Retention In Field-flow Fractionation Research Articles

Improved terminology for experimental field-flow fractionation

The terminology for sample analyte retention in field-flow fractionation (FFF) rests on the same fundamental theoretical concepts as for chromatography [1–6]. The most basic phenomenon to start reasoning from is the migration velocity νa of the sample analyte zones. If retention occurs, which is a prerequisite for separation, these velocities will be lower than the carrier (mobile phase) velocity, the latter taken as the axial velocity averaged over the cross-sectional dimension, i.e. the cross-sectional mean carrier velocity . It is natural that quantitative treatment of retention be based on the relationship between these two velocities. For this purpose, the symbol R was introduced by Martin and Synge [1] and LeRosen [2, 3], who noted that “the term R is a fundamental quantity” [3]. It was defined as the relative rate of movement of the analyte zone quantified by

Can slip walls improve field-flow fractionation or hydrodynamic chromatography?

One way to potentially modify the performance of field-flow fractionation (FFF) would be to move the position of the maximum flow velocity away from the mid-point of the channel, for example by using walls with non-zero slip lengths. In this short communication, we extend the ideal theory of FFF to include the effects of two slip walls. Our calculations demonstrate that while the hydrodynamic chromatography limit of FFF (weak fields) is not improved by engineering devices with slip-walls, the performance of Normal-Mode FFF can be enhanced by having slip at the depletion wall in moderate fields. We also introduce a new regime, which we call Slip-Mode FFF, where a large external field (typical of Normal-Mode FFF) and a large slip at the accumulation wall lead to sharp separations characterized by an elution order that is similar to that of hydrodynamic chromatography.

Size Selectivity in Field-Flow Fractionation: Lift Mode of Retention with Near-Wall Lift Force

A simple theoretical model for the size selectivity, S(d), in the lift mode of retention in field-flow fractionation (FFF) is developed on the basis of the near-wall lift force expression. S(d) is made up of two contributions: the flow contribution, S(d,f), arising from the variation of the flow velocity at center of particle due to a change in particle position with particle size, and a slip contribution, S(d,s), arising from the concomitant change in the extent of retardation due to the presence of a nearby channel wall. The slip contribution is minor, but not negligible, and amounts to 10-20% of the overall size selectivity. It contributes to reduce S(d) in sedimentation FFF but to enhance it in flow FFF. S(d) would steadily increase with particle size if the flow profile was linear (Couette flow). Because of the curvature of the flow profile encountered in the classical Poiseuille flow, S(d) exhibits a maximum at some specific particle size. The model predicts a significant difference in S(d) between sedimentation FFF and flow FFF, arising from the different functional dependences of the field force with particle size between these two methods. The predictions are in good agreement with the various S(d) values reported in the literature in both sedimentation and flow FFF. On the basis of the model, guidelines are given for adjusting the operating parameters (carrier flow rate and field strength) to optimize the size selectivity. Finally, it is found that S(d) generally decreases with decreasing channel thickness.

Effect of Particle-Particle Attractive and Repulsive Interactions on the Retention in Field-Flow Fractionation

The experimental data obtained by using various field-flow fractionation techniques and by other experimental methods are compared with the published results. All available and relevant data are analyzed with regard to the particle-particle interactions occurring in polarization and focusing mechanisms governing the field-flow fractionation of the macromolecular and colloidal species. It is concluded that these interactions strongly depend on the experimental conditions. The appearance of the organized structures, such as colloidal crystals, formed under the usual field-flow fractionation conditions is revealed. All these interactions must be taken into account for a rigorous interpretation of the experiments, especially with regard to the effect of the experimental conditions on the retention.

Influence of operating parameters on the retention of chromatographic particles by thermal field-flow fractionation.

We have investigated the retention behavior of chromatographic particles in thermal field-flow fractionation (FFF). Retention time is found to increase with increasing temperature drop across the channel thickness, as expected for species exhibiting a thermophoretic mobility. Experiments have been performed with a vertically oriented channel rather than by using the classical horizontal configuration as this leads to much more reproducible retention data. In acetonitrile, silica-based particles are more retained than octadecyl-bonded silica particles, which confirms our previous finding, by means of a different method, that the thermophoretic mobility of the latter is smaller than that of the former. Whatever the type of particles and the nature of the carrier liquid, the relative retention time is observed to decrease with increasing carrier flow rate. This indicates that a hydrodynamic lift force acts on particles to move them away from the accumulation wall, as is usually observed in all FFF experiments with micrometer-sized particles. However, upward and downward flow directions in the vertical channel lead to similar retention data, indicating that inertial lift forces have a minor influence on retention. In addition, the relative retention time steadily decreases with increasing sample concentration, suggesting that the hydrodynamic lift force increases significantly with sample concentration. Accordingly, we speculate that a new transport phenomenon, called shear-induced hydrodynamic diffusion, not previously accounted for in the modeling of retention in FFF, is controlling the migration of the particles in the FFF channel. Implications of the influence of this phenomenon in other FFF experiments are discussed.

Coupling gravitational and flow field-flow fractionation, and size-distribution analysis of whole yeast cells.

This work continues the project on field-flow fractionation characterisation of whole wine-making yeast cells reported in previous papers. When yeast cells are fractionated by gravitational field-flow fractionation and cell sizing of the collected fractions is achieved by the electrosensing zone technique (Coulter counter), it is shown that yeast cell retention depends on differences between physical indexes of yeast cells other than size. Scanning electron microscopy on collected fractions actually shows co-elution of yeast cells of different size and shape. Otherwise, the observed agreement between the particle size distribution analysis obtained by means of the Coulter counter and by flow field-flow fractionation, which employs a second mobile phase flow as applied field instead of Earth's gravity, indicates that yeast cell density can play a major role in the gravitational field-flow fractionation retention mechanism of yeast cells, in which flow field-flow fractionation retention is independent of particle density. Flow field-flow fractionation is then coupled off-line to gravitational field-flow fractionation for more accurate characterisation of the doubly-fractionated cells. Coupling gravitational and flow field-flow fractionation eventually furnishes more information on the multipolydispersity indexes of yeast cells, in particular on their shape and density polydispersity.

Determination of thermal diffusion coefficients using thermal field-flow fractionation and Mark-Houwink constants.

In this paper, a new approach for determination of thermal diffusion coefficient D(T) values using thermal field-flow fractionation retention data and Mark-Houwink constants is reported. The method utilizes the availability of Mark-Houwink constants from the literature together with thermal field-flow fractionation retention data to calculate D(T) values for both narrowly and broadly dispersed polymeric samples. The proposed method was tested with thermal field-flow fractionation data from a number of published papers. In general, D(T) results obtained from the new approach agree well with those reported from the literature. Since Mark-Houwink constants have been extensively tabulated, the new method can be used to generate a broad database of D(T) values for use in the characterization of polymers and in studies of the thermal diffusion phenomenon.

Onset of sample concentration effects on retention in field‐flow fractionation

AbstractIn field‐flow fractionation (FFF), when the sample amount is increased, a shift in retention occurs. Positive as well as negative deviations of the retention have been observed, depending on the type of sample and on the experimental conditions. Two concentration effects have been invoked to explain this retention shift: the viscosity effect (the velocity profile of the analyte suspension or solution is modified by the concentration dependence of the viscosity) and the mean distance effect (the mean distance of the analyte molecules or particles to the accumulation wall and the steady state cross‐sectional concentration profile of the analyte are affected by concentration‐dependent interactions between analyte molecules or particles). The general FFF retention equation has been solved in the case where the reciprocal of the viscosity and the FFF analyte‐field interaction parameter, λ, are linearly related to concentration. The two proportionality coefficients involved in these relationships are identified as the intrinsic viscosity and twice the second virial coefficient of the osmotic pressure, respectively. A general expression of the derivative of the analyte average relative velocity, R, with respect to the cross‐sectional average analyte concentration, 〈c〉, has been obtained as a function of these two coefficients, and of λo, the limit of λ for infinite dilution. A survey of retention effects reported in FFF literature for various sample types and FFF methods is presented.

Sample Mass Effects on Thermal Field-Flow Fractionation Retention and Universal Calibration

The effect of sample mass on retention in thermal field-flow fractionation (FFF) can be large, becoming more important as the molecular weight increases. Greater retention with increased sample mass is observed beginning at the detection limit of 0.1 μg of polystyrene in tetrahydrofuran (THF) with higher molecular weight material. Empirical equations have been developed to correct for changes in calibration parameters for the thermal FFF universal calibration curves due to the sample mass effect on polymer retention for polystyrene, polymethylmethacrylate, and polyisoprene in THF.

Retention in Field-Flow Fractionation with a Moderate Nonuniformity in the Field Force

In some field-flow fractionation (FFF) techniques, the basic analyte-field interaction parameter, λ, is not constant but varies within the channel cross section as a result of the nonuniformity of the force exerted by the field on the analyte. This is the case, for instance, in thermal FFF, because of the temperature dependence of the relevant physicochemical transport parameters. To account for this effect, a new FFF retention model is developed, allowing a linear variation of λ from the accumulation to the depletion wall, which is assumed to describe correctly moderate nonuniformity in λ in the vicinity of the accumulation wall. A methodology for sample characterization on the basis of this model is proposed. It associates λ(app), the apparent λ value derived from the retention ratio by means of the classical retention model, with a specific distance from the accumulation wall. An empirical relationship between that distance and λ(app) is derived. In the high retention limit, it is found that this specific distance is not equal, as sometimes intuitively believed, to the mean distance of the molecule or particle cloud to the accumulation wall but is approximately equal to twice this mean distance. The validity of the proposed approach is checked.

Entropic Contribution to the Retention of Nonspherical Particles in Field-Flow Fractionation

In this paper the effect of particle shape on the entropy of nonspherical particles adjacent to a plane surface is considered. The subsequent influence on particle retention in field-flow fractionation (FFF) has been estimated. New retention equations for thin rod and disc shaped particles have been derived to cover this steric-entropic region of FFF. As particle size increases relative to the mean cloud thickness, the retention ratio for nonspherical particles is predicted to increase compared to small spherical particles of the same mass. This could result in a significant underestimation of the calculated equivalent spherical diameter (d) by FFF methods. The steric-entropic FFF equations could be used to calculate accuratedvalues if the large particle dimension is estimated independently (e.g., by microscopy). Alternatively, run conditions could be designed to minimize steric-entropic perturbations to the ideal retention equation.

Study of thermal diffusion of polybutadiene and polytetrahydrofuran in various organic solvents

The thermal diffusion coefficient (Dτ) was determined for polybutadiene and polytetrahydrofuran (i.e., polyoxytetramethylene) standards of different molecular masses in various organic solvents. The combination of thermal field-flow fractionation retention data with diffusion data from dynamic light scattering or diffusion NMR experiments yielded the thermal diffusion coefficients. In agreement with earlier work on thermal diffusion of polymers in solution, the thermal diffusion coefficient was found to be independent of the molecular mass of the polymers. However, this work once more indicates that the Dτ value is strongly influenced by the chemical composition of both the polymer and the solvent. © 1996 John Wiley & Sons, Inc.

Study of thermal diffusion of polybutadiene and polytetrahydrofuran in various organic solvents

The thermal diffusion coefficient (Dτ) was determined for polybutadiene and polytetrahydrofuran (i.e., polyoxytetramethylene) standards of different molecular masses in various organic solvents. The combination of thermal field-flow fractionation retention data with diffusion data from dynamic light scattering or diffusion NMR experiments yielded the thermal diffusion coefficients. In agreement with earlier work on thermal diffusion of polymers in solution, the thermal diffusion coefficient was found to be independent of the molecular mass of the polymers. However, this work once more indicates that the Dτ value is strongly influenced by the chemical composition of both the polymer and the solvent. © 1996 John Wiley & Sons, Inc.

Retention in field-flow fractionation with secondary chemical equilibria

The application range of field-flow fractionation (FFF) can be extended to low molecular weight solutes, as demonstrated a few years ago by Berthod et al., by taking profit of secondary chemical equilibria (SCE) occurring between the bulk carrier and a retained carrier component. The theory of solute retention in this SCE-FFF method is developed for any value of the solute distribution coefficient and of the retention ratio of the retained carrier component, provided that the Brownian mode of retention applies for this component and that the flow velocity profile is parabolic. This removes some of the limitations of the model previously developed by Berthod and Armstrong and sheds light on the potentialities of the SCE-FFF method for physico-chemical studies about secondary chemical equilibria in colloidal systems. Remaining assumptions in the model are discussed.

Outlet Stream Splitting for Sample Concentration in Field-Flow Fractionation

Abstract Retention in field-flow fractionation is a result of a field-induced sample concentration in the vicinity of a channel wall. By splitting the channel flow at the exit, so that sample containing flow lines leave the channel at a different port than those free of sample, one may markedly enhance the detectability of a given sample. The enhancement is discussed for the cases of “ideal splitting,” where an obstacle inserted in the flow stream affects the split, and “nonideal splitting,” where the flow is caused to split at some level in the channel specified by the relative resistance to flow through the two outlets. The proposed increase in detector response through stream splitting is experimentally verified.

Characterization of T4D virus by sedimentation field-flow fractionation

Sedimentation field-flow fractionation (a non-destructive separation method) has been used to measure the molecular weight and density of T4D virus. The molecular weight was obtained under experimental conditions in which the major parameters governing retention in field-flow fractionation were varied to examine their effect on the calculated result. The parameters varied are (1) carrier density, (2) carrier velocity, (3) field strength, (4) injected sample volume and (5) column volume. Retention was not affected by flow-rate or sample volume injected. Effective molecular weights, calculated from a large number of experiments run under different conditions, had a mean and standard deviation of (100 ± 1)·10 6. The actual molecular weight, subject to greater uncertainty, was determined to be 317·10 6. The density calculated for the virus was 1.47 g/ml. A study of whether the sedimentation field-flow fractionation technique is destructive or not showed that the infectivity of the virus is affected very little by passage through the channel and detector system.

Non-Newtonian Flow for Retention Control in Field-Flow Fractionation

Abstract Retention in field-flow fractionation (FFF) can be altered and controlled by the introduction of different kinds of velocity profiles in the FFF channel. Here we propose the use of non-Newtonian fluid flow to manipulate retention in FFF. The flexible, three parameter Ellis equation, describing non-Newtonian behavior, is used to derive the dependence of retention ratio R on the dimensionless mean solute layer thickness λ. Numerical calculations show the way in which changes in the parameters of the Ellis equation change the velocity profile in the channel and therefore the shape of the R versus λ functions.