Surface Barriers for Retention Enhancement in Field-Flow Fractionation

Abstract

It is proposed to modify the surface of a field-flow fractionation (FFF) channel by introducing small barriers perpendicular to flow. Possible advantages include increased retention, sample capacity, and selectivity. It is shown that this approach brings FFF into a closer relationship with chromatography and countercurrent distribution. Approximate theories are developed for retention, plate height, and selectivity, and sources of departure from theory are discussed. Two experimental thermal FFF systems are described, one with barriers established by cutting grooves in a Mylar sheet and another with grooves cut in a copper bar. Despite an observed deviation from the assumed rectangular groove shape, retention enhancement was considerable, and was in reasonable agreement with theory. Plate height, however, greatly exceeded the values observed for nongrooved systems.     

A Pinched Inlet System for Reduced Relaxation Effects and Stopless Flow Injection in Field-Flow Fractionation

Abstract

The concept of a pinched inlet system for field-flow fractionation (FFF), in which the channel thickness at the inlet end is reduced to hasten relaxation, is introduced and its advantages in simplifying FFF operation and increasing analysis speed are noted. Three forms of FFF operation are described for taking advantage of the split inlet: stopless flow injection, slow flow injection, and stopflow injection. Stopless flow operation is the simplest because flow is neither stopped nor changed to accommodate relaxation. However, stopless flow operation causes band broadening. It is found that the time-based variance of band broadening for many FFF systems is proportional to the fourth power of channel thickness w. Therefore, by reducing w at the inlet end where relaxation occurs, this band broadening can be controlled. The implementation of this concept is discussed for different forms of FFF.     

Parameters Affecting Magnetic Field-Flow Fractionation of Metal Oxide Particles

Abstract

Magnetic field-flow fractionation (FFF) is a new separation technique particularly suited to separations involving particulate materials of high magnetic permeability. In this technique a magnetic field, generated by an electromagnet, is used to induce retention of particles in the FFF flow stream. This paper discusses the theoretical basis for magnetic FFF in terms of the fundamental retention equation of FFF and the magnetic force equation. Experimental data are presented which characterizes the magnetic field and the retention process. The resolution of single particles from dimeric aggregates is demonstrated based upon their difference in volume.     

Magnetic Field-Flow Fractionation: Theoretical Basis

Abstract

The application of high gradient magnetic fields to field-flow fractionation (FFF) is quite attractive based on the preliminary calculations presented here. The theoretical basis has defined the parameters of force on the individual particles and normal FFF parameters to evaluate the separation capability. The possible application of magnetic FFF to dynamic systems as well as a tool for separation of nominally paramagnetic and magnetically tagged molecules provides an impetus for further development.     

Field-Flow Fractionation: Methodological and Historical Perspectives

Abstract

Field-flow fractionation (FFF) is briefly introduced with respect to its working nature, mechanism of retention, types of applications, and relationships to chromatography. Nine fundamental characteristics of FFF are then outlined. The nine characteristics are used (Table I) to distinguish FFF from other separation methods which employ an external field perpendicular to the flow axis.

Three brief accounts of the early history of FFF are given. These accounts relate the individual experiences of the three authors of this articles.     

Shear Field-Flow Fractionation: Theoretical Basis of a New,Highly Selective Technique

Abstract

Shear field-flow fractionation (shear FFF) is described as an FFF system in which shear forces are responsible for migration perpendicular to flow. It is shown that a desirable configuration for shear FFF is a concentric cylinder system with one cylinder rotating. After providing the relevant theoretical framework of FFF, the equations of Shafer et al. describing shear migration are simplified and applied to the limiting case of very thin annular spaces to get tractable retention expressions. On this basis the maximum selectivity is predicted to be 3 or greater, a value considerably higher than that for any other macromolecular separation technique. This high selectivity is confirmed using an alternate shear migration theory developed by Tirrell et al. However, it is shown that shear FFF is only applicable to macromolecules of high molecular weight, perhaps ～10⁷ and above. It may also be applicable to globular particles.     

Nonequilibrium Theory for Field-Flow Fractionation in Annular Channels

Abstract

The principles of field-flow fractionation (FFF) and reasons for extending the FFF methodology from parallel-plate channels to annular channels (ANNCs) are briefly reviewed. A theory for the nonequilibrium plate height H of FFF zones in ANNCs is developed by extending the nonequilibrium theory of FFF to polar coordinates. The principal assumption in the theory is that component zones are localized near the ANNC walls by the general force F = A/rⁿ , where A and n are constants and r is the radial coordinate. Equations for H are developed as functions of n, the inner-to-outer radius ratio of the ANNC, and the fundamental FFF parameter, γ. A closed-form analytical solution to H is obtained when n = 1; the n ≠ 1 solution must generally be expressed as a ratio of the integrals involved. These integrals can be approximated analytically, however, when γ ? 1. The functions for H are compared to their parallel-plate counterpart, and differences are rationalized.     

Fast Algorithm for the Conversion of R to Lambda Values in Field-Flow Fractionation

Abstract

The numerical methodology needed to convert the retention ratio, R, to the dimensionless mear layer thickness, Λ, commonly encountered in field-flow fractionation (FFF), is reviewed and a rigorous interpolation scheme which is suitable for any of the FFF techniques is presented. Computer implementation and error estimates are discussed in detail.     

Hyperlayer Field-Flow Fractionation

Abstract

Hyperlayer field-flow fractionation is proposed as a method designed to overcome some of the limitations of conventional field-flow fractionation (FFF). In hyperlayer FFF, steady-state particle layers are formed above the channel wall by the combination of a primary field (e.g., sedimentation or electrical) and a secondary gradient (such as density of pH). Such zones could be separated along the flow axis in FFF even if they strongly overlap in the field or lateral direction. An approximate theory is derived for sedimentation hyperlayer FFF, showing both the rate of zone migration and the extent of peak broadening. Calculations are presented which show that the system should be highly effective for the separation of particles in the vicinity of μm in diameter or larger.     

Steric Field-Flow Fractionation: A New Method for Separating I to 100 μm Particles

Abstract

Steric field-flow fractionation (steric FFF) is described as a high field limit of normal FFF that separates particles according to their diameter or radius. Retention equations are used to describe the phenomenon; these equations lead to the suggestion that steric FFF is applicable to particles from 1 to 100 μm as a minimum range. Conditions that control resolution are discussed, and fundamental similarities and differences between steric FFF on one hand and normal FFF and hydrodynamic chromatography on the other hand are noted. Optimum flow conditions are discussed and the complication of describing the migration of irregular particles is noted. Preliminary experiments with glass beads of 10 to 30 μm diameter demonstrate the existence of fractionation.     

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.     

Peak Capacity in Field-Flow Fractionation

Abstract

Field-flow fractionation (FFF) peak capacity values have been computed with only two major assumptions: first, the plate height is supposed the sum of only two contributions, axial molecular diffusion and transversal nonequilibrium, and second, the steric effect has been neglected in the equations of retention and peak broadening.

Several reduced parameters have been defined to generalize the equations and limit the number of variable parameters. It appears that among the already implemented FFF subtechniques for which the elution spectrum is an explicit function of the principal dimension, or mass, of the retained sample (which excludes electrical FFF), sedimentation FFF has some peculiar characteristics due to the fact that the field-induced velocity depends on a particular sample, while in thermal and flow FFF it is the same for all samples of a given type under fixed experimental conditions. For example, in sedimentation FFF, the axial diffusion contribution to the plate height persists at a much larger reduced eluant velocity than for the other techniques.

The effect on the peak capacity of the retention volume, the channel length, the eluant velocity as well as the influence of detection limit and analysis time have been studied. Simple relationships between peak capacity and these parameters are established in the high retention and negligible axial diffusion limits which previal in most experimental situations, and deviations from these limits are discussed. It is shown that for all three     

Diffusion Coefficients and Molecular Weight Distributions of Humic and Fulvic Acids Determined by Flow Field-Flow Fractionation

Abstract

The ability to characterize molecules whose physical and chemical properties are intimately linked to their diffusion coefficients and molecular weight is important to further understanding of chemical transport in the environment. Flow field-flow fractionation (flow FFF) was used to obtain separations of water-soluble macromolecules of varying molecular weight, including polystyrene sulfonates and humic substances. The separation occurs due to differing diffusion rates for chemical species of differing molecular weight in aqueous solution. Flow FFF uses fluid flow as the mechanism of separation. A model that yields liquid phase diffusion coefficients as a function of molecular weight was utilized to determine molecular weights from degree of separation. Separations of polystyrene sulfonates, a humic acid, and two fulvic acids of known molecular weight were accomplished using flow FFF. The separations obtained were used to develop a relationship between flow FFF separation and species molecular weight. Separations were obtained for humic and fulvic acids of unknown molecular weight.     

Field-Flow Fractionation

Abstract

Field-flow fractionation (FFF) is a separation method first described in 1966 (I). FFF is an elution technique, like chromatography, and the experimental sequence of pump, column, detector, and fraction collector is much like that used in chromatographic operations (2-4). However, FFF appears to be unique in its ability to separate an extremely broad range of molecules, macromolecules, supramacromolecular structures, colloidal particles, and larger particles at a high level of resolution. In dealing with these complex and often refractory materials, FFF has a number of unique advantages (5). Along with its intrinsically high resolving power, FFF is a versatile technique in which experimental conditions can be varied widely to optimize the range, speed, and power of the separation. FFF is also unusual in that the characteristics of the separation can be calculated rather exactly in terms of well-defined physicochemical parameters such as molecular weight, size, charge, etc. The equations used for this purpose can be inverted to yield molecular weight and other important parameters for the components of complex mixtures (3-6).     

Thermogravitational Field-Flow Fractionation: An Elution Thermogravitational Column

Abstract

The major operating characteristics of thermal field-flow fractionation (thermal FFF) and of thermogravitational columns are compared, and it is shown that the two approaches can be advantageously combined in a method we call thermogravitational FFF. The theory of this technique is developed, with primary attention given to a change in the velocity profile under different flow conditions and its effect on component retention, column efficiency, resolution, and selectivity. Experimental results are shown to be in good overall accord with theory. It is shown that the potential of thermogravitational FFF lies in the fractionation of low molecular weight polymers or of other species having weak thermal diffusion.     

Feasibility Study of Dielectrical Field-Flow Fractionation

Abstract

The possible use of dielectrophoretic forces for the development of a new subtechnique of field-flow fractionation (FFF) termed dielectrical FFF is examined. Dielectrical FFF is based on the dielectrophoresis of neutral particles in the nonuniform electric field of an annular channel (or charged coaxial capacitor). The feasibility of the subtechnique is assessed by estimating the magnitudes of retention ratio R predicted from theory for select species representative of several classes of particle/fluid mixtures. Minimum attainable R values are calculated using estimates of the maximum electric field strengths applicable to the mixtures. Calculations show that. the dielectrophoretic force is strong enough to retain and separate ultrahigh-molecular-weight polymers and submicron-diameter particles dissolved or suspended in organic liquids of high dielectric constant Evidence suggests that pearl-chain formation may impose a fundamental limitation on particle retention at the inner cylinder of the annular channel, especially in aqueous suspensions.     

Parameters for Optimum Separations in Field-Flow Fractionation

Abstract

Parameters that yield optimum separations in field-flow fractionation (FFF) are investigated. Expressions for minimum plate height and optimum velocity are derived. It is shown that a typical FFF column is theoretically capable of yielding 12,000 plates per foot. With increasing retention, plate height decreases and optimum velocity increases. Minimum time conditions, analyzed next, are related to the rate of generation of theoretical plates. The latter increases with the rate of molecular transport and, surprisingly, with retention. Practical hurdles to achieving an infinite rate of generation of plates by going to infinite retention are discussed. Finally, a comparison is made between optimum separations using FFF and using direct fields (electrophoresis, sedimentation, and related methods.)     

Exact Analysis of Field-Flow Fractionation

Abstract

A rigorous convective diffusion theory is formulated for the predictive modeling of field-flow fractionation (FFF) columns used for the separation of colloidal mixtures. The theory is developed for simulating the behavior of a colloid introduced into fluid in time-dependent flow in a parallel plate channel across which a transverse field is applied. The methodology of generalized dispersion theory is used to solve the model equations. The theoretical results show that the cross-sectional average concentration of the colloid satisfies a dispersion equation with time-dependent coefficients. The results of this work, in principle, are valid for all values of time since the introduction of the colloid. It is shown that these results asymptotically approach those of the nonequilibrium theory formulated by Giddings for large values of time.

Illustrative numerical results are obtained for the case of steady laminar flow and a uniform initial distribution. The behavior of the coefficients in the dispersion equation is explained on physical grounds. Of particular interest is the fact that at large values of the transverse Peclet number P, Taylor dispersion in the FFF column is very small. Under these conditions, axial molecular diffusion as well as Taylor dispersion in the connecting tubing could make a substantial contribution to the axial dispersion observed in practical FFF columns.

The theoretical predictions are compared with the experimental data of Caldwell et al. and Kesner et al. on electrical FFF columns. The comparisons indicate that the theory has potential in predicting the performance of such systems.     

Antibacterial Photodynamic Inactivation of Fagopyrin F from Tartary Buckwheat (Fagopyrum tataricum) Flower against Streptococcus mutans and Its Biofilm

The objective of this study was to determine reactive oxygen species (ROS) produced by fagopyrin F-rich fraction (FFF) separated from Tartary buckwheat flower extract exposed to lights and to investigate its antibacterial photodynamic inactivation (PDI) against Streptococcus mutans and its biofilm. ROS producing mechanisms involving FFF with light exposure were determined using a spectrophotometer and a fluorometer. S. mutans and its biofilm inactivation after PDI treatment of FFF using blue light (BL; 450 nm) were determined by plate count method and crystal violet assay, respectively. The biofilm destruction by ROS produced from FFF after exposure to BL was visualized using confocal laser scanning microscopy (CLSM) and field emission scanning electron microscope (FE-SEM). BL among 3 light sources produced type 1 ROS the most when applying FFF as a photosensitizer. FFF exposed to BL (5 and 10 J/cm²) significantly more inhibited S. mutans viability and biofilm formation than FFF without the light exposure (p < 0.05). In the PDI of FFF exposed to BL (10 J/cm²), an apparent destruction of S. mutans and its biofilm were observed by the CLSM and FE-SEM. Antibacterial PDI effect of FFF was determined for the first time in this study.