毛细管反相电色谱法分离行为的研究

对乙睛-水-磷酸二氢销体系毛细管反相电色谱分离行为进行了研究。采用柱上紫外检测,在75μmi.d.×30cm的毛细管ODS（3μm）填充柱上获得了小于2.0的折合培板高度。同时还研究了乙睛的比例、电解质的浓度和电场强度等因素对电渗流和往效的影响。     

Capillary electrochromatography using continuous-bed columns of sol-gel bonded silica particles with mixed-mode octadecyl and propylsulfonic acid functional groups

Continuous-bed columns containing sol-gel bonded 3 microm silica particles with mixed-mode octadecyl and propylsulfonic acid functional groups (ODS/SCX) were prepared by first packing the ODS/SCX particles into a fused-silica capillary, then filling the packed capillary with a siliceous sol-gel, curing the sol-gel, and finally drying the column with supercritical carbon dioxide. The performance of the sol-gel bonded ODS/SCX columns was evaluated for capillary electrochromatography using acetonitrile aqueous mobile phase containing phosphate buffer. The columns were mechanically strong and permeable. Both EOF velocity and current increased linearly with elevation of the applied electric field strength. The EOF velocity was high at low pH and nearly constant over a range of pH 2-9. Higher buffer concentration resulted in higher current and lower EOF velocity. The acetonitrile content had no significant effect on the EOF. Without thermosetting the column, no bubble formation was noticed with currents up to 2.5 microA. The minimum plate height of a 25/34 cm x 75 microm I.D. sol-gel bonded 3 microm ODS/SCX column was 5.7 microm (1.75 x 10(5) plates per meter) at an optimum EOF velocity of 0.92 mm s(-1). Mixtures of test aromatic compounds and aromatic hydrocarbon homologues gave symmetrical peaks when using a low pH mobile phase. The retention and elution order of aromatic compounds represented a typical reversed-phase separation mechanism similar to conventional ODS columns. The run-to-run and column-to-column retention factor reproducibility was better than 2.5% and 8.0% RSD, respectively.     

Influence of the unpacked section on the chromatographic performance of duplex strong anion-exchange columns in capillary electrochromatography

This work describes initial investigations of strong anion-exchange (SAX) packing materials for capillary electrochromatography (CEC). The use of SAX phases in CEC is theoretically appealing for the analysis of negatively charged species. The reversed direction of the electroosmotic flow (EOF) generated by SAX phases (in comparison to reversed phases and strong cation-exchange phases) means that negative species can migrate with the EOF, not against it, hence the analysis times, of such species should be decreased and efficiencies improved. Duplex CEC columns (the standard for instruments using UV detection) consist of a packed and an unpacked section. Using common reversed-phase packing materials the direction of the EOF in both sections is co-linear, however when normal fused-silica capillaries are packed with SAX material the direction of the EOF in the two sections oppose one another. It has been shown, using conventional duplex CEC columns and fully packed CEC-MS columns that the opposing direction of EOF causes a massive degradation in column performance. Consequentially, it is demonstrated that if the EOF in the open section of the duplex SAX column can be controlled via pH or capillary derivatisation then good, reproducible CEC can be performed on anionic species using SAX packed CEC columns.     

Enhancement of electroosmotic flow in capillary electrochromatography.

A major impediment to enhancing the speed of separation in capillary electrochromatography (CEC) is the upper limit on the electroosmotic flow (EOF) velocity by the maximal zeta potential of the chromatographic surface. Here, a new approach to speeding up EOF, suggested by Yang and El Rassi (Electrophoresis 1999, 20,18-23), is examined critically. It entails the use of a tandem arrangement of a separating column and an auxiliary column, the sole function of which is to boost EOF velocity in the separating column and thus facilitate faster analysis by CEC. Based on the principle of conservation of mass and current and using experimental data obtained in a wide range of conditions, the flow velocities in the separating and auxiliary columns were evaluated. The results show that an equidiameter open tubular auxiliary column offers a greater enhancement of EOF velocity than a packed column. Nevertheless, within the scope of the experiments the enhancement of EOF velocity by as much as 50% by using open tubular auxiliary columns has been obtained.     

Capillary electrochromatography with segmented capillaries for controlling electroosmotic flow

Capillaries consisting of two segments each packed with a different stationary phase were introduced for the control and manipulation of the electroosmotic flow (EOF) in capillary electrochromatography (CEC). This kind of column configuration was called segmented capillary where one segment was packed with octadecyl silica (ODS) and served as the separation segment while the other segment was packed with bare silica and functioned as the EOF accelerator segment. The average flow in the segmented capillary increased linearly with increasing fractional length of the EOF accelerator segment, and consequently the analysis time was reduced. Under a given set of conditions, the average flow can be varied over a certain range that extends from the EOF in the individual ODS capillary at the lower end to the EOF in the individual bare silica capillary at the higher end. The pore size of the bare silica in the EOF accelerator segment influenced the average flow in the segmented capillary. Because of the difference in the EOF of the individual segments, the average flow across the segmented capillary is partially degenerated from EOF to viscous flow. Furthermore, the retaining frits in CEC columns are restrictive points which slow down the average flow, thus furthering the degeneration of the flow from EOF to viscous flow. In other words, in CEC columns containing retaining frits, the flow of the mobile phase is not only based on electroosmosis but is contaminated by a viscous component.     

Novel approach for fritless capillary electrochromatography

At present, the main limitation for the further adoption of capillary electrochromatography (CEC) in the (routine) laboratory is caused by the lack of reproducible and stable columns. The main source of column instability is concentrated in the frits needed to retain the packed bed inside the CEC capillary. The sintering process used to prepare the frits can be rather problematic and irreproducible, particularly for small stationary phase particles and wide column diameters. Since the (surface) composition of the frits is different from the bulk stationary phase packing, different electroosmotic flow (EOF) velocities are generated. This effect is assumed to be primarily responsible for rapid column destruction. In this contribution, a novel approach for the preparation of fritless CEC capillaries is presented and evaluated. Using 5 microm Hypersil ODS particles, separation efficiencies in the range of 130,000-200,000 plates/m were obtained. In a 100 microm inner diameter packed column, electrical currents up to 50 microA could be tolerated without negative effects such as bubble formation. The prepared CEC columns were found to be stable and could easily be operated continuously for several days without column damage. An additional advantage of the proposed tapering approach is that application of pressure on the in- and outlet vial during separation was not required to prevent bubble formation.     

Capillary electrochromatography using a fluoropolymer as the chromatographic support material

Fused-silica capillary columns were packed with ethylene chlorotrifluoroethylene (ECTFE) particles for use in capillary electrochromatography (CEC). Electroosmotic flow (EOF) was generated in these columns using acetonitrile-water mixtures as the mobile phase. Electroosmotic mobilities of 1.6 x 10(-4) cm2 V(-1) s(-1) (linear velocities of 1 mm s(-1)) were observed using a mobile phase without an electrolyte present. The EOF in the ECTFE-packed columns is enhanced when using trifluoroacetic acid (TFA) as a mobile phase additive; electroosmotic mobilities of 3.65 x 10(-4) cm2 (V-1) s(-1) (linear velocity of 2.5 mm s(-1)) were observed. This enhancement of EOF is attributed to dynamic coating of the ECTFE particles by TFA. Other electrolytes (i.e., Tris/Tris-HCl buffer and H3PO4) in the mobile phase did not have such an enhancement of EOF. However, a slight enhancement of EOF is observed, for example, if small quantities of TFA are added to the mobile phase containing Tris buffer. The potential of ECTFE for CEC is demonstrated by separating a mixture of amino acids.     

Packing capilllary electrochromatography columns using vacuum--a preliminary study

This paper introducesa novel method for packing Capillary Electrochromatography Columns (CEC). Using vacuum packing methodology, silica particles as small as 1 microm were successfully packed into the capillary columns with 75 microm inner diameter. The columns are verystable and show no noticeable loss in efficiency after 200 sample injections. The performance of these vacuum packed capillary columns was evaluated with a mixture of aromatic and non-aromatic compounds. A 24 cm long capillary column can produce peak efficiencies of around 45,000 plates for benzene.     

Capillary electrochromatography of 1-phenyl-3-methyl-5-pyrazolone derivatives of some mono-and disaccharides

Summary A packing procedure was adopted for capillary electrochromatography (CEC) that produces capillary columns with high separation efficiencies. The columns were fully packed, 50 cm long, with UV detection being performed through the packed section 30 cm from the inlet end. The CEC experiments were run at ambient pressure, with no additional pressure applied to the ends of the column. The stationary phase (octadecyl silica (ODS), 5 μm) promoted a high velocity electroosmotic flow (EOF), enabling rapid and efficient separation of a hydrocarbon test mixture. Some 1-phenyl-3-methyl-5-pyrazolone (PMP) derivatives of mono- and disaccharides were baseline separated, using a 5 mM NaH₂PO₄ in 80% acetonitrile and 20% water (v/v) buffer solution. CEC shows promise for future applications in carbohydrate analysis. Presented at Balaton Symposium on High Performance Separation Methods, Siófok, Hungary, September 1–3, 1999     

Towards the ultimate minimum particle diameter of silica packings in capillary electrochromatography

Porous silica beads with an average particle diameter between 0.2 and 3 microm have been applied as packing material in capillary electrochromatography (CEC). The experiments were directed to investigate whether it is really feasible and as promising as expected to use such small particles. In CEC, plate heights of H approximately/= 1-2 d(p) can be achieved which is smaller than the plate heights usually attained in high-performance liquid chromatography. Using a capillary packed with 0.5 microm silica beads we achieved a plate height of H = 3 d(p) indicating the presence of dispersive effects like Joule heating. Calculations demonstrate that at a field strength of about 800 V cm(-1) one third of the plate height can be lost by Joule heating effects if the heat is not removed by a cooling system. Additionally, the H(u) curve is still descending at the maximum electroosmotic flow (EOF) velocity we generated with the modified capillary electrophoresis instrument. To fully exploit the potential of submicron size silicas higher field strengths, i.e., higher EOF velocities, must be attained. To study the influence of the kind of packing on the EOF porous as well as nonporous silicas have been applied. The experiments clearly indicate that the EOF of porous and nonporous silicas is the same. Since the EOF is more or less exclusively generated by the packing material the zeta potential of n-octyl bonded 0.5 microm silica has been determined. The dependence of the zeta potential on the pH is identical to the dependence of the EOF on the pH in a packed capillary. The point of zero charge of the silica is at pH 2-3.     

Modeling the velocity field of the electroosmotic flow in charged capillaries and in capillary columns packed with charged particles: interstitial and intraparticle velocities in capillary electrochromatography systems

Mass transfer systems based on electrokinetic phenomena (i.e., capillary electrochromatography (CEC)) have shown practical potential in becoming powerful separation methods for the biotechnology and pharmaceutical industries. A mathematical model has been constructed and solved to describe quantitatively the profiles of the electrostatic potential, pressure, and velocity of the electroosmotic flow (EOF) in charged cylindrical capillaries and in capillary columns packed with charged particles. The results obtained from model simulations (i) provide significant physical insight and understanding with regard to the velocity profile of the EOF in capillary columns packed with charged porous particles which represent systems employed in CEC, (ii) provide the physical explanation for the experimental results which indicate that the velocity of the EOF in capillary columns packed with charged porous particles is a very weak function (it is almost independent) of the diameter of the particles, and (iii) indicate that the intraparticle velocity, nu(p,i), of the EOF can be greater than zero. The intraparticle Peclet number, Pe(int rap), for lysozyme was found to be greater than unity and this intraparticle convective mass transfer mechanism could contribute significantly, if the appropriate chemistry is employed in the mobile liquid phase and in the charged porous particles, in (a) decreasing the intraparticle mass transfer resistance, (b) decreasing the dispersive mass transfer effects, and (c) increasing the intraparticle mass transfer rates so that high column efficiency and resolution can be obtained. Furthermore, the results from model simulations indicate that for a given operationally permissible value of the applied electric potential difference per unit length, Ex, high values for the average velocity of the EOF can be obtained if (1) the zeta potential, zeta(p), at the surface of the particles packed in the column has a large negative magnitude, (2) the value of the viscosity, mu, of the mobile liquid phase is low, (3) the magnitude of the dielectric constant, epsilon, of the mobile liquid phase is reasonably large, and (4) the combination of the values of the concentration, C(infinity), of the electrolyte and of the dielectric constant, epsilon, provide a thin double layer. The theoretical results for the velocity of the EOF obtained from the solution of the model presented in this work were compared with the experimental values of the velocity of the EOF obtained from a fused-silica column packed with charged porous silica C8 particles. Systems with four different particle diameters and three different concentrations of the electrolyte were considered, and the magnitude of the electric field was varied widely. The agreement between theory and experiment was found to be good.     

Life-time studies with capillary electrochromatography columns operated under different conditions

A test system has been established to permit the monitoring of the life-time performance of several reversed- phase capillary electrochromatography (CEC) columns. The retention factors, k(cec), peak symmetry coefficients, lambda(sym), and column efficiencies, N, of three neutral n-alkylbenzene analytes, namely ethyl-, n-butyl- and n-pentylbenzenes, were determined for Hypersil 3 microm n-octylsilica and n-octadecylsilica packed into CEC capillary columns of 100 microm I.D., with a packed length of 250 mm, and a total length of 335 mm. The performances of these CEC capillary columns were examined for a variety of eluents with pH values ranging between pH 2.0 - 8.0, similar to those employed to study the retention behaviour of peptides that we have previously reported. The relative standard deviation (RSD) of the retention factors (k(cec) values) of these n-alkylbenzenes, acquired with an eluent of (25 mM Tris-HCl, pH 8.0,)-acetonitrile (1:4, v/v), when the CEC capillary columns were used for the first time (virgin values), were 4% (based on data acquired with 4 CEC capillary columns) for the n-octyl bonded silica capillary columns, and 6% (based on 8 columns) for n-octadecyl bonded silica capillary columns. The RSD values of the k(cec) values of the n-alkylbenzenes for one set of replicates (n=6) with one CEC capillary column was < 0.5%. The theoretical plate numbers, N, for the virgin CEC capillary columns were ca. 60,000, whilst the observed N values for all new CEC capillary columns were > or = 40,000 for n-octyl bonded silica capillary columns and > or = 50,000 for n-octadecyl bonded silica capillary columns. The peak symmetry coefficients, lambda(sym), of the n-alkylbenzenes for virgin CEC capillary columns and for CEC capillary columns used for more than 1,000 injections were always in the range 0.95-1.05. The experimental results clearly document that the life-time performance of the CEC capillary columns depends on the eluent composition, as well as the nature of the analytes to which the CEC capillary columns are exposed.     

Short communication performance of octadecylsilylated monolithic silica capillary columns of 530 microm inner diameter in HPLC

Monolithic silica capillary columns were successfully prepared in a fused silica capillary of 530 microm inner diameter and evaluated in HPLC after octadecylsilylation (ODS). Their efficiency and permeability were compared with those of columns pakked with 5-microm and 3-microm ODS-silica particles. The monolithic silica columns having different domain sizes (combined size of through-pore and skeleton) showed 2.5-4.0-times higher permeability (K= 5.2-8.4 x 10(-14) m2) than capillary columns packed with 3-mm particles, while giving similar column efficiency. The monolithic silica capillary columns gave a plate height of about 11-13 microm, or 11 200-13 400 theoretical plates/150 mm column length, in 80% methanol at a linear mobile phase velocity of 1.0 mm/s. The monolithic column having a smaller domain size showed higher column efficiency and higher pressure drop, although the monolithic column with a larger domain size showed better overall column performance, or smaller separation impedance (E value). The larger-diameter (530 microm id) monolithic silica capillary column afforded a good peak shape in gradient elution of proteins at a flow rate of up to 100 microL/min and an injection volume of up to 10 microL.     

Tryptic digest mapping by gradient capillary electrochromatography

A high performance liquid chromatography (HPLC) system complemented with T-split, capillary detection cell and a high voltage power supply was used for peptide mapping by gradient electrochromatography and nanoliquid chromatography (nano-LC). With capillary columns of 100 microm ID, 6 cm packed with octadecylated 1.5 microm silica particles, the typical analysis time was approximately 10-15 min. The resolution of a tryptic digest of cytochrome c obtained by electrochromatography at 100 kV/m was superior compared to the analysis by nano-LC. Bubble formation caused by Joule heating at currents up to 100 microA was successfully suppressed by using a resistor capillary of 25 microm ID connected to the outlet of the packed column.     

In-line coupling of low-pressure short capillary electrochromatography columns to electrospray ionisation mass spectrometry

A new in-house designed and constructed injection valve for capillary electrochromatography (CEC) based on a rotating injection part with compartments for the eluent as well as for the sample has been coupled to a mass spectrometer via a sheath flow electrospray ionisation (ESI) interface, using short capillary columns of 15 cm length. The CEC columns were packed with 3 microm C(18) bonded silica particles, and a mixture of peptides was analysed using an ammonium acetate/acetonitrile eluent. A significant increase in the signal-to-noise ratio was obtained when the peptides were dissolved in water with the same content of organic modifier as in the eluent with an addition of 0.5% (v/v) acetic acid. When the CEC analysis was performed without any additional pressure, the separation current sometimes dropped tremendously due to bubble formation, caused by different permeability in the first and packed part of the column causing an extremely low electroosmotic flow. The separation current was restored to its original value by applying only 7 bar at the inlet of the CEC column, and the separation performance for the test peptides was recovered. A comparison of the CEC performance of peptides in pure CEC mode and in low-pressure CEC mode is reported.     

Dynamics of capillary electrochromatography. II. Comparison of column efficiency parameters in microscale high-performance liquid chromatography and capillary electrochromatography.

In capillary electrochromatography (CEC) the flow of the mobile phase is generated by electrosmotic means in high electric field. This work compares band spreading measured experimentally in several packed capillaries with electrosmotic flow (EOF) and viscous flow under otherwise identical conditions. The data were fitted to the simplified van Deemter equation for the theoretical plate height, H = A + B/u + Cu, in order to evaluate parameters A and C in each mode of flow in the different columns. The ratio of these two parameters obtained with the same column in microscale HPLC (mu-HPLC) and CEC was used to quantify the attenuation of their contribution to band spreading upon changing from viscous flow (in mu-HPLC) to electrosmotic flow (in CEC). The capillary columns used in this study were packed with stationary phases of different pore sizes as well as retentive properties and measurements were carried out under different mobile phase conditions to examine the effects of the retention factor and buffer concentration. In the CEC mode, the value of both column parameters A and C was invariably by a factor of two to four lower than in the mu-HPLC mode. This effect may be attributed to the peculiarities of the EOF flow profile in the interstitial space and to the generation of intraparticle EOF inside the porous particles of the column packing. Thus, band spreading due to flow maldistribution and mass transfer resistances is significantly lower when the mobile phase flow is driven by voltage as in CEC, rather than by pressure as in mu-HPLC.     

Column technology for capillary electrochromatography

Column technologies for capillary electrochromatography (CEC) are reviewed. To achieve high efficiency, the inner diameters of open-tubular and packed columns should be less than 25 and 200 μm, respectively. To obtain acceptable separation speed under typical CEC conditions (e.g. 30 kV, 1 mm s⁻¹ electroosmotic flow velocity, and 2–4×10⁻⁸ m² V⁻¹ s⁻¹ electroosmotic mobility) the column lengths for open-tubular and packed columns should be less than 120 and 60 cm, respectively. Capillary CEC columns are generally classified into three types: packed, open-tubular, and continuous-bed or monolithic. The various column preparation procedures and the advantages and disadvantages of each column type are discussed in detail.     

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.     

Ultrahigh-pressure liquid chromatography using a 1-mm id column packed with 1.5-microm porous particles

The evolution of chromatography has led to the reduction in the size of the packing materials used to fabricate HPLC columns. The increase in the backpressure required has led to this technique being referred to as ultrahigh-pressure liquid chromatography (UHPLC) when the column backpressure exceeds 10000 psi (approximately 700 bar). Until recently, columns packed with sub-2-microm materials have generally fitted into two classes; either short (less than 5 cm) columns designed for use on traditional HPLC systems at pressures less than 5000 psi (350 bar), or capillary columns (inner diameters less than 100 microm). By using packing materials with diameters <2 microm to fabricate UHPLC columns, there is an increase in efficiency and a decrease in the analysis time that are directly proportional to the size of the packing material. In order to realize and exploit the increase in efficiency, however, the columns must maintain lengths typically associated with analytical columns (15-25 cm). We have packed 1 mm diameter, 150 mm in length columns with 1.5 microm packing material, and evaluated their performance in UHPLC. The pressure required to achieve optimum linear velocities in plots of plate height versus linear velocity was in the vicinity of 1104 bar (16000 psi). The 1.5 microm particle-packed column was compared with the more traditional 150 mm long analytical columns packed with 3 microm materials. This column showed an efficiency that was approximately twice that observed with the 3 microm packed column and a concomitant reduction in the analysis time, theoretically predicted.     

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.