Chemometrics in capillary electrophoresis. Part A: Methods for optimization

Application of Experimental Design Methodologies in the Enantioseparation of Pharmaceuticals by Capillary Electrophoresis: A Review

Chirality is one of the major issues in pharmaceutical research and industry. Capillary electrophoresis (CE) is an interesting alternative to the more frequently used chromatographic techniques in the enantioseparation of pharmaceuticals, and is used for the determination of enantiomeric ratio, enantiomeric purity, and in pharmacokinetic studies. Traditionally, optimization of CE methods is performed using a univariate one factor at a time (OFAT) approach; however, this strategy does not allow for the evaluation of interactions between experimental factors, which may result in ineffective method development and optimization. In the last two decades, Design of Experiments (DoE) has been frequently employed to better understand the multidimensional effects and interactions of the input factors on the output responses of analytical CE methods. DoE can be divided into two types: screening and optimization designs. Furthermore, using Quality by Design (QbD) methodology to develop CE-based enantioselective techniques is becoming increasingly popular. The review presents the current use of DoE methodologies in CE-based enantioresolution method development and provides an overview of DoE applications in the optimization and validation of CE enantioselective procedures in the last 25 years. Moreover, a critical perspective on how different DoE strategies can aid in the optimization of enantioseparation procedures is presented.

Cyclodextrins as Chiral Selectors in Capillary Electrophoresis Enantioseparations

Due to their structural variability and their commercial availability, cyclodextrins are the most frequently used chiral selectors in capillary electrophoresis. A variety of migration modes can be realized depending on the characteristics of the cyclodextrins and the analytes. The basic considerations regarding the development of a chiral CE method employing cyclodextrins as chiral selectors are briefly discussed. The presented examples illustrate the separation modes of an acidic and a basic analyte with native and charged cyclodextrin derivatives as a function of the pH of the background electrolyte and the concentration of the cyclodextrin.

Recent Advances in the Determination of Biogenic Amines in Food Samples by (U)HPLC

The determination of biogenic amines (BAs) in food products stirs increasing interest because of the implications in toxicological and food quality issues. Apart from these aspects, in recent years, the relevance of BAs because of some organoleptic and descriptive concerns has been pointed out by several researchers. This overview aims at revising recent advances in the determination of BAs in food samples based on liquid chromatography. In particular, papers published in the past five years have been commented. Special attention has been paid to the great possibilities of ultrahigh-performance liquid chromatography and high-resolution mass spectrometry. With regard to applications, apart from the determination of BAs in a wide range of food matrices, novel lines of research focused on the characterization, classification, and authentication of food products based on chemometrics have also been discussed.

Separation of Peptides by Capillary Electrophoresis

Peptides are an important class of analytes in chemistry, biochemistry, food chemistry, as well as medical and pharmaceutical sciences including biomarker analysis in peptidomics and proteomics. As a high-resolution technique, capillary electrophoresis (CE) is well suited for the analysis of polar compounds such as peptides. In addition, CE is orthogonal to high-performance liquid chromatography (HPLC) as both techniques are based on different physicochemical separation principles. For the successful development of peptide separations by CE, operational parameters including puffer pH, buffer concentration and buffer type, applied voltage, capillary dimensions, as well as background electrolyte additives such as detergents, ion-pairing reagents, cyclodextrins, (poly)amines, and soluble polymers have to be considered and optimized.

Optimizing the relationship between chromatographic efficiency and retention times in temperature-programmed gas chromatography

A methodology that can maximise the chromatographic efficiency that can be achieved within a defined time frame in temperature-programmed gas chromatography is described. The efficiency can be defined as the inverse of peak widths measured in retention index units. This parameter can be described by a model similar to the van Deemter equation, which is expanded to account for the effect of the temperature rate in addition to the effect of carrier gas velocity. The model of efficiency is found by response surface methodology, where the temperature rates and the carrier gas velocities are systematically varied in the experiments. A second model that accurately explains the retention time of the last eluting compound can be found from the same experiments, and optimal conditions are found by combining the two models. The methodology has been evaluated with four capillary columns and three carrier gases, using fatty acid methyl esters as analytes. All experiments showed that there is a fairly linear decrease in efficiency with increasing temperature rates. At any temperature rate, optimal velocity is only marginally higher than the velocity that maximises chromatographic efficiency, since the carrier gas velocity has a limited effect on the retention times.

Multivariate optimization of capillary electrophoresis methods: a critical review

In this article a review on the recent applications of multivariate techniques for optimization of electromigration methods, is presented. Papers published in the period from August 2007 to February 2013, have been taken into consideration. Upon a brief description of each of the involved CE operative modes, the characteristics of the chemometric strategies (type of design, factors and responses) applied to face a number of analytical challenges, are presented. Finally, a critical discussion, giving some practical advices and pointing out the most common issues involved in multivariate set-up of CE methods, is provided.

Enantioseparations by capillary electrophoresis using cyclodextrins as chiral selectors

Due to their commercial availability, cyclodextrins are the most frequently used chiral selectors in capillary electrophoresis as documented by the numerous publications in the field. A variety of migration modes can be realized depending on the characteristics of the cyclodextrins and the analytes. The basic considerations regarding the development of a chiral CE method employing cyclodextrins as chiral selectors are briefly discussed. The presented examples illustrate the separation modes of an acidic and a basic analyte with native and charged cyclodextrin derivatives as a function of the pH of the background electrolyte and the cyclodextrin concentration.

Advanced separation methods of food anthocyanins, isoflavones and flavanols

In recent years, increasing knowledge of the positive health effects of food polyphenols has prompted the need to develop new separation techniques for their extraction, fractionation and analysis. This article provides an updated and exhaustive review of the application of counter-current chromatography, high performance liquid chromatography, capillary electrophoresis, and their hyphenation with mass spectrometry to the study of food polyphenols. Flavonoids constitute the largest class of polyphenols, widely spread in the plant kingdom and common in human diet which has been the most widely studied with respect to their antioxidant and biological activities. The main subgroups are anthocyanins, catechins, isoflavones, flavonols and flavones. They are reported to exhibit antioxidant, anti-carcinogenic, anti-inflammatory, anti-atherogenic, anti-thrombotic, and immune modulating functions, among others. Since red fruit anthocyanins, soy isoflavones and flavanols from grapes and teas are currently the most used phenolic compounds for producing new nutraceuticals and functional foods, this review is focused on these three flavonoid groups.

Optimization of capillary electrophoresis conditions for a glucagon competitive immunoassay using response surface methodology

The capillary electrophoresis (CE) conditions for a competitive immunoassay of glucagon were optimized for highest sensitivity of the immunoassay and resolution of the electrophoretic peaks using a Box-Behnken design. Injection time, voltage ramp time, and separation voltage were varied between three levels and two responses, bound-to-free (B/F) ratio of the immunoassay peaks and resolution between the peaks, were measured. Analysis of variance was applied to fit a predictive model, and a desirability function was used to simultaneously optimize both responses. A 10-s injection, 1.6-min ramp time, and a 22-kV separation voltage were the conditions found when high B/F was given more emphasis than high resolution. To test the model, calibration curves of a glucagon immunoassay were measured at the optimum and least optimum CE conditions. Optimal conditions increased the sensitivity of the immunoassay by 388% compared to the least optimum conditions while maintaining adequate resolution.

Enantioselective analysis of the antipsychotic 9-hydroxyrisperidone, main metabolite of risperidone, by chiral capillary EKC using dual CDs

An analytical method for the enantioseparation of the 9-hydroxyrisperidone, main metabolite of the antipsychotic risperidone (Risp), was developed by CD-EKC in the dual CDs mode using anionic and neutral CDs at acidic pH 2.5. Preliminary experiments allowed us to select the more suitable couple of CDs composed of sulfated-alpha-CD and hydroxypropylated-beta-CD. The optimization of the main experimental parameters (concentrations of both CDs and concentration of the phosphate buffer) was based on a central composite design through the response surface methodology. Then, the influence of the voltage and the temperature on the enantioseparation was studied using the classical univariate approach. The final method permits to resolve the enantiomers of the 9-hydroxyrisperidone with a resolution of 3.13 and an analysis time of about 13 min. Finally, this method was successfully applied for the simultaneous determination of Risp and the enantiomers of 9-hydroxyrisperidone (9-OHRisp).

Chemometric experimental design based optimization techniques in capillary electrophoresis: a critical review of modern applications

A critical review of recent developments in the use of chemometric experimental design based optimization techniques in capillary electrophoresis applications is presented. Current advances have led to enhanced separation capabilities of a wide range of analytes in such areas as biological, environmental, food technology, pharmaceutical, and medical analysis. Significant developments in design, detection methodology and applications from the last 5 years (2002-2007) are reported. Furthermore, future perspectives in the use of chemometric methodology in capillary electrophoresis are considered.

Implementation of chemometric methodology in ACE: Predictive investigation of protein–ligand binding

An ACE predictive investigation of protein-ligand binding using a highly effective chemometric response surface design technique is presented. Here, K(d) was estimated using one noninteracting standard which relates to changes in the electrophoretic mobility of carbonic anhydrase B (CAB, EC 4.2.1.1) on complexation with the ligand 4-carboxybenzenesulfonamide (CBSA) present in the electrophoresis buffer. Experimental factors including injection time, capillary length, and applied voltage were selected and tested at three levels in a Box-Behnken design. Statistical analysis results were used to create a mathematical model for response surface prediction via contour and surface plots at a given target response (K(d) = 1.19x10(-6) M). As expected, there were a number of predicted solutions that reached our target response based on the significance of each factor at appropriate levels. The adequacy of the model was validated by experimental runs with the predicted model solution (capillary length = 47 cm, voltage = 11 kV, injection time = 0.01 min) presented in detail as an example.

Use of multivariate analysis for optimization of separation parameters and prediction of migration time, resolution, and resolutionper unit time in micellar electrokinetic chromatography

The optimization of separation parameters in chromatography for better separation and resolution of analytes continues to be a labor intensive procedure usually performed by a trial and error method. A multivariate analysis in the form of multilinear regression (MLR) is used to optimize separation parameters and predict the migration behavior, resolution, and resolution per unit time of achiral (4-chlorophenol, pentachlorophenol, clonazepam, and diazepam) and chiral (1,1'-binaphthyl 2,2'-dihydrogen phosphate (BNP), and 1,1'-bi-2-naphthol (BOH)) compounds in MEKC. Separations of achiral and chiral analytes were performed using an achiral (poly(sodium N-undecylenic sulfate)) molecular micelle and chiral (poly(sodium N-undecanoyl-L-leucylvalinate) or poly(sodium N-undecanoyl-L-isoleucylvalinate)) molecular micelle, respectively, at various operating temperatures, applied voltages, pH values, and molecular micelle concentrations in the BGE. The separation parameters were subsequently used as input variables for MLR models. The models were validated with independent samples. The root-mean-square percent relative error (RMS%RE) is used as a figure of merit for characterizing the performance of the migration time, resolution, and resolution per unit time models. The RMS%RE obtained for predicted migrated times, resolutions, and resolution per unit time of 4-chlorophenol, pentachlorophenol, clonazepam, diazepam, BNP, and BOH ranged between 8 and 19%. The same experimental procedure was used to optimize the separation parameters of six other chiral analytes of different compound class. The predicted migration times, resolutions, and resolution per unit time of the chiral as well as the achiral analytes compare favorably with the experimental migration times and resolutions, indicating versatility and wide applicability of the technique in MEKC.

Sequential uniform designs for fingerprints development of Ginkgo biloba extracts by capillary electrophoresis

A sequential procedure for the method of development of fingerprints based on a uniform design approach has been described in this paper. The sequential uniform design is used to reach the global optimum for a separation. The procedure is illustrated through developing the fingerprint of a Ginkgo biloba extract by capillary electrophoresis (CE) with diode array detection (DAD). The local overlap index (LOVI) is proposed as a criterion to evaluate the separation quality of two-dimensional hyphenated data in the optimization process. The successful application of the described techniques, including a sequential procedure, the CE-DAD hyphenated method and the LOVI criterion, shows their practicality in the fingerprint development of herbal medicines. However, some critical remarks on their use can also be made.

Enantiomeric separation of acidic compounds using single-isomer amino cyclodextrin derivatives in nonaqueous capillary electrophoresis

The enantiomeric separation of a series of acidic pharmaceuticals (mostly nonsteroidal anti-inflammatory drugs) has been investigated in NACE systems using single-isomer amino beta-CD derivatives. The first part of this study consisted of the selection of the basic experimental conditions to separate efficiently the enantiomers of acidic drugs. Several parameters, such as the nature of the ionic BGE components, were studied and a methanolic solution of ammonium acetate containing the cationic CD was selected as BGE. A D-optimal design with 20 experimental points was then applied and the nature and concentration of the CD were found to have a significant effect on the enantiomeric resolution for all studied compounds. Resolution (R(s)) values were always higher with 6-monodeoxy-6-mono(3-hydroxy)propylamino-beta-CD (PA-beta-CD) compared to those obtained with 6-monodeoxy-6-mono(2-hydroxy)propylamino-beta-CD (IPA-beta-CD). However, the latter led to shorter migration times. Generic NACE conditions were then selected by means of the multivariate approach in order to obtain the highest R(s) values in a minimum amount of time. Finally, dependence of separation selectivity, resolution, as well as mobility difference on chiral selector concentration was discussed and binding constants with PA-beta-CD were estimated for the two enantiomers of one of the model compounds, suprofen in these NACE systems.

Mixture design in the optimization of a microemulsion system for the electrokinetic chromatographic determination of ketorolac and its impurities: Method development and validation

Microemulsion EKC (MEEKC) was used for the determination of ketorolac and its three impurities. The microemulsion system was optimized, for the first time in the literature, using a multivariate strategy involving a mixture design. A 13-run experimental plan covering an experimental domain defined by the components aqueous phase (10 mM borate buffer pH 9.2), oil phase (n-heptane) and surfactant/cosurfactant (SDS/n-butanol) was carried out. Good results were obtained with all microemulsions tested considering as responses analysis time and resolution, and according to the desirability function the best microemulsion system was constituted by 90.0% 10 mM borate buffer, 2.0% n-heptane, 8.0% of SDS/n-butanol in 1:2 ratio. Finally, with the aim of reducing analysis time, a response surface study was carried out in the experimental domain defined by the process variables temperature and voltage and the best values were 17 degrees C and -17 kV, respectively. Applying the optimised conditions, a complete resolution among the analytes was obtained in about 3 min using the short-end injection method. The method was validated for both drug substances and drug product and was applied to the quality control of ketorolac in coated tablets. A comparison of MEEKC, MEKC and CEC for assaying ketorolac and its related substances has been made.

Capillary electrophoresis determination of biogenic amines by field-amplified sample stacking and in-capillary derivatization

A sensitive CE method for determining biogenic amines in wines based on in-capillary derivatization with 1,2-naphthoquinone-4-sulfonate is presented. In this method, reagent and buffer solutions are introduced hydrodynamically into the capillary whereas the sample is injected electrokinetically, thus, allowing a selective preconcentration of the analytes by field-amplified sample stacking. Amines are labeled inside the capillary using a zone-passing derivatization approach in mixed tandem mode. The most relevant variables influencing on the derivatization and separation as well as significant interactions have been evaluated using experimental design. Multi-criteria decision making is utilized for the simultaneous optimization of interacting variables through overall desirability response surfaces. The validation of the method has proven an excellent separation performance and accuracy for the determination of biogenic amines such as histamine, tryptamine, phenylethylamine, tyramine, agmatine, ethanolamine, serotonin, cadaverine, and putrescine in red wines. Detection limits range from 0.02 mg/L for ethanolamine to 0.91 mg/L for serotonin. The RSDs for migration time and peak area are around 1.2 and 6.2%, respectively. Red wines from different Spanish regions have been analyzed using the proposed method.

Approach to method development and validation in capillary electrophoresis for enantiomeric purity testing of active basic pharmaceutical ingredients

A chiral capillary electrophoresis system allowing the determination of the enantiomeric purity of an investigational new drug was developed using a generic method development approach for basic analytes. The method was optimized in terms of type and concentration of both cyclodextrin (CD) and electrolyte, buffer pH, temperature, voltage, and rinsing procedure. Optimal chiral separation of the analyte was obtained using an electrolyte with 2.5% carboxymethyl-beta-CD in 25 mM NaH2PO4 (pH 4.0). Interchanging the inlet and outlet vials after each run improved the method's precision. To assure the method's suitability for the control of enantiomeric impurities in pharmaceutical quality control, its specificity, linearity, precision, accuracy, and robustness were validated according to the requirements of the International Conference on Harmonization. The usefulness of our generic method development approach for the validation of robustness was demonstrated.

Determination of B2 and B6 vitamers in serum by capillary electrophoresis-molecular fluorescence-charge coupled detector

A method for the separation by capillary zone electrophoresis and determination of vitamers of two important water-soluble vitamins, i.e., B2 and B6, is proposed here. The working conditions for optimal separation were obtained by a multivariate methodology in order to succeed in the best resolution in the shortest analysis time. The optimization of the buffer composition together with other variables, such as analysis and injection voltage, temperature of the capillary cassette, and injection time, resulted in a solution of 30 mM KH2PO4 adjusted to pH 8.5 with formic acid. Concerning the detection step, the target analytes were quantified by molecular fluorescence, for which two different detectors, a photomultiplier tube (PMT) and a charge-coupled detector (CCD), were compared in terms of resolution, sensitivity, and precision. A Xe-Hg lamp was used as an irradiation source in the two cases. The best option was the CCD, which provides three-dimensional electropherograms and enables to solve the overlapped peaks. Besides, the sensitivity of the CCD was similar to that of the PMT, due to the treatment data, obtaining limits of detection and quantification from 1.16 to 27.1 ng/g and from 3.83 to 89.4 ng/g, respectively. The method was applied to the serum samples for which a prior liquid-liquid extraction using ethanol in an acid medium was mandatory for eliminating the interferences and concentrating the analytes by a factor of 5. The rapidity of the analysis (13 min for the electrophoretic separation) and the excellent precision (repeatability and within-laboratory reproducibility between 2.86 and 4.11% and 7.03 and 8.45%, respectively, both expressed as relative standard deviation) demonstrated the capability of the proposed method for a clinical routine analysis.

Simplex maximization of the correlation coefficient for DNA sizing analysis by capillary electrophoresis

The separation of DNA molecules in polymeric solution by capillary electrophoresis involves the optimization of several variables, such as polymer solution concentration, electric field separation, temperature, etc. The optimization of each variable individually usually is a time-consuming process and the results may reach a false optimum point. Chemometric methods are suitable to be applied in such cases in which a number of variables can be optimized simultaneously. The simplex is a chemometric method that can perform such a task easily and efficiently. In this study, a simplex method was carried out to maximize the correlation coefficient (r(2)) of a logarithmic plot of mobility (mu) vs. base pair (bp), which was obtained from the separation of DNA fragments of size between 75 and 4072 bp. The simplex showed three vertexes with r(2) > 0.98 and the vertex 21 showing the highest resolution. For the fragments between 201 and 2036 bp, the r(2) increased to 0.992 with and relative standard deviation (RSD) lower than 0.2% (inter- and intra-day variation). The precision of the method in determining the size of a PCR DNA fragment was carried out using a 1 kbp DNA ladder. With the addition of an internal standard to the sample, the precision could be further improved.

Resolution of overlapping capillary electrophoresis peaks by using chemometric analysis: Quantification of the components in compound reserpine tablets

The application of multivariate curve resolution with alternating least squares (MCR-ALS) methods to second-order data from capillary electrophoresis with diode array detector (CE-DAD) is reported. Initial qualitative solutions obtained by evolving factor analysis (EFA) and pure-variable detection method can be further optimized by a simultaneous analysis of multiple electrophoresis run data with ALS regression. While unknown samples are analyzed simultaneously against the corresponding standards in different composition ratios, the exact amounts of common components in different CE runs can be determined by the traditional calibration curve method, and quantification can thus be achieved. The above methods are applied to the determination of the components in compound reserpine tablets in overlapping peaks from CE. The quantification results are compared with those of the first derivative of the electropherogram method and artificial neural network (ANN) method.

Capillary electrophoresis of inorganic ions: An update

This review as a sequel of three earlier similar reports gives a summary of the progress and significant methodological developments, starting from 2002, in the use of capillary electrophoresis (CE) for inorganic ion analysis. As substantiated by the illustrative number of relevant references, improvements in sensitivity achieved both in and outside a CE system, advances in manipulating the separation selectivity, novel hardware configurations, and system performance innovations are continually being reported over the review period. Specifically viewed are the recent advancements in elemental (bio)speciation analysis, which remains one of the most fertile areas of CE research, as well as in three recently booming research topics: contactless conductivity detection, separations on microchips, and transient isotachophoretic preconcentration. A state-of-the-art picture of technique's potentialities within the field of interest presented here demonstrates that CE has become recognized and is growing in acceptance as a reliable alternative to traditional analytical methods such as high-performance liquid chromatography (HPLC).

Fast separation and determination of phenolic compounds by capillary electrophoresis–diode array detection

A dynamic approach has been proposed for the ultrasound-assisted extraction of twenty phenolic compounds from alperujo, a semisolid waste from the olive oil industry, that is a representative example of samples with a complex matrix. Multivariate methodology was used to carry out a detailed optimisation study of both the separation-determination and extraction steps in terms of resolution-analysis time and extraction efficiency, respectively. Consequently, the proposed method was able to extract the target analytes in 13 min; then, after dilution and centrifugation, the extract was injected into the capillary electrophoresis-diode array detection system for individual separation determination in 11 min. No cleanup of the extract was required. This method is less time-consuming, more selective and provides a larger information level than the Folin-Ciocalteau spectrophotometric method. Alperujo was demonstrated to be a powerful source of phenolic compounds, particularly as compared with olive oil--8680 versus 50-1200 microg/g.

A chemometric study of active parameters and their interaction effects in a nebulized sheath-liquid electrospray interface for capillary electrophoresis-mass spectrometry

A chemometrics approach has been used for evaluating the effect of four experimental parameters when coupling capillary electrophoresis (CE) to electrospray ionization-mass spectrometry (ESI-MS). Electrospray voltage, sheath-liquid flow rate, nebulizing gas flow rate, and spray needle position in respect to the MS orifice were varied according to a full factorial design. In addition to main effects, two interaction effects could be identified as significant when measuring the peak intensity of the analytes, from a sample mixture containing peptides and pharmaceuticals. The first interaction effects, between the nebulizing gas flow rate and the sheath-liquid flow rate, and the second interaction effect, between the nebulizing gas flow rate and the spray position, could further explain the impact that these variables have on the spray performance. The number of theoretical plates and the baseline noise were also measured. The sheath-liquid flow was found to significantly affect the separation efficiency, while the noise level mainly was controlled by the nebulizing gas flow. The same factorial design was also used for a CE capillary with lower internal diameter (ID) and the effects of the same variables were compared on those capillaries using equal injection volume for both capillaries. Similar trends were obtained in both capillaries but capillary ID was shown to be a significant variable when evaluating both capillaries in a single model. It was found that a capillary with 25 microm ID provided improved CE-MS performance over than corresponding 50 microm ID capillary. Enhanced sensitivity was obtained using the narrow-bore capillary, and at lower sheath-liquid flow rate the 25 microm ID capillary also gave rise to more efficient peaks.