Imaging solute distribution in capillary electrochromatography with laser scanning confocal microscopy

[Recent advances in capillary electrochromatography and its coupling techniques]

As a novel micro-separation technique, capillary electrochromatography (CEC) has the merits of high efficiency, high selectivity, high resolution and rapid analysis. However, the small-volume injection manipulated in capillary dimensions poses a great challenge for detectors in achieving high sensitivity. Currently, one of the major researches into CEC involves the development of some sensitive detection modes. The general introduction, which includes the historical perspectives and the principles of CEC, is briefly described. The recent advances about CEC coupled with various detectors and its applications in the separation of complex samples are summarized. A total of 141 references are reviewed.

Microscopic Origins of Band Broadening in Chromatography. Polarity Distribution in C18 Stationary Phase Probed by Confocal Ratiometric Imaging of Nile Red

Band broadening is a major factor that influences the efficiency and resolution of chromatographic separations. Studies of microscopic origins of band broadening, such as the micropolarity distribution of chromatographic stationary phase, can provide a better understanding of many chromatographic phenomena and retention behavior. In this work, we probe the chemical environments of C18 chromatographic stationary phase with quantitative confocal fluorescence microscopy under real reversed-phase liquid chromatography conditions. Ratiometric imaging of C18 interface is achieved by loading the stationary phase with a polarity-sensitive dye, Nile red, and optical sectioning with confocal microscopy. The results reveal that there are uniform micropolarity distributions inside individual chromatographic beads, but the polarity may differ between stationary-phase particles. The homogeneity of micropolarity of individual beads suggests that there are not any spatially large exposed silica sites beyond the optical resolution in C18 stationary phase. The strong adsorption sites are smaller in size than the optical resolution of a few hundred nanometers. The heterogeneity between chromatographic beads indicates that the interactions of Nile red with C18 bonded phase are different between beads. This contributes to the broad overall polarity distribution of the C18 stationary phase and can be one of the factors that cause band broadening in separations. With its high spatial resolution and optical sectioning capabilities, confocal fluorescence imaging is shown to be an ideal method to probe the chromatographic stationary phase. The distribution of micropolarity sheds light on the microscopic heterogeneity in chromatographic processes and its influence on chemical separations.

Quantification of Binary Diffusion in Protein Crystals

The use of confocal laser scanning microscopy for visualization and quantification of binary diffusion within anisotropic porous material is described here for the first time. The dynamics of adsorption profiles of dianionic fluorescein, zwitterionic rhodamine B, and their mixture in the cationic native orthorhombic lysozyme crystal were subsequently analyzed. All data could be described by a classical pore diffusion model. There was no change in the adsorption characteristics, but diffusion decreased with the introduction of a second solute in the solution. It was found that diffusion is determined by the combination of steric and electrostatic interactions,while adsorption is dependent on electrostatic and hydrophobic interactions. Thus, it was established that the outcome of binary transport depends on the solute, protein, and crystal characteristics.

Probing Strong Adsorption of Solute onto C18-Silica Gel by Fluorescence Correlation Imaging and Single-Molecule Spectroscopy under RPLC Conditions

Understanding molecular adsorption at a chromatographic interface is of great interest for addressing the tailing problem in chemical separations. Single-molecule spectroscopy and confocal fluorescence correlation imaging are used to study the adsorption sites of C(18) silica beads under RPLC chromatographic conditions. The experiments show that cationic molecule rhodamine 6G laterally diffuses through the chromatographic interface of a C(18) hydrocarbon monolayer and acetonitrile with occasional reversible strong adsorptions. Fluorescence correlation imaging extracts the rare strong adsorption events from large data sets, revealing that the strong adsorption sites are randomly distributed throughout the silica beads. Virtually every imaging pixel of silica beads adsorbs molecules. Single-molecule spectroscopy of the 584 strong adsorption events observed indicates that the strong adsorptions persist on the time scales from several milliseconds to seconds, having an average desorption time of 61 ms. The strong adsorption events are rare, comprising 0.3% of the total observation time. The sizes of strong adsorption sites are within the optical resolution of confocal imaging.

Investigation of the stability of polyelectrolyte multilayer coatings in open-tubular capillary electrochromatography using laser scanning confocal microscopy

A simple polyelectrolyte multilayer (PEM) coating procedure was used for the development of stable modified capillaries. PEM coatings were constructed in fused-silica capillaries using alternating rinses of cationic and anionic polyelectrolytes. The multilayer coatings investigated in this study consisted of two and twenty layer pairs, or bilayers. A bilayer is one layer of a cationic polymer and one layer of an anionic polymer. Poly(diallyldimethylammonium chloride) was used as the cationic polymer, and the polymeric surfactant poly(sodium N-undecanoyl-L-leucylvalinate) was used as the anionic polymer. Previous studies for both chiral and achiral separations have shown that PEM-coated capillaries have excellent reproducibilities, remarkable endurance, and strong stabilities against extreme pH values when used in open-tubular capillary electrochromatography (OT-CEC). In this study, the stability of the coatings was further investigated after exposure to 0.1 M and 1.0 M NaOH. Structural changes of these coatings were monitored using laser scanning confocal microscopy (LSCM) after flushing the capillaries with NaOH. This technique allowed observation of the degradation of the coatings. Observations are discussed in terms of separations using OT-CEC. Electropherograms obtained from the chiral separation of 1,1'-binaphthyl-2,2'-dihydrogenphosphate in OT-CEC showed a decrease in selectivity and an increase in electroosmotic mobility after long exposure to NaOH. The ability to recover the capillaries by exposure to NaOH was also demonstrated. Measurements of electroosmotic mobility and selectivity showed that 2-bilayer and 20-bilayer PEM coatings could be completely removed from the capillary surface after approximately 3.5 and 9.5 h, respectively, of continuous exposure to 1 M NaOH.

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.

Nonequilibrium electrokinetic effects in beds of ion-permselective particles

Electrokinetic transport of fluorescent tracer molecules in a bed of porous glass beads was investigated by confocal laser scanning microscopy. Refractive index matching between beads and the saturating fluid enabled a quantitative analysis of intraparticle and extraparticle fluid-side concentration profiles. Kinetic data were acquired for the uptake and release of electroneutral and counterionic tracer under devised conditions with respect to constant pressure-driven flow through the device and the effect of superimposed electrical fields. Transport of neutral tracer is controlled by intraparticle mass transfer resistance which can be strongly reduced by electroosmotic flow, while steady-state distributions and bead-averaged concentrations are unaffected by the externally applied fields. Electrolytes of low ionic strength caused the transport through the charged (mesoporous) beads to become highly ion-permselective, and concentration polarization is induced in the bulk solution due to the superimposed fields. The depleted concentration polarization zone comprises extraparticle fluid-side mass transfer resistance. Ionic concentrations in this diffusion boundary layer decrease at increasing field strength, and the flux densities approach an upper limit. Meanwhile, intraparticle transport of counterions by electromigration and electroosmosis continues to increase and finally exceeds the transport from bulk solution into the beads. A nonequilibrium electrical double layer is induced which consists of mobile and immobile space charge regions in the extraparticle bulk solution and inside a bead, respectively. These electrical field-induced space charges form the basis for nonequilibrium electrokinetic phenomena. Caused by the underlying transport discrimination (intraparticle electrokinetic vs extraparticle boundary-layer mass transfer), the dynamic adsorption capacity for counterions can be drastically reduced. Further, the extraparticle mobile space charge region leads to nonlinear electroosmosis. Flow patterns can become highly chaotic, and electrokinetic instability mixing is shown to increase lateral dispersion. Under these conditions, the overall axial dispersion of counterionic tracer can be reduced by more than 2 orders of magnitude, as demonstrated by pulse injections.

Quantifying anisotropic solute transport in protein crystals using 3-D laser scanning confocal microscopy visualization

The diffusion of a solute, fluorescein into lysozyme protein crystals has been studied by confocal laser scanning microscopy (CLSM). Confocal laser scanning microscopy makes it possible to non-invasively obtain high-resolution three-dimensional (3-D) images of spatial distribution of fluorescein in lysozyme crystals at various time steps. Confocal laser scanning microscopy gives the fluorescence intensity profiles across horizontal planes at several depths of the crystal representing the concentration profiles during diffusion into the crystal. These intensity profiles were fitted with an anisotropic model to determine the diffusivity tensor. Effective diffusion coefficients obtained range from 6.2 x 10(-15) to 120 x 10(-15) m2/s depending on the lysozyme crystal morphology. The diffusion process is found to be anisotropic, and the level of anisotropy depends on the crystal morphology. The packing of the protein molecules in the crystal seems to be the major factor that determines the anisotropy.