J.R. Torres-Lapasió

Modelling of retention behaviour of solutes in micellar liquid chromatography

In micellar liquid chromatography (MLC), the resolution for a given multi-component mixture can be optimized by changing several variables, such as the concentrations of surfactant and organic modifier, the pH and temperature. However, this advantage can only be fully exploited with the development of mathematical models that describe the retention and the separation mechanisms. Several reports have appeared recently on the possibilities of accurately predicting the solute retention in MLC. Although the retention and selectivity may strongly change with varying concentrations of surfactant, organic modifier and/or pH, the observed changes are very regular, and are well described by simple models. This characteristic enables a successful prediction of retention times and compensates the negative effect of the broad and tailed chromatographic peaks obtained for some solutes when micellar eluents are used. An overview of the models proposed in the literature to describe the retention behaviour in pure micellar eluents and micellar eluents containing an organic modifier, at a fixed pH or at varying pH, is given. The equations derived permit the evaluation of the strength of micelle-solute and stationary phase-solute interactions. The prediction of the retention based on molecular properties and the use of neural networks, together with the factors affecting the prediction capability of the models (linearization of the equations, dead time, critical micellar concentration, ionic strength and temperature) are commented on. The strategies used for the optimization of resolution are also given.

Retention mechanisms in micellar liquid chromatography

Micellar liquid chromatography (MLC) is a reversed-phase liquid chromatographic (RPLC) mode with mobile phases containing a surfactant (ionic or non-ionic) above its critical micellar concentration (CMC). In these conditions, the stationary phase is modified with an approximately constant amount of surfactant monomers, and the solubilising capability of the mobile phase is altered by the presence of micelles, giving rise to diverse interactions (hydrophobic, ionic and steric) with major implications in retention and selectivity. From its beginnings in 1980, the technique has evolved up to becoming a real alternative in some instances (and a complement in others) to classical RPLC with hydro-organic mixtures, owing to its peculiar features and unique advantages. This review is aimed to describe the retention mechanisms (i.e. solute interactions with both stationary and mobile phases) in an MLC system, revealed in diverse reports where the retention behaviour of solutes of different nature (ionic or neutral exhibiting a wide range of polarities) has been studied in a variety of conditions (with ionic and non-ionic surfactants, added salt and organic solvent, and varying pH). The theory is supported by several mechanistic models that describe satisfactorily the retention behaviour, and allow the measurement of the strength of solute-stationary phase and solute-micelle interactions. Suppression of silanol activity, steric effects in the packing pores, anti-binding behaviour, retention of ionisable compounds, compensating effect on polarity differences among solutes, and the contribution of the solvation parameter model to elucidate the interactions in MLC, are commented.

Modelling of the retention behaviour of solutes in micellar liquid chromatography with organic modifiers

Abstract Most of the reported procedures for the determination of compounds by micellar liquid chromatography make use of micellar mobile phases containing an alcohol. The retention of a solute in a purely micellar eluent has been adequately described by the linear equation 1/ k′ vs . micelle concentration. This equation seems also to be valid for mobile phases with the same alcohol concentration and varying micelle concentrations. A model to describe the retention behaviour of solutes in any mobile phase of surfactant and alcohol is proposed, which makes use of the elution data in five mobile phases of surfactant with different amounts of alcohol. A function of the type 1/ k′ = A μ + B ϕ + C μϕ + D , where μ and ϕ are surfactant and alcohol concentration, respectively, proved to be satisfactory for different solutes (catecholamines, amino acids, phenols and other aromatic compounds).

Prediction of the retention in reversed-phase liquid chromatography using solute–mobile phase–stationary phase polarity parameters

A previously reported algorithm, based on the equation: log k = (log k)o + p(PN(m) - PN(s)), that relates the retention in reversed-phase liquid chromatography with solute (p), mobile phase (PN(m)) and stationary phase (PN(s)) relative polarity parameters, is improved. The retention data reported by several authors for different sets of compounds, eluted with acetonitrile-water and methanol-water mixtures, are used to test the algorithm and elaborate a database of p values. The methodology is successfully applied to predict the retention using PN(m), values calculated as PN(m) = 1.00 - (2.13phi)/(1+1.4phi) for acetonitrile-water and PN(m) = 1.00 - (1.33phi)/(l1 + 0.47phi) for methanol-water, phi being the organic solvent volumetric fraction. The polarity parameters are demonstrated to be useful to transfer retention data between solvent systems and between columns. Accordingly, the retention in a solvent system is predicted by characterising the working column with a small training set of compounds having diverse polarities, and using the p values known for another solvent system or column. The p polarity parameter is found to be a good descriptor of the retention, allowing the prediction of the expected elution order and peak overlaps.

Description of the partitioning behaviour of solutes and data treatment in micellar liquid chromatography with modifiers

The physico-chemical meaning of an empirical model, previously reported for describing the retention of solutes in reversed-phase liquid chromatography with micellar mobile phases of surfactant and alcohol, is given. The partition coefficient between stationary phase and water, and the solute-micelle association constant are considered as conditional parameters that depend on the concentration of modifier. An extension of the elution model is proposed for solutes having a strong hydrophobic character, that takes the strong interaction of these solutes with the stationary phase into account. Several fitting procedures (non-linear regression, and non-weighted and weighted linear regression) of the experimental data to the model are compared. The effect of errors in the value of the critical micellar concentration during the calculation of the retention physico-chemical parameters and prediction of the capacity factors, is also studied.

Global treatment of chromatographic data with MICHROM

Abstract The program MICHROM for the general treatment of chromatographic data is presented. MICHROM takes part in all the stages of the analytical process. It allows determination of dead time, smoothing of chromatograms, measurement of peak parameters, fitting of skewed peaks, and deconvolution of overlapped peaks. Tools for the experimental design, optimization of the mobile phase composition to resolve a mixture of analytes, and simulation of chromatograms in several experimental conditions, are implemented. Routines for the graphical representation of chromatograms, resolution surfaces, contour maps, management of data series, optimization and regression analysis, are also included. The resolution of chromatographic problems of diverse nature and complexity is possible.

Interpretive optimisation strategy applied to the isocratic separation of phenols by reversed-phase liquid chromatography with acetonitrile–water and methanol–water mobile phases

An optimisation protocol is presented for the resolution of complex mixtures in isocratic RPLC with binary mobile phases of organic solvent and water, which is based on the prediction of peak position and shape of the individual compounds. A good description of the retention was achieved through the application of statistical weights to the widely used linear or quadratic relationships between the logarithm of the retention factor (log k) and the organic solvent concentration in the mobile phase. The maximisation of the product of peak purities for each compound is shown as a competitive resolution strategy versus the worst value of a selectivity parameter. Peak purities allow one to associate a single resolution value to each compound, which is not affected by the identity of the interfering peaks. It is shown how when full resolution is not achieved with a single mobile phase, the same experimental data set (retention factors, asymmetries and efficiencies) can be used for finding two or three optimal complementary mobile phases (CMPs). Each CMP resolves fully some compounds in the mixture, while the remaining compounds can overlap among them. The elementary limiting resolutions, which measure the maximal separation degree for each compound, are also given as a useful guide in the selection of the elution conditions. A mixture of 13 phenols (phenol, chloro-, bromo-, nitro- and methyl-derivatives), eluted with acetonitrile-water or methanol-water mobile phases, is used to show the proposed methodology.

Retention Mechanisms for Basic Drugs in the Submicellar and Micellar Reversed-Phase Liquid Chromatographic Modes

The reversed-phase liquid chromatographic (RPLC) behavior (retention, elution strength, selectivity, efficiency, and peak asymmetry) for a group of basic drugs (beta-blockers), with mobile phases containing the anionic surfactant sodium dodecyl sulfate (SDS) and acetonitrile, revealed different separation environments, depending on the concentrations of both modifiers: hydro-organic, submicellar at low surfactant concentration and high concentration of organic solvent, micellar, and submicellar at high concentration of both surfactant and organic solvent. In the surfactant-mediated modes, the anionic surfactant layer adsorbed on the stationary phase interacts strongly with the positively charged basic drugs increasing the retention and masks the silanol groups that are the origin of the poor efficiencies and tailing peaks in hydro-organic RPLC with conventional columns. Also, the strong attraction between the cationic solutes and anionic SDS micelles or monomers in the mobile phase enhances the solubility and allows a direct transfer mechanism of the cationic solutes from micelles to the modified stationary phase, which has been extensively described for highly hydrophobic solutes.

Description of the retention behaviour in micellar liquid chromatography as a function of pH, surfactant and modifier concentration

Abstract Micellar liquid chromatography permits the elution of solutes of diverse polarity. One of the most outstanding advantages of the technique is its capability of predicting the retention with high accuracy, as a function of different experimental variables. The separation of a group of compounds is usually optimized by varying the concentrations of surfactant and modifier in the mobile phase. The pH is, however, for many solutes, a variable that should be considered in the description of their elution behaviour. A global model that takes into account, simultaneously, the concentrations of surfactant and modifier, and the pH as chromatographic variables, is proposed for ionizable solutes. The mean relative errors obtained in the prediction of the retention of seven compounds eluted with 81 mobile phases of sodium dodecyl sulphate (SDS) and 1-propanol, using equations fitted with the experimental data measured in 9 to 12 mobile phases was, usually, lower than 6%. The ranges of the variables considered were 0.05–0.15 M SDS, 0–0.08 (v/v) 1-propanol and pH 3–7.

Resolution assessment and performance of several organic modifiers in hybrid micellar liquid chromatography

The performance of four criteria that measure the elementary resolution (modified selectivity, modified RS, peak purity, and orthogonal valley-to-peak ratio) was critically assessed using as global resolution function, the product of elementary measurements. The peak purities and valley-to-peak criteria yielded the best description of the overall separation according to the shape of the resolution surfaces compared to the peak arrangements in the chromatograms, the capability of defining unambiguously the composition regions of complete resolution, and the resolution achieved in the predicted optimums. Peak purities were used to compare the effect of five organic modifiers (1-propanol, 1-butanol, 1-pentanol, acetonitrile and tetrahydrofuran) on the separation quality of micellar mobile phases of sodium dodecyl sulphate (SDS). Acetonitrile, a common solvent in reversed should read reversed-phase liquid chromatography but scarcely used in micellar liquid chromatography, allowed the most satisfactory resolution in an extensive composition region with very small overlapping and sufficiently low retention times. The enhanced resolution was produced by the improved selectivity, and larger efficiencies and asymmetries of the chromatographic peaks.