The analysis of basic pharmaceutical compounds by capillary electrochromatography using continuous bed stationary phase

The challenges of the analysis of basic compounds by high performance liquid chromatography: some possible approaches for improved separations

This review considers some of the difficulties encountered with the analysis of ionised bases using reversed-phase chromatography, such as detrimental interaction with column silanol groups, and overloading which both lead to poor peak shapes. Methods of overcoming these problems in reversed-phase (RP) separations, by judicious selection of the column and mobile phase conditions, are discussed. Hydrophilic interaction chromatography is considered as an alternative method for the separation of some basic compounds.

Simultaneous analysis of basic, acidic and neutral compounds on an endcapped octadecylsilane silica-based monolith by pressure-assisted capillary electrochromatography

This simultaneous separation of basic, acidic and neutral analytes by pressure-assisted CEC (pCEC) using a hybrid (tetramethoxysilane and methyltrimethoxysilane) silica-based monolith, chemically modified with octadecyldimethylchlorosilane followed by endcapping with hexamethyldisilazane is described. The endcapping resulted in near Gaussian peaks for highly basic analytes such as nortriptyline without a significant loss in the EOF. The migration behaviour of analytes on this phase could be rationalised based on hydrophobicity, electrophoretic mobility and ion-exchange interactions. The high porosity of the monolith allowed manipulation of the linear velocity of mobile phases by the addition of varying amounts of pressure at the inlet to reduce analysis times and overcome the reversed migration of anionic species towards the detection window in cathodic EOF mode. The concomitant programmed application of pressure (2-4 bar) and voltage (27 kV) facilitated the simultaneous separation of four cationic, four neutral and two anionic compounds in 6 min with efficiencies ranging from 41 000 to 94 000, 57 000 to 77 000 and 180 000 to 210 000 theoretical plates/metre, respectively. The % RSD values of migration times and efficiencies in pCEC mode were less than 3.6 and 7.9%, respectively (n = 5).

CEC separation of heterocyclic amines using methacrylate monolithic columns

Two methacrylate-based monolithic columns, one with a negatively charged group (sulfonic group) and another with a new monomer N,N-dimethylamino ethyl acrylate (DMAEA), were prepared and tested for the separation of basic compounds by CEC. This new monolithic stationary phase was prepared by the in situ polymerization of DMAEA with butyl methacrylate and ethylene dimethacrylate, using a ternary porogenic solvent consisting of water, 1-propanol and 1,4-butanediol. The performance of this column was evaluated by means of the analysis of a family of heterocyclic amines. Separation conditions such as pH, amount of organic modifier, ionic strength and elution mode (normal or counterdirectional flow) were studied. At the optimal running electrolyte composition, and using the counterdirectional mode, symmetrical electrochromatographic peaks were obtained, with the number of theoretical plates up to 30,000 and a good resolution between closely related peaks. The 2-acrylamido-2-methyl-1-propane-sulfonic acid column was used for CEC-MS, taking advantage of the compatibility of its elution mode (normal flow) with the MS coupling.

On-line preconcentration of organic anions in capillary electrophoresis by solid-phase extraction using latex-coated monolithic stationary phases

Quaternary ammonium functionalised polymeric latex particles were coated onto the wall of a fused-silica capillary or onto a methacrylate monolithic bed synthesised inside the capillary in order to create ion-exchange stationary phases of varying ion-exchange capacity. These capillaries were coupled in-line to a separation capillary and used for the solid-phase extraction (SPE), preconcentration and subsequent separation of organic anions by capillary electrophoresis. A transient isotachophoretic gradient was used for the elution of bound analytes from the SPE phase using two modes of separation. The first comprised a low capacity SPE column combined with a fluoride/octanesulfonate discontinuous electrolyte system in which peak compression occurred at the isotachophoretic gradient front. The compressed anions were separated electrophoretically after elution from the SPE preconcentration phase and resolution was achieved by altering the pH of the electrolyte in which the separation was performed. In the second approach, a latex-coated monolithic SPE preconcentration stationary phase was used in combination with a fluoride/perchlorate electrolyte system, which allowed capillary electrochromatographic separation to occur behind the isotachophoretic gradient front. This method permitted the removal of weakly bound anions from the SPE phase, thereby establishing the possibility of sample clean-up. The effect of the nature of the strong electrolyte forming the isotachophoretic gradient on the separation and also on the preconcentration step was investigated. Capillary electrochromatography of inorganic and organic species performed on the latex-coated monolithic methacrylate column highlighted the presence of mixed-mode interactions resulting from the incomplete coverage of latex particles onto the monolithic surface. Analyte preconcentration prior to separation resulted in compression of the analyte zone by a factor of 300. Improvement in the limit of detection of up to 10400 times could be achieved when performing the preconcentration step and the presented methods had limits of detection (S/N=3) ranging between 1.5 and 12 nM for the organic anions studied.

Capillary electrochromatography of therapeutic peptides on mixed-mode butylmethacrylate monoliths

In this study, a porous mixed-mode n-alkyl methacrylate-based monolith has been used in the separation of therapeutic peptides. While the sulfonic acid (SCX) moiety derived from 2-acrylamido-2-methyl-1-propanesulfonic acid supports the generation of a stable electroosmotic flow (EOF) at both acidic and basic pH values, the butyl ligands provide the nonpolar sites for chromatographic resolution. The performance of the monolith was evaluated regarding the influence of pH on chromatographic resolution of peptides. The suitability of the butylmethacrylate/SCX monolith for the analysis of therapeutic peptides containing basic centres, for example arginine, at moderately high pH 9.5 and the stability to repeat injections of a mixture of peptides was demonstrated. Separations with efficiencies as high as 5.0 x 10(5) plates/m were obtained and the migration behaviour of the peptides at both low (2.8) and high (9.5) pH values could be rationalised based on their charge, molecular mass/shape and relative hydrophobicities.

Latex-Coated Polymeric Monolithic Ion-Exchange Stationary Phases. 1. Anion-Exchange Capillary Electrochromatography and In-Line Sample Preconcentration in Capillary Electrophoresis

A sulfonated methacrylate monolithic polymer has been synthesized inside fused-silica capillaries of diameters 50-533-microm i.d. and coated with 65-nm-diameter fully functionalized quaternary ammonium latex particles (AS18, Dionex Corp.) to form an anion-exchange stationary phase. This stationary phase was used for ion-exchange capillary electrochromatography of inorganic anions in a 75-microm-i.d. capillary with Tris/perchlorate electrolyte and direct UV detection at 195 nm. Seven inorganic anions (bromide, nitrate, iodide, iodate, bromate, thiocyanate, chromate) could be separated over a period of 90 s, and the elution order indicated that both ion exchange and electrophoresis contributed to the separation mechanism. Separation efficiencies of up to 1.66 x 10(5) plates m(-1) were achieved, and the monoliths were stable under pressures of up to 62 MPa. Another latex-coated monolith in a 250-microm-i.d. capillary was used for in-line preconcentration by coupling it to a separation capillary in which the EOF had been reversed using a coating of either a cationic polymer or cationic latex particles. Several capillary volumes of sample were loaded onto the preconcentration monolith, and the analytes (inorganic anions) were then eluted from the monolith with a transient isotachophoretic gradient before being separated by electrophoresis in the separation capillary. Linear calibration curves were obtained for aqueous mixtures of bromide, nitrite, nitrate, and iodide. Recoveries of all analytes except iodide were reduced significantly when the sample matrix contained high levels of chloride. The preconcentration method was applied to the determination of iodide in open ocean water and provided a limit of detection of 75 pM (9.5 ng/L) calculated at a signal-to-noise ratio of 3. The relative standard deviation for migration time and peak area for iodide were 1.1 and 2.7%, respectively (n = 6). Iodide was eluted as an efficient peak, yielding a separation efficiency of 5.13 x 10(7) plates m(-1). This focusing was reproducible for repeated analyses of seawater.

Polymerised bicontinuous microemulsions as stationary phases for capillary electrochromatography

Monolithic columns for capillary electrochromatography (CEC) were prepared by in situ polymerisation of bicontinuous microemulsions containing butyl methacrylate. The resulting monoliths were found to be permeable to mobile phase flow and their behaviour as CEC stationary phases was investigated. It was found that the monoliths were able to separate a simple test mixture of phthalates, but that efficiencies were low. However, several advantages of the monoliths compared to conventional ODS packed columns were found: preparation time is short, many columns can be prepared from the same batch of microemulsion and column conditioning is much faster. The columns show promise as stationary phases for CEC, but more development is required to improve efficiencies.

Development and application of polymeric monolithic stationary phases for capillary electrochromatography

Monolithic columns for capillary electrochromatography are receiving quite remarkable attention. This review summarizes results excerpted from numerous papers concerning this rapidly growing area with a focus on monoliths prepared from synthetic polymers. Both the simplicity of the in situ preparation and the large number of readily available chemistries make the monolithic separation media a vital alternative to capillary columns packed with particulate materials. Therefore, they are now a well-established stationary phase format in the field of capillary electrochromatography. A wide variety of synthetic approaches as well as materials used for the preparation of the monolithic stationary phases are presented in detail. The analytical potential of these columns is demonstrated with separations involving various families of compounds and different chromatographic modes.