Improved column preparation and performance in capillary electrochromatography

Integrated liquid chromatography-heated nebulizer microchip for mass spectrometry

A new integrated microchip for liquid chromatography-mass spectrometry (LC-MS) is presented. The chip is made from bonded silicon and glass wafers with structures for a packed LC column channel, a micropillar frit, a channel for optional optical detection, and a heated vaporizer section etched in silicon and platinum heater elements on the glass cover. LC eluent is vaporized and mixed with nebulizer gas in the vaporizer section and the vapor is sprayed out from the chip. Nonpolar and polar analytes can be efficiently ionized in the gas phase by atmospheric pressure photoionization (APPI) as demonstrated with polycyclic aromatic hydrocarbons (PAHs) and selective androgen receptor modulators (SARMs). This is not achievable with present LC-MS chips, since they are based on electrospray ionization, which is not able to ionize nonpolar compounds efficiently. The preliminary quantitative performance of the new chip was evaluated in terms of limit of detection (down to 5 ng mL(-1)), linearity (r>0.999), and repeatability of signal response (RSD=2.6-4.0%) and retention time (RSD=0.3-0.5%) using APPI for ionization and PAHs as standard compounds. Determination of fluorescent compounds is demonstrated by using laser-induced fluorescence (LIF) for detection in the optical detection channel before the vaporizer section.

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.

Preparation of fritless capillary using avidin immobilized magnetic particles for electrochromatographic chiral separation

In capillary electrochromatography (CEC), magnetic particles (MPs) were packed in a fused silica capillary by using the magnetic field to be retained without frits. For a chiral CEC separation, avidin was immobilized onto the surface of the MPs (AVI-MPs) as a stationary phase by using the physical adsorption technique. The injected AVI-MPs into the capillary were stably captured with the magnet (surface magnetic flux density, 250 mT) under the separation voltage of 10 kV (190 V/cm). By employing the fritless AVI-MPs packed capillary, the chiral separation of ketoprofen was successfully attained with the packing length of only 5 cm. Effects of the modification condition of avidin, pH of background solution, and the packing length on the enantioseparation were also investigated. Under the optimal condition, furthermore, the repeatability for the retention time of ketoprofen was better than 1.5% in the relative standard deviation and the capillary-to-capillary reproducibility was also acceptable in the prepared fritless capillaries.

Practical aspects of using methacrylate-ester-based monolithic columns in capillary electrochromatography

Methacrylate-ester-based monoliths containing quaternary ammonium groups were prepared in situ in capillary columns and in simultaneous experiments in vials, employing thermal initiation. The chromatographic properties of the monoliths were determined with capillary electrochromatography (CEC), and their morphology was studied with mercury-intrusion porosimetry on the bulk materials. Materials with different, well repeatable pore-size distributions could be prepared. A satisfactory column-to-column and run-to-run repeatability was obtained for the electro-osmotic mobility, the retention characteristics (k-values) and the efficiency on the columns prepared and tested in the CEC mode. A relatively high electro-osmotic flow was observed in the direction of the positive electrode. The electro-osmotic mobility was found to be influenced only marginally by mobile-phase parameters such as the pH, ionic strength, and acetonitrile content. The retention behavior of the monolithic columns was similar to that of columns packed with C18-modified silica particles. Columns could be prepared with optimum plate heights ranging from 6 microm for unretained compounds to 20 microm for well retained (k=2.5) polyaromatic hydrocarbons. However, for specific analytes a - still unexplained - lower chromatographic column efficiency was observed.

Fabrication of Internally Tapered Capillaries for Capillary Electrochromatography Electrospray Ionization Mass Spectrometry

In this study, we report a novel procedure for fabricating internally tapered capillary columns suitable for the coupling of capillary electrochromatography (CEC) to electrospray mass spectrometry (ESI-MS). The internal tapers were prepared by slowly heating the capillary end in a methane/O2 flame. Due to continuous self-shrinking of the inner channel of the capillary, the inside diameter of the opening was reduced to 7-10 microm. The procedure is easy to handle, with no requirement for expensive equipment as well as elimination of problematic grinding of the tip. Several advantages of these new internal tapers, as compared to using externally tapered columns, are described. First, the problems of poor durability and tip breakage associated with external tapering were successfully overcome with the internal taper. A comparison of the online CEC/ESI-MS between external versus internal tapers showed that the latter provides enhanced electrospray stability, resulting in significantly lower short-term noise and very short-term noise values. In turn, the more rugged design of internal tapers allows performing CEC/MS utilizing a harsh polar organic mobile phase, which was not previously successful using an external taper due to higher operating current and electrospray arcing. Next, data on the reproducibility of the internally tapered CEC/MS column using warfarin and beta-blockers as model analytes are presented. For example, when comparing the reproducibility for separation of warfarin under reversed-phase conditions, the internal taper demonstrated superior intraday % RSD (1.6-3.4) as compared to the external taper intraday % RSD (5-6). Last, the applicability of performing quantitative CEC/MS with internally tapered capillaries is demonstrated for simultaneous enantioseparation of beta-blockers. Impressive quantitative results include good linearity of calibration curves (e.g., R2 = 0.9940-0.9988) and limit of detection as low as 30 nM. The sensitive detection of a minor impurity of one enantiomer at the 0.1% level in a major chiral entity buttresses the suitability of compliance with FDA guidelines.

Separation of small peptides by electrochromatography on silica-based reversed phases and hydrophobic anion exchange phases

Peptide separations are regarded as a promising application of capillary electrochromatography (CEC) and, at the same time, a suitable model to elucidate its mixed separation mechanism when charged analytes are involved. In this paper, studies on the separation of small peptides (2-4 amino acids) on a Spherisorb octadecyl silane (ODS) phase at acidic pH and on a strong anion exchange (SAX)/C18 mixed mode phase at weakly basic pH are reported. For the ODS phase a comparison of CEC, capillary zone electrophoresis (CZE) and high-performance liquid chromatography (HPLC) under identical buffer/eluent conditions is presented. The predicted retention factors for CEC under the assumption of simple superposition of HPLC retention and CZE migration matched the measured results for the peptides that had small retention factors in HPLC. For both types of stationary phases, a variation of the acetonitrile content in the mobile phase led to a wide range of retention factors, including negative values when co-electroosmotic migration was dominant. Though both the ODS and the SAX/C18 phase offer unique advantages, the SCX/C18 phase at pH 9 provides more flexibility to alter separation selectivity for the selected peptides.

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.

Comparison of the efficiency of microparticulate and monolithic capillary columns

A comparison is made between the efficiency of microparticulate capillary columns and silica and polymer-based monolithic capillary columns in the pressure-driven (high-performance liquid chromatography) and electro-driven (capillary electrochromatography) modes. With packed capillary columns similar plate heights are possible as with conventional packed columns. However, a large variation is observed in the plate heights for individual columns. This can only be explained by differences in the quality of the packed bed. The minimum plate height obtained with silica monolithic capillary columns in the HPLC mode is approximately 10 microm, which is comparable to that of columns packed with 5-microm particles. The permeability of wide-pore silica monoliths was found to be much higher than that of comparable microparticulate columns, which leads to much lower pressure drops for the same eluent at the same linear mobile phase velocity. For polymer-based monolithic columns (acrylamide, styrene/divinyl benzene, methacrylate, acrylate) high efficiencies have been found in the CEC mode with minimum plate heights between 2 and 10 microm. However, in the HPLC mode minimum plate heights in the range of 10 to 25 microm have been reported.

Use of coupled open-tubular capillaries for in-line ion-exchange preconcentration of anions by capillary electrochromatography with elution by a transient isotachophoretic gradient

Open-tubular capillaries have been joined together for use in on-column ion-exchange preconcentration of anions by capillary electrochromatography (CEC) with elution by a transient isotachophoretic gradient. This involved the coupling of a preconcentration capillary and a separation capillary using a PTFE sleeve. Such coupling allowed precise lengths of differently coated capillaries to be joined in-line to form a single multi-mode column. The different segments could be tailored to optimize a separation by either altering the length of each segment to precisely manipulate the amount of stationary phase present or by changing the internal diameter of each segment to alter the phase ratio in the chromatographic column without affecting the path length for UV detection. In this work, a segmented in-line capillary was used in conjunction with a fluoride-octanesulfonate discontinuous electrolyte system to increase the number of anions that could be preconcentrated and separated. Quaternary ammonium functionalised latex particles were used for creating the preconcentration segment and the separation segment was coated with poly(diallyldimethylammonium chloride). This allowed the detection of trace anions in drinking water and in situ sampling of river water for the analysis of trace inorganic anions. The repeatability of producing the quaternary ammonium functionalized latex-coated segments was assessed and the effect of segmentation on peak efficiency was investigated.

On the nature of the forces controlling selectivity in the high performance capillary electrochromatographic separation of peptides

In this minireview, the nature of the forces controlling selectivity in the high performance capillary electrochromatographic (HP-CEC) separation of peptides has been examined. For uncharged and charged peptides, a synergistic interplay occurs in HP-CEC systems between adsorptive/partitioning events and electrokinetically driven motion. Moreover, at high field strengths, both bulk electrophoretic migration and surface electrodiffusion occur. Thus, the migration behavior of peptides in different HP-CEC systems can be rationalized in terms of the combined consequences of these various processes. Moreover, in HP-CEC, the buffer electrolyte interacts with both the peptide analytes and the sorbent as bulk phenomena. These buffer-mediated processes control the solvational characteristics, ionization status and conformational behavior of the peptides as well as regulate the double-layer properties of the sorbent, and the ion flux and electro-osmotic flow characteristics of the HP-CEC system per se. These buffer electrolyte effects mediate mutual interactions between the peptide and the sorbent, irrespective of whether the interaction occurs at the surface of microparticles packed into a capillary, at the surface of a contiguous monolithic structure formed or inserted within the capillary or at the walls of the capillary as is the case with open tubular HP-CEC. Diverse molecular and submolecular forces thus coalesce to provide the basis for the different experimental modes under which HP-CEC can be carried out. As a consequence of this interplay, experimental parameters governing the separation of peptides in HP-CEC can be varied over a wide range of conditions, ensuring numerous options for enhanced selectivity, speed, and resolution of peptides. The focus of the peptide separation examples presented in this minireview has been deliberately restricted to the use of HP-CEC capillaries packed with n-alkyl-bonded silicas or mixed-mode strong ion exchange sorbents, although other types of sorbent chemistries can be employed. From these examples, several conclusions have been drawn related to the use of HP-CEC in the peptide sciences. These observations confirm that variation of a specific parameter, such as the pH or the content of the organic solvent modifier in the buffer electrolyte, simultaneously influences all other physicochemical aspects of the specific HP-CEC separation. Peptide selectivity in HP-CEC thus cannot be fine-tuned solely through the use of single parameter optimization methods. In this context, HP-CEC differs significantly from the analogous reverse phase high performance liquid chromatography (RP-HPLC) procedures with peptides. Rather, more sophisticated multiparameter optimization procedures, involving knowledge of (a) the field strength polarity, (b) its contour and flux characteristics, (c) effects of buffer electrolyte composition and pH, (e) the influence of the temperature, and (f) the impact of the sorbent characteristics, are required if the full capabilities offered by HP-CEC procedures are to be exploited. In this minireview, the HP-CEC migration behavior of several different sets of synthetic peptides has been examined, and general guidelines elaborated from these fundamental considerations to facilitate the interpretation and modulation of peptide selectivity in HP-CEC.

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.

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.

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.

Recent advances in capillary electrophoresis/electrospray-mass spectrometry

In this review, the progress in hyphenation of capillary electrophoresis (CE) with electrospray ionization-mass spectrometry (ESI-MS) since the article of Banks (Banks, J. F., Electrophoresis 1997, 18, 2255-2266) is reported. In all capillary-based electromigration techniques, such as capillary gel electrophoresis (CGE), capillary isotachophoresis (CITP), capillary isoelectric focussing (CIEF), micellar electrokinetic chromatography (MEKC), affinity capillary electrophoresis (ACE), as well as in the hybrid techniques capillary electrochromatography (CEC), and pressurized capillary electrochromatography (pCEC) progress has been made in experimental setups, and for many groups of analytes, such as peptides, proteins, nucleotides, saccharides, drugs and their metabolites, CE/ESI-MS has been successfully applied. Electromigration is further miniaturized. New preconcentration methods allow the investigation of compounds, which are not sensitively detected with ESI-MS. Coordination ion spray (CIS) MS is another method for sensitivity enhancement by on-line formation of charged coordination compounds.