Search

It is well known that high-speed/high-efficiency separations in nano-flow liquid chromatography (LC) are very sensitive to the quality of the connections between the column and the rest of the instrument. In the present study, two types of connection errors (capillary misalignment and the occurrence of an inter-capillary gap) have been investigated using computational fluid dynamics. Interestingly, it has been found that large degrees of capillary misalignment (assuming an otherwise perfect contact between the capillary end-faces) can be afforded without introducing any significant dispersion over the entire range of investigated relative misalignment errors (0 ≤ ε/ d cap ≤ 75%), even at the largest flow rates considered in nano-LC. On the other hand, when an inter-capillary gap is present, the dispersion very rapidly increases with the radial width D c of this gap (extra variance ∼ D c n with n even reaching values above 4). The dependency on the gap length L c is however much smaller. Results show that, when D c ≤ 30 μm and L c ≤ 200 μm, dispersion losses can be limited to the order of 1 nL 2 at a flow of 1.5 μL/min, which is generally very small compared to the dispersion in the capillaries (20 μm i.d.) themselves. This result also reconfirms that zero-dead volume connectors with a sufficiently narrow bore can in theory be used without compromising peak dispersion in nano-LC, at least when the capillaries can be matched perfectly to the connector in- and outlet faces. The results are also indicative of the extra dispersion occurring inside microfluidic chips or in the connections between a microfluidic chip and the outer world.

In this contribution, we review the recent literature relating to the measurement and modelling of all diffusion-dominated processes contributing to the efficiency of a chromatographic column. In first instance, this involves the measurement and modelling of the overall effective diffusion coefficient D eff (determining the so-called B-term band broadening). The latter manifests itself most clearly during a so-called peak parking experiment. Using effective medium theory modelling, the measured D eff -value can subsequently be decomposed into its constituent contributions, of which the intra-particle or the mesoporous zone and the surface diffusion coefficient are the most important ones. As an accurate estimation of the diffusion processes also allows computing the C-term plate height contribution terms, the review ends with some recent insights obtained when using the established B- and C-term contributions to compute the degree of eddy-dispersion in contemporary packed bed columns., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Over the past years viscous heating band broadening occurring in high pressure liquid chromatography has been studied extensively. In the present numerical study, we investigate the fine details of this band broadening contribution under extreme high-pressure conditions (2500 bar). To analyze the results, we first show that viscous heating leads to two clearly distinguishable band broadening effects, one originating from the radial differences in the species migration velocity and the other from the axial variations. It was found that the radial contribution is independent of the intrinsic band broadening of the bed (i.e. band broadening in absence of viscous heating) while it strongly depends on the radial dispersion coefficient and the retention enthalpy of the analytes. On the other hand, the axial contribution is strongly dependent on the bed intrinsic band broadening and it is found to be 4 to 5 times lower than the radial contribution. We also show the strong effect of the endfittings on the temperature gradients inside the column thus on the resulting viscous heating band broadening., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

We report on a numerical simulation study of a number of potential column technology solutions to minimize the plate height contribution (H vh ) originating from the use of ultra-high pressures and their concomitant viscous heating effect. Looking as far as possible into the future of UHPLC, all main results are obtained for the case of a 2500 bar pressure gradient. However, to generalize the result, a correlation is given that can be used to interpolate the results to lower pressures with some 10% accuracy. For the considered case of a 2.1mm column, a liquid flow rate of 0.45 ml/min, an analyte with retention factor k(25°C)=3 and a retention enthalpy chosen such that ΔH R /R= -1000 K, it is found that, in order to keep the global plate height as measured at the column outlet (H vh,glob,out ) below 1 μm, the bed conductivity would need to be raised to λ bed =2.4 W/m•K, i.e., 4 times higher than a typical packed bed of fully-porous or core-shell silica particles. An equivalent effect on the band broadening could be obtained if it would be possible to replace the steel column wall with a low conductivity material. In this case, a wall conductivity of 0.25 W/m•K, i.e., 64 times smaller than the conductivity of steel, would be needed to keep H vh,glob,out below 1 μm. Results are also interpreted based on contour plots of the axial and radial velocity variation of a retained analyte., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021. Published by Elsevier B.V.)

The present study proposes a ready-to-use analytical expression to calculate the mobile zone mass transfer contribution (h Cm ) in packed bed columns. For this purpose, first high-accuracy computations of the band broadening in a perfectly ordered sphere array (fcc-arrangement, external porosity ε=0.40) were made using computational fluid dynamics (CFD), covering a broad range of zone retention factors (2≤k''≤18) and reduced velocities (0≤ν i ≤48). Subsequently, these data were used to determine the value of the geometrical constants in a number of possible analytical expressions for the h Cm -contribution. This fitting exercise showed the traditional literature approach, using the Wilson-Geankoplis correlation to calculate the dimensionless Sherwood (Sh) number for the mass transfer, leads to fitting errors on the h Cm -term as large as 150%. Instead, a new correlation for Sh is established. In addition, we also explored the difference in fitting accuracy between h Cm -expressions based on either a plug-flow or a laminar flow profile assumption. Surprisingly, no significant difference in fitting accuracy between both assumptions was observed. Finally, a best-fit analytical expression is proposed that can represent the CFD-computed band broadening data with an average absolute fitting error of Δh=0.005, corresponding to a relative error of 2.5% on the h Cm -term and of only 0.3% on the total plate height in a perfectly ordered sphere packing. Defining the presently investigated fcc-ordered sphere array with external porosity=40% as the reference geometry for a perfect sphere packing, the established expression can be used as a new yardstick expression against which the degree of eddy-dispersion can be measured., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

We report on a series of high-accuracy computational fluid dynamics band broadening simulations in three different 2-D flow systems: a 2-D pillar array and 2-D lumped packed bed geometries with different checkerboard velocity bias patterns. These media display a local maximum in the relationship between the eddy-dispersion plate height and the mobile phase velocity. The occurrence of such a dispersion maximum has not been reported before but appears to be a characteristic of regular chromatographic media with alternating velocity bias, at least in 2-D geometries. This newly observed behavior can be fully understood and modelled using the checkerboard model established in part I of the present study., Competing Interests: Declaration of Competing Interest None., (Copyright © 2020 Elsevier B.V. All rights reserved.)

The additivity assumption underlying Giddings' coupling model for the eddy-dispersion in laminar flows through heterogeneous media is critically analyzed and a potential solution for its non-additivity in the high velocity limit is presented. Whereas the unit cell in Giddings' model only consists of a single velocity bias step, the unit dispersion cell of the newly proposed model comprises two consecutive velocity bias steps. Consequently, the unit cell of this new model allows to account for the occurrence of an internal velocity bias rectification at high reduced velocities and is therefore additive in both the low and high velocity limit. First, a mathematical expression for the velocity- and diffusion-dependency of the model's dispersion characteristics has been established. Subsequently, the physical behavior of the model is discussed. It is shown the relation between the eddy-dispersion plate height h and the reduced velocity ν can be expected to display a local maximum in systems where the transversal dispersion purely occurs by molecular diffusion, as is the case in perfectly ordered flow-through media. In disordered media, where the transversal dispersion also contains a significant advective component, the model predicts a velocity-dependency that is qualitatively similar to that described by Giddings' coupling model but, all other conditions being equal, converges to a significantly smaller horizontal asymptote at high reduced velocity. The latter might shed new light on earlier eddy-dispersion studies pursuing a quantitative agreement between experimental data and the Giddings model., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Background: Blood eosinophil counts (BEC) were recently included in the 2019 Global Initiative for Obstructive Lung Disease (GOLD) guideline as an easily accessible theragnostic biomarker for Chronic Obstructive Pulmonary Disease (COPD). However, the stability of BEC remains insufficiently studied., Methods: We conducted a retrospective study in six primary care practices in Belgium on data from Electronic Health Records of stable COPD patients, to characterise the stability of blood eosinophils over time. We report the percentage of patients with BEC persistently below or above the 2019 GOLD guideline thresholds (100 and 300 cells/μL). For each patient the mean, standard deviation (SD) and relative standard deviation (RSD) of the BEC were calculated to determine the intra-patient variability., Results: Ninety-eight patients were included, yielding 1082 eosinophil measurements (median 8 measurements/patient), with BEC ranging between 0 and 1504 cells/μL. Four (4.1%) patients had BEC persistently below 100 cells/μL, 34 (34.7%) had measurements persistently above this threshold. Approximately half of the patients (51.0%) had BEC persistently below 300 cells/μL and 3 (3.1%) patients had counts persistently above this threshold. 28.6% of patients crossed both threshold values throughout the registration period. The mean BEC per patient ranged between 15 and 846 cells/μL with an intra-patient SD between 5 and 658 cells/μL. The mean intra-patient RSD was 0.46. There was a significant strong positive correlation (Pearson analyses) between the mean BEC and SD (r = 0.765; n = 98). Simple linear regression was used to further describe the influence of the mean eosinophil count on the SD (B = 0.500; 95%CI 0.415-0.586; n = 98; p < 0.001)., Conclusion: BEC can be variable in individual COPD patients. Therefore, the use of a single measurement to guide therapeutic decisions remains debatable. Further prospective research remains necessary to validate the reproducibility of this biomarker.

We report on a numerical study of the thermal conductivity and temperature distribution in analytical packed bed and monolithic HPLC columns to assess the feasibility of a number of potential solutions to the viscous heating problem that would normally impede high efficiency separations when moving to extreme operating pressures (e.g., 2500 bar). Computational fluid dynamic (CFD) simulations were employed to study heat transfer on three hierarchical levels of the column: meso‑pore level, through-pore level and column level. At the first level, realistic values were determined for the conductivity of the porous zone (k pz ), depending on the internal structure of the porous zone and the mobile phase used (acetonitrile, water or a mixture of both). These k pz -values were in turn used at the second level to determine realistic values for the effective conductivity of the bed (k eff ). It was shown that the presence of a solid core only has a minor effect on the packed bed conductivity. Using highly conducting materials as core material can be expected to maximally lead to a 60% increase in bed conductivity. Contrarily, in monolithic beds, the presence of a core material would form one continuous phase of highly conducting material, thus greatly enhancing the conductivity of the bed. At the third level, the temperature field in the entire column (bed and column housing) was resolved for three typical boundary conditions: isothermal, adiabatic and still-air oven. The effect of different physical properties (inlet pressure, mobile phase composition, bed conductivity, wall conductivity and column ID) on these temperature fields was investigated. It was shown that, theoretically, besides 1 mm ID columns also "core-shell monoliths" can provide a solution to viscous heating (by increasing the bed conductivity). Other possible solutions are proposed and discussed., Competing Interests: Declaration of Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

In spatial multi-dimensional liquid chromatography (LC) devices the flow of each dimension has to remain in the corresponding region, otherwise the separation efficiency is undermined. Adequate flow-confinement measures are necessary. Here, the use of permeability differences across different compartments of spatial two-dimensional (2D) and three-dimensional (3D) LC devices as a method to guide fluid flow and reduce analyte loss during the first, second- and third-dimension development was investigated with computational fluid dynamics (CFD) simulations. In case of 2DLC devices, it was shown that porous barriers with a permeability on the order of 10 -12 m 2 suffice to keep the total sample spillage from an open 1 D channel under 1%. In case of 3DLC devices, it was shown that flow confinement could be achieved using an open 1 D channel in combination with a highly-permeable monolith (permeability on the order of 10 -12 m 2 ) in the second-dimension ( 2 D) and a less permeable packing with a permeability on the order of 10 -15 m 2 (e.g. 1 μm particles) in the third-dimension ( 3 D). Additionally, the impact of the 3 D flow-distributor has been studied and a novel design, capable of limiting the spillage to the other dimensions to the absolute minimum, is proposed., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2019 Elsevier B.V. All rights reserved.)

The Groningen Institute Model for Management in Care Services aims to prepare medical students for their complex tasks as family physicians, based on the CanMEDS framework. Although initially developed for pharmacy students, the present paper reports on the eight-year experience with GIMMICS for family physician students at the Vrije Universiteit Brussel. The Groningen Institute Model for Management in Care Services is a training game that simulates real-life situations in a structured and supervised setting. It offers students the possibility to practice clinical, practical, and communicational skills. Students install and manage their group practices, hold consultations with simulated patients, participate in several assignments and collaborate with pharmacy students. Feedback sessions showed that the training game is well-received by the students. A self-assessment questionnaire comprised of 23 questions on significant aspects of the seven CanMEDS roles showed significantly higher scores at the end of the game for 17 questions (p<0.05, Wilcoxon signed-rank test ). GIMMICS is a valuable linking pin between the different learning methods in medical education and clinical practice, helping students to improve themselves in the CanMEDS roles. However, simulation-based medical education requires significant time and resource investment., (©Carol Davila University Press.)

Conventional one-dimensional column-based liquid chromatographic (LC) systems do not offer sufficient separation power for the analysis of complex mixtures. Column-based comprehensive two-dimensional liquid chromatography offers a higher separation power, yet suffers from instrumental complexity and long analysis times. Spatial two-dimensional liquid chromatography can be considered as an alternative to column-based approaches. The peak capacity of the system is ideally the product of the peak capacities of the two dimensions, yet the analysis time remains relatively short due to parallel second-dimension separations. Aspects affecting the separation efficiency of this type of systems include flow distribution to homogeneously distribute the mobile phase for the second-dimension ( 2 D) separation, flow confinement during the first-dimension ( 1 D) separation, and band-broadening effects during analyte transfer from the 1 D separation channel to the 2 D separation area. In this study, the synergy between computational fluid dynamics (CFD) simulations and rapid prototyping was exploited to address band broadening during the 2 D development and analyte transfer from 1 D to 2 D. Microfluidic devices for spatial two-dimensional liquid chromatography were designed, simulated, 3D-printed and tested. The effects of presence and thickness of spacers in the 2 D separation area were addressed and leaving these out proved to be the most efficient solution regarding band broadening reduction. The presence of a stationary-phase material in the 1 D channel had a great effect on the analyte transfer from the 1 D to the 2 D and the resulting band broadening. Finally, pressure limit of the fabricated devices and printability are discussed., (Copyright © 2019 The Author(s). Published by Elsevier B.V. All rights reserved.)

Spatial comprehensive two-dimensional liquid chromatography ( x LC× x LC) may be an efficient approach to achieve high peak capacities in relatively short analysis times, thanks to parallel second-dimension separations [1,2]. A key issue to reach the potential of x LC× x LC is to achieve adequate flow control and confinement of the analytes to the desired regions, i.e. confinement in the first-dimension direction and subsequently homogeneous flow in the second dimension. To achieve these goals we propose the TWIST concept (TWo-dimensional Insertable Separation Tool), a modular device that includes an internal first-dimension ( 1 D) part that is cylindrical and rotatable. This internal part features a series of through-holes, each of which is perpendicular to the direction of the 1 D flow. The internal part is inserted in the cylindrical casing of the external part. The internal diameter of the casing is marginally larger than the external diameter of the internal part. The external part also comprises a flow distributor and second-dimension ( 2 D) channels. During the 1 D injection and development, the channel is placed in a position where the through-holes are facing the wall of the external part, such that the liquid remains confined within the 1 D channel. Thereafter, to realize the transfer to the second dimension ( 2 D injection), the 1 D channel is rotated, so that the holes of the internal part are aligned with the holes on the external part, allowing a transversal flow of the 2 D mobile phase from the distributor through the 1 D channel and eventually into the 2 D area., (Copyright © 2018 The Authors. Published by Elsevier B.V. All rights reserved.)

We report on a numerical study of the thermal conductivity of core-shell particle packed bed columns. Covering a variety of packing structures and a broad range of mobile phase and porous zone conductivities, it was in all cases found that switching to particles with a highly conducting core (e.g., with a gold or copper core instead of a silica core) would produce a much smaller increase of the effective heat conductivity of the bed (k eff ) than previously expected in literature. We found maximal increases on the order of some 20-70%, which is much lower than the potential increases up to 2000% assumed in literature. The overestimation in literature could be attributed to the fact that this literature was based on an incorrect extrapolation of the Zarichnyak-model which was the heat conductivity model predominantly used up till now. On the other hand, the computed relationships between k eff and the core conductivity obtained in the present study are in good agreement with an analytical solution derived from the effective medium theory, a theory which is physically much more relevant for the case at hand than the Zarichnyak-model. The results also show that the observed increase in effective bed conductivity between fully porous and core-shell particle beds frequently observed in literature is not only due to the presence of the core, but that differences in the shell layer conductivity can play an equally important role. In addition, it could also be demonstrated that, if ways could be found to increase the conductivity of the shell layer, this would produce a much stronger increase of the overall bed conductivity than will ever be possible by increasing the conductivity of the cores., (Copyright © 2018 Elsevier B.V. All rights reserved.)

The present study reports on a computational fluid dynamics study of the band broadening occurring in injector systems frequently used in contemporary liquid chromatography instruments. The aim of this work is to determine band broadening originating purely from the injection volume in absence of any other possible contribution (e.g. band broadening due to the injection valve) and to unravel the mechanism behind it. Simulations of the dispersion process in flow through needle injectors were performed. In addition, fixed loop injectors were also simulated and comparison with flow through needle injectors was made. The results are also represented in a dimensionless form, allowing to generalize them to different experimental conditions. It was shown that two different injection regimes exist (the convection regime and the diffusion regime), both leading to narrow injection bands, while operating the injection needle in between the two regimes leads to broad injections bands. It was also found that the band broadening in the flow-through needle injector is strongly affected by the holding time between sample uptake and the actual injection. As a result, fixed, full loop injectors produced narrower injection bands compared to flow-through needle injectors operated with a realistic holding time., (Copyright © 2018 Elsevier B.V. All rights reserved.)

A new type of microfluidic flow distributor (referred to as the mixed mode or MM-distributor) is proposed. Its performance characteristics are determined using computational fluid dynamics (CFD), both in the absence and the presence of clogging, which is an important problem in microfluidic systems. A comparison is made with two existing, well-performing distributor types: the bifurcating (BF) distributor and an optimized diverging distributor, the so-called radially interconnected (RI) distributor. It was found that, in the absence of clogging, the MM-distributor produces only a little more dispersion than the bifurcating (BF) distributor, but much less than the radially interconnected (RI) distributor. The dispersion in an MM-distributor also follows a similar dependency on its width (power ≅ 2) as the BF-distributor. The dispersion in the RI-distributor on the other hand displays a very disadvantageous 4th-order dependency on its width, prohibiting its use to distribute the flow across wide beds (order of millimeters or centimeters). These observations hold independently of the flow rate. With increasing degree of clogging, the MM-distributor rapidly becomes advantageous over the BF-distributor, owing to the fluid contact zones that are provided after each bifurcation step. This means that overall, and when the occurrence of clogging cannot be excluded, the MM-type distributor seems to offer the best possible compromise between the ability to cope with local clogging events and the dispersion in the absence of clogging., (Copyright © 2018 Elsevier B.V. All rights reserved.)

We report on a first-principles numerical study explaining the potential advantage of core-shell particles with strictly radially-oriented mesopores. Comparing the efficiency of these particles with fully porous and core-shell particles with a conventional (i.e., randomly oriented) mesopore network, the present numerical study shows a similar strong reduction in minimal reduced plate height (hmin) as was very recently observed in an experimental study by Wei et al. (respectively a hmin-reduction on the order of about 1 and 0.5 reduced plate height-units). As such, the present work provides a theoretical basis to understand and confirm their experimental findings and quantifies the general advantage of "radial-diffusion-only" particles. Determining the effective longitudinal diffusion (B-term contribution) in a series of dedicated, independent simulations, it was found that this contribution can be described by a very simple, yet fully exact mathematical expression for the case of "radial- diffusion-only" particles. Using this expression, the significant increase in efficiency of these particles can be fully attributed to their much smaller B-term band broadening, while their C-term band broadening (representing the mass transfer resistance) remains unaffected., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Objective: To predict the impact of thyroid autoimmunity (TAI) on the probability of delivery after a defined number of treatment cycles, using analysis of cumulative delivery rates in patients with and without TAI., Design: Retrospective cohort study performed at the Center for Reproductive Medicine and Department of Endocrinology, University Hospital of Brussels, approved by the institutional review board of the hospital., Setting: University hospital., Patient(s): All patients who started their first IVF/intracytoplasmic sperm injection cycle at our fertility center between January 1, 2010 and December 31, 2011 were included., Main Outcome Measure(s): Live birth delivery after 25 weeks' gestation was taken as the primary endpoint of our study Cumulative delivery rates were calculated for both groups until six treatment cycles., Intervention(s): All patients (in both groups) received the usual IVF treatment protocols (i.e., antagonist or agonist protocol)., Result(s): In total 2,406 women who consulted our center were included. We included 333 patients with TAI and 2019 patients without TAI. In the TAI group the crude cumulative delivery rate after six cycles was 47%, whereas the expected cumulative delivery rate was 65%. In our control the crude cumulative delivery rate after six cycles was 47%, whereas the expected cumulative delivery rate was 76%., Conclusion(s): Our study did not confirm an influence of TAI status in patients undergoing fertility treatment on cumulative delivery rates after six IVF/intracytoplasmic sperm injection cycles., (Copyright © 2016 American Society for Reproductive Medicine. Published by Elsevier Inc. All rights reserved.)

Various designs of chips for comprehensive two-dimensional spatial liquid chromatography were investigated. The performance of these chips was initially evaluated using computational fluid dynamics (CFD). A bifurcating distributor with an angle of 140° between branches was implemented in order to achieve a homogeneous velocity field. The cross-sectional area of the channels of the flow distributor was fixed at 0.5 × 0.5 mm, which allows a robust micromilling technique to be used for chip manufacturing. Experiments were performed with chips featuring purposely introduced imperfections in the structure of the bifurcating flow distributor to study its capacity of overcoming potential local clogging. Split peaks were observed when 75% of one of the flow channels was obstructed, in line with the CFD predictions. The main bottlenecks for the performance of the spatial two-dimensional chips were identified, viz. sample injected in the first dimension diverging into the flow distributor and channel discretization (i.e., remixing of first-dimension separation peaks because of finite number of second-dimension channels). Solutions to the former problem were studied by applying a flow resistance in the vertical segments that formed the outlets of the flow distributor and by simulating the presence of constrictions. It was found that a flow resistance of 1.0×10(11) m(-2) reduced the amount of sample diverging into the flow distributor by a factor of 10. The presence of a constriction of 90% of the segment area and 50% of the segment length decreased the diverging flow by a factor of 5. The influence of the linear velocity was significant. Solutions to the channel discretization problem were sought by investigating different designs of spatial two-dimensional chips., (Copyright © 2016 Elsevier B.V. All rights reserved.)

In this manuscript, the band broadening contribution of a generic flow distributor and collector has been calculated using computational fluid dynamics (CFD). The effects of distributor design and operating conditions on distributor performance have been studied. The non uniform flow fields in the distributor cause band deformation and an increase in volumetric band variance (σv(2)). This volumetric band variance was calculated to be about 0.2 μl(2) at the inlet of a 4.6 mm ID column and 0.3 μl(2) at the outlet for a flow rate of 2 ml/min. For a 2.1 mm column the volumetric band variance is about 0.002 μl(2) for a flow rate of 0.41 ml/min. For current column performance the measured contributions are negligible in practice, probably because designs have been empirically improved together with improving column performance. None the less, the simulations show that bad distributor design can be detrimental for column performance., (Copyright © 2014 Elsevier B.V. All rights reserved.)

We used numerical simulations to investigate the recent observation that slip flow chromatography can obtain reduced plate heights as low as 0.032. The simulations were carried out for a 2D pillar array and a 3D face centred cubic particle stacking. We found that slip flow can only affect the mobile phase mass transfer contribution, which reduces the minimal total plate height by no more than 0.02 reduced plate height units for totally porous particles. Slip flow by itself can thus not fully explain the observed extremely low plate height values and further research is necessary to understand these results., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Effective medium theory (EMT) expressions for the B-term band broadening in monolithic silica columns are presented at the whole-column as well as at the mesoporous skeleton level. Given the bi-continuous nature of the monolithic medium, regular as well as inverse formulations of the EMT-expressions have been established. The established expressions were validated by applying them to a set of experimental effective diffusion (Deff)-data obtained via peak parking on a number of 1st and 2nd generation monolithic silica columns, as well as to a set of numerical diffusion simulations in a simplified monolithic column representation (tetrahedral skeleton model) with different external porosities and internal diffusion coefficients. The numerically simulated diffusion data can be very closely represented over a very broad range of zone retention factors (up to k″=80) using the established EMT-expressions, especially when using the inverse variant. The expressions also allow representing the experimentally measured effective diffusion data very closely. The measured Deff/Dmol-values were found to decrease significantly with increasing retention factor, in general going from about Deff/Dmol=0.55 to 0.65 at low k″ (k″≅1.5-3.8) to Deff/Dmol=0.25 at very high k″ (k″≅40-80). These values are significantly larger than observed in fully-porous and core-shell particles. The intra-skeleton diffusion coefficient (Dpz) was typically found to be of the order of Dpz/Dmol=0.4, compared to Dpz/Dmol=0.2-0.35 observed in most particle-based columns. These higher Dpz/Dmol values are the cause of the higher Deff/Dmol values observed. In addition, it also appears that the higher internal diffusion is linked to the higher porosity of the mesoporous skeleton that has a relatively open structure with relatively wide pores. The observed (weak) relation between Dpz/Dmol and the zone retention factor appears to be in good agreement with that predicted when applying the regular variant of the EMT-expression directly to the mesoporous skeleton level., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Using a fully transparent micropillar array chip and an optical "injection" setup capable of writing pulsed and continuous patterns into the flow by uncaging a fluorescent dye, highly detailed measurements of the transversal dispersion process in two-dimensional (2D) chromatographic beds could be made. With the use of water-glycerol mobile phase spanning a wide range of viscosities, the obtained data cover a broad range of reduced velocities (0 < ν < 3000) and show a clear leveling-off of the transversal dispersion coefficient at large reduced velocities. With dependence on the packing density, this leveling-off occurs at a value of about Dtrans/Dmol = 10 (ε = 0.4), Dtrans/Dmol = 4 (ε = 0.6) and Dtrans/Dmol = 2.5 (ε = 0.8). Another interesting observation that could be made is that the effect of the bed order on the observed transversal dispersion process is relatively small. The observed leveling-off in the relation between the measured Dtrans values and the reduced liquid velocity furthermore clearly invalidates the classical Galton-board model, predicting a linear increase of Dtrans with the reduced velocity. On the other hand, it corroborates a recently proposed series-connection transport model for Dtrans in 2D porous media.

The kinetic performance of commercially available first generation and prototype second generation silica monoliths has been investigated for 2.0mm and 3.0-3.2mm inner diameter columns. It is demonstrated that the altered sol-gel process employed for the production of second generation monoliths results in structures with a smaller characteristic size leading to an improved peak shape and higher efficiencies. The permeability of the columns however, decreases significantly due to the smaller throughpore and skeleton sizes. Scanning electron microscopy pictures suggest the first generation monoliths have cylindrical skeleton branches, whereas the second generation monoliths rather have skeleton branches that resemble a single chain of spherical globules. Using recently established correlations for the flow resistance of cylindrical and globule chain type monolithic structures, it is demonstrated that the higher flow resistance of the second generation monoliths can be entirely attributed to their smaller skeleton sizes, which is also evident from the external porosity that is largely the same for both monolith generations (ɛe∼0.65). The recorded van Deemter plots show a clear improvement in efficiency for the second generation monoliths (minimal plate heights of 13.6-14.1μm for the first and 6.5-8.2μm for the second generation, when assessing the plate count using the Foley-Dorsey method). The corresponding kinetic plots, however, indicate that the much reduced permeability of the second generation monoliths results in kinetic performances (time needed to achieve a given efficiency) which are only better than those of the first generation for plate counts up to N∼45,000. For more complex samples (N≥50,000), the first generation monoliths can intrinsically still provide faster analysis due to their high permeability. It is also demonstrated that - despite the improved efficiency of the second generation monoliths in the practical range of separations (N=10,000-50,000) - these columns can still not compete with state-of-the-art core-shell particle columns when all columns are evaluated at their own maximum operating pressure (200bar for the monolithic columns, 600bar for core-shell columns). It is suggested that monolithic columns will only become competitive with these high efficiency particle columns when further improvements to their production process are made and their pressure resistance is raised., (Copyright © 2013 Elsevier B.V. All rights reserved.)

Background: Influenza causes a substantial socioeconomic burden. In Belgium, only 54% of the target group receives an annual vaccination. Patient reminder/recall systems are effective in improving vaccination rates in primary care, but little is known about patients' preferences on notification of influenza vaccination., Aims: To evaluate whether general practice patients wish to be notified of the possibility of receiving influenza immunisation, and how., Methods: In January 2008, 750 questionnaires were handed out to all consecutive patients aged >18 years in three Belgian general practices. Main outcome measures were the percentage wanting to be notified, demographic and medical factors influencing the information needs of the patients and the specific way in which patients wanted to be notified., Results: About 80% of respondents wanted to be notified of the possibility of influenza vaccination. Logistic regression analysis showed that those who had previously been vaccinated particularly wished to be notified, both in the total population (OR 4.45; 95% CI 2.87 to 6.90; p<0.0001) and in the subgroup of high-risk individuals (OR 9.05; 95% CI 4.47 to 18.33; p<0.0001). More than 85% of the participants wanted to be informed by their family physician, mostly during a consultation regardless of the reason for the encounter. The second most preferred option was a letter sent by the family physician enclosing a prescription., Conclusions: The majority of general practice patients want to be notified of the possibility of influenza vaccination. More than 85% of participants who wanted to be notified preferred to receive this information from their family physician, mostly by personal communication during a regular visit. However, since a large minority preferred to be addressed more proactively (letter, telephone call, e-mail), GPs should be encouraged to combine an opportunistic approach with a proactive one.

Numerical calculations of the mobile zone mass transfer rate in a variety of ordered 2D and 3D structures are presented. These calculations are in line with earlier theoretical and experimental findings made in the field of chemical engineering and suggest that the Sherwood-number (Sh(m)) appearing in the mobile phase mass transfer term of the general plate height expression of liquid chromatography is not correctly predicted by the Wilson-Geankoplis--or the Kataoka--or the penetration model expression that have been used up to now to in the field of LC, and that at least more research is needed before these expressions can be continued to be used with confidence. The aforementioned expressions were obtained by neglecting the effect of axial dispersion on the mass transfer process, and it seems that they therefore underestimate the true Sh(m)-number by a factor of 2-5 around the minimum of the van Deemter-curve. New correlations describing the variation of the Sh(m)-coefficient as a function of the reduced velocity for a number of other packing geometries (tetrahedral monolith, 2D pillar array) are proposed. These correlations are in agreement with earlier theoretical and experimental studies showing that at low velocities the local-driving force-based Sh(m)-value is of the order of 10-20 in a packed bed column with an external porosity on the order of 35-40%., (Copyright © 2012 Elsevier B.V. All rights reserved.)

Using computational fluid dynamics (CFD), the effective B-term diffusion constant γ(eff) has been calculated for four different random sphere packings with different particle size distributions and packing geometries. Both fully porous and porous-shell sphere packings are considered. The obtained γ(eff)-values have subsequently been used to determine the value of the three-point geometrical constant (ζ₂) appearing in the 2nd-order accurate effective medium theory expression for γ(eff). It was found that, whereas the 1st-order accurate effective medium theory expression is accurate to within 5% over most part of the retention factor range, the 2nd-order accurate expression is accurate to within 1% when calculated with the best-fit ζ₂-value. Depending on the exact microscopic geometry, the best-fit ζ₂-values typically lie in the range of 0.20-0.30, holding over the entire range of intra-particle diffusion coefficients typically encountered for small molecules (0.1 ≤ D(pz)/D(m) ≤ 0.5). These values are in agreement with the ζ₂-value proposed by Thovert et al. for the random packing they considered., (Copyright © 2011 Elsevier B.V. All rights reserved.)

The present article reports on a numerical study of the permeability and flow resistance of ordered and random-structured polymer monolith mimics with different structures. Comparing the permeability of the different monolith structures with the same domain size and the same external porosity, it is found that the larger the degree of globule clustering, the lower is the resulting permeability. More or less independently of the external porosity, a 40% decrease in permeability is observed when comparing a purely cylindrical branch-type skeleton with a strongly clustered globular skeleton with the same external porosity. Correlations allowing to estimate the flow resistance as a function of the external porosity are presented for each considered degree of globule clustering., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Using the permeability analogue of the diffusion and partitioning processes occurring in a chromatographic column, the different Effective Medium Theory (EMT) models that exist in literature for the electrical and thermal conductivity have been transformed into expressions that accurately predict the B-term band broadening in chromatographic columns. The expressions are written in such a form that they hold for both fully porous and porous-shell particles, and both spherical and cylindrical particles are considered. Mutually comparing the established EMT-expressions, it has been found that the most basic variant, i.e., the Maxwell-based expression, is already accurate to within 5% for the typical conditions encountered in liquid phase chromatography, independently of the exact microscopic morphology of the packing. For most typical values of the intra-particle diffusion rate and the species retention factors, it is even accurate to within 1%. If even higher accuracies are needed, more elaborate EMT-expressions are available. The modelling accuracy of all explicit EMT-expressions is much better than the residence time weighted (RTW) B-term expressions that have been used up to now in the field of chromatography, where the error is typically on the order of 10% and more. The EMT-models have also been used to establish expressions for the obstruction and tortuosity factor in packings of non-porous particles. The EMT has also been applied to the meso-porous zone only, yielding an expression for the intra-particle diffusion coefficient that can be used without having to specify any obstruction factor. It has also been shown that the EMT also provides a very simple but exact expression to represent the way in which the solid core obstructs the effective intra-particle diffusion in the case of porous-shell particles. This obstruction factor is given by γ(part)=2/(2+ρ³) for spherical particles and γ(part)=1/(1+ρ³) for cylinders. Back-transforming the obtained expressions, a set of simple explicit expressions has been obtained that allow to directly obtain the intra-particle diffusion coefficient (D(part)) from peak parking or B-term constant measurements. Using these expressions, it could be demonstrated that the traditionally employed RTW-model yields D(part)-values that display an erroneous retention factor dependency, even in cases where the RTW-model appears to be able to closely fit the peak parking measurements., (Copyright © 2010 Elsevier B.V. All rights reserved.)

The results of a numerical simulation study of the diffusion and retention in fully porous spheres and cylinders are compared with some of the high order accuracy analytical solutions for the effective diffusion coefficient that have been derived from the effective medium theory (EMT) theory in part I of the present study. A variety of different ordered (spheres and cylinders) and disordered (cylinders) packings arrangements has been considered. The agreement between simulations and theory was always excellent, lying within the (very tight) accuracy limits of the simulations over the full range of retention factor and diffusion constant values that is practically relevant for most LC applications. Subsequently filling up the spheres and cylinders with a central solid core, while keeping the same packing geometry and the same mobile phase (same thermodynamic retention equilibrium), it was found that the core induces an additional obstruction which reduces the effective intra-particle diffusion coefficient exactly with a factor γ(part)=2/(2+ρ³) for spherical particles and γ(part)=1/(1+ρ²) for cylinders (ρ is the ratio of the core to the particle diameter, ρ=d(core)/d(part)). These expressions hold independently of the packing geometry, the value of the diffusion coefficients and the equilibrium constant or the size of the core. The expressions also imply that, if considering equal mobile phase conditions, the presence of the solid core will never reduce the particle contribution to the B-term band broadening with more than 33% (50% in case of cylindrical pillars)., (Copyright © 2010 Elsevier B.V. All rights reserved.)

The Salmonella enterica serotype Typhimurium genome contains 13 operons with homology to fimbrial gene sequences. Here we investigated the role of 11 serotype Typhimurium fimbrial proteins, including FimA, AgfA (CsgA), BcfA, StbA, SthA, LpfA, PefA, StdA, StcA, StiA, and StfA, as antigens during the infection of genetically resistant mice (CBA). Upon the growth of serotype Typhimurium in standard laboratory broth culture, only the expression of FimA could be detected by Western blot analysis. The infection of mice with serotype Typhimurium grown in broth culture, followed by at least one subsequent infection, resulted in seroconversion of animals to FimA, AgfA, BcfA, StbA, SthA, LpfA, PefA, StdA, StcA, StiA, and StfA positivity. Most animals seroconverted to only a subset of these fimbrial antigens. The immunization of mice with glutathione S-transferase (GST)-FimA, GST-AgfA, GST-BcfA, GST-StbA, GST-SthA, GST-LpfA, GST-PefA, GST-StdA, GST-StcA, GST-StiA, and GST-StfA fusion proteins resulted in reduced fecal shedding of serotype Typhimurium during a challenge compared to that by a control group immunized with purified GST protein. Collectively, these data suggest that the expression of serotype Typhimurium fimbrial antigens is induced during the infection of mice.