Ulrich Tallarek

Chromatographic performance of monolithic and particulate stationary phases. Hydrodynamics and adsorption capacity.

Monolithic chromatographic support structures offer, as compared to the conventional particulate materials, a unique combination of high bed permeability, optimized solute transport to and from the active surface sites and a high loading capacity by the introduction of hierarchical order in the interconnected pore network and the possibility to independently manipulate the contributing sets of pores. While basic principles governing flow resistance, axial dispersion and adsorption capacity are remaining identical, and a similarity to particulate systems can be well recognized on that basis, a direct comparison of sphere geometry with monolithic structures is less obvious due, not least, to the complex shape of theskeleton domain. We present here a simple, widely applicable, phenomenological approach for treating single-phase incompressible flow through structures having a continuous, rigid solid phase. It relies on the determination of equivalent particle (sphere) dimensions which characterize the corresponding behaviour in a particulate, i.e. discontinuous bed. Equivalence is then obtained by dimensionless scaling of macroscopic fluid dynamical behaviour, hydraulic permeability and hydrodynamic dispersion in both types of materials, without needing a direct geometrical translation of their constituent units. Differences in adsorption capacity between particulate and monolithic stationary phases show that the silica-based monoliths with a bimodal pore size distribution provide, due to the high total porosity of the material of more than 90%, comparable maximum loading capacities with respect to random-close packings of completely porous spheres.

Morphology and separation efficiency of a new generation of analytical silica monoliths

The heterogeneous morphology of current silica monoliths hinders this column type to reach its envisioned performance goals. We present a new generation of analytical silica monoliths that deliver a substantially improved separation efficiency achieved through several advances in monolith morphology. Analytical silica monoliths from the 1st and 2nd Chromolith generation are characterized and compared by chromatographic methods, mercury intrusion porosimetry, scanning electron microscopy, and confocal laser scanning microscopy. The latter method is instrumental to quantify morphological differences between the monolith generations and to probe the radial variation of morphological properties. Compared with the 1st generation, the new monoliths possess not only smaller macropores, a more homogeneous macropore space, and a thinner silica skeleton, but also radial homogeneity of these structural parameters as well as of the local external or macroporosity. The 66.5% reduction in minimum plate height observed between silica monoliths of the 1st and 2nd Chromolith generation can thus be attributed to two key improvements: a smaller domain size at simultaneously increased macropore homogeneity and the absence of radial morphology gradients, which are behind the considerable peak asymmetry of the 1st generation.

Statistical analysis of packed beds, the origin of short-range disorder, and its impact on eddy dispersion

We quantified the microstructural disorder of packed beds and correlated it with the resulting eddy dispersion. For this purpose we designed a set of bulk (unconfined) monodisperse random sphere packings with a systematic, protocol-dependent degree of microstructural heterogeneity, covering a porosity range from the random-close to the random-loose packing limit (ε = 0.366-0.46). With the precise knowledge of particle positions, size, and shape we conducted a Voronoiï tessellation of all packings and correlated the statistical moments of the Voronoiï volume distributions (standard deviation and skewness) with the porosity and the protocol-dependent microstructural disorder. The deviation of the Voronoiï volume distributions from the delta function of a crystalline packing describes the origin of short-range disorder of the investigated random packings. Eddy dispersion was simulated over a wide range of reduced velocities (0.5 ≤ ν ≤ 750) and analyzed with the comprehensive Giddings equation. Transient dispersion was found to correlate with the spatial scales of heterogeneity in the packings. The analysis of short-range disorder based on the Voronoiï volume distributions revealed a strong correlation with the short-range interchannel contribution to eddy dispersion, whereas transchannel dispersion was relatively little affected. The presented approach defines a strictly scientific route to the key morphology-transport relationships of current and future chromatographic supports, including their morphological reconstruction, statistical analysis, and the correlation with relevant transport phenomena. It also guides us in our understanding, comparison, and optimization of the diverse packing algorithms and protocols used in simulations and experimental studies.

Transient Effects on Microchannel Electrokinetic Filtering with an Ion-Permselective Membrane

The electrokinetics and hydrodynamics in a hybrid microfluidic/nanofluidic pore network configuration and its effect on the concentration enrichment of charged analytes are described. A hydrogel microplug, photopolymerized in a microfluidic channel, with negative surface charge serves as a nanoporous membrane and dictates the electrokinetic behavior within the adjoining microchannel compartments. The nanoporous hydrogel with a mean pore size on the order of the electrical double layer thickness imparts ion-permselectivity (cation-selectivity) to the migration of ionic species which, under the influence of an applied electrical field, drives concentration polarization in bulk solution near the interfaces between the two microchannel compartments and the hydrogel-based nanopores. The concentration enrichment efficiency for charged analytes depends on this concentration polarization, which strongly affects the distribution of local electrical field strength. In addition, electroosmotic flow in the device plays a critical role in determining the location of the analyte enrichment zone. A theoretical model and simulations are presented to explain the interplay of concentration polarization and electroosmotic flow with respect to the observed concentration enrichment of negatively charged analytes at the cathodic hydrogel plug-microchannel solution interface.

Bipolar Electrode Focusing: Simultaneous Concentration Enrichment and Separation in a Microfluidic Channel Containing a Bipolar Electrode

A method for simultaneously concentrating and separating analytes in a buffer-filled microfluidic channel is reported. The approach is based on modulation of the local electric field within the channel and the corresponding opposition of electrophoretic and electroosmotic flow (EOF) velocities. Dye molecules having different electrophoretic mobilities are focused at different locations within the channel where concentration takes place. At least three species, all small dye molecules, can be simultaneously concentrated and separated, with localized enrichment factors up to approximately 600 achieved within 400 s. The enrichment zones affect the electric field profile, as evidenced by significant differences in focusing of single versus multiple analytes. The EOF could be modulated by modifying the channel walls with an appropriate polymer, and this had the effect of increasing both the enrichment factors and resolution of the separation. Numerical simulations provide insights into the underlying fundamental principles for the experimental findings.

Capillary HPLC−NMR Coupling: High-Resolution 1H NMR Spectroscopy in the Nanoliter Scale

Coupling HPLC and NMR is one of the most powerful techniques for simultaneous separation and structural elucidation of unknown compounds in mixtures. To date, however, minimizing the detection volume, as is required when coupling NMR with miniaturized separation techniques, has been accompanied by a dramatic loss in resolution of the NMR spectra. Here, we report on the coupling of gradient capillary HPLC with on-column, high-resolution NMR detection. On-line stopped-flow and static (1)H NMR spectra were acquired with capillary columns of 75-315 μm i.d. With detection over a length of 1.2 cm, cell volumes cover a range of 50-900 nL. An on-line-detected NMR separation of dansylated amino acids was carried out in a 315 μm i.d. fused silica capillary packed to a length of 12 cm with C(18) stationary phase. The low solvent consumption makes the use of fully deuterated solvents economically feasible. NMR spectra with resolution on the order of 3 Hz were obtained using a 50 nL detection cell to measure 1.1 nmol of dansylated γ-aminobutyric acid under static conditions in a 75 μm i.d. capillary.

Physical reconstruction of packed beds and their morphological analysis: Core–shell packings as an example

We report a fast, nondestructive, and quantitative approach to characterize the morphology of packed beds of fine particles by their three-dimensional reconstruction from confocal laser scanning microscopy images, exemplarily shown for a 100μm i.d. fused-silica capillary packed with 2.6μm-sized core-shell particles. The presented method is generally applicable to silica-based capillary columns, monolithic or particulate, and comprises column pretreatment, image acquisition, image processing, and statistical analysis of the image data. It defines a unique platform for fundamental comparisons of particulate and monolithic supports using the statistical measures derived from their reconstructions. Received morphological data are column cross-sectional porosity profiles and chord length distributions from the interparticle macropore space, which are a descriptor of local density and can be characterized by a simplified k-gamma distribution. This distribution function provides a parameter of location and a parameter of dispersion which can be correlated to individual chromatographic band broadening processes (i.e., to transchannel and short-range interchannel contributions to eddy dispersion, respectively). Together with the transcolumn porosity profile the presented approach allows to analyze and quantify the packing microstructure from pore to column scale and therefore holds great promise in a comparative study of packing conditions and particle properties, particularly for characterizing and minimizing the packing process-specific heterogeneities in the final bed structure.

Electrokinetics in Microfluidic Channels Containing a Floating Electrode

Electrokinetic transport within a buffer-filled microchannel incorporating a flat bipolar electrode is investigated. The key finding is that the presence of the electrode disrupts the passage of electrical current through the microchannel and thereby alters the uniformity of the local electric field. Electroosmotic flow further modulates the local field gradient. These dynamics are demonstrated experimentally by utilizing the field gradient for concentration enrichment of negatively charged tracer molecules, and a set of computer simulations is presented to interpret the underlying electrokinetics.

The influence of membrane ion-permselectivity on electrokinetic concentration enrichment in membrane-based preconcentration units

The performance of nanoporous hydrogel microplugs with varying surface charge density is described in concentrating charged analytes electrokinetically in a microfluidic device. A neutral hydrogel plug with a mean pore size smaller than the size of charged analytes acts as a simple size-exclusion membrane. The presence of fixed charges on the backbone of a nanoporous hydrogel creates ion-permselectivity which results in charge-selective transport through the hydrogel. This leads to the development of concentration polarization (CP) in the adjoining bulk electrolyte solutions under the influence of an applied electrical field. CP strongly affects the distribution of the local electrical field strength, in particular, in the vicinity of the hydrogel plug which can significantly reduce the concentration enrichment factors compared to the neutral hydrogel. A theoretical model and simulations are presented, together with experimental data, to explain the interplay of hydrogel or membrane cation-selectivity, electrical field-induced CP, and the distribution of the local electrical field strength with respect to concentration enrichment of negatively charged analytes at the cathodic membrane-solution interface.

NMR imaging of the chromatographic process migration and separation of bands of gadolinium chelates

Abstract The presence of gadolinium nuclei accelerates the relaxation of protons in their neighborhood. Using the imaging techniques of nuclear magnetic resonance developed for medical diagnosis, it is possible to observe the chromatographic bands of Gadolinium compounds which appear in shades of grey turning lighter with increasing gadolinium concentration. The existence of unexpectedly wide distributions of the local mobile phase velocity and the local height equivalent to a theoretical plate in an efficient chromatographic column is illustrated. The formation and resorption of fissured and/or compact zones in the column where the external porosity appears lower or higher than average is documented for the first time.