Advances in column technology and instrumentation in capillary electrochromatography

Rapid Formation of Polymer Frits in Fused Silica Capillaries Using Spatially defined Thermal Free-Radical Initiated Polymerization

Column preparation in capillary chromatography commonly relies upon the generation of on-column porous frits. Here, we report a simple, robust and low-cost approach for preparing polymer frits on-column, in a rapid and spatially controlled manner using thermal free-radical initiated polymerization. In this approach, a simple, temperature-controlled heating apparatus is positioned adjacent to a 100 μm i.d. fused-silica capillary for a defined duration. Frits were synthesized in 3-(trimethoxysilyl)propyl methacrylate modified capillaries using a monomer solution of 2,2-azobisisobutyronitrile, glycidyl methacrylate, ethylene glycol dimethacrylate, and decanol. Frit length and stability were investigated as a function of polymerization time and temperature. Frit length was easily controlled via a combination of polymerization time and temperature and position was readily controlled using a simple mechanical placement jig. Thermal initiated frits were stable throughout column packing and did not require removal of the capillary polyimide coating. The thermal initiation approach offers higher throughput, with polymerization times of < 2 min compared to ≥ 30 min for UV-initiated polymerization and significantly reduces the cost, enabling broader access to on-column frit technology for a variety of capillary separation applications.

Cholesterol-based polymeric monolithic columns for capillary liquid chromatography

A novel, cholesterol-based polymeric monolithic stationary phase for capillary liquid chromatography, was prepared by thermally initiated in-situ polymerization. Cholesteryl methacrylate (CholMA) was used as a functional monomer and trimethylolpropane trimethacrylate (TRIM) was a cross-linker, while azobisisobutyronitrile (AIBN) was an initiator. Isooctane and toluene were chosen as "poor" and "good" solvent, respectively, as constituents of the porogen solvent. Isocratic elutions of alkylbenzenes and separation of the testing mixture of o-terphenyl and triphenylene were conducted for all of the monoliths to assess their hydrophobicity and planar selectivity characteristic for cholesterol-based stationary phases. The synthesized columns demonstrated efficiency exceeding N=10,000 plates and a plate height of ca. H=30 μm. Column preparation was found to be highly reproducible; the relative standard deviation (RSD) values (n=3) for day-to-day and column-to-column were less than 4.08 and 2.02%, respectively, based on retention factor of alkylbenzenes.

Recent advances on capillary columns, detectors, and two-dimensional separations in capillary electrochromatography

As a typical miniaturized analytical technique, CEC has attracted much attention because of its low sample and solvent consumption, high efficiency, high selectivity, high resolution, and fast speed. In this review, we mainly cover the development of capillary columns and detection techniques in the CEC since 2009. Herein, three types of capillary columns, namely, open-tubular capillary columns, monolithic columns and packed columns, and several types of detectors are reviewed in detail. Moreover, a 2D separation system based on CEC is also reported.

Chiral selectors in CE: recent developments and applications

This review article provides an overview of the recent advances in enantioanalysis by use of electrophoretic techniques. Due to the big number of publications in the subject mentioned above, this article is focused on chiral method developments and applications published from 2008 until 2011, and it demonstrates chiral selectors used in CE. Numerous chiral selectors have been used over the years, and these include the cyclic and the linear oligo- and polysaccharides, the branched polysaccharides, the polymeric and monomeric surfactants, the macrocyclic and other antibiotics, and the crown ethers. Different dual-selector systems are also presented in this article, and the results are compared with those obtained by use of a single chiral selector. Finally, several pharmaceutical and biomedical applications based on chiral recognition are summarized.

Silica-based zwitterionic monolithic stationary phase for separation of neutral and ionized solutes using pressurized CEC

A porous zwitterionic monolith was prepared by in situ covalent attachment of lysine to a gamma-glycidoxypropyltrimethosysilane-modified silica monolith. The prepared column was used to perform neutral and ionized solutes separations by pressurized (pCEC). Due to the zwitterionic nature of the resulting stationary phase, the monolithic column provided both electrostatic attraction and repulsion sites for electrochromatographic retention for ionized solutes. Separation of several nucleotides was investigated on the monolithic column. It was shown that the nucleotides could be separated based on hydrophilic and electrostatic interactions between the stationary phase and analyte. Besides, the separation property of the zwitterionic silica monolith was compared with the use of diamine-bonded silica monolith as stationary phase. As expected, the lysine monolith exhibited a lower retention for the five nucleotides, which was due to the dissociation of the external carboxylic acid groups, leading to electrostatic repulsion with negatively charged solutes. Under the same experimental conditions, separation of the five nucleotides on the zwitterionic column was in less than 8 min, while that on the diamine column was in approximately 60 min.

Magnetically immobilized frits for the preparation of packed columns used in capillary electrochromatography

Fabrication of porous frits to retain stationary phases is a critical issue in column preparation for capillary electrochromatography (CEC). In this work, porous frits were prepared by applying an external magnetic field to magnetically responsive particles placed inside a fused-silica capillary. Three batches of uniform magnetite spheres with particle diameters of 0.3, 0.4, and 0.6 microm and saturation magnetization values of 73.03, 74.41, and 77.83 emu/g, respectively, were used as frit particles and octadecyl- and phenyl-bonded silica gels were packed successfully into frit-containing capillaries. The performance of the resulting magnetically immobilized frits and packed columns was evaluated. The electroosmotic mobilities in capillaries containing outlet frit only were found to be reduced by 2-4% whereas the plate heights of an unretained marker increased by 30-50% as compared to those in open capillaries. These variations are believed to be associated with the inhomogeneities of the packed structure of the frits. The magnetically immobilized frits showed adequate mechanical strength to withstand the flow drag force, allowing separation in capillaries packed with 5-mum stationary phases up to 10-15 cm, thus rendering column efficiency and reproducibility comparable with those obtained with sintered frits. Taken together, retaining frits made of uniform magnetite particles serves as a viable alternative to sintered frits for column preparation, which offers several distinct advantages such as ease of preparation, improved durability as compared to sintered frits where the removal of the polyimide coating makes the packed column susceptible to breakage, and use of large-bore capillaries for semipreparative separations.

Methacrylate-based monolithic column with mixed-mode hydrophilic interaction/strong cation-exchange stationary phase for capillary liquid chromatography and pressure-assisted CEC

A novel porous polymethacrylate-based monolithic column by in situ copolymerization of 3-sulfopropyl methacrylate (SPMA) and pentaerythritol triacrylate in a binary porogenic solvent consisting of cyclohexanol/ethylene glycol was prepared. The monolith possessed in their structures bonded sulfonate groups and hydroxyl groups and was evaluated as a hydrophilic interaction and strong cation-exchange stationary phases in capillary liquid chromatography (cLC) and pressure-assisted CEC using small polar neutral and charged solutes. While the SPMA was introduced as multifunctional monomer, the pentaerythritol triacrylate was used to replace ethylene glycol dimethacrylate as cross-linker with much more hydrophilicity due to a hydroxyl sub-layer. The different characterization of monolithic stationary phases were specially designed and easily prepared by altering the amount of SPMA in the polymerization solution as well as the composition of the porogenic solvent for cLC and pressure-assisted CEC. The resulting monolith showed the different trends about the effect of the permeabilities on efficiency in the pressure-assisted CEC and cLC modes. A typical hydrophilic interaction chromatography mechanism was observed at higher organic solvent content (ACN%>70%) for polar neutral analytes. For polar charged analytes, both hydrophilic interaction and electrostatic interaction contributed to their retention. Therefore, for charged analytes, selectivity can be readily manipulated by changing the composition of the mobile phase (e.g., pH, ionic strength and organic modifier). With the optimized monolithic column, high plate counts reaching greater than 170 000 plates/m for pressure-assisted CEC and 105 000 plates/m for cLC were easily obtained, respectively.

Preparation of a mixed-mode hydrophilic interaction/anion-exchange polymeric monolithic stationary phase for capillary liquid chromatography of polar analytes

A novel cationic hydrophilic interaction monolithic stationary phase based on the copolymerization of 2-(methacryloyloxy)ethyltrimethylammonium methyl sulfate (META) and pentaerythritol triacrylate (PETA) in a binary porogenic solvent consisting of cyclohexanol/ethylene glycol was designed for performing capillary liquid chromatography. While META functioned as both the ion-exchange sites and polar ligand provider, the PETA, a trivinyl monomer, was introduced as cross-linker. The monolithic stationary phases with different properties were easily prepared by adjusting the amount of META in the polymerization solution as well as the composition of the porogenic solvent. The hydrophilicity of the monolith increased with increasing content of META in the polymerization mixture. A typical hydrophilic interaction chromatography mechanism was observed when the content of acetonitrile in the mobile phase was higher than 20%. The poly(META-co-PETA) monolith showed very good selectivity for neutral, basic and acidic polar analytes. For polar-charged analytes, both hydrophilic interaction and electrostatic interaction contributed to their retention. Peak tailing of basic compounds was avoided and the efficient separation of benzoic acid derivatives was obtained.

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.

Preparation and evaluation of C18-bonded 1-µm silica particles for pressurized capillary electrochromatography

Nonporous silica spheres (1 microm) were synthesized and bonded with octadecylsilane functionality. These stationary phase particles were packed electrokinetically into fused-silica capillaries with 100 microm id for a length of 20 cm, which was evaluated by using pressurized CEC (pCEC). The efficiency of the C18 RP column was characterized through the theoretical plates of thiourea, benzyl alcohol, toluene, styrene, and naphthalene. The effects of experimental parameters such as the applied voltage, sample size, pump flow rate, pH value and the concentration of the buffer solution, and the content of methanol in the mobile phase, on-column efficiency were evaluated. Column efficiency as high as 200 000 theoretical plates per meter for naphthalene was obtained with the optimal condition of 70% v/v methanol and 30% v/v of 10 mmol/L phosphate buffer (pH 7.8) at an applied voltage of 10 kV and a supplementary pressure of 500 psi.

Reversed-phase and weak anion-exchange mixed-mode silica-based monolithic column for capillary electrochromatography

A silica-based CEC monolithic column with mixed modes of RP and weak anion-exchange (WAX) was successfully prepared by using the sol-gel technique at mild temperature. The synthesizing procedure was optimized by changing the ratios of tetraethoxysilane (TEOS), aminopropyltriethoxysilane (APTES), and octyltriethoxysilane (C(8)-TEOS) in the mixture. While serving as WAX group, the amino group dominated the charge on the surface of the capillary column and generated an EOF from cathode to anode at low pH. At pH above 7.5, a cathodic EOF was observed due to the full ionization of silanol group and the suppression in the ionization of amino group. The morphology of monolithic columns was examined by SEM, and the performance of column was evaluated in detail by separating different kinds of compounds. As expected, the monolithic column exhibited RP chromatographic behavior for neutral solutes. Fast and efficient separation of six aromatic acids was obtained using acidic mobile phase with column efficiency up to 160,000 plates/m. Symmetrical peaks can be obtained for aromatic amines because positively charged amino groups on the surface can effectively minimize the adsorption of positively charged analytes to the stationary phase.

Preparation of particle-fixed silica monoliths used in capillary electrochromatography

Fused-silica capillarieswere packed with porous 1 microm bare silica microspheres and immobilized by potassium silicate-formamide in order to obtain columns with silica-based monolithic packing. After curing, the particle-fixed monolithic columns were octadecylated insitu with dimethyloctadecylchlorosilane. The columns were mechanically strong and permeable. No noticeable loss in efficiency was found after using a column continuously for 1 month. The performances of the particle-fixed silica monolithic columns were evaluated for CEC under RP conditions. High separation efficiency (about125 200 plates/m) was obtained by using these new types of columns.

Self-made frits for nanoscale columns in proteomics

We report here the production of self-made frits for nano-columns. The frits introduce a minor dead volume and can be placed in capillaries with a wide range of diameters (20-250 microm tested) in an extremely simple and low-cost procedure. The obtained columns appear to be comparable to "no-frit" columns with near-ideal chromatographic characteristics. We expect that this frit will be useful for the spotting of gradients onto MALDI plates but also where special ESI set-ups do not allow for "no-frit" solutions.

Selection of two reliable parameters to evaluate the impact of the mobile-phase composition on capillary electrochromatography performance with monolithic and particle-packed capillary columns

Different models have been described in the literature to evaluate the total porosity of CEC columns: gravimetric, flow as well as conductivity-based methods. In this study, these models have been compared for two kinds of CEC columns: two mixed-mode silica particle stationary phases and different monolithic columns (acrylate or polystyrene divinylbenzene-based). The total porosities measured from the conductivity-based methods were lower than the total column porosities obtained by gravimetric or flow methods for all the investigated columns while the wide distribution of observed values shows that conductivity-based methods discriminate columns more efficiently with very different properties. We propose a conductivity-based method taking into account the actual length proposed by Horvath, to evaluate what we call an "actual electrokinetic" porosity (AEP). This parameter, based on electrokinetic theory only, affords the most consistent evaluation of porosity under experimental CEC conditions for the packed- and acrylate-based monolithic columns. To illustrate the potential of AEP and actual EOF for the estimation of the performances of a CEC system (stationary and mobile phases) we studied the influence of the mobile-phase composition on these parameters for CEC separations with an ammonium embedded packed stationary phase. The AEP and the actual electroosmotic mobility should allow a better understanding of the perfusive EOF and stationary-phase wettability. For neutral compounds (substituted phenols), AEP evaluation allowed us to predict the mobile-phase conditions able to enhance the efficiency while both AEP and actual EOF had to be considered in the case of peptide analysis.

Capillary Electrochromatography−Mass Spectrometry of Zwitterionic Surfactants

This work describes the on-line hyphenation of a packed capillary electrochromatography (CEC) column with an internally tapered tip coupled to electrospray ionization-mass spectrometry (ESI-MS) and atmospheric pressure chemical ionization-mass spectrometry (APCI-MS) for the analysis of betaine-type amphoteric or zwitterionic surfactants (Zwittergent). A systematic investigation of the CEC separation and MS detection parameters comparing ESI and APCI is shown. First, a detailed and optimized manufacturing procedure for fabrication of the CEC-MS column with a reproducible internally tapered tip (7-9 microm) is presented. Next, the optimization of the separation parameters by varying the C(18) stationary-phase particle size (3 versus 1.5 microm), as well as mobile-phase composition including acetonitrile (ACN) volume fraction, ionic strength, and pH is described. The optimized separation is achieved using 3-microm C(18) packing with 75% ACN (v/v), 5 mM Tris at pH 8.0. Optimization for on-line CEC-ESI-MS detection is then done varying both the sheath liquid and spray chamber parameters while evaluating the use of random versus structured factorial table experimental designs. The more structured approach allows fundamental analysis of individual ESI-MS parameters while minimizing CEC and MS equilibration time between settings. A comparison of CEC-ESI-MS to CEC-APCI-MS using similar sheath and spray chamber conditions presents new insight for coupling of CEC to APCI-MS. The sheath liquid flow rate required to maintain adequate sensitivity is much higher in APCI source (50 microL/min) as compared to the ESI source (3 microL/min). The on-line mass spectra obtained in the full scan mode show that fragmentation in the two sources occurs at different positions on the Zwittergent molecules. For ESI-MS, the protonated molecular ion is always highest in abundance with minor fragmentation occurring due to the loss of the alkyl chain. In contrast, the APCI-MS spectra show that the highest abundant ion resulted by elimination of propane sulfonate from the Zwittergent molecule. A comparison of the sensitivity between the two sources in positive ionization SIM mode shows that CEC-ESI-MS provides an impressive limit of detection (LOD) of 5 ng/mL, which is at least 3 orders of magnitude lower than CEC-APCI-MS (LOD 100 microg/mL). Finally, the optimized CEC-MS methods comparing ESI and APCI are applied for separation and structural characterization of a real industrial zwittergent sample, Rewoteric AM CAS.

Review coupling of capillary electrochromatography to mass spectrometry

This review discusses the development of capillary electrochromatography (CEC) coupled to mass spectrometric (MS) detection over the last few years. Major topics addressed are instrumental setups employed and applications of this technology published in the recent literature. The instrumental section includes a discussion of the most commonly used interfaces for the hyphenation of CEC and MS as well as ionization techniques. Applications reviewed in this paper come from a variety of different fields such as the analysis of biomolecules like proteins, peptides, amino acids or carbohydrates, chiral separations or the analysis of pharmaceutical an their metabolites in a series of matrices.

Direct observation of frits and dynamic air bubble formation in capillary electrochromatography using confocal fluorescence microscopy

Confocal fluorescence microscopy has been used to study the capillary electrochromatography (CEC) frits and dynamic air bubble formation under real chromatographic conditions. Confocal fluorescence microscopy provides a nondestructive way to view the three-dimensional structure of the frits with high spatial resolution. Frits prepared with four different procedures were studied: (1) sintering bare silica beads with sodium silicate; (2) sintering bare silica beads wetted with water; (3) sintering C18 beads wetted with water; and (4) sintering C18 beads wetted with water and then surfaced-recovered with C18. Frits prepared with sintering silicate-wetted beads have a high degree of heterogeneity, while the other three types of frits have similar, more homogeneous packing structures. Confocal fluorescence microscopy also provides sufficient temporal resolution for in situ observation of the dynamic processes in air bubble formation. In this study, air bubble formation is imaged during the reorganization process of the packing bed and is shown to occur close to the border between the packing bed and the outlet frit. Confocal fluorescence microscopy opens a new avenue in studying dynamic processes in situ in CEC separations.

Exact electric field strength over the packed capillary electrochromatography column bed, theoretical considerations and practical determination

On the basis of the experiments, it was proved that Ohm's law holds true in a capillary electrochromatography (CEC) column. By using the additivity of the potential drop over the packed and unpacked sections, the exact values of the electric field strength over the packed CEC column bed were determined experimentally. The ratio of the resistance of packed CEC column sections to that of an open capillary of the same length (Rp/Ro,Lp) were calculated. Some theoretical calculations were made to show how some structural parameters of particle lattice affect the resulting Rp/Ro,Lp value of the column bed formed accordingly. It is suggested that to correctly elucidate the EOF phenomenon in a CEC column, the potential drop over the packed and unpacked column sections, Ep and Eo, should be accounted for respectively, rather than using the average field strength over the total column length.

Sample injection in capillary electrochromatography by heart-cut technique

The splitter working in heart-cut regime was used for sample injection in capillary electrochromatography. The principle was implemented in an automated microgradient system allowing to inject from microlitre down to nanolitre volumes with high repeatability and minimal extra-column band broadening. The apparatus is able to deliver discrete volumes of liquids at a preset volumetric flow rate and to stop and restore the flow at any moment. This brings a high degree of liquid manipulation flexibility. An extremely low split ratio is sufficient during the analysis, which saves mobile phase consumption substantially. The key parameters influencing the function of the heart-cut splitter were characterised. The function of the apparatus was demonstrated under isocratic, preconcentration and gradient capillary electrochromatography separation conditions. In all cases the statistic evaluation of the main parameters was performed, showing that high repeatability of retention times, peak heights and areas was achieved.

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.

Comparison of Frits Used in the Preparation of Packed Capillaries for Capillary Electrochromatography

The performance of several types of frits, including sintered frits, photopolymerized frits, and frits made by sol-gel technologies, is evaluated. The frits are formed in open capillaries, and the electroosmotic mobilities are calculated and compared to those obtained in an open capillary. Run-to-run, day-to-day, and column-to-column reproducibilities are evaluated. In all cases, the run-to-run reproducibility varied by < 4%. The greatest mobility was through the capillaries with the sol-gel frits (1.29 x 10(-3) cm2/V x s) which also exhibited the best day-to-day reproducibility. The capillaries with the photopolymerized frits had the best column-to-column reproducibility, yet the slowest mobility (1.00 x 10(-3) cm2/V x s). The second central moment of benzene as a solute is calculated and plotted as a function of time to estimate the differences in peak dispersion among the various frits studied. Although the mobilities through the capillaries with the sintered frits were the least reproducible, these frits are associated with the least amount of band-broadening. An estimation of the dispersion caused by each of the different types of frits is reported.

Automated microgradient system for capillary electrochromatography

A microprocessor controlled gradient elution system suitable for capillary electrochromatography has been developed and tested. It is based on a liquid handling device described previously which is capable of liquid transport with both low and high fluid dispersion. The low dispersion region formed by stainless steel needle 250 microm I.D. serves for sample injection, while the high dispersion region, created by steep extension of tube diameter, is used for continuous mobile phase gradient generation. A homologous series of seven alkylphenones was electrochromatographically separated on a monolithic polyacrylic column under gradient conditions. An S-shaped acetonitrile gradient (30-70%) was applied. A high reproducibility of retention times (RSD about 0.1%) was obtained, indicating accuracy of automated gradient operations.

Separation of peptides by pressurized capillary electrochromatography

A pressurized electrochromatography (pCEC) instrument with gradient capability was used in this work for separation of peptides. Three separation modes, namely, pCEC, high-performance liquid chromatography and capillary electrophoresiscan be carried out with the instrument. In pCEC mode, the mobile phase is driven by both electroosmotic flow and pressurized flow, facilitating fine-tuning in selectivity of neutral and charged species. A continuous gradient elution can be carried out conveniently on this instrument, which demonstrates that it is more powerful than isocratic pCEC for separation of complicated samples. The effects of applied voltage, supplementary pressure and ion-pairing agents on separation of peptides in gradient pCEC were investigated. The effects of flow-rate of the pump and the volume of the mixer on resolution were also evaluated.

Packing structure and self-heating in capillary electrochromatography

The origin of bubble formation during operation of capillary electrochromatography (CEC) has been an issue of debate. Ohmic heating resulted from current passed through a packed column was proposed as the primary cause. However, this explanation has been questioned on the ground that the current measured in CEC is much lower than that measured with open-tubular separation systems where no bubble formation occurs. To resolve this issue, we carried out a theoretical study correlating self-heating of the electrolyte with packing structure of the column. We used a bundle of capillary tubes, a bundle of two types of capillary tubes and two bundles of capillary tubes connected serially to model, respectively, the flow channels in the column of non-porous particles, in the column of porous particles and in the column of various packing densities. The results from this study indicate that, for columns of homogeneous packing density, the heat output is indeed smaller than that in open-tubular columns of the same dimensions. In this case, the self-heating cannot be a key factor responsible for the bubble formation in CEC. However, for columns of heterogeneous packing density, a large excess of heat release may be produced in column sections of high packing density and, in turn, over-heating in such sections may become the primary cause for the formation of bubbles. It follows from this study that preparation of columns of homogeneous packing structure is essential to obtain reproducible and bubble-free CEC systems.

Measurement of packing tortuosity and porosity in capillary electrochromatography

The application of conductivity measurements for packing structure characterization has been extended to a column consisting of a packed section and an open section as typically used in capillary electrochromatography (CEC). Because of the difference in electric conductivity between the packed and open sections, the electric fields applied across the two sections vary, depending on the length of the packed section relative to that of the total column. On the basis of mass conservation law, it can be shown that the ratio of the electric current measured in such a duplex column to that without packing is a function of the length and the geometric structure of the packing bed. Thus, knowing the lengths of the packed section and the whole column, we can readily calculate the obstructive factors, such as the porosity and the tortuosity factor, from the measured conductivity ratio. An example is given to demonstrate the application of this method, with experimental data taken from published work.

Nanoparticles as pseudostationary phase in capillary electrochromatography/ESI-MS

A novel technique that uses polymer nanoparticles as pseudostationary phase in capillary electrochromatography with electrospray ionization mass spectrometry detection is described. A continuous full filling technique in which the nanoparticles were suspended in the entire electrolyte volume as well as a conventional partial filling technique is presented. No nanoparticles entered the mass spectrometer, which was fitted with an orthogonal electrospray interface, despite the continuous flow of nanoparticles into the interface. Nanoparticles (average diameter 160 nm) were prepared from methacrylic acid, methyl methacrylate, and trimethylolpropane trimethacrylate by utilizing a precipitation polymerization technique. Salbutamol, nortriptyline, and diphenhydramine were used as analytes. The interaction between analytes and nanoparticles was found to be predominantly ionic.

A new type of capillary column for open-tubular electrochromatography

A new type of open-tubular C(18) ester-bonded electrochromatographic column was prepared with sol-gel technology, followed by an on-column octadecyl silylation reaction. Glycidoxypropyltrimethoxysilane, a widely used and important silane agent, was used as the sol-gel precursor to form a thin coating layer on the wall of the fused-silica capillary. The C(18) groups were introduced into the coating layer by on-column esterification reaction with stearic acid. The electrochromatography behavior of the column was evaluated in terms of the separation of peptides. A high efficiency of 4.8x10(5) plates/m was achieved for a basic pentapeptide using the C(18 )ester-bonded column. In comparison with bare capillaries and glycidoxypropyltrimethoxysilane sol-gel-coated capillaries, the C(18) ester-bonded column showed the best separation of a mixture of seven pentapeptides under identical conditions of buffer, pH, and applied voltage.

Metal complex-substituted polysiloxanes as novel coatings for capillary electrophoresis and capillary electrochromatography

Two novel polysiloxane-based polymers, which contain metal complexes, have been prepared. To prepare the Co(TACN)3+(2) (TACN= 1,4,7-triazacyclononane) based polymers, an orthoamide derivative of TACN was added to bromobutane-substituted methylpolysiloxane and hydrolyzed with base. Co(II) was then coordinated to the TACN, followed by cobalt oxidation to make polymer A or followed by N-octyl TACN coordination and cobalt oxidation to make polymer B. In both materials, TACN forms thermodynamically and kinetically stable Co(TACN)3+(2) complexes in which the six coordination sites of the Co(III) are occupied by nitrogens from the TACN. The polymers were coated on fused-silica capillary columns and spherical silica particles, which were used for capillary electrophoresis and capillary electrochromatography, respectively. The open and packed columns showed strong and pH-independent reversed electroosmotic flow.