Dispersion In Mobile Phase Research Articles

Characterization and correlation of mobile phase dispersion of aqueous-organic solvent systems in centrifugal partition chromatography

To reach a high separation efficiency using Centrifugal Partition Chromatography (CPC), the fluid dynamical behavior of the liquid-liquid two-phase systems must be clearly understood. The fluid dynamics, namely the dispersion, the coalescence, and the stationary phase retention, have a high impact on a separation. Especially the mobile phase dispersion influences the mass transfer during a separation. In this study, the mobile phase dispersion of different aqueous-organic solvent systems was characterized for ascending and descending mode via video analysis. Thereby the influence of the physical properties of the solvent systems, the operating parameters, and the geometry of the chamber inlet was investigated systematically using dimensional analysis. With the help of the dimensionless numbers Ohnesorge number (OhCPC), Eötvös number (EoCPC), and Weber number (WeCPC) the impact of the solvent system, the plant parameters, and the operating parameters on the mobile phase dispersion could be described. Inside the three dimensional area, spanned by the dimensionless numbers, each state of mobile phase dispersion (undispersed, low dispersed, highly dispersed, and atomized) could be allocated to an individual region for both operating modes. Moreover, differences in mobile phase deflection depending on the operating mode and a possible reason for these were described.

Numerical Elucidation of Flow and Dispersion in Ordered Packed Beds: Nonspherical Polygons and the Effect of Particle Overlap on Chromatographic Performance.

Spherical particles are widely considered as the benchmark stationary phase for preparative and analytical chromatography. Although this has proven true for randomly packed beds in the past, we challenge this paradigm for ordered packings, the fabrication of which are now feasible through additive manufacturing (3D printing). Using computational fluid dynamics (Lattice Boltzmann Method) this work shows that nonspherical particles can both reduce mobile-phase band broadening and increase permeability compared with spheres in ordered packed beds. In practice, ordered packed beds can only remain physically stable if the particles are fused to form a contiguous matrix, thus creating a positional overlap at the points of fusion between what would otherwise be discrete particles. Overlap is shown to decrease performance of ordered packed beds in all observed cases, thus we recommend it should be kept to the minimum extent necessary to ensure physical stability. Finally, we introduce a metric to estimate column performance, the mean deviated velocity, a quantitative description of the spread of the velocity field in the column. This metric appears to be a good indicator of mobile-phase dispersion in ordered packed bed media, including overlapped beds, and is a useful tool for screening new stationary-phase morphologies without having to perform computationally expensive simulations.

Modeling pH-zone refining countercurrent chromatography: A dynamic approach

A model based on mass transfer resistances and acid–base equilibriums at the liquid–liquid interface was developed for the pH-zone refining mode when it is used in countercurrent chromatography (CCC). The binary separation of catharanthine and vindoline, two alkaloids used as starting material for the semi-synthesis of chemotherapy drugs, was chosen for the model validation. Toluene/CH3CN/water (4/1/5, v/v/v) was selected as biphasic solvent system. First, hydrodynamics and mass transfer were studied by using chemical tracers. Trypan blue only present in the aqueous phase allowed the determination of the parameters τextra and Pe for hydrodynamic characterization whereas acetone, which partitioned between the two phases, allowed the determination of the transfer parameter k0a. It was shown that mass transfer was improved by increasing both flow rate and rotational speed, which is consistent with the observed mobile phase dispersion. Then, the different transfer parameters of the model (i.e. the local transfer coefficient for the different species involved in the process) were determined by fitting experimental concentration profiles. The model accurately predicted both equilibrium and dynamics factors (i.e. local mass transfer coefficients and acid–base equilibrium constant) variation with the CCC operating conditions (cell number, flow rate, rotational speed and thus stationary phase retention). The initial hypotheses (the acid–base reactions occurs instantaneously at the interface and the process is mainly governed by mass transfer) are thus validated. Finally, the model was used as a tool for catharanthine and vindoline separation prediction in the whole experimental domain that corresponded to a flow rate between 20 and 60mL/min and rotational speeds from 900 and 2100 rotation per minutes.

Methodology for optimally sized centrifugal partition chromatography columns

Centrifugal Partition Chromatography (CPC) is a separation process based on the partitioning of solutes between two partially miscible liquid phases. There is no solid support for the stationary phase. The centrifugal acceleration is responsible for both stationary phase retention and mobile phase dispersion. CPC is thus a process based on liquid–liquid mass transfer. The separation efficiency is mainly influenced by the hydrodynamics of the phases in each cell of the column. Thanks to a visualization system, called “Visual CPC”, it was observed that the mobile phase can flow through the stationary phase as a sheet, or a spray. Hydrodynamics, which directly governs the instrument efficiency, is directly affected during scale changes, and non-linear phenomena prevent the successful achievement of mastered geometrical scale changes. In this work, a methodology for CPC column sizing is proposed, based on the characterization of the efficiency of advanced cell shapes, taking into account the hydrodynamics. Knowledge about relationship between stationary phase volume, cell efficiency and separation resolution in CPC allowed calculating the optimum cell number for laboratory and industrial scale CPC application. The methodology is highlighted with results on five different geometries from 25 to 5000mL, for two applications: the separation of alkylbenzene by partitioning with heptane/methanol/water biphasic system; and the separation of peptides by partitioning with n-butanol/acetic acid/water (4/1/5) biphasic system. With this approach, it is possible to predict the optimal CPC column length leading to highest productivity.

Computer Tool for Calibration of Chromatography Models

Model based engineering require good models to make analysis, optimisation and design. There is a need of tools for systematic parameter estimation and model calibration. In this paper a computer tool that supports the calibration of chromatography models for protein purification is presented. A model for protein purification has to describe a set of behaviours that can be present during the optimisation. Examples of behaviours are mobile phase dispersion, diffusion in stationary phase, multi component adsorption and elution dependency. A systematic model calibration procedure is needed to find all these parts in the model. Each step in the procedure involves a parameter estimation application for a given model structure and specific experimental data. There is a need for using both single data responses and multiple data responses to calibrate the given model structure. Experimental data is imported to the computer tool directly from a chromatography set-up. The model response is computed in a simulator solving a set of partial differential equations. A regression method is used to minimize the residuals to find the best parameter set. The tool is developed in MATLAB and has a graphical user interface to support the user through a model calibration activity.

MODELING SOLUTE TRANSPORT IN MICELLAR LIQUID CHROMATOGRAPHY

To better understand the causes of reduced efficiencies in micellar liquid chromatography (MLC), a mathematical model that includes both solute–stationary phase and solute–micelle interactions has been developed. Solute mass transfer between mobile and stationary phases and kinetic limitations within the stationary phase are incorporated in the model equations. Equilibrium is assumed between the micelles and the surfactant monomer in the bulk solvent of the mobile phase as well as for the solute distributed between the micelles and the mobile phase solvent. It is also assumed that only free surfactant is found in the pores of the alkyl-bonded phase since the micelles are larger than the typical stationary phase pore sizes. The increase in mobile phase dispersion due to partitioning of the solute between the micelle and the bulk solvent is incorporated in this model. Solution of the solute mass balance equations developed for the mobile phase, the pores of the silica gel, and the surface of the stationary phase yields an explicit expression for the number of plates as a function of the physical and chemical parameters governing the kinetics and transport in MLC separations. An examination of changes in predicted plate numbers with different mobile phase conditions helps to understand the observed efficiencies with MLC systems. Model predictions were compared to experimental observations from a series of vanillin compounds injected onto a BDS-C18 column with mobile phases containing different concentrations of sodium dodecyl sulfate. This comparison showed that an equilibrium model provides a reasonable prediction of the number of plates for all of the solutes considered. However, the experimental results for vanillin, isovanillin, and coumarin indicate that stationary phase kinetics also play a minor role in column efficiency. The results from this analysis suggest that it is the secondary equilibrium between micellar and bulk mobile phases, which is the primary contributor to band broadening in MLC.

Monte Carlo model of nonlinear chromatography

A stochastic approach to the nonlinear chromatography theory, based on the Monte Carlo simulation method, is presented. A computer program, acting as a "virtual chromatograph" and performing a discrete event simulation, is described. Such a program allows one to choose the column type, operating conditions, sample composition, injection method, mobile-phase dispersion model, and stationary-phase sorption-desorption kinetics. Nonlinearity is accounted for by continuously monitoring and updating both the column and the solute status and by moving individual molecules step by step along the column according to specific random modes. The program has been validated through a series of statistical tests and comparing the results with the well-known achievements of the classical stochastic theory. A first application is presented, referred to a real case benzene elution on a gas solid capillary column, where the Langmuir adsorption isotherm is assumed. The effect of both the sorption modes and the site capacity are investigated. Possible applications to investigate open problems in several fields of separation science are emphasized. In addition, several specific points such as the down-scaling of a real case and the correspondence of specific adsorption dynamics with the equilibrium Langmuir isotherm are described.

Stochastic−Dispersive Theory of Chromatography

The stochastic model of chromatography has been combined with mobile-phase dispersion. With the combined model, both the effect of slow mass transfer or adsorption-desorption kinetics and dispersion on the band profile can be characterized. The stochastic model of chromatography is addressed with the characteristic function method. The moments of the peaks are calculated analytically for homogeneous and heterogeneous surfaces. It is shown that even in cases when the characteristic function cannot be calculated in closed form, the moments of the peak, and therefore the retention time, the number of theoretical plates, the peak asymmetry, can be calculated with simple expressions. Therefore, a full description of the chromatographic peak is available for homogeneous and any heterogeneous surfaces provided that the distribution of the sorption energies is known.

Separation of Uranium Isotopes by the Chemical Exchange Method

Abstract The basic chemical process and technology for producing 3-5% enriched uranium has been established, through advances which allowed increases in the electron-exchange and adsorption-desorption reaction rates, effective uranium adsorption band formation and maintenance, reduction of the mobile-phase dispersion, and reduction in the height of the separation unit, which is largely determined by the diffusion coefficient, the electron exchange reaction rate of uranium ions, and the non-uniform flow pattern in the adsorption band. Physical theory and experimental results show the attainment of a specific separation power of approximately 500 SWU/m3·yr for the process, and the possibility of an enrichment cost of $41/SWU in its commercial-scale application as calculated with depreciation terms of 15 years for equipment and 45 years for buildings and interest payments at 8% on investment capital. Inherent advantages of the process, in addition to low enrichment cost, are simple, stable operation and facilitation of the nuclear fuel cycle, with efficient separation of uranium-235 from the other uranium isotopes of spent nuclear fuel and elimination of the need for UF6 conversion.

Theoretical Consideration and Recent Progress of Chemical Enrichment Process for Uranium Enrichment

Research and development for uranium enrichment by oxidation-reduction chromatography, begun in 1972 despite the prevailing view that a chemical process would require several centuries of operation to attain fuel-grade uranium enrichment and would thus be impractical, led to the establishment of basic process technology permitting attainment of 3% enrichment within several months of operation by the mid 1980, through advance which brought increased electron exchange and oxidation-reduction reaction rates, effective uranium adsorption band formation and maintenance, and equilibrium plate height reductions based on elucidation of mobile-phase dispersion. Theoretical descriptions of the enrichment factor and the isotopic abundance curve of the adsorption band were developed, and the theoretical and experimental development and verification of an intracolumn mechanism of redox agent self-regeneration led to a new “Super” process version characterized by greater simplicity and efficiency than previously thought possible. A pilot plant with enrichment columns of 1 m diameter and 3 m height, constructed at Hyuga City in Miyazaki Prefecture, demonstrated the recovery of 3% enriched uranium in April 1988. Subsequent development work has brought further advances in process economy and continuous-operation performance.

Theoretical consideration and recent progress of chemical enrichment process for uranium enrichment.

Research and development for uranium enrichment by oxidation-reduction chromatography, begun in 1972 despite the prevailing view that a chemical process would require several centuries of operation to attain fuel-grade uranium enrichment and would thus be impractical, led to the establishment of basic process technology permitting attainment of 3% enrichment within several months of operation by the mid 1980, through advance which brought increased electron exchange and oxidation-reduction reaction rates, effective uranium adsorption band formation and maintenance, and equilibrium plate height reductions based on elucidation of mobile-phase dispersion. Theoretical descriptions of the enrichment factor and the isotopic abundance curve of the adsorption band were developed, and the theoretical and experimental development and verification of an intracolumn mechanism of redox agent self-regeneration led to a new “Super” process version characterized by greater simplicity and efficiency than previously though...

High-performance gel permeation chromatography with silica microspheres: mass-transfer dispersion and polydispersity of polystyrene

Separations of polystyrene standards by gel permeations chromatography have been performed with columns containing silica microspheres having a particle diameter of 8 μm. Experimental plate-height data for polystyrene have been interpreted in terms of an expression in which the only mobile-phase dispersion term results from eddy diffusion. This simplified expression permits the evaluation of the contribution to chromatogram broadening from solute mass transfer in the stationary phase. Values of the diffusion coefficient of polystyrene in the stationary phase have been calculated from the mass-transfer contribution, suggesting that restricted diffusion occurs. A procedure is proposed for evaluating the mobile-phase dispersion term, which then permits the determination of the polydispersity for polystyrene from experimental plate-height data.

High-performance gel permeation chromatography of polystyrene with silica microspheres

Gel permeation chromatography separations have been performed with short low capacity columns containing silica microspheres (particle diameter ≈ 20 μm) having narrow particle size distributions and mean pore diameters in the range 8–120 nm. Plate height data for non-permeating polystyrene standards permitted an evaluation of chromatogram broadening due to mobile phase dispersion. These results together with plate height data for permeating polystyrene standards, tetraphenylethylene and toulene gave an assessment of chromatogram broadening due to mass transfer dispersion as a function of eluent flow-rate and solute molecular weight. It was found that mass transfer dispersion increased at higher flow-rates, the flow-rate dependence increasing as the diffusion coefficient of permeating polystyrene decreased. At very low flow-rates (0.05 cm 3 min −1, mobile phase dispersion is the major contributor to the chromatogram broadening,of permeating polystyrene. From theoretical considerations of chromatogram broadening, a relation is derived permitting the approximate determination of the true polydispersity of a permeating polymer at very low eluent flow-rates from the plate height for that polymer and from the plate height arising from mobile phase dispersion. The results show that fast separations may be accomplished in several minutes and that the most precise determinations of polydispersity are obtained at slow eluent flow-rates with separation times of about 1 h.

High performance g.p.c. with crosslinked polystyrene gels: influence of particle size distribution

Spherical crosslinked polystyrene gel particles have been separated by air classification into fractions having average particle diameters in the range 10–40 μm. The particle size distributions have been shown to be narrow by Coulter Counter measurements. G.p.c. column performance improves as the average particle diameter decreases and columns packed with gel fractions having a number-average particle diameter below 20 μm give plate counts in excess of 3000 plates per foot. Plate height results as a function of eluent flow rate suggest that chromatogram broadening due to mobile phase dispersion is reduced in columns packed with spherical particles having a narrow size distribution.

Mobile-phase dispersion in liquid chromatography

Literature data on dispersion of inert components in liquid flow through chromatographic columns packed with glass beads are compatible with a plate-height equation based on a combination of radial and axial dispersion. It also appears that the measurements can be described by means of a known empirical equation whose usefulness in liquid chromatography has not been verified so far. Both equations contain constants which depend on the ratio of the column diameter to the particle diameter of the packing material.