Liquid chromatography at critical conditions in ternary mobile phases: Gradient elution along the critical line

In ternary mobile phases consisting of acetone, methanol, and water, the retention of PEG on reversed‐phase columns is independent on molar mass at certain compositions of the mobile phase. Along this critical adsorption line, the retention of polypropylene glycol varies quite strongly, which can be utilized in the separation of block copolymers. Gradient elution along the critical line allows a baseline separation of all oligomers in polypropylene glycol up to approximately 25 propylene oxide units. The same resolution can be achieved in the separation of ethylene oxide‐propylene oxide block copolymers, regardless of the length of the ethylene oxide block.     

Studies of Association Effects in Liquid Adsorption Chromatography with Ternary Mobile Phases

Abstract

In this paper a model of chromatographic process, which takes into account molecular association in the mobile phase is discussed. This model is applied to study the association effects in liquid adsorption chromatography with ternary mobile phases. Efects of nature of the eluent and structure of the chromatographed substance on the type of association is also investigated. Association effects in the chromatographic system containing ternary mobile phase can be evaluated by means of equations describing chromatographic process with the binary mobile phases.     

Prediction of Retention in Liquid-Solid Chromatography with Ternary Mobile Phases

The method for the prediction of capacity factors in ternary mobile phases is presented. The adsorption mechanism of retention is considered. The simple theoretical equations are proposed for mobile phases for which the ratio of mole fractions of the weaker solvents remains fixed. The relations between parameters characterizing retention in ternary and binary mobile phases are discussed. The theoretical model is verified for numerous solutes and different mobile phases.     

Adsorption model for retention in normal-phase liquid chromatography with ternary mobile phases

The review gives a link between the theory of adsorption from multicomponent solutions and liquid chromatography. The article surveys the methods developed to describe the retention in normal-phase chromatography with ternary mobile phases with emphasis on the results of the authors. In the model used the driving force for the separation is the difference in adsorption of a solute and all solvents onto the solid surface. The general equation generates a series of simple linear relationships to predict the retention factor in ternary mobile phase for which certain parameters remain fixed. Theoretical concepts are tested by comparison with experimental data. The correlations between parameters characterizing retention in ternary, binary and pure solvents are discussed.     

A study of the relationship between the thermodynamic quality of mobile phases and their strength for the critical conditions of liquid chromatography of polystyrenes

The relationship between the thermodynamic quality of mobile phases and their strength ε was studied for the critical conditions of liquid chromatography. The chromatography of reference polystyrene samples was carried out in the adsorption and exclusion modes, and also under critical conditions, with binary mobile phases and a silica gel column used. The thermodynamic quality of solvent mixtures with respect to one another was evaluated by the intrinsic viscosity [η].     

Analysis of polyethyleneoxide macromonomers by liquid chromatography along the critical adsorption line

Polyoxyethylene macromonomers are analyzed by one-dimensional liquid chromatography under different conditions, depending on the required information. These samples may contain polyethylene glycol (PEG) and the corresponding di(meth)acrylate besides the desired mono(meth)acrylate. The molar mass distribution (MMD) of the PEG and the monoester can be obtained by liquid adsorption chromatography (LAC) on a reversed-phase column in acetone–water with a gradient from 10% to 20% acetone. The MMD of the diesters can be obtained with isocratic elution by liquid chromatography at critical conditions (LCCC) on a reversed-phase column in 31% acetone, or using size-exclusion conditions for PEG and LAC conditions for the end groups, which is the case in 40–55% acetone. The absolute amount of the series with different functionality can be obtained by LCCC in ternary mobile phases consisting of acetone, methanol, and water along the critical adsorption line. Under such conditions, all series elute as narrow peaks (regardless their MMD), which can easily be integrated and quantified.     

Obtimization of separation conditions in reversed phase liquid chromatography using complex solvent mixtures

Summary Four different methods of calculation of retention in ternary mobile phases were compared and it was found that the simple calculation based only on two values of the capacity factors (one for each binary system that composes the ternary mobile phase) provides the accuracy of prediction that is, at least, comparable to the other methods of calculation that require a large number of preliminary experiments. The deviations from ideal behaviour in ternary solvent mixtures are discussed; some sources of errors can be avoided, at least partially, using binary systems of adequate compositions for preparation of ternary mobile phases. Several examples show the comparison of calculated and experimental selectivities in ternary solvent systems. A simple calculation can be used for rapid selection of an adequate ternary (or more complex) mobile phase, if it is necessary to achieve the separation of a given sample mixture.Presented at the 14th International Symposium on Chromatography London, September, 1982     

Liquid chromatography of polyoxyethylenes under critical conditions: a thermodynamic study

Polyethylene glycols (PEG), fatty alcohol ethoxylates (FAE) and fatty acid polyglycol esters were analyzed by liquid chromatography under critical conditions (LCCC) on typical reversed phase columns in mobile phases consisting of methanol-water and acetone-water in the region of the critical adsorption point (CAP) for the oxyethylene unit. Critical conditions can be adjusted by varying the mobile phase composition or temperature. In methanol-water the temperature dependence is almost negligible, while it is much more pronounced in acetone-water. Critical conditions for the oxyethylene unit were realized on octadecyl phases in methanol-water containing 85.8 wt.% methanol and in acetone-water at 89.6 wt.% acetone. In the van't Hoff plots the logarithm of the distribution coefficient was plotted versus the reciprocal temperature. In all plots straight lines were found, from slope and intercept of which the entropy and enthalpy changes were calculated. The relation of the thermodynamic parameters -deltaG/RT, -deltaH/RT, and deltaS/R to the numbers of A and B chain units in block copolymers AB and BAB at the CAP for A is studied both theoretically and experimentally. Various approaches for obtaining the critical temperature are discussed.     

Polydispersity Effect on the Adsorption of Polymer Mixtures

The changes in molecular mass distribution (MMD) after adsorption from solution in concentration regions below and above the critical concentration of overlapping have been studied for various polymers and their mixtures. It was found that the formation of entanglements in both binary and ternary solutions affects the changes in MMD. For binary and ternary solutions above the critical concentration of overlapping, C*, the entanglements lead to diminishing selectivity of adsorption of high-molecular-weight fractions. Simultaneously, transition through C* diminishes adsorption of the fractions of higher surface activity. It may be assumed that the entanglements play the role of an additional energetic barrier which macromolecules must overcome to be adsorbed. The transition through the critical concentration of overlapping for the case of binary solutions diminishes the selectivity of adsorption of high molecular fractions. Adsorption of polar polymers from the ternary solutions both below and above C* leads to the appearance of the bimodality of MMD due to adsorption of surface-active fractions. Copyright 2000 Academic Press.     

Binary mobile phases for the analysis of poly(methyl methacrylates) via thin-layer chromatography

Tetrahydrofuran-containing binary mobile phases used for the thin-layer chromatography study of poly(methyl methacrylates) are comprehensively investigated. The peculiarities of the transition from the adsorption regime to the exclusion regime through the conditions critical for PMMA are studied in a wide range of molecular masses at different macromolecule-to-pore size ratios. Binary phase compositions corresponding to the critical conditions are determined, and chromatographic results are compared with viscometry data. In the adsorption region near the critical conditions, the dependence of the retention parameter on the molar fraction of tetrahydrofuran in the mixtures is approximated by a linear function. The slope of this dependence is shown to grow with both the molecular mass of the polymer and the thermodynamic quality of the mixed solvent.     

Thin-layer adsorption chromatography with mixed mobile phases. 3. Correlation between parameters characterizing the chromatographic systems with binary and ternary mobile phases

TLC experimental data have been measured for polycyclic hydrocarbons in binary and ternary mobile phases on silica gel. These data have been interpretated using the equation discussed by us in Part 1 of this series [HRC & CC 2 (1979) 236]. The heterogeneity parameter appearing in this equation has been evaluated and compared for different ternary and binary mobile phases.     

Adsorption-driven retention in normal-phase chromatography with ternary mobile phases

The model for retention in chromatographic systems using ternary mobile phases is considered. The driving force for the separation is assumed to be only a difference in adsorptive properties of a solute and all solvents. The effects of mobile phase composition are discussed. A series of very simple dependences between parameters characterizing retention in ternary and binary solvents is presented. The linear relationship is proposed to predict the capacity factor in the ternary mobile phase, for which the ratio of mole fractions of two less-polar solvents remains fixed. Theoretical concepts are verified by comparing the calculated values with experimental data measured in the entire concentration region.     

Retention in liquid chromatography with multicomponent mobile phases: Relationship with binary systems

Summary Using the concept of an effective concentration a method is proposed for predicting retention in liquid chromatography with multicomponent mobile phases based on experiments with corresponding binary ones. The method is verified for normal and reversed-phase systems with ternary mobile phases and agrees closely with experimental data.     

Optimisation of multilinear gradient elutions in reversed-phase liquid chromatography using ternary solvent mixtures

The multilinear gradient elution theory for binary mobile phases in reversed-phase liquid chromatography presented in [P. Nikitas, A. Pappa-Louisi, A. Papageorgiou, J. Chromatogr. A 1157 (2007) 178] is extended to ternary gradients. For the evaluation of this theory and the performance of the various fitting and optimisation algorithms we used 13 o-phthalaldehyde (OPA) derivatives of amino acids with mobile phases modified by acetonitrile and methanol. It is shown that the theory can lead to high quality predictions of the retention times under gradients elutions and optimisation of ternary gradients provided that we use a six-parameter expression for the logarithm of the retention factor, lnk, and the adjustable parameters of this expression are determined from ternary isocratic data.     

Adsorption isotherms and retention behavior of 1,1'-bis(2-naphthol) on CHIRIS AD1 and CHIRIS AD2 columns

The separation of the atropoisomers of 1,1'-bis(2-naphthol) was studied on CHIRIS AD1 and CHIRIS AD2, two Pirkle-type chiral stationary phases. Satisfactory selectivity was found only on CHIRIS AD2. The ternary mobile phases comprised hexane, dichloromethane and methanol. The effects of their composition and of the temperature on the retention under analytical conditions and on the single-component and competitive isotherms were investigated. The retention of the R- and S-isomers on CHIRIS AD1 and CHIRIS AD2 is controlled by the enthalpic contribution to adsorption, but the effect of the mobile phase on the retention should be attributed mainly to the entropic contribution. The adsorption of the less retained R-isomer is controlled by the achiral interactions, which are the same as for the S-isomer. The single-component and competitive isotherms of the R- and S-isomers are adequately described by the sum of a Langmuir term for the achiral contribution to adsorption and a linear-term characterising the selective or chiral adsorption of the S-isomer in the concentration range experimentally available, i.e. within the solubility limit of 1,1'-bis(2-naphthol).     

Correlation between excess adsorption data measured for solvent mixture/adsorbent systems and RM values obtained for different solutes chromatographed in binary mobile phases

This paper concerns the application of excess adsorption isotherms, measured for solvent mixture/adsorbent systems, to the characterization of TLC data. For this purpose the excess adsorption isotherms for three liquid mixtures: cyclohexane/ benzene, benzene/acetone, and carbon tetrachloride/ethyl acetate on silica gel at 20°C have been measured. These mixtures have been used as binary mobile phases in TLC measurements. It has been shown for a given solute in binary mobile phase that the quantity R_M is a simple function of the excess adsorption. Parameters of this function have been used to characterize chromatographic systems with binary mobile phases.     

Screening of macrocycles retention for microplanar analytical devices involving host-guest interactions and silica or octadecylsilica adsorbents

This paper reports planar chromatographic behavior and multivariate characterization of three types of macrocyclic compounds: cyclodextrins, calixarenes and macrocyclic antibiotics. Additionally two non-macrocyclic chemicals (estriol and pyrene) were analyzed as the reference retention markers. Target compounds were chromatographed using normal and reversed-phase systems and involving mono as well as binary water-organic liquids mobile phases based on methanol, ethanol and acetonitrile. Non-forced flow TLC/HPTLC elution was performed under isothermal (elevated temperature?=?303?K) and mobile phase vapor saturated conditions using 10?cm long glass plates positioned vertically inside temperature controlled Dewar type chambers. It is hoped that presented retention data, which were collected for the whole range of binary mobile phases compositions (0–100%) may be useful for designing of hybrid active layers for extraction, separation and detection systems involving host-guest interactions. Retention profiles were inspected using multivariate statistics (principal component analysis, PCA). Based on reported data various extraction and separation systems can be created where weak or strong interactions of macrocycles with chromatographic stationary phases or given micro device active supports should be expected. Particularly, studied macrocycles can be applied as selective additives for solid phase adsorption (SPE, SPME), microfluidic paper devices (μPAD) as well as micro-total analytical systems (μTAS), where bars formed active layers are fabricated.     

Chromatographic study of adsorption from binary and ternary solutions

Adsorption of benzene, anisole, and nitrobenzene on hydroxylated silica gel from binary and ternary solutions (adsorbate--n-hexane and adsorbate--n-hexane--terahydrofuran) was studied by HPLC. The equation that describes the adsorbate retention as a function of its concentration in binary and ternary solutions was proposed. The equation makes it possible to calculate the adsorption isotherms and adsorption equilibrium constants directly from chromatograms using the retention parameters and equilibrium concentrations of the adsorbate in the mobile phase.     

Retention prediction in ternary solvent reversed-phase liquid chromatography systems based on the variation of retention with binary mobile phase composition

An extension of the treatment adopted in a recent paper [P. Nikitas, A. Pappa-Louisi, P. Agrafiotou, J. Chromatogr. A 946 (2002) 33] was used to derive expressions describing the variation of solute retention k with composition in ternary reversed phase liquid chromatography, RP-LC, solvent systems. The equation of the partition model obtained in this way for a ternary mobile phase was identical to that previously derived using the solubility parameter concept. This equation as well as two new expressions of In k versus organic modifiers content were tested in a variety of ternary solvent systems in order to examine the possibility of predicting retention behavior of solutes under ternary solvent mixture elution conditions from known retention characteristics in binary mobile phases. It was demonstrated the superiority of both new equations derived in this paper to that previously proposed and applied to date in ternary solvent mixtures.     

Adsorption from binary solvent mixtures onto silica gel by HPLC frontal analysis

Summary The chromatographic technique of frontal analysis is applied to measuring adsorption from binary liquid mixtures by silica gel. The complete adsorption isotherm of a solvent mixture is obtained by measuring the break-through curves for a series of small concentration steps of the mobile phase. This method offers a direct way to determine the composition of the stationary phase in liquid-solid chromatography with mixed mobile phases. The surface excess isotherms of all binary systems formed by benzene, cyclohexane, and 1,2-dichloroethane, at the solution-silica gel interface at 25 °C are presented. The data of the three systems are shown to be thermodynamically mutually consistent.