Multiple-open-tubular column enabling transverse diffusion. Part 3: Simulation of solute dispersion along a real three dimensional-printed column with quadratic channels.

Multiple-open-tubular columns enabling transverse diffusion (MOTTD) consist of straight and parallel flow-through channels separated by a mesoporous stationary phase. In Part 1, a stochastic model of band broadening along MOTTD columns accounting for longitudinal diffusion, trans-channel velocity bias, and mass transfer resistance in the stationary phase was derived to demonstrate the intrinsic advantage of MOTTD columns over classical particulate columns. In Part 2, the model was refined for the critical contribution of the channel-to-channel polydispersity and applied to address the best trade-off between analysis speed and performance. In this Part 3, a MOTTD column with a square array of quadratic channels is fabricated by 3D-printing (combining polymer stereolithography with photolithography using photomasks) to deliver unprecedently small apparent channel diameters of 117.6 ± 5.0 µm. The colors in the microscopy photographs of the actual 3D-printed channels are binarized to delimitate the mobile phase volume from the stationary phase volume. The same numerical simulations as those in Part 2 are then performed for two MOTTD columns (external porosity ϵe=31.7%, same apparent channel diameter 117.6 µm): one containing 16 virtual perfect quadratic channels and the other 16 real 3D-printed channels. The reduced velocities (or Peclet numbers) are varied over a wide range from 0.2 to 5000 and the zone retention factors were fixed at k1=1.04, 5, and 25. The results demonstrate that smoothing the edges of the targeted quadratic channels by the 3D-printed technique is advantageous in terms of solute dispersion. It outperforms the negative effect of the channel-to-channel polydispersity which is mitigated by transverse diffusion of the analyte in the stationary phase. For Peclet numbers larger than 50, the HETP of the 3D-printed MOTTD column is found 7%, 15%, and 16% smaller than that of the MOTTD column consisting of a square array of perfect quadratic channels. This confirms the known effect of channel geometry on solute dispersion in microfluidic systems. Flow channels in fabricated MOTTD columns are preferred to be circular so that the distribution of transverse diffusion lengths across the open channels remains as tight as possible. Finally, the general theory of nonuniform columns of Giddings reveals that the polydispersity of the cross-sectional area (RSD 8.4%) along a single 3D-printed channel has no negative impact on solute dispersion in MOTTD columns. Overall, MOTTD columns could become a serious alternative technology to conventional particulate columns. This implies a novel fabrication process that delivers circular channel diameters smaller than 10 µm, cross-sectional area polydispersity no larger than 25%, external porosities in a range from 15% (high speed separations) to 75% (high performance separations), and conventional mesoporous silica as the stationary phase. It adresses new synthesis routes based on either organic fibers or tubular micelle templating agents in suspension with silica gel solutions.

Quantitative analysis of mesoporous structures by electron tomography: A phantom study.

Electron tomography (ET) has attracted significant attention for a quantitative analysis of mesoporous materials, especially for complex disordered pore structures, as no priori assumption on the pore shape is needed, which is normally inevitable when using traditional bulk characterization techniques. However, a reliable quantification of such pore structures from ET critically depends on the fidelity of the segmented reconstruction, which can be significantly affected, e.g. by the raw data quality, the limited tilting range, artifacts introduced during alignment and further depends on the reconstruction algorithm. Therefore, we systematically investigated the reconstruction reliability of three main-stream algorithms including simultaneous iterative reconstruction technique (SIRT), total variation minimization (TVM) and discrete algebraic reconstruction technique (DART) for mesoporous materials using different imperfect (realistic) tilt-series based on a set of phantom simulations. We found that DART outperforms the other two methods in reliably revealing small pores and narrow channels, especially when the number of projections is strongly constrained. The accurately segmented reconstruction from DART makes it possible to achieve reliable quantification of the pores structure, which in turn leads to reliable evaluation of effective diffusion coefficients. We discuss the influence of different acquisition and reconstruction parameters on the reconstructed 3D volume and the quantitative analysis of pore features. We aim to provide a practical guideline for optimizing acquisition and reconstruction parameters and how to evaluate the accuracy when describing the mesoporous structure.

Multiple-open-tubular column enabling transverse diffusion. Part 2: Channel size distribution and structure optimization.

Multiple-open-tubular columns enabling transverse diffusion (MOTTD) are made of straight, parallel, and cylindrical flow channels separated by a mesoporous stationary phase. In Part 1, a model of band broadening along MOTTD columns accounting for longitudinal diffusion, the trans-channel velocity bias, and mass transfer resistance in the stationary phase was proposed and validated. In this Part 2, the model is completed by considering the impact of short-range inter-channel velocity biases on the MOTTD plate number. These velocity biases are caused by the wide distribution of the channel diameters. Different ratios, ρ, of the average inner diameter, 2, of the flow channels to their closest center-to-center distance d (d= 5 µm, ρ= 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9) with a relative standard deviation (RSD) increasing from 0 to 50% are considered. The zone retention factor k1 was increased from 1 to 25. The complete model of band broadening is validated after adjustment to dispersion data obtained by 1) the lattice-Boltzmann method for modeling fluid flow, 2) a random-walk particle-tracking (RWPT) technique to address advective-diffusive transport, and 3) by considering two distinct populations of flow channels (inner radii rc,1=(1-RSD) and rc,2=(1+RSD)) arranged at the nodes of a hexagonal compact array. The completed model of band broadening in MOTTD columns reveals that the RSD of the channel diameters has only a moderate impact on the optimum plate number of MOTTD columns: the relative increase of the minimum plate height do not exceed 30% even for the largest RSDs. However, when the mass transfer of the analyte is governed by its slow rate of transverse diffusion across the MOTTD column, the plate height can be increased by up to 100% at high average velocities. Regarding the best trade-off between analysis speed and column performance at a fixed pressure drop of 400 bar, irrespective of the zone retention factor and RSD of the distribution of the channel diameters, the fastest analyses are recommended for MOTTD columns having a small structural parameter ρ. In contrast, for the longest analysis times, the largest values of ρ are required to maximize the performance of MOTTD columns.

Multiple-open-tubular column enabling transverse diffusion. Part 1: Band broadening model for accurate mass transfer predictions.

We derive a model of band broadening in multiple-open-tubular columns enabling transverse diffusion (MOTTD). In MOTTD columns, the flow channels are straight, parallel, cylindrical tubes arranged in a hexagonal compact array. A mesoporous material or stationary phase (130 Å bridged-ethyl hybrid (BEH) silica support) is filling the volume between the flow channels. The model is based on Giddings' random-walk theory of non-equilibrium chromatography. It is calibrated for the unknown configuration factor, qs, related to the specific geometry of the stationary phase in MOTTD columns. qs values are found based on the best fit of the model to simulated dispersion data obtained by the lattice-Boltzmann method for modelling fluid flow and a random-walk particle-tracking technique to address advective-diffusive transport of the analytes. For the model calibration, simulations are performed for different ratios, ρ, of the average inner diameter of the flow channels to their closest center-to-center distance under retained and non-retained conditions. The model is successfully validated (average relative errors below 10%) under both retained and non-retained conditions. For the same column format (4.6 mm i.d.  ×  150 mm), external porosity, zone retention factor, and relative standard deviation of the distribution of the inner diameters of the flow channels, the derived model reveals the intrinsic advantage of MOTTD columns (center-to-center distance between flow channels of 5 µm and ρ = 0.62) over a conventional column packed with 5 µm 130 Å BEH silica particles and the same multiple porous-layer open-tubular column (MPLOT) disabling transverse dispersion. MOTTD columns are weakly affected by the polydispersity of the inner diameter of the flow channels. Provided MOTTD columns could be prepared at a small feature size of 5 µm or less, they are expected to deliver a significant improvement in column technology relative to current particulate and silica monolithic columns.

Morphology-transport relationships in liquid chromatography: Application to method development in size exclusion chromatography.

We present relationships between the multiscale structure and the separation properties of size exclusion chromatography (SEC) columns. Physical bed reconstructions of wall and bulk regions from a 2.1 mm i.d. column packed with fully porous 1.7 µm bridged-ethyl hybrid (BEH) particles, obtained by focused ion-beam scanning electron microscopy, serve as geometrical models for the packing microstructure in wall and central regions of a typical narrow-bore SEC column. In addition, the intraparticle mesopore space morphology of the BEH particles is reconstructed using electron tomography, to ultimately construct a realistic multiscale model of the bed morphology from mesopore level via interparticle macropore space to transcolumn scale. Complemented by the results of eddy dispersion simulations in computer-generated bulk packings, relationships between packing microstructure and transchannel, short-range interchannel, as well as transcolumn eddy dispersion are used to analyze the fluid dynamics in the interparticle macropore space of the model. Further, we simulate hindered diffusion and accessible porosity for passive, finite-size tracers in the intraparticle mesopore space, to finally determine the effective particle and bed diffusion coefficients of these tracers in the hierarchical (macro-mesoporous) bed. Retention and transport properties of polystyrene standards with hydrodynamic diameters from 5 to 95 Å in tetrahydrofuran are subsequently predicted without introducing bias from arbitrary models. These properties include the elution volumes of the polystyrene standards, the global peak capacity (over the entire separation window), and the rate of peak capacity at any fixed elution volume. Optimal flow rates yielding maximal global peak capacity and a nearly uniform rate of peak capacity over the entire separation window are close to 0.04 and 0.20 mL/min, respectively. SEC column performance obtained for fully porous and superficially porous particles is compared by varying the core-to-particle diameter ratio ρ from 0 to 0.95. Because the separation window is narrowing more rapidly than the rate of peak capacity is growing with increasing ρ, core-shell particles always provide smaller global peak capacity; they still can be advantageous but only for simple sample mixtures. The presented morphology-performance approach holds great promise for method development in SEC.

Faster dewetting of water from C8- than from C18-bonded silica particles used in reversed-phase liquid chromatography: Solving the paradox.

For comparable surface coverage of alkyl-bonded chains (∼3 µmol/m2), the dewetting of 100% aqueous mobile phases from the mesopores of octyl(C8)-bonded silica particles is found 70 times faster than that from the same but octadecyl(C18)-bonded silica particles. This observation was made in this work for both fully porous (5 µm Symmetry) and superficially porous (2.7 µm CORTECS) particles. This experimental result is paradoxical because (1) the average pore size of C8-bonded materials is 10-15 Šlarger than that of C18-bonded materials for the same unbounded silica gel and (2) the contact angle of water measured on smooth and planar C8-bonded surface is about 6° smaller than that on the same but C18-bonded surface (104° versus 110°). The equilibrium Laplace pressure is then expected to be smaller and the kinetics of water dewetting to be slower for silica-C8 than for silica-C18 stationary phases used in RPLC. The solution to this riddle is investigated based on (1) the calculation of the dewetting time assuming that the pores are monosized and the process is driven by the Laplace pressure, (2) the measurement of the advancing and receding contact angles of three different C18- and C8-bonded silica gels (4 µm NovaPak, 5 µm Symmetry, and 2.7 µm CORTECS) from the water porograms measured in a range of water pressure from normal pressure to 500 bar, and (3) on the calculation of the pore connectivity for both C8 and C18-bonded silica. First, the experimental results show that the observed dewetting times are of the order of minutes or even hours instead of millisecond as predicted by the dewetting model. Secondly, the advancing and receding contact angles of water onto the C8-bonded silicas are found larger (by an average of +7° and +2°, respectively) than those measured for the same but C18-bonded silica (average of 112° and 92°). Finally, the calculated pore connectivity is decreasing by about 30% for 90 Šunbounded silica materials from C8 to C18-bonded RPLC phases. Overall, the observed and much faster dewetting of water from C8 column than that from C18 column is primarily explained by a higher internal pore connectivity due to the thinner thickness of the alkyl-bonded layer (7 Šversus 15 Å) and, to a lesser extent, by a higher extrusion Laplace pressure of water (≃+10 bar).

Relationship between bed heterogeneity, chord length distribution, and longitudinal dispersion in particulate beds.

We analyse a relationship between the bulk microstructure of randomly packed beds, which we quantify through chord length distribution (CLD) analysis of the interparticle void space, and the associated flow heterogeneity, as expressed by the longitudinal dispersion coefficient at a Péclet number of Pe = 10. A random collection of physically reconstructed packings is complemented with a systematic set of computer-generated packings of monosized spheres, for which the packing-generation algorithm has been carefully adjusted to realize a monotonic variation of the bed porosity and microstructural heterogeneity. The most relevant difference in the morphology between these computer-generated and the physically reconstructed packings are structural defects present in the real packings, such as particle oligomers and larger voids as well as contaminations and particle debris. These defects influence the pore space morphology and introduce additional structural heterogeneity. Hydrodynamic dispersion coefficients for all packings are derived by implementing the lattice-Boltzmann method to simulate fluid flow and a random-walk particle tracking technique to record the transport of passive, point-like tracers in the flow fields. We propose a morphological descriptor, σ/µ, based on statistical parameters of a CLD (standard deviation σ and mean chord length µ) that can be used to predict the dispersion coefficient in packed beds, independent from the underlying particle size distribution, packing-generation protocol, bed porosity, and the occurrence of structural defects.

Analysis of microstructure-effective diffusivity relationships for the interparticle pore space in physically reconstructed packed beds.

We evaluate the effective diffusion coefficient in the interparticle pore space of eight physically reconstructed chromatographic packings of fully porous and core-shell particles, employing an analytical formula that involves the three-point microstructural parameter calculated from two-point and three-point correlation functions through an approach based on sampling templates. Diffusivities calculated by the approximate analytical formula are close to those obtained from pore-scale simulations in the reconstructions using a random-walk particle-tracking technique. Diffusivities are affected, apart from the interparticle porosity, by the actual bed microstructure and packing defects like particle oligomers, spalled shells, larger voids, and debris. Importantly, the data for the microstructural parameter and effective diffusion coefficient over the porosity range spanned by the individual packings (0.363-0.444) reveals non-monotonic behavior. Our analysis demonstrates that the three-point microstructural parameter reflects this morphological specificity of the packings and, as a result, Eq. (3) provides accurate estimates of the effective diffusion coefficients. The presented numerical approach can therefore be applied to evaluate diffusivities in packed beds with all their salient features just from the geometrical information embodied in the bed porosity and the three-point microstructural parameter.

Morphology of Fluids Confined in Physically Reconstructed Mesoporous Silica: Experiment and Mean Field Density Functional Theory.

Three-dimensional physical reconstruction of the random mesopore network in a hierarchically structured, macroporous-mesoporous silica monolith via electron tomography has been used to generate a lattice model of amorphous, mesoporous silica. This geometrical model has subsequently been employed in mean field density functional theory (MFDFT) calculations of adsorption and desorption. Comparison of the results with experimental sorption isotherms for nitrogen at 77 K shows a good qualitative agreement, with both experiment and theory producing isotherms characterized by type H2 hysteresis. In addition to the isotherms, MFDFT provides the three-dimensional density distribution for the fluid in the porous material for each state studied. We use this information to map the phase distribution in the mesopore network in the hysteresis region. Phase distributions on the desorption boundary curve are compared to those on the adsorption boundary curve for both fixed pressure and fixed density, revealing insights into the relationship between phase distribution and hysteresis.

Impact of diffusion on transverse dispersion in two-dimensional ordered and random porous media.

Solute dispersion in fluid flow results from the interaction between advection and diffusion. The relative contributions of these two mechanisms to mass transport are characterized by the reduced velocity ν, also referred to as the Péclet number. In the absence of diffusion (i.e., when the solute diffusion coefficient D_{m}=0 and ν→∞), divergence-free laminar flow of an incompressible fluid results in a zero-transverse dispersion coefficient (D_{T}=0), both in ordered and random two-dimensional porous media. We demonstrate by numerical simulations that a more realistic realization of the condition ν→∞ using D_{m}≠0 and letting the fluid flow velocity approach infinity leads to completely different results for ordered and random two-dimensional porous media. With increasing reduced velocity, D_{T} approaches an asymptotic value in ordered two-dimensional porous media but grows linearly in disordered (random) structures depending on the geometrical disorder of a structure: a higher degree of heterogeneity results in a stronger growth of D_{T} with ν. The obtained results reveal that disorder in the geometrical structure of a two-dimensional porous medium leads to a growth of D_{T} with ν even in a uniform pore-scale advection field; however, lateral diffusion is a prerequisite for this growth. By contrast, in ordered two-dimensional porous media the presence of lateral diffusion leads to a plateau for the transverse dispersion coefficient with increasing ν.

Computational investigation of longitudinal diffusion, eddy dispersion, and trans-particle mass transfer in bulk, random packings of core-shell particles with varied shell thickness and shell diffusion coefficient.

In recent years, chromatographic columns packed with core-shell particles have been widely used for efficient and fast separations at comparatively low operating pressure. However, the influence of the porous shell properties on the mass transfer kinetics in core-shell packings is still not fully understood. We report on results obtained with a modeling approach to simulate three-dimensional advective-diffusive transport in bulk random packings of monosized core-shell particles, covering a range of reduced mobile phase flow velocities from 0.5 up to 1000. The impact of the effective diffusivity of analyte molecules in the porous shell and the shell thickness on the resulting plate height was investigated. An extension of Giddings' theory of coupled eddy dispersion to account for retention of analyte molecules due to stagnant regions in porous shells with zero mobile phase flow velocity is presented. The plate height equation involving a modified eddy dispersion term excellently describes simulated data obtained for particle-packings with varied shell thickness and shell diffusion coefficient. It is confirmed that the model of trans-particle mass transfer resistance of core-shell particles by Kaczmarski and Guiochon [42] is applicable up to a constant factor. We analyze individual contributions to the plate height from different mass transfer mechanisms in dependence of the shell parameters. The simulations demonstrate that a reduction of plate height in packings of core-shell relative to fully porous particles arises mainly due to reduced trans-particle mass transfer resistance and transchannel eddy dispersion.

Effect of adsorption on solute dispersion: a microscopic stochastic approach.

We report on results obtained with a microscopic modeling approach to Taylor-Aris dispersion in a tube coupled with adsorption-desorption processes at its inner surface. The retention factor of an adsorbed solute is constructed by independent adjustment of the adsorption probability and mean adsorption sojourn time. The presented three-dimensional modeling approach can realize any microscopic model of the adsorption kinetics based on a distribution of adsorption sojourn times expressed in analytical or numerical form. We address the impact of retention factor, adsorption probability, and distribution function for adsorption sojourn times on solute dispersion depending on the average flow velocity. The approach is general and validated at all stages (no sorption; sorption with fast interfacial mass transfer; sorption with slow interfacial mass transfer) using available analytical results for transport in Poiseuille flow through simple geometries. Our results demonstrate that the distribution function for adsorption sojourn times is a key parameter affecting dispersion and show that models of advection-diffusion-sorption cannot describe mass transport without specifying microscopic details of the sorption process. In contrast to previous one-dimensional stochastic models, the presented simulation approach can be applied as well to study systems where diffusion is a rate-controlling process for adsorption.

Comparison of first and second generation analytical silica monoliths by pore-scale simulations of eddy dispersion in the bulk region.

We present the first quantitative comparison of eddy dispersion in the bulk macropore (flow-through) space of 1st and 2nd generation analytical silica monoliths. Based on samples taken from the bulk region of Chromolith columns, segments of the bulk macropore space were physically reconstructed by confocal laser scanning microscopy to serve as models in pore-scale simulations of flow and dispersion. Our results cover details of the 3D velocity field, macroscopic Darcy permeability, transient and asymptotic dispersion behavior, and chromatographic band broadening, and thus correlate morphological, microscopic, and macroscopic properties. A complete set of parameters for the individual eddy dispersion contributions in the bulk was obtained from a Giddings analysis of the simulated plate height data. The identified short-range structural heterogeneities correspond to the average domain size of the respective monoliths. Our plate height curves show that structural improvements in the bulk morphology of 2nd generation monoliths play only a minor role for the observed improvement in overall column efficiency. The results also indicate a topological dissimilarity between 1st and 2nd generation analytical silica monoliths, which raises the question how the domain size of silica monoliths can be further decreased without compromising the structural homogeneity of the bed.

Comments on "hydrodynamic and dispersion behavior in a non-porous silica monolith through fluid dynamic study of a computational mimic reconstructed from sub-micro-tomographic scans".

We comment on a recently published paper by Loh and Vasudevan [J. Chromatogr. A 1274 (2013) 65], which reported the physical reconstruction of the bulk macropore space of an analytical silica monolith by X-ray computed microtomography and the subsequent computational fluid dynamics simulations of flow and mass transport in the reconstructed monolith model. Loh and Vasudevan claim that their combined reconstruction and simulation approach offers a significant reduction of computational expenses without significant loss in accuracy in characterizing the macropore space heterogeneity of the monolith and predicting its transport properties. We challenge their claim and question the validity and validation of their results by discussing the employed scanning resolution, the characterization of macropore space heterogeneities, the interpretation of the simulated dispersion data, as well as the comparison of computational expenses with previous work.

Enrichment of cations via bipolar electrode focusing.

We have previously demonstrated up to 5 × 10(5)-fold enrichment of anionic analytes in a microchannel using a technique called bipolar electrode focusing (BEF). Here, we demonstrate that BEF can also be used to enrich a cationic fluorescent tracer. The important point is that chemical modification of the microchannel walls enables reversal of the electroosmotic flow (EOF), enabling cations, instead of anions, to be enriched via an electric field gradient focusing mechanism. Reversal of the EOF has significant consequences on the formation and shape of the region of the buffer solution depleted of charge carriers (depletion zone). Electric field measurements and numerical simulations are used to elucidate the factors influencing the depletion zone. This information is used to understand and control the location and shape of the depletion zone, which in turn influences the stability and concentration of the enriched band.

From random sphere packings to regular pillar arrays: analysis of transverse dispersion.

We study the impact of microscopic order on transverse dispersion in the interstitial void space of bulk (unconfined) chromatographic beds by numerical simulations of incompressible fluid flow and mass transport of a passive tracer. Our study includes polydisperse random sphere packings (computer-generated with particle size distributions of modern core-shell and sub-2 µm particles), the macropore space morphology of a physically reconstructed silica monolith, and computer-generated regular pillar arrays. These bed morphologies are analyzed by their velocity probability density distributions, transient dispersion behavior, and the dependence of asymptotic transverse dispersion coefficients on the mobile phase velocity. In our work, the spherical particles, the monolith skeleton, and the cylindrical pillars are all treated as impermeable solid phase (nonporous) and the tracer is unretained, to focus on the impact of microscopic order on flow and (particularly transverse) hydrodynamic dispersion in the interstitial void space. The microscopic order of the pillar arrays causes their velocity probability density distributions to start and end abruptly, their transient dispersion coefficients to oscillate, and the asymptotic transverse dispersion coefficients to plateau out of initial power law behavior. The microscopically disordered beds, by contrast, follow power law behavior over the whole investigated velocity range, for which we present refined equations (i.e., Eq.(13) and the data in Table 2 for the polydisperse sphere packings; Eq.(17) for the silica monolith). The bulk bed morphologies and their intrinsic differences addressed in this work determine how efficient a bed can relax the transverse concentration gradients caused by wall effects, which exist in all confined separation media used in chromatographic practice. Whereas the effect of diffusion on transverse dispersion decreases and ultimately disappears at increasing velocity with the microscopically disordered chromatographic beds, it dominates in the pillar arrays. The pillar arrays therefore become the least forgiving bed morphology with macroscopic heterogeneities and the engendered longitudinal dispersion in chromatographic practice. Wall effects in pillar arrays and the monolith caused by their confinement impact band broadening, which is traditionally observed on a macroscopic scale, more seriously than in the packings.

Propagating concentration polarization and ionic current rectification in a nanochannel-nanofunnel device.

We study ionic current rectification observed in a nanofluidic device with a nanofunnel positioned between two straight nanochannels. Ion transport is simulated by resolving the coupled three-dimensional Nernst-Planck, Poisson, and Navier-Stokes equations. In the modeled system, the electric double layer extends into the channel, and consequently, the funnel tip exhibits charge-selective properties, which results in the formation of enriched and depleted concentration polarization (CP) zones within the nanofunnel in the high- and low-conductance states, respectively. This scenario is similar to the one observed for ion transport through a charged conical nanopore connecting two macroscopic reservoirs. However, the presence of the adjacent straight nanochannels allows the CP zones to propagate out of the funnel into the adjoining channels. The condition for propagation of the CP zones is determined by several parameters, including the electroosmotic flow velocity. We demonstrate that in the high-conductance regime the modeled system is characterized by increased ionic concentrations in the entire cathodic nanochannel, whereas in the low-conductance state the depleted CP zone does not propagate out of the funnel and remains localized. The required three-dimensional modeling scheme is implemented on a parallel computational platform, is general as well as numerically efficient, and will be useful in the study of more advanced nanofluidic device designs for tailoring ionic current rectification.

Label-free electrochemical monitoring of concentration enrichment during bipolar electrode focusing.

We show that a label-free electrochemical method can be used to monitor the position of an enriched analyte band during bipolar electrode focusing in a microfluidic device. The method relies on formation of a depleted buffer cation region, which is responsible for concentration enrichment of the charged analyte. However, this depletion region also leads to an increase in the local electric field in the solution near a bipolar electrode (BPE), and this in turn results in enhanced faradaic reactions (oxidation and reduction of water) at the BPE. Therefore, it is possible to detect the presence of the concentrated analyte band by measuring the current passing through the BPE used for concentration enrichment, or the concentrated band can be detected at a secondary BPE dedicated to that purpose. Both experiments and simulations are presented that fully elucidate the underlying phenomenon responsible for these observations.

Morphology-transport relationships for silica monoliths: from physical reconstruction to pore-scale simulations.

This work describes individual steps of an approach toward quantitative correlations between morphological and mass transport properties of capillary silica monoliths. The macropore space morphology of the central core region of the capillary monolith is visualized by a fast, nondestructive, and quantitative method using three-dimensional reconstruction from confocal laser scanning microscopy images. The reconstructed 60 µm×60 µm×12 µm monolith domain consisted of 1.6×10(9) cubic voxels with 30 nm edge length. The received morphological data were chord length distributions for the bulk macropore space and skeleton of the monolith, which we characterized by k-gamma distributions. This analysis provides parameters that can be correlated with the mass transport properties obtained by macropore-scale simulations of flow and transport in the reconstructed monolith. These simulations were realized on a supercomputing platform and comprised the lattice-Boltzmann method for fluid flow and a random-walk particle-tracking method for advective-diffusive mass transport. The characteristic length scales of eddy dispersion correlate with the statistical measures of the chord length distributions. Simulated plate height curves demonstrate that the bulk monolith is very homogeneous, and that the intraskeleton transport properties and a stochastic variation of macropore space characteristics can be neglected compared with the importance of reducing column radial heterogeneity in chromatographic practice.