Retention In Micellar Liquid Chromatography Research Articles

Application of micellar liquid chromatography to model ecotoxicity of pesticides. Comparison with immobilized artificial membrane chromatography and n-octanol-water partitioning

The potential of Micellar Liquid Chromatography (MLC) to model ecotoxicological endpoints for a series of pesticides was investigated. To exploit the flexibility in MLC conditions, different surfactants were employed and retention mechanism was tracked and compared to Immobilized Artificial Membrane (IAM) chromatographic retention and n-octanol- water partitioning, logP. Neutral polyoxyethylene (23) lauryl ether (Brij-35), anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethylammonium bromide (CTAB) were used in presence of PBS at pH=7.40 and acetonitrile as organic modifier when necessary. Similarities/ dissimilarities between MLC retention and IAM or logP were investigated by Principal Component Analysis (PCA) and Liner Solvation Energy Relationships (LSER). LSER revealed that hydrogen bonding acidity is the most important factor for differentiation between MLC and IAM or logP. The impact of hydrogen bonding is exemplified in the relationships of MLC retention factors with IAM or logP, which necessitate the inclusion of a relevant descriptor. PCA further revealed that MLC retention factors are clustered together with IAM indices and logP within a broader ellipse formed by ecotoxicological endpoints, involving LC50/ EC50 values of six aquatic organisms namely Rainbow Trout, Fathead Minnow, Bluegill Sunfish, Sheepshead Minnow, Eastern Oyster and Water Flea as well as LD50 values of Honey Bee, thus justifying their use to construct relevant models. Satisfactory specific models for individual organisms, as well as general fish models, were obtained, in most cases, upon combination of MLC retention factors with Molecular Weight (MW) or/ and hydrogen bond parameters. All models were evaluated and compared to previously reported IAM and logP based models using an external validation data set. Predictions with Brij-35 and SDS based models were comparable, although slightly inferior than those obtained with IAM, while they were in all cases better than those obtained with logP. CTAB led to a satisfactory prediction model for Honey Bee, but it was found less suitable for aquatic organisms.

Retention and Partition Behaviors of Solutes in a Surfactant-Based Mobile Phase at Concentrations Approaching the Critical Micelle Concentration in Liquid Chromatography

Previously developed three-phase retention model has been extensively used to understand the partition behaviors of solutes in micellar liquid chromatography (MLC). The model has been used to explain the linear plots of the reciprocal of the solute retention factor versus the micelle concentration in the mobile phase. Because a phase equilibrium model of micellization is assumed, the retention model is generally applied to systems at surfactant concentrations above the critical micelle concentration (CMC). This study investigated the retention behaviors of five solutes, namely acetone, benzaldehyde, acetophenone, benzene, and benzyl alcohol, as a function of the concentration of dodecyltrimethylammonium bromide (DTAB) in water on a C18 sorbent. Distinct retention behaviors were observed for DTAB concentrations below and above the CMC, which have not been adequately explained by the retention models described in the literature. Thus, a new MLC retention model based on a mass action model of micellization was developed herein. Retention and partition behaviors were found to be determined by three key dimensionless groups, representing the partition coefficients of the solutes between the DTAB-uncomplexed aqueous phase and the stationary, DTAB-complexed aqueous, and micellar phases. Thermodynamic equilibrium constants were estimated from fitting the model to the retention factor data. The new model showed a good fit to the HPLC data below and above the CMC and was reliable for describing the transition of distinct retention behaviors near the CMC.

Performance comparison of nonlinear and linear regression algorithms coupled with different attribute selection methods for quantitative structure - retention relationships modelling in micellar liquid chromatography

In micellar liquid chromatography (MLC), the addition of a surfactant to the mobile phase in excess is accompanied by an alteration of its solubilising capacity and a change in the stationary phase's properties. As an implication, the prediction of the analytes’ retention in MLC mode becomes a challenging task. Mixed Quantitative Structure – Retention Relationships (QSRR) modelling represents a powerful tool for estimating the analytes’ retention.This study compares 48 successfully developed mixed QSRR models with respect to their ability to predict retention of aripiprazole and its five impurities from molecular structures and factors that describe the Brij - acetonitrile system. The development of the models was based on an automatic combining of six attribute (feature) selection methods with eight predictive algorithms and the optimization of hyper-parameters. The feature selection methods included Principal Component Analysis (PCA), Non-negative Matrix Factorization (NMF), ReliefF, Multiple Linear Regression (MLR), Mutual Info and F-Regression. The series of investigated predictive algorithms comprised Linear Regressions (LR), Ridge Regression, Lasso Regression, Artificial Neural Networks (ANN), Support Vector Regression (SVR), Random Forest (RF), Gradient Boosted Trees (GBT) and K-Nearest neighbourhood (k-NN).A sufficient amount of data for building the model (78 cases in total) was provided by conducting 13 experiments for each of the 6 analytes and collecting the target responses afterwards. Different experimental settings were established by varying the values of the concentration of Brij L23, pH of the aqueous phase and acetonitrile content in the mobile phase according to the Box-Behnken design. In addition to the chromatographic parameters, the pool of independent variables was expanded by 27 molecular descriptors from all major groups (physicochemical, quantum chemical, topological and spatial structural descriptors). The best model was chosen by taking into consideration the Root Mean Square Error (RMSE) and cross-validation (CV) correlation coefficient (Q2) values.Interestingly, the comparative analysis indicated that a change in the set of input variables had a minor impact on the performance of the final models. On the other hand, different regression algorithms showed great diversity in the ability to learn patterns conserved in the data. In this regard, testing many regression algorithms is necessary in order to find the most suitable technique for model building. In the specific case, GBT-based models have demonstrated the best ability to predict the retention factor in the MLC mode. Steric factors and dipole-dipole interactions have proven to be relevant to the observed retention behaviour. This study, although being of a smaller scale, is a most promising starting point for comprehensive MLC retention prediction.

Micellar and sub-micellar ultra-high performance liquid chromatography of hydroxybenzoic acid and phthalic acid positional isomers

Micellar liquid chromatography (MLC) has been used primarily for the separation of neutral analytes of varying polarities, most commonly phenols and polyaromatic hydrocarbons, but does not seem to have been used to study aromatic hydroxy acids in detail. We have studied the separation of hydroxybenzoic acid mixtures, including monohydroxybenzoic and dihydroxybenzoic acid positional isomers by MLC. Sodium dodecylsulfate (SDS) is investigated as the modifying surfactant on a C18 ultra-high performance liquid chromatography (UHPLC) column (100×2.1mm, 1.8μm). The addition of only SDS (no organic solvent) to the mobile phase reduced the influence of hydrophobic interactions while improving the retention times, resolution, and peak shapes, even at concentrations below the critical micellization concentration (CMC). The UHPLC separation of 7 hydroxybenzoic acids, including 6 dihydroxybenzoic acid positional isomers and one trihydroxybenzoic acid, is achieved with high efficiency using 0.1% SDS in 1.84mM sulfuric acid (pH 2.43) mobile phase, in less than 6min with a flow rate of 0.3mLmin−1, and in less than four min with a flow rate of 0.7mLmin−1. Six monohydroxybenzoic acid isomers are also effectively separated by MLC, using a 0.5% SDS mobile phase modifier, in less than 20min with a flow rate of 0.3mLmin−1, and in less than 14min with a flow rate of 0.7mLmin−1. The 3 phthalic acid isomers could be separated using a similar mobile phase and flow rates in less than 6 and 4min. Solute-micelle equilibrium constants and partition coefficients are calculated for 6 monohydroxybenzoic acids based on a plot of MLC retention factor vs. mobile phase micelle concentration. All aromatic acid isomers studied can be classified as binding solutes in the MLC retention mechanism. Less effective separations are observed with shorter chain surfactants, leading to higher retention times and poor peak shapes. It is concluded that increasing chain length led to more efficient MLC separations, and SDS is the preferred modifying surfactant for the examined separation.

COSMO-RS for the prediction of the retention behavior in micellar liquid chromatography based on partition coefficients of non-dissociated and dissociated solutes

Several methods for the description of the retention behavior in micellar liquid chromatography (MLC) were described previously. Thereby, the most common are the linear solvation energy relationships (LSER). However, for the evaluation of the LSER, a number of experimental data are necessary. In this work, the retention data are predicted based on a single data point for a given solute/surfactant combination. The prediction of micelle/water partition coefficients with the COSMO-RS model was evaluated for the nonionic surfactant Brij 35 and the cationic CTAB for 21 solutes. The predicted partition coefficients along with the retention data of a single measurement were combined with a common retention model for the evaluation of partition coefficients. Thus, the description of the retention data based on a minimum of experimental data was realized in this work. The COSMO-RS model appears to be suitable for the flexible and qualitative prediction of the retention data, which is of special benefit, when a large number of solutes are studied. Furthermore, the introduced method is a promising alternative to describe the retention behavior of ionizable solutes, which is of great interest for e.g. pharmaceutical processes and analyses. To evaluate the method, lipophilicity profiles of several solutes were determined with MLC. While for the nonionic surfactant the lipophilicity is similar to the octanol/water system, a contrary phenomenon, namely a significant increase of the partition coefficient with progressive dissociation can be observed with the ionic surfactant. The MLC method proved to be an efficient method for the determination of micelle/water partition coefficients, excluding non-binding and “overbinding” solutes. The introduced COSMO-RS based approach for the estimation of the retention in MLC is especially promising for highly retarding solutes, since it is time and cost saving compared to the experimental determination.

Retention Behaviour in Micellar Liquid Chromatography

Retention in micellar liquid chromatography is highly reproducible and can be modelled using empirical or mechanistic models with great accuracy to predict the retention changes when the mobile phase composition varies (surfactant and organic solvent concentrations), thus facilitating the optimisation of separation conditions. In addition, the different equilibria inside the column among the solute, the mobile phase, and the modified stationary phase by monomers of surfactant have been exhaustively studied. In a sequential strategy, the retention of the solutes is not known a priori, and each set of mobile phases is designed by taking into account the retention observed with previous eluents. By contrast, in an interpretative strategy, the experiments are designed before the optimization process and used to fit a model that will allow the prediction of the retention of each solute. This strategy is more efficient and reliable. The sequential strategy will be inadequate when several local and/or secondary maxima exist, as frequently occurs in chromatography, and may not give the best maximum, that is to say, the optimum. More often than not, the complexity of the mixtures of compounds studied and the relevant modification of their chromatographic behaviour when changing the mobile phase composition requires the use of computer-assisted simulations in MLC to follow the modifications in the chromatograms in detail. These simulations can be done with sound reliability thanks to the use of chemometrics tools.

Optimization of Artificial Neural Networks for Modeling of Atorvastatin and Its Impurities Retention in Micellar Liquid Chromatography

Artificial Neural Networks (ANNs) present a powerful tool for the modeling of chromatographic retention. In this paper, the main objective was to use ANNs as a tool in modeling of atorvastatin and its impurities’ retention in a micellar liquid chromatography (MLC) protocol. Factors referred to MLC were evaluated through 30 experiments defined by the Central Composite Design. In this manner, 5–x–3 topology as a starting point for ANNs’ optimization was defined too. In the next step, in order to set the network with the best performance, network optimization was done. In the first part, the number of nodes in the hidden layer and the number of experimental data points in training set were simultaneously varied, and their importance was estimated with suitable statistical parameters. Furthermore, a series of training algorithms was applied to the current network. The Back Propagation, Conjugate Gradient-descent, Quick Propagation, Quasi-Newton, and Delta-bar-Delta algorithms were used to obtain the optimal network. Finally, the predictive ability of the optimized neural network was confirmed through several statistical tests. The obtained network showed high ability to predict chromatographic retention of atorvastatin and its impurities in MLC.

Study of Retention in Micellar Liquid Chromatography on a C18 Column on the Basis of Linear Solvation Energy Relationships

In this study, 8 solutes (aniline, caffeine, p-cresol, ethyl benzene, methylparaben, phenol, pyridine, and toluene) have been tested in terms of linear solvation energy relationships (LSER). Several micellar liquid chromatography (MLC) systems using cationic surfactant cetyltrimethylammonium bromide (CTAB) and a mixture of water with (methanol, n-propanol, and n-butanol) modifiers were characterized using the LSER solvation parameter model. The effects of the surfactant and modifier concentration on the retention in MLC were discussed. LSER model had demonstrated high potential to predict retention factors with high squared correlation coefficients (<TEX>$r^2$</TEX> > 0.99). A comparison of predicted and experimental retention factors suggests that LSER formalism is able to reproduce adequately the experimental retention factors of the solutes studied in the different experimental conditions investigated. This model is a helpful tool to understand the solute-surfactant interactions and evaluate the retention characteristic of micellar liquid chromatography.

Study of retention in micellar liquid chromatography on a C 8 column by the use of linear solvation energy relationships

Linear solvation energy relationships (LSERs) are used to investigate the fundamental chemical interactions governing the micellar liquid chromatographic retention of 22 aromatic compounds (11 benzene derivatives and 11 aromatic polycyclic hydrocarbons) in 80 mobile phases on a C 8 column. The systems studied involve combinations of 0.050 to 0.140 M sodium dodecyl sulfate or cetyltrimethylammonium bromide, with 0 to 10% methanol, n-propanol, and n-butanol as mobile phase modifiers. The ability of the LSERs to account for the chemical interactions underlying solute retention is shown. A comparison of predicted and experimental retention factors suggests that LSER formalism is able to reproduce adequately the experimental retention factors of the solutes studied in the different experimental conditions investigated.

Quantitation of hydrophobicity in micellar liquid chromatography

Micellar liquid chromatography (MLC) is shown to be a promising technique for measuring the hydrophobicity of solutes. The presence of micelles has a profound effect on the chromatographic characteristics of reversed-phase columns. The linear relationships between the logarithm, log k, of the retention factor and such diverse properties as the number of carbon atoms in homologous series, octanol–water partition coefficients and solvatochromic parameters, which are observed in conventional reversed-phase liquid chromatography (RPLC), are not usually valid in MLC. For series of compounds exhibiting a wide range of hydrophobicity, k itself is linearly related to these properties. The reasons for this behavior, and the advantages of MLC over conventional RPLC in the quantitation of hydrophobicity, are discussed. The values of k in mobile phases extrapolated to zero micelle concentration are shown to give a better measure of hydrophobicity than k with 100% water in conventional RPLC. The ability of MLC to characterize bioactive compounds is also discussed. A single MLC retention parameter can describe the bioactivity of phenols, while three conventional structural descriptors are needed to achieve a similar correlation. The correlation between the retention in MLC and the sites of action of diuretics within the nephron provides another interesting example of its mimetic value for physiological systems, where hydrophobic and electrostatic interactions, and kinetic phenomena are important.

Influence of the addition of modifiers on solute-micelle interaction in hybrid micellar liquid chromatography

In reversed-phase micellar liquid chromatography (MLC) organic modifiers are usually added to the mobile phase to modify the eluent strength and to increase the efficiency of the chromatographic peaks. The effect of the modifiers methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, acetonitrile and tetrahydrofuran on the critical micelle concentration (cmc) of the anionic surfactant sodium dodecylsulphate (SDS) has been studied to enable understanding of the interactions between solute, micelles and bulk liquid in such hybrid eluents. Methanol, acetonitrile and tetrahydrofuran increased thecmc, whereas the other alcohols reduced its value. For butanol and pentanol, which partition into the micelle, thecmc barely changed for alcohol concentrations above 4% and 1.5%, respectively. This behaviour indicated that the micelle was mainly modified at lower alcohol concentrations by introduction of the molecule into the micelle palisade. When the ratio (number of molecules of alcohol in the micelle)/(number of molecules of surfactant), was >3, new molecules of the alcohol probably dissolved in the micelle core. The dependence of the capacity factors of several solutes on the concentration of modifier was similar to that found for thecmc, implying that the effects that change thecmc values upon addition of alcohol to a surfactant solution are, at least partially, those that induce reduced retention in MLC—modification of the nature of bulk water and micelle. The change in retention observed with SDS mobile phases, when butanol and pentanol are added, are mainly a result of modification of the structure of the micelle.

Chromatographic quantification of hydrophobicity using micellar mobile phases

In micellar liquid chromatography (MLC), the hydrophobicity of a compound is the predominant effect on its retention and interaction with micelles; however, some contradictory results have been obtained concerning whether k or log k best correlates with the logarithm of partition coefficients (log P) in the biphasic solvent system octanol-water. An empirical model which describes the relationship between retention in MLC and log P is presented. The retention data for series of neutral compounds eluted with different pure and mixed mobile phases and alkyl-bonded stationary phases were used to test the model. The results indicate that non-linear relationships between k or log k and log P are to be expected and only in particular circumstances can linear relationships be obtained. In contrast, in all the series studied, excellent correlations between the logarithm of the retention factor at zero micellar concentration, k m, and log P, were found ( r 2 in the range 0.965–0.987 and F in the range 277–608). Log k m is proposed as the best chromatographic index for the quantification of the hydrophobicity of solutes using micellar mobile phases.

Correlation between hydrophobicity of amino acids and retention data in reversed-phase liquid chromatography with micellar eluents

Hydrophobic character is usually expressed in terms of the partition coefficient in 1-octanol-water (log PO/W). However, measurement of this coefficient is often problematic. Retention in micellar liquid chromatography is mainly due to hydrophobic interactions and can also be used as an index of hydrophobicity. A hydrophobicity scale was established with retention data foro-phthalaldehyde (OPA)-N-acetyl-L-cysteine (NAC) amino acid derivatives, using the glycine derivative as reference. Since the OPA-NAC derivatives only differ in the nature of R1 in the amino acid (R1CH(COOH)NH2), in the absence of electrostatic interactions the hydrophobic character of the substituent was responsible for retention. Linear relationships were obtained between log of the ratiok′ of amino acid derivatives:k′ of the glycine derivative for a given mobile phase, and logP O/W for the R1 substituent. Good correlations were also found for phenylthiohydantoin amino acid derivatives.

Comparison of the models describing the retention in micellar liquid chromatography with hybrid eluents for a group of benzene derivatives and polycyclic aromatic hydrocarbons

Some models for predicting capacity factors of benzene derivatives and some polycyclic aromatic hydrocarbons as a function of surfactant and organic modifier concentrations in micellar liquid chromatography with hybrid eluents have been tested. The surfactants used in this study were hexadecyltrimethylammonium bromide and sodium dodecyl sulphate and as organic modifiers n-propanol and n-butanol were employed both on C 8 and C 18 columns. The equation that best explains the experimental results is 1 / k' = Aμ + Bϕ 2 + Cϕ + Dμϕ + E so we propose the use of this model in conjunction with the appropriate factorial design to predict the solute retention behaviour in micellar liquid chromatography with hybrid eluents.

On the Retention of Diuretics in Micellar Liquid Chromatography and Their Site of Action Within the Nephron

Abstract The therapeutical action of diuretics has been attributed to their hydrophobic character, among other properties. Therapeutical and physiological classifications of diuretics are correlated. The site of action of high efficacay diuretics is the loop of Henle, that of intermediate efficacy diuretics is the distal tubule and that of low efficacy diuretics is the proximal and distal tubules, and the collecting duct. Partition coefficients between water and micelles offer a scale for hydrophobicity, which leads to a further correlation between retention in Micellar Liquid Chromatography (MLC), with a sodium dodecyl sulphate mobile phase and an ODS-C18 column, and the sites of action within the nephron. The site of action of those diuretics showing the lower retention in MLC is the proximal tubule (acetazolamide) and the loop of Henle (loop diuretics), for those diuretics with an intermediate retention is the distal tubule (thiazides), and for those with a long retention is the collecting duct (potass...

Quantitative retention-biological activity relationship study by micellar liquid chromatography

In a previous paper, the usefulness of micellar liquid chromatography (MLC) in predicting octanol-water partition coefficients of organic compounds was reported. This paper is the first successful report of a quantitative retention-activity relationship study using the retention factor in MLC for predicting the biological activity of a group of phenolic compounds. Excellent correlation was obtained between the capacity factor in MLC and the bioactivity (measured as log 1/C, where C is the 50% inhibitory growth concentration) of 26 para-substituted phenols. A single MLC retention parameter is capable of describing the bioactivity of phenols, while three conventional molecular descriptors (log P(ow), pKa, and R) are needed to achieve a similar correlation. This indicates that both hydrophobic and electronic interactions are incorporated in a single MLC retention parameter, which is due to the amphiphilic nature of surfactants in the system. In situations like this, QRAR is a suitable alternative to QSAR since measuring MLC retention is much easier than measuring different molecular descriptors needed to build the QSAR model. Addition of 10% 2-propanol to a micellar system (hybrid system) proved to be the best chromatographic system for the best estimation of the phenols bioactivity. Other chromatographic factors such as pH and stationary phase also showed significant effect on the correlation between capacity factor k' and log 1/C.

Solute retention in micellar liquid chromatography

A study of the retention behavior of aromatic compounds in micellar and hydro-organic mobile phases was undertaken to better understand the differences in retention and selectivity between micellar liquid chromatography (MLC) and reversed-phase liquid chromatography. The capacity factor for 21 aromatic compounds was measured on Microsorb C 18 using both micellar and hydro-organic mobile phases. Through correlation analysis it was shown that solute retention in MLC is influenced in some measure by the net surface charge of the stationary phase as well as by the unusual nature of the micelle—solute interaction.

Additive effects on surfactant adsorption and ionic solute retention in micellar liquid chromatography

Etudes de l'adsorption du laurylsulfate de Na et du bromure de palmityltrimethylammonium sur 2 silices modifiees par des restes octadecyle (Hypersil ODS) ou des restes methyl (Hypersil SAS) dans des solutions aqueuses de methanol et de NaCl. Donnees sur l'influence de l'addition du methanol et de NaCl