Capillary electrochromatography with novel stationary phases. 3. Retention behavior of small and large nucleic acids on octadecyl-sulfonated-silica

History, advancement, bottlenecks, and future of chiral capillary electrochromatography

Capillary electrochromatography (CEC) represents a technique with less than 30 years of intense development and in this period this technique has seen huge promise, fast development, stagnation, and significant decline of innovative activity. The major goal of the present overview is not to present an extensive review of the literature on chiral CEC but to analyze the reasons for this dramatic development and attempting to answer questions such as: 1) Was the potential of CEC reasonably evaluated in 1990s before starting the explosive development in this field? 2) Did the development of this technique take the right track? 3) What other developments and competitive trends led to stagnation in the advancement of CEC? 4) Why is the activity in this field currently decreasing? 5) What are the current challenges and promises and what is the future of chiral CEC?

Pressurized CEC of neutral and charged solutes using silica monolithic stationary phases functionalized with 3-(2-aminoethylamino)propyl ligands

A novel silica monolithic stationary phase functionalized with 3-(2-aminoethylamino)propyl ligands for pressurized CEC has been presented. The monolithic capillary columns were prepared by a sol-gel process in 75 microm id fused-silica capillaries and followed by a chemical modification. The diamino groups on the surface of the stationary phase are meant to generate the chromatographic surface and a substantial anodic EOF as well as to provide electrostatic interaction sites for charged solutes. The electrochromatographic characterization and column performance were evaluated by a variety of neutral and charged solutes. It was observed that the anodic EOF for the diamine-bonded monolith was greatly affected by the reaction time with 3-(2-aminoethylamino)propyltrimethoxysilane and the PEG amount in the sol-gel reaction mixture in addition to the mobile phase conditions. The monolithic stationary phase exhibited hydrophilic interaction chromatographic behavior toward neutral solutes. Good separations of various solutes including phenols, nucleic acid bases, nucleosides and nucleotides were achieved under different experimental conditions. Fast and efficient separations were obtained with high plate counts reaching more than 130,000 plates/m.

Preparation of polymethacrylate monolithic stationary phases having bonded octadecyl ligands and sulfonate groups: electrochromatographic characterization and application to the separation of polar solutes for pressurized capillary electrochromatography

In this report, the preparation of porous polymethacrylate-based monolithic columns by in situ copolymerization of octadecyl methacrylate (OMA), 3-sulfopropyl methacrylate (SPMA) and ethylene dimethacrylate (EDMA) in a binary porogenic solvent consisting of cyclohexanol/1,4-butanediol are proposed. These monoliths possess in their structures bonded octadecyl ligands and sulfonate groups and are evaluated in pressurized capillary electrochromatography (pCEC) system using small neutral and charged solutes. While the sulfonate groups are meant to generate the electroosmotic flow (EOF) necessary for transporting the mobile phase through the monolithic capillary; the octadecyl ligands are introduced to provide the nonpolar sites for chromatographic retention for neutral solutes. However, incorporating the sulfonate groups in the monoliths does not only support the EOF but also exhibit hydrophilic interaction as well as electrostatic interaction/repulsion with the monoliths in addition to electrophoretic migration with polar charged solutes (e.g., nucleotides). The monolithic stationary phases at different EOF velocities are easily prepared by altering the amount of SPMA in the polymerization solution as well as the composition of the porogenic solvent. Optimum EOF velocity, the highest efficiency and adequate chromatographic retention are obtained when 0.6% SPMA is added to the reaction mixture. Under these conditions, rapid separation and high plate counts reaching greater than 170,000 plates/m are readily obtained.

Theory and practical understanding of the migration behavior of proteins and peptides in CE and related techniques

CEC is defined as an analytical method, where the analytes are separated on a chromatographic column in the presence of an applied voltage. The separation of charged analytes in CEC is complex, since chromatographic interaction, electroosmosis and electrophoresis contribute to the experimentally observed behavior. The putative contribution of effects such as surface electrodiffusion has been suggested. A sound theoretical treatment incorporating all effects is currently not available. The question of whether the different effects contribute in an independent or an interdependent manner is still under discussion. In this contribution, the state-of-the-art in the theoretical description of the individual contributions as well as models for the retention behavior and in particular possible dimensionless 'retention factors' is discussed, together with the experimental database for the separation of charged analytes, in particular proteins and peptides, by CEC and related techniques.

Negatively charged sol-gel column with stable electroosmotic flow for online preconcentration of zwitterionic biomolecules in capillary electromigration separations

A negatively charged sol-gel coating was developed for on-line preconcentration of zwitterionic biomolecules in capillary electrophoresis (CE), using asparagine and myoglobin as representative zwitterionic bioanalytes. The sol-gel coating was created by using a solution containing three precursors: mercaptopropyltrimethoxysilane (MPTMS), tetramethoxysilane (TMOS), and n-octadecyltriethoxysilane (C18-TEOS). The resulting sol-gel coating contained chemically bonded mercaptopropyl functional groups that were further oxidized by hydrogen peroxide to the corresponding sulfonic acid moieties. Such a surface-bonded sol-gel coating can carry a negative charge over a wide range of pH due to the presence of deprotonated sulfonic acid groups. Under favorable pH conditions, the negatively charged sol-gel coating can facilitate the extraction of positively charged analytes from a zwitterionic sample through electrostatic interaction. This principle was employed to extract myoglobin and asparagine by passing aqueous samples of these zwitterionic analytes through a negatively charged sol-gel column. The extracted analytes were then desorbed and focused via local pH change and stacking. The local pH change was accomplished by passing a buffer solution with a pH above the solute p/ value, while a dynamic pH junction between the sample solution and the background electrolyte was utilized to facilitate solute focusing. The sorption/desorption phenomena could, perhaps, also be explained on the basis of ion-exchange and local pH junction effects. On-line preconcentration and analysis results obtained on sulfonated sol-gel columns were compared with those obtained on an uncoated fused silica capillary of identical dimensions using conventional sample injections. Using UV detection, the presented sample preconcentration technique provided a sensitivity enhancement factor (SEF) on the order of 3 x 10(3) for myoglobin, and 7 x 10(3) for asparagine.

Use of mixed-mode sorbents for the electrochromatographic separation of thrombin receptor antagonistic peptides

In this study, the thrombin receptor antagonistic peptide TRAP-1 and its alanine-scan analogues, TRAP 2-6, have been employed as probes to characterise the performance of C18/SCX mixed-mode capillary electrochromatographic (CEC) columns. It was found that the resolution of this group of peptides could only be achieved in a narrow pH range with phosphate-based running electrolytes. The influence of the running electrolyte composition, e.g. the buffer choice, the ionic strength, the pH and the organic solvent content, on the electroosmotic flow (EOF) of these mixed-mode CEC columns was investigated. In addition, the retention mechanism for this group of peptide probes in the electrochromatographic process was studied by examining the effect of varying the running electrolyte composition. As a result, it can be concluded that the electrochromatographic separation of this set of peptides was mediated by a combination of electrophoretic migration and chromatographic retention involving both hydrophobic as well as ion exchange interactions. By modulating the running electrolyte composition, the hydrophobic or ion exchange components of the interaction process could be made to dominate the chromatographic retention of the peptides. Based on this strategy, a high-resolution separation of six closely related synthetic peptides was demonstrated with this mixed-mode CEC system.

Sol–gel stationary phases for capillary electrochromatography

A review is presented on the current state of the art and future trends in the development of sol-gel stationary phases for capillary electrochromatography (CEC). The design and synthesis of stationary phases with prescribed chromatographic and surface charge properties represent challenging tasks in contemporary CEC research. Further developments in CEC as a high-efficiency liquid-phase separation technique will greatly depend on new breakthroughs in the area of stationary phase development. The requirements imposed on CEC stationary phase performance are significantly more demanding compared with those for HPLC. The design of CEC stationary phase must take into consideration the structural characteristics that will provide not only the selective solute/stationary phase interactions leading to chromatographic separations but also the surface charge properties that determine the magnitude and direction of the electroosmotic flow responsible for the mobile phase movement through the CEC column. Therefore, the stationary phase technology in CEC presents a more complex problem than in conventional chromatographic techniques. Different approaches to stationary phase development have been reported in contemporary CEC literature. The sol-gel approach represents a promising direction in this important research. It is applicable to the preparation of CEC stationary phases in different formats: surface coatings, micro/submicro particles, and monolithic beds. Besides, in the sol-gel approach, appropriate sol-gel precursors and other building blocks can be selected to create a stationary phase with desired structural and surface properties. One remarkable advantage of the sol-gel approach is the mild thermal conditions under which the stationary phase synthesis can be carried out (typically at room temperature). It also provides an effective pathway to integrating the advantageous properties of organic and inorganic material systems, and thereby enhancing and fine-tuning chromatographic selectivity of the created hybrid organic-inorganic stationary phases. This review focuses on recent developments in the design, synthesis, characterization, properties, and applications of sol-gel stationary phases in CEC.

Investigation of the novel mixed-mode stationary phase for capillary electrochromatography

A novel packing material, 3-(4-sulfo-1,8-naphthalimido)propyl-modified silyl silica gel (SNAIP), was prepared for the use as a stationary phase of capillary electrochromatography (CEC). The sulfonic acid groups on SNAIP stationary phase contributed to the generation of electroosmotic flow (EOF) at low pH and served as a strong cation-exchanger. In CEC with SNAIP, a mixed-mode separation was predicted, comprising hydrophobic and electrostatic interactions as well as electrophoretic migration process. In order to understand the retention mechanism on SNAIP, effects of buffer pH, concentration, and mobile phase composition on EOF mobility and the retention factors of barbiturates and benzodiazepines were systematically investigated. Moreover, the retention behavior of barbiturates on SNAIP was investigated and compared with those on octadecyl silica (ODS), phenyl-bonded silica, and 3-(1,8-naphthalimido)propyl-modified silyl silica gel to confirm the presence of pi-pi interaction on its retention mechanism. It was observed that a column efficiency was more than 85,000 N/m for retained compounds and the relative standard deviations for the retention times of EOF marker, thiourea, and five barbiturates were below 2.5% (n = 4). Under an applied voltage of 20 kV and a mobile phase consisted of 5 mM phosphate (pH 3.8) and 40% methanol, the baseline separation of five barbiturates was achieved within 3 min.

Capillary electrochromatography with monolithic stationary phases

This article, which is closely related to part II, is concerned with the evaluation of the retentive properties of cationic stearyl-acrylate monoliths (i.e. cationic C17 monoliths) over a wide range of elution conditions with various uncharged and charged solutes including proteins. The retention parameters for charged solutes including the retention factor k* observed under capillary electrochromatography conditions and the velocity factor k(ep)*, which reflects the electrophoretic process, were measured for weak, moderate and strong basic compounds. These retention parameters allowed the assessment of the respective contributions from electrophoretic and partitioning separation mechanisms. The cationic C17 monoliths exhibited sufficient hydrophobic interactions with relatively weak basic solutes. Moderate and strong bases showed migration behaviors dominated by their relatively strong electrophoretic mobility with marginal chromatographic partitioning. At low pH, the cationic C17 monoliths allowed the separation of proteins with minimum electrostatic interactions between proteins and the cationic sites on the surface of the stationary phase. The utility of the cationic C17 monoliths was demonstrated in the rapid and efficient separation of two crude extracts of membrane proteins, namely galactosyl transferase and cytochrome c reductase. Short capillary columns (8.5 cm effective length) of the cationic C17 monoliths allowed rapid and efficient separations of neutral and charged pesticides and metabolites, phenylthiohydrantoin amino acids and proteins at the time scale of seconds at relatively high flow velocity.

The separation of biomolecules using capillary electrochromatography

The unique properties of capillary electrochromatography such as high performance, high selectivity, minimum consumption of both reagents and samples, and good compatibility with mass spectrometry make this technique an attractive one for the analysis of biomolecules including peptides, proteins, carbohydrates, nucleosides and nucleotides. Irreversible adsorption between the biomolecules and the charged packing surface leads to a lack of reproducibility and serious peak tailing, so various approaches have been taken to overcome this and to improve the technique for future challenges.

Capillary electrochromatography with monolithic stationary phases: 1. Preparation of sulfonated stearyl acrylate monoliths and their electrochromatographic characterization with neutral and charged solutes

A novel monolithic stationary phase having long alkyl chain ligands (C17) was introduced and evaluated in capillary electrochromatography (CEC) of small neutral and charged species. The monolithic stationary phase was prepared by the in situ copolymerization of pentaerythritol diacrylate monostearate (PEDAS) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) in a ternary porogenic solvent consisting of cyclohexanol/ethylene-glycol/water. While AMPS was meant to support the electroosmotic flow (EOF) necessary for transporting the mobile phase through the monolithic capillary, the PEDAS was introduced to provide the nonpolar sites for chromatographic retention. Monolithic columns at various EOF velocities were readily prepared by conveniently adjusting the amount of AMPS in the polymerization solution as well as the composition of the porogenic solvent. The monolithic stationary phases thus obtained exhibited reversed-phase chromatography behavior toward neutral solutes and yielded a relatively strong EOF. For charged solutes (e.g., dansyl amino acids), nonpolar as well as electrostatic interaction/repulsion with the monoliths were observed in addition to electrophoretic migration. Therefore, for charged solutes, selectivity and migration can be readily manipulated by changing various parameters including the nature of the monolith and the composition of the mobile phase (e.g., pH, ionic strength and organic modifier). Ultrafast separation on the time scale of seconds of 17 different charged and neutral pesticides and metabolites were performed using short capillary columns of 8.5 cm x 100 microm ID.

Capillary electrochromatography: a rapidly emerging separation method

This overview concerns the new chromatographic method--capillary electrochromatography (CEC)--that is recently receiving remarkable attention. The principles of this method based on a combination of electroosmotic flow and analyte-stationary phase interactions, CEC instrumentation, capillary column technology, separation conditions, and examples of a variety of applications are discussed in detail.

Surfactant-mediated capillary electrochromatography with octadecyl-silica- packed capillary columns for the separation of nonpolar compounds. Case of pyrethroid insecticides

Capillary electrochromatography (CEC) with octadecyl-silica-packed capillary columns was evaluated in the separation of nonpolar compounds, e.g., pyrethroid insecticides, using surfactant-rich mobile phases. This novel concept is referred to as surfactant-mediated capillary electrochromatography (SM-CEC), and is based on including a charged surfactant, namely sodium di-2-ethylhexyl sulfosuccinate (DOSS), in the mobile phase. Under these conditions, DOSS plays the role of a slowly moving pseudostationary phase so that solutes are partitioned between a mobile phase, a fixed stationary phase and a slowly moving pseudostationary phase. The SM-CEC system was investigated with pyrethroid insecticides over a wide range of DOSS and acetonitrile concentrations in the mobile phase. Pyrethroid insecticides, which are very hydrophobic solutes consisting of geometric isomers and diastereomers, were better resolved in SM-CEC than in straight CEC.

Recent applications of capillary electrochromatography

A review is presented of the most important recent applications of capillary electrochromatography (CEC) for the analysis of acidic, basic, and neutral compounds, of biomolecules, environmental substances, natural products, pharmaceuticals, and chiral compounds. Packed-column CEC (packed-CEC), open-tubular (OT-CEC), as well as pressure-assisted CEC (pseudo-CEC) are hereby considered. Papers published between July 1999 and April 2001 were taken into account. Applications before July 1999 have been reviewed in Electrophoresis 1999, 20, 3027-3065.

Capillary electrochromatography with polyacrylamide monolithic stationary phases having bonded dodecyl ligands and sulfonic acid groups: evaluation of column performance with alkyl phenyl ketones and neutral moderately polar pesticides

In this report, we describe the preparation of porous polyacrylamide-based monolithic columns via vinyl polymerization. These monoliths possess in their structures bonded dodecyl ligands and sulfonic acid groups. While the sulfonic acid groups are meant to support the electroosmotic flow (EOF) necessary for moving the mobile phase through the monolithic capillary, the dodecyl ligands are introduced to provide the nonpolar sites for chromatographic retention. However, incorporating the sulfonic acid groups in the monoliths does not only support the EOF but also exhibit hydrophilic interaction with moderately polar compounds such as urea herbicides and carbamates insecticides. Consequently, mixed-mode (reversed-phase/normal phase) retention behavior is observed with neutral and moderately polar pesticides. The amount of sulfonic acid group in the monolith can be conveniently adjusted by changing the amount of vinylsulfonic acid added to the polymerization reaction. Optimum EOF velocity and adequate chromatographic retention are obtained when 15% vinylsulfonic acid is added to the reaction mixture. Under these conditions, rapid separation and high plate counts reaching greater than 400000 plates/m are readily obtained.

Application of hydrophobic anion-exchange phases in capillary electrochromatography

Capillary electrochromatography (CEC) requires stationary phases that enable appropriate electroosmotic propel under various conditions. Analyte retention can be controlled through hydrophobic or electrostatic interaction with the packing material. The development and characterization of new strong anion-exchange materials with additional hydrophobic moieties (SAX/C18 mixed-mode phases) is described. The synthesis was based on polymer encapsulation of porous silica. The phases were systematically characterized by means of elemental analyses, HPLC frontal analyses and CEC experiments. The studies focused on the influence of various parameters (e.g., pH, kind of buffer, capillary wall) on the electroosmotic flow (EOF). Phases with high anion-exchange capacity generated a fast and constant EOF over a wide pH range. Long-time stability of EOF and hydrophobic retention under CEC conditions were demonstrated within the course of 100 consecutive injections. The applicability of the SAX/C18 phases in appropriate buffer systems is demonstrated for neutral, acidic and basic compounds.

Separation of 4-dimethylamino-6-(4-methoxy-1-naphthyl)-1,3,5-triazine-2-hydrazine derivatives of carbonyl compounds by reversed-phase capillary electrochromatography

4-Dimethylamino-6-(4-methoxy-1-naphthyl)-1,3,5-triazine-2-hydrazine (DMNTH) is a novel derivatizing reagent specially designed for the determination of carbonyl compounds. In this work, we describe the separation of DMNTH-derivatized carbonyl compounds by reversed-phase capillary electrochromatography (CEC). After systematic investigations of the effects of experimental conditions viz. pH and concentration of buffer, type of stationary phase, injection volume of sample, organic modifier, and temperature, optimal conditions were found. The sample compounds, which were separated with gradient high performance liquid chromatography (HPLC), were separated by CEC under isocratic elution due to the high efficiency. Comparisons of separations by CEC and micellar electrokinetic chromatography (MEKC) were made.

Use of vancomycin silica stationary phase in packed capillary electrochromatography I. Enantiomer separation of basic compounds

Chiral separation of basic compounds was achieved by using 75 or 100 microm ID fused-silica capillaries packed with a vanoomycin-modified diol silica stationary phase. The capillary was firstly packed for about 12 cm with a slurry mixture composed of diolsilica (3:1) then with the vancomycin modified diol-silica (3:1) (23 cm), and finally with diol-silica (3:1) for about 2 cm. Frits were prepared by a heating wire at the two ends of the capillary; the detector window was prepared at 8.5 cm from the end of the capillary where vancomycin was not present. The influence of the mobile phase composition (pH and concentration, organic modifier type and concentration) on the velocity of the electroosmotic flow, chiral resolution and enantioselectivity was studied. Good enantiomeric resolution was achieved for atenolol, oxprenolol, propranolol, and venlafaxine using a mobile phase composition of 100 mM ammonium acetate solution (pH 6)/water/acetonitrile (5:5:90 v/v/v) while for terbutaline a mixture of 5:15:80 v/v/v provided the best separations. The use of methanol instead of acetonitrile caused a general increase of enantiomer resolution of the studied compounds together with a reduction of efficiency and detector response. However, the combination of acetonitrile and methanol in the mobile phase (as, e.g., 10% methanol and 80% acetonitrile) allowed to improve the enantiomer resolution with satisfactory detector response.

Recent progress in capillary electrochromatography

Capillary electrochromatography (CEC) continues to captivate many separation scientists. A remarkable activity is apparent from the numerous publications in the literature using CEC. A review of the most recent progress in CEC is presented herein, covering an extensive fraction of the literature on CEC published from the year 1997 until the beginning of 2000. Most of the recent developments have concentrated on column technology.

Capillary electrochromatography with physically and dynamically absorbed stationary phases

Adsorption is always considered a troublesome effect in capillary electrophoresis (CE) and capillary electrochromatography (CEC). However, the adsorption effect can also be exploited to prepare or optimize the stationary phase in CEC. Compared with the chemical synthesis of new stationary phase materials for CEC, this method is simpler and more convenient. This review is focused on CEC with physically and dynamically adsorbed stationary phases. Separation of some acidic, basic and neutral solutes as well as enantiomers in CEC with dynamically adsorbed stationary phases are presented. The theory for the migration of charged solutes and the stationary phases currently used in CEC are also briefly reviewed.

Enantiomeric separation by capillary electrochromatography. II. Chiral separation of dansyl amino acids and phenoxy acid herbicides on sulfonated silica having surface-bound hydroxypropyl-beta-cyclodextrin

A chiral silica-based stationary phase having surface-bound hydroxypropyl-beta-cyclodextrin (HP-beta-CD) with a relatively strong electroosmotic flow (EOF) was introduced for enantioseparation by capillary electrochromatography (CEC). The stationary phase contained a hydrophilic sulfonated sublayer to which a chiral top layer of HP-beta-CD was immobilized. While the sulfonated sublayer was to provide a relatively strong EOF, the top HP-beta-CD was to confer the desired chiral recognition towards enantiomeric solutes. This HP-beta-CD sulfonated silica (CDSS) stationary phase proved useful for the rapid separation of anionic enantiomers such as dansyl amino acids and phenoxy acid herbicides. The effects of the organic modifier content, pH, and ionic strength of the mobile phase on enantioseparation were investigated. Under the optimized separation conditions, ten dansyl amino acids and six phenoxy acid herbicides were enantioseparated with a resolution greater than unity.

Sol-gel monolithic columns with reversed electroosmotic flow for capillary electrochromatography

Sol-gel chemistry was used to prepare porous monolithic columns for capillary electrochromatography. The developed sol-gel approach proved invaluable and generates monolithic columns in a simple and rapid manner. Practically any desired column length ranging from a few tens of centimeters to a few meters may be readily obtained. The incorporation of the sol-gel precursor, N-octadecyldimethyl[3-(trimethoxysilyl)propyl]ammonium chloride, into the sol solution proved to be critical as this reagent possesses an octadecyl moiety that allows for chromatographic interactions of analytes with the monolithic stationary phase. Additionally, this reagent served to yield a positively charged surface, thereby providing the relatively strong reversed electroosmotic flow (EOF) in capillary electrochromatography. The enhanced permeability of the monolithic capillaries allowed for the use of such columns without the need for modifications to the commercial CE instrument. There was no need to pressurize both capillary ends during operation or to use high pressures for column rinsing. With the developed procedure, no bubble formation was detected during analysis with the monolithic capillaries when using electric field strengths of up to 300 V cm(-1). The EOF in the monolith columns was found to be dependent on the percentage of organic modifier present in the mobile phase. Separation efficiencies of up to 1.75 x 10(5) plates/m (87,300 plates/column) were achieved on a 50 cm x 50 microm i.d. column using polycyclic aromatic hydrocarbons and aromatic aldehydes and ketones as test solutes.

Enantiomeric separation by capillary electrochromatography. I. Chiral separation of dansyl amino acids and organochlorine pesticides on a diol-silica dynamically coated with hydroxypropyl-beta-cyclodextrin

In this work, a commercially available diol-silica stationary phase was converted in situ to a chiral stationary phase by dynamically coating it with hydroxypropyl-beta-cyclodextrin (HP-beta-CD). This stationary phase was shown useful for the capillary electrochromatography (CEC) separation of neutral and anionic enantiomers such as some organochlorine pesticides and dansyl amino acids, respectively. The inclusion of HP-beta-CD in the mobile phase to produce the in situ chiral stationary phase allowed the rapid separation of the anionic dansyl amino acid enantiomers at relatively low electroosmotic flow (EOF). The formation of host-guest complexes between the dansyl amino acids and the neutral HP-beta-CD in the mobile phase lowered the actual charge-to-mass ratios of the anionic solutes, thus speeding up their transport by the EOF across the packed capillary column. Several parameters affecting enantioseparation were investigated, including the concentration of HP-beta-CD, ionic strength, pH, and organic modifier content of the mobile phase.

Investigations on the behaviour of acidic, basic and neutral compounds in capillary electrochromatography on a mixed-mode stationary phase

This work describes the separation of acidic, basic and neutral organic compounds as well as inorganic anions in a single run by capillary electrochromatography employing a stationary phase which exhibits both strong anion-exchange and reversed-phase chromatographic characteristics. The positive surface charge of this stationary phase provided a substantial anodic electroosmotic flow. The analytes were separated by a mixed-mode mechanism which comprised chromatographic interactions (hydrophobic interactions, ion-exchange) as well as electrophoretic migration. The influence of ion-exchange and hydrophobic interactions on the retention/migration of the analytes could be manipulated by varying the concentration of a competing ion and/or the amount of organic modifier present in the background electrolyte. Additionally the effects of pH changes on both the chromatographic interactions as well as the electrophoretic migration of the analytes were investigated.

Separation of acidic compounds by strong anion-exchange capillary electrochromatography

Separation of the acidic compounds in the ion-exchange capillary electrochromatography (IE-CEC) with strong anion-exchange packing as the stationary phase was studied. It was observed that the electroosmotic flow (EOF) in strong anion-exchange CEC moderately changed with increase of the eluent ionic strength and decrease of the eluent pH, but the acetonitrile concentration in the eluent had almost no effect on the EOF. The EOF in strong anion-exchange CEC with eluent of low pH value was much larger than that in RP-CEC with Spherisorb-ODS as the stationary phase. The retention of acidic compounds on the strong anion-exchange packing was relatively weak due to only partial ionization of them, and both chromatographic and electrophoretic processes contributed to separation. It was observed that the retention values of acidic compounds decreased with the increase of phosphate buffer and acetonitrile concentration in the eluent as well as the decrease of the applied voltage, and even the acidic compounds could elute before the void time. These factors also made an important contribution to the separation selectivity for tested acidic compounds, which could be separated rapidly with high column efficiency of more than 220000 plates/m under the optimized separation conditions.

Capillary electrochromatography with novel stationary phases. IV. Retention behavior of glycosphingolipids on porous and non-porous octadecyl sulfonated silica

In this investigation, capillary electrochromatography (CEC) with a novel stationary phase proved useful for the separation of neutral and acidic glycosphingolipids (GSLs). Four different gangliosides, namely G(M1a), G(D1a), G(D1b) and G(T1b), served as the acidic GSLs model solutes. The following four GSLs: galactosylceramide (GalCer), lactosylceramide (LacCer), globotriaosylceramide (Gb3Cer) and globotetraosylceramide (Gb4Cer) served as the typical neutral GSLs. The stationary phase, octadecyl sulfonated silica (ODSS), consisted of octadecyl functions bonded to a negatively charged layer containing sulfonic acid groups. Porous and non-porous ODSS stationary phases were examined. The retention behavior of the acidic and neutral GSLs was examined over a wide range of elution conditions, including the nature of the electrolyte and organic modifier and the pH of the mobile phase. The porous ODSS stationary phase yielded the separation of the four different gangliosides using a hydro-organic eluent of moderate eluent strength whereas the non-porous ODSS stationary phase permitted the separation of the four neutral GSLs with a mobile phase of relatively high eluent strength.

Proceedings of the 10th Annual Frederick Conference on Capillary Electrophoresis. Frederick, Maryland, USA. October 18-20, 1999

Since the introduction of the first commercial capillary electrophoresis (CE) instrument a decade ago, CE applications have become widespread. Today, CE is a versatile analytical technique which is successfully used for the separation of small ions, neutral molecules, and large biomolecules and for the study of physicochemical parameters. It is being utilized in widely different fields, such as analytical chemistry, forensic chemistry, clinical chemistry, organic chemistry, natural products, pharmaceutical industry, chiral separations, molecular biology, and others. It is not only used as a separation technique but to answer physicochemical questions. In this review, we will discuss different modes of CE such as capillary zone electrophoresis, micellar electrokinetic chromatography, capillary gel electrophoresis, capillary isoelectric focusing, and capillary electrochromatography, and will comment on the future direction of CE, including array capillary electrophoresis and array microchip separations.

Electrically driven microseparation methods for pesticides and metabolites: III. Capillary electrochromatography with novel silica-based stationary phases having a surface-bound surfactant moiety

A silica-based stationary phase with surface bound silylpropyl trialkylammonium functions was introduced and evaluated in the capillary electrochromatography of alkylbenzenes and pesticides. This stationary phase is referred to as octadecyldimethyl(3-trimethoxysilylpropyl) ammonium-silica (ODAS) and has quaternary amine functions that generate an anodic electroosmotic flow (EOF) and octadecyl functions that are responsible for solute retention by a reversed-phase chromatography mechanism. The ODAS stationary phase was characterized over a wide range of elution conditions in term of EOF and retention behavior of alkylbenzene homologous series. The ODAS stationary phase proved useful in the separation of pesticides as well as in the on-column preconcentration of dilute pesticide samples, thus permitting the detection of solution at 7 x 10(-7) M using a UV detector.