Compounds In Micellar Electrokinetic Chromatography Research Articles

Theoretical models of separation selectivity for charged compounds in micellar electrokinetic chromatography

Equations and theoretical models for MEKC separation selectivity (α(MEKC) ) were established to explain a change in separation and electrophoretic mobility order of fully charged analytes, in which α(MEKC) is related to the dimensionless values of mobility selectivity in CZE (α(CZE)) and retention selectivity (α(k)) in MEKC, and where α(CZE) and α(k) are defined as the ratio of electrophoretic mobility in CZE and the ratio of retention factor (k) in MEKC for two charged analytes, respectively. Using four alkylparabens as test analytes, excellent agreement was found between the observed α(MEKC) and the proposed α(MEKC) models of test analytes in MEKC over a wide range of SDS concentrations and values of k. For example, in comparison with CZE separation of charged analytes, MEKC separation can enhance separation selectivity up to the maximum value when the selectivity ratio (ρ) is greater than 1.0 (ρ=α(k)/α(CZE)), while lower separation selectivity is obtained with ρ<1.0 (α(CZE) >α(k) >1).

Optimization of the separation of ionizable compounds in micellar electrokinetic chromatography by simultaneous change of pH and SDS concentration

A method to optimize the separation in micellar EKC (MEKC) of mixtures of acidic compounds as a function of two parameters, pH and concentration of sodium dodecyl sulfate, has been developed. The method considers the prediction of the retention time and the shape of the peaks. The retention time is predicted from the retention factor model and the peak shape by a polynomically modified Gaussian function that considers peak width, asymmetry factor, and height. An algorithm to calculate the global resolution of the separation at any experimental pH and [SDS] has been applied. This algorithm provides a 3-D resolution map to easily detect the areas in which resolution for the separation of the compounds is maximum. Initial experiments to fit the models have been performed with a set of ten phenolic compounds with different hydrophobicities and pK(a) values, and therefore, expected to behave in a different way with changes of pH and surfactant concentration. The experiments encompassed a pH range from 6.7 to 11.1, and a sodium dodecyl sulfate concentration range from 40 to 80 mM. Through the proposed methodology, chromatograms have been simulated at different pH and [SDS] very accurately. Furthermore, the resolution at any experimental point within the studied ranges have been also calculated, giving an optimum resolution value at pH 6.7 and [SDS] = 72 mM.

Prediction of retention factors in micellar electrokinetic chromatography from theoretically derived molecular descriptors

An artificial neural network (ANN) was constructed and trained for the prediction of the retention factors of some benzene derivatives and heterocyclic compounds in micellar electrokinetic chromatography (MEKC) based on quantitative structure-property relationship. The inputs of this network are theoretically derived descriptors, which were chosen by the stepwise multiple linear regressions features selection technique. These descriptors are; molecular surface area, Kier shape index, dipole moment and maximum positive charge on the Carbon atom which were used as inputs for constructed 4:2:1 ANN. By comparing of the results obtained from multiple linear regression and ANN models, it can be seen that statistical parameters (Fisher ratio, correlation coefficient and standard error of the model) of the ANN model are better than that regression model, which indicates that nonlinear model can simulate the relationship between the structural descriptors and the MEKC retention of the investigated molecules more accurately. Also the cross-validation test was used for the evaluation of the predictive power of the ANN model. The statistical parameters obtained were Q2 = 0.57 and PRESS = 0.55, which reveals the credibility of ANN model.

Artificial neural network prediction of retention factors of some benzene derivatives and heterocyclic compounds in micellar electrokinetic chromatography

A 5-4-1 artificial neural network (ANN) was constructed and trained for prediction of the retention factors of some benzene derivatives and heterocyclic compounds in micellar electrokinetic chromatography (MEKC) based on quantitative structure-property relationship (QSPR). The inputs of this network are theoretically derived descriptors that were chosen by the stepwise variable selection techniques. These descriptors are: molecular surface area, maximum value of electron density on atom in molecule, path four connectivity index, average molecular weight, and sum of atomic polarizability which were selected by using stepwise multiple linear regression as a feature selection technique. The standard errors of training, test, and validation sets for the ANN model are 0.091, 0.119, and 0.114, respectively. Results obtained showed that nonlinear model can simulate the relationship between the structural descriptors and the retention factors of the molecules in data set accurately. Also the appearance of these descriptors in QSPR models reveals the role of electronic and steric interactions in solute retention in MEKC.

Verification of statistical-overlap theory in micellar electrokinetic chromatography

The limited peak capacity of neutral compounds in micellar electrokinetic chromatography (MEKC) causes peak overlap in a simple 38-compound sample that is predicted by statistical-overlap theory (SOT). The low-concentration sample was prepared in-house from several compound classes to span the entire migration-time range and was resolved partially in a pH=7 phosphate buffer containing 50 mM sodium dodecyl sulfate. Peaks, singlets, doublets, and other multiplets were identified on the basis of known migration times and were counted at 13 voltages spanning 4 - 26 kV. These numbers agreed well with predictions of a simple SOT based on the assumption of an inhomogeneous Poisson distribution of migration times. Because the dispersion theory of MEKC is simple, the standard deviations of single-component peaks were modeled theoretically. As part of a new way to implement SOT, probability distributions of the numbers of peaks, singlets, and so on, were computed by Monte Carlo simulation. These distributions contain all theoretical information on peak multiplicity predictable by SOT and were used to evaluate the agreement between experiment and theory. The peak capacity of MEKC was calculated numerically and substituted into the simplest equations in SOT, affirming that peak overlap arises from limited peak capacity.

Enantioselectivity of structurally modified poly(sodium undecenoyl‐L‐leucinate) by insertionof Triton X‐102 surfactant molecules

The effectiveness of Triton X-102 (TX-102), as a structural modifier of the polymeric surfactant sodium undecanoyl-L-leucinate (L-SUL) was investigated for enhanced enantiomeric recognition of various chiral compounds in micellar electrokinetic chromatography (MEKC). Increasing concentrations of TX-102 were separately added into the micellar solutions of L-SUL and then polymerized to form poly-L-SUL. The resulting polymers were purified by use of 3500 molecular-weight-cutoff (MWCO) dialysis membranes. Fluorescence and pulsed field gradient-nuclear magnetic resonance (PFG-NMR) techniques were used to elucidate the structural effects of TX-102 on poly-L-SUL. Evaluation of data from fluorescence measurements suggested an increase in polarity with increasing concentration of TX-102. However, the polarity decreased at higher concentrations of TX-102. Evaluation of data from PFG-NMR suggested an increase in hydrodynamic radius upon increasing the concentration of TX-102. The racemates of coumarinic and phenythiohydantoin amino acid derivatives, and pindolol were used as test analytes in MEKC. A notable increase in resolution and capacity factors of the test analytes was observed when the modified poly-L-SUL was used in MEKC measurements. Examination of the data obtained from fluorescence, PFG-NMR, and MEKC suggests a strong correlation between the polarity and the hydrodynamic radii of TX-102 modified micelles and the enantiomeric resolution of the test analytes.

Influence of the polydispersity of polymeric surfactants on the enantioselectivity of chiral compounds in micellar electrokinetic chromatography.

Poly(sodium undecenoyl-L-leucinate) (poly-L-SUL) was fractionated by the use of different molecular weight cutoff (MWCO) filters to narrow the polydispersity of the macromolecular sizes of the polymeric surfactant. The resulting polymeric surfactant fractions were characterized by the use of three techniques: (1) pulsed field gradient nuclear magnetic resonance (PFG-NMR) was used to determine the hydrodynamic radii, (2) analytical ultracentrifugation (AUC) was used to determine the molecular weights, and (3) steady-state fluorescence was used to determine the polarity of the nonfractionated and fractionated polymeric surfactants. From the data acquired from PFG-NMR, AUC, and fluorescence, it was noted that the hydrodynamic radii and molecular weight of the fractionated poly-L-SUL increased, while the polarity decreased with the increase in the size of the MWCO filter. However, a similarity in physical properties was observed between the nonfractionated and 10-30K fractionated poly-L-SUL except for the hydrodynamic radius and diffusion coefficients. The influence of different macromolecular sizes of poly-L-SUL on the chiral separation of phenylthiohydantion (PTH)-amino acids and coumarinic derivatives, as test analytes, was elucidated by the use of micellar electrokinetic chromatography (MEKC). The size of polymeric surfactants as a prerequisite for chiral separation was demonstrated by comparing the separation properties of fractionated versus nonfractionated polymeric surfactants. Fractionated poly-L-SUL resulted in enhanced resolution and separation efficiency of the test analytes as compared to the case of the nonfractionated poly-L-SUL. This observation indicates that minimizing polydispersity of polymeric surfactants may be important for some chiral separation applications.

Use of poly(sodium oleyl-L-leucylvalinate) surfactant for the separation of chiral compounds in micellar electrokinetic chromatography.

A chiral amino acid-based monomeric and polymeric surfactant, sodium oleyl-L-leucylvalinate) (L-SOLV) and poly(sodium oleyl-L-leucylvalinate) (poly-L-SOLV) were synthesized and used for chiral separations in micellar electrokinetic chromatography (MEKC). Poly-L-SOLV was used successfully in the separation of various enantiomers of neutral, acidic, and basic analytes such as 1,1'-bi-2-napthol, 1,1'-binaphthyl-2,2'-diamine, benzoin, hydrobenzoin, benzoin methylether, warfarin, and coumachlor obtaining well-resolved peaks but with only partial separation of temazepam. In addition, the atropisomer 1,1'-binaphthyl-2, 2'-dihydrogen phosphate was chosen to study the applicability of the polymeric surfactant over a wide range of parameters such as concentration, temperature, voltage, and pH. The most striking characteristic of this new surfactant is its high hydrophobicity. It is favorable to interactions with hydrophobic chiral analytes, and thus may provide better chiral recognition for the compounds.

Application of Temperature and Applied Voltage Programming in Micellar Electrokinetic Capillary Chromatography

Temperature and voltage programming modes were utilized to optimize selectivity and increase the eluting rate of strongly retained compounds in micellar electrokinetic chromatography. Separations obtained by applying temperature, voltage, and a simultaneous combination of temperature and voltage gradient in micellar electrokinetic capillary chromatography were compared with separations performed under isothermal and constant voltage conditions. A complete separation of all the constituents of the test mixture was only achieved in the temperature programming run and in a combination of temperature and voltage programming modes. Simultaneous variations of column temperature and applied voltage during a separation run, yielded a 30% reduction in the total analysis time when compared to a temperature gradient alone. Temperature programming and voltage programming modes may be the gradient methods of choice because of the considerable technical difficulties involved in performing linear solvent gradient elution in micellar electrokinetic chromatography.

Application of Temperature and Applied Voltage Programming in Micellar Electrokinetic Capillary Chromatography

Temperature and voltage programming modes were utilized to optimize selectivity and increase the eluting rate of strongly retained compounds in micellar electrokinetic chromatography. Separations obtained by applying temperature, voltage, and a simultaneous combination of temperature and voltage gradient in micellar electrokinetic capillary chromatography were compared with separations performed under isothermal and constant voltage conditions. A complete separation of all the constituents of the test mixture was only achieved in the temperature programming run and in a combination of temperature and voltage programming modes. Simultaneous variations of column temperature and applied voltage during a separation run, yielded a 30% reduction in the total analysis time when compared to a temperature gradient alone. Temperature programming and voltage programming modes may be the gradient methods of choice because of the considerable technical difficulties involved in performing linear solvent gradient elution in micellar electrokinetic chromatography.

New assessments of dispersion in micellar electrokinetic chromatography

A recent theory for plate number N in micellar electrokinetic chromatography (MEKC) is reevaluated using more accurate measurements of micellar diffusion coefficients and instrumental contributions to dispersion. These measurements are determined using theory outlined here from the low-field dispersion of peaks of micellar markers. The theory for N accurately predicts the experimental N's of weakly and intermediately retained analytes over wide ranges of field strength and concentration of the surfactant, sodium dodecylsulfate. The departure from theory of experimental N's of highly retained species developed at high field strengths in buffers having high surfactant concentrations is shown by additional theory to be consistent with the radial variation of micellar electrophoretic mobility resulting from Joule heat. These calculations are the most accurate reported to date that reconcile with theory the observed high-field dispersion of highly retained neutral compounds in MEKC. © 1998 John Wiley & Sons, Inc. J Micro Sep 10: 479–489, 1998

New assessments of dispersion in micellar electrokinetic chromatography

A recent theory for plate number N in micellar electrokinetic chromatography (MEKC) is reevaluated using more accurate measurements of micellar diffusion coefficients and instrumental contributions to dispersion. These measurements are determined using theory outlined here from the low-field dispersion of peaks of micellar markers. The theory for N accurately predicts the experimental N's of weakly and intermediately retained analytes over wide ranges of field strength and concentration of the surfactant, sodium dodecylsulfate. The departure from theory of experimental N's of highly retained species developed at high field strengths in buffers having high surfactant concentrations is shown by additional theory to be consistent with the radial variation of micellar electrophoretic mobility resulting from Joule heat. These calculations are the most accurate reported to date that reconcile with theory the observed high-field dispersion of highly retained neutral compounds in MEKC. © 1998 John Wiley & Sons, Inc. J Micro Sep 10: 479–489, 1998

Migration behaviour of monovalent weak acids in micellar electrokinetic chromatography mobility model versus retention model

Migration of charged compounds in micellar electrokinetic chromatography (MEKC) is based on micellar solubilization as well as electrophoretic migration. Consequently, the migration behaviour of these compounds can be described with a retention model or a mobility model. Both models are discussed and evaluated for the migration behaviour of monovalent weak acids in a sodium dodecyl sulphate micellar system. We have shown that the calculation of retention factors may be troublesome for hydrophobic compounds, due to interaction with surfactant molecules in the aqueous phase. Therefore, the mobility model is preferable. The simultaneous determination of dissociation constants by spectroscopic and electrophoretic methods is discussed and apparent dissociation constants in micellar media are determined, showing that p K a shifts may occur in MEKC analyses, due to micellar solubilization. Furthermore, a sample stacking procedure is described, based on differences in overall effective mobilities between aqueous and micellar electrolyte systems.

Correlation between the logarithm of capacity factors for aromatic compounds in micellar electrokinetic chromatography and their octanol-water partition coefficients

The correlation between the logarithm of the capacity factors (log k′) and the logarithm of the octanol-water partition coefficients (log Pow) for a group of 20 organic compounds (ten benzene derivatives and ten polycyclic aromatic hydrocarbons) was studied in micellar electrokinetic chromatography. Sodium dodecyl sulphate micelles in different aqueous and hydro-organic (n-propanol and n-butanol) buffers were used as electrolyte solutions. A linear relationship between log k′ and log Pow was found. This correlation improved when the percentage of alcohol in the buffer system was increased and when the polarity of the alcohol was decreased.

Effect of organic modifier on resolution of hydrophobic compounds by micellar electrokinetic chromatography.

Three organic modifiers, methanol, acetonitrile, and dimethylformamide, were used as organic modifiers to improve the resolution of hydrophobic compounds in micellar electrokinetic chromatography. The capacity factor decreased with increasing concentration of organic modifier. Acetonitrile and dimethylformamide were more effective modifiers to decrease the capacity factor than methanol. Maximum resolution was obtained when the capacity factor was reduced to about 2. Methanol enhanced the resolution for less hydrophobic compounds, while acetonitrile and dimethylformamide were better modifiers to improve the resolution for strongly hydrophobic compounds.