadecane, it is shown that dispersive interactions are almost exclusively controlled by the volume fraction of free unassociated methanol. It is also shown that the other components (the methanol–water associate and the free water) exert significant control only over polar interactions.

Effect of pressure-induced ionization, partitioning, and complexation on solute retention in reversed-phase liquid chromatography

In contrast to supercritical fluid chromatography, pressure is not commonly considered an important parameter affecting solute retention in liquid chromatography. While it is true that the bulk compressibility of polar mobile phases is minimal for the modest pressures encountered in reversed-phase LC (<5000 psi; <350 bar), recent studies in our laboratory have demonstrated that pressure-induced shifts in interaction equilibria can lead to systematic perturbations in solute retention. In this study, we address the theoretically predicted impact of pressure on several primary equilibria of importance in separations. Comparison with experimentally determined capacity factor changes is accomplished for reversed-phase separations with and without a mobile-phase additive. Without a mobile-phase additive, capacity factors for the nitrophenol model solutes exhibit a systematic increase of 6–8% for an average pressure increase from 65 to 280 bar. Perturbations in solute ionization are predicted to have a minor impact under these separation conditions, and pressure-induced shifts in the partitioning equilibria are implicated. When β-cyclodextrin is added to the mobile phase, pressure-induced changes in solute retention are exacerbated, leading to capacity factor shifts of up to 12%. This experimental observation is consistent with predictions based on a Le Chatelier model of the coupled partitioning/complexation equilibria. These results have pragmatic implications for the practice of liquid chromatography, especially in quality control situations where retention reproducibility is of key importance. Moreover, pressure-controlled liquid chromatography is demonstrated as a fundamental measurement tool for determining molar volume changes upon partitioning and complexation.

Theoretical and Experimental Studies of the Effect of Pressure on Solute Retention in Liquid Chromatography

Pressure is often assumed to have a negligible influence on solute retention in liquid chromatography because of the small compressibility of the mobile and stationary phases. The range of pressures commonly encountered in reversed-phase separations is considerable, however, and may give rise to significant changes in solute capacity factor. In this study, the retention of model solutes is measured directly along the chromatographic column as a function of the local pressure. The model solutes, a homologous series of derivatized fatty acids, exhibit a significant increase in capacity factor ranging from +9.3% for n-C(10) to +24.4% for n-C(20) for inlet pressures from 1500 to 5000 psi. These experimental results are compared with a thermodynamic model derived from regular solution theory. This model suggests that state effects alone are not sufficient to describe the measured change in solute retention and that variations in interaction energy with density must also be considered. By using the simple relationship of van der Waals for the interaction energy (E ∝ 1/V), the change in capacity factor with density is slightly underestimated. However, by using an extended relationship that better describes polar fluids (E ∝ 1/V(2)), good agreement is observed. Finally, the correlation of experimental results with this thermodynamic model reveals that all components in the chromatographic system, including the solute, mobile phase, and stationary phase, must be considered compressible. The results of this study have clear implications for the determination of fundamental physicochemical parameters, as well as for the everyday practice of liquid chromatography.

Pressure-Dependent Retention and Selectivity in Reversed-Phase Liquid Chromatographic Separations Using β-Cyclodextrin Stationary Phases

The influence of pressure on solute retention in liquid chromatography is commonly ignored due to the small compressibility of polar mobile phases. However, the equilibrium processes driving solute retention may be significantly affected by pressure, even under the modest conditions commonly encountered in HPLC (<350 bar). In this paper, we examine the role of pressure in separations where the primary mechanism for solute retention is inclusion complexation. Using the positional isomers of nitrophenol as model solutes, pressure-induced decreases in solute capacity factor ranging from −2.1% to −35.1% are observed experimentally for pressures from 40 to 340 bar. Individual contributions of pressure-induced solute ionization and complexation to this pressure-dependent solute retention are isolated by controlling mobile-phase pH. Pressure-induced dissociation of the cyclodextrin−solute complex appears to play the primary role in determining the pressure dependence of solute retention. Experimentally observed selective perturbation in solute retention with pressure is shown to have a direct impact on chromatographic resolution. In addition, the magnitude of the pressure-induced decrease in solute retention is demonstrated to be a function of the mobile-phase solvent strength. This previously underappreciated pressure effect has clear implications for the practical application of cyclodextrin stationary phases, as well as for the fundamental interpretation of those thermodynamic parameters central to the separation process.

Influence of pressure on solute retention in liquid chromatography

Confirming recent, independent experimental results, simple considerations of thermodynamics show that the logarithm of the retention factors of solutes vary linearly with increasing pressure, in the pressure range most commonly used (0–200 atm). The coefficient of this dependence increases linearly with the difference between the partial molar volumes of the solute in the stationary and the mobile phases. For homologues, this coefficient increases linearly with the number of carbon atoms and with the difference, Δ V CH 2 , between the partial molar volumes of a CH 2 group in the two phases. The pressure dependence of the retention factor may be significant because for a C 18—methanol system Δ V CH 2 / V CH 2 is of the order of 6%.

Calculation and Experimental Verification of Solute Retention in Liquid Chromatography Using Binary Eluents

Abstract The chromatographic transport problem is a kinematic wave problem that generally leads to a set of coupled, nonlinear partial differential equations. In the linear elution mode of chromatography an approximation can be made which allows linearization of the set of equations and hence solution by the matrix eigenvalue decomposition method. To make the solution explicit, an expression for a multicomponent distribution isotherm is required. Generally, Langmuir isotherms are used for this purpose. In this work a recently published multicomponent isotherm, that can be derived from first principles, is applied to the problem. For the case of binary eluents the capacity factors and peak compositions of both system peaks and analyte peaks are calculated using the elution mode approximation and the isotherm referred to above. Experimental results pertain to the ion-exchange separation of alkali ions. A good agreement between calculated and measured quantities is observed. An important consequence of interactions among sample and eluent constituents for the practice of chromatography lies in the field of indirect detection. The possibility of universal detection in LC is briefly discussed.

Influence of pressure on solute retention in liquid chromatography

AbstractAlthough the mobile phase is generally considered to be incompressible in liquid chromatography, the local pressure appears to have a significant effect upon solute retention. In this investigation, laser fluorescence detection was utilized to measure solute retention directly along a reversed‐phase packed capillary column with a methanol mobile phase. For derivatized fatty acids, n‐C10:0 to n‐C20:0, the capacity factors increased systematically by +9.3% to + 24.4% under normal operating pressure conditions (1,500 to 5,000 psi). Whereas the magnitude of this increase in solute retention seems surprising, it appears to correlate well with the unified theoretical model developed by Martire.

Methanol-water association and its effect on solute retention in liquid chromatography

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMethanol-water association and its effect on solute retention in liquid chromatographyE. D. Katz, C. H. Lochmuller, and R. P. W. ScottCite this: Anal. Chem. 1989, 61, 4, 349–355Publication Date (Print):February 15, 1989Publication History Published online1 May 2002Published inissue 15 February 1989https://doi.org/10.1021/ac00179a013RIGHTS & PERMISSIONSArticle Views524Altmetric-Citations84LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (686 KB) Get e-Alerts Get e-Alerts

Solvent modulation: An accurately modeled method for controlling solute retention in liquid chromatography

AbstractAn innovative solvent delivery method termed solvent modulation has been developed to control solute retention in liquid chromatography. Solvent modulation is the technique whereby individual solvent zones are introduced onto the chromatographic column in a known random orrepeating sequence. Because the solvent zones are of constant composition and are spatially separated from one another, solute retention is controlled independently in each zone. Hence, the overall retention of a solute is a time‐weighted average of the capacity factors in the solvent zones it has encountered. Solvent modulation offers a simple, versatile, and accurately modeled means to control and predict solute retention in liquid chromatography.

Description of Non-Specific and Specific Solute-Solvent Interactions in Liquid Chromatography with Mixed Eluents

Abstract A new description of solute retention in liquid chromatography with mixed eluents is proposed. This description involves non-specific solute-solvent and solvent-solvent interactions in the mobile and surface phases as well as association equilibria occuring in these phases. The general equation derived for the distribution coefficient is discussed with respect to the displacement and partition mechanisms of solute distribution between the mobile and surface phases.

Models of solute and solvent distribution for describing retention in liquid chromatography

Models of solute and solvent distribution between the mobile phase and surface-influenced stationary phase are presented and defined in terms of classical thermodynamics. Combination of these models makes possible formulation of models describing solute retention in liquid chromatography. This approach enables the possibility for classification of the majority of known models of solute retention and reveals, in many cases, their thermodynamic foundations.