Recovery, overloading, and protein interactions in asymmetrical flow field-flow fractionation

In asymmetrical flow field-flow fractionation (AF4), similar to other separation techniques, mass recovery and overloading require special attention in order to obtain quantitative results. We conducted a systematic study with five globular proteins of different molecular weight (36.7–669 kDa) and isoelectric point (4.0–6.5), and ultrafiltration membranes that are commonly used in aqueous AF4, regenerated cellulose (RC) and polyethersulfone (PES). Phosphate-buffered saline (PBS) with ionic strength 0.15 M and pH 7.2 was used as the carrier liquid in this study. The actual molecular weight cutoff (MWCO) was found to be higher than the nominal value and varied between membranes of different chemistry but the same nominal MWCO. Adsorption on the membrane was found to be dependent on the membrane chemistry (RC had lower adsorption compared to PES), and independent of the protein standard for the examined proteins. On the other hand, the mass overloading effects (i.e., higher retention times, peak broadening, and fronting peaks) were significantly more pronounced for γ-globulin than for the other proteins. The overloading effects could be rationalized with the increase of the local viscosity close to the membrane, depending on the properties of the proteins, and we derived theoretical equations that related the dependency of the migration velocity on the protein concentration through this non-ideal viscosity effect.

Equilibrium separation and filtration of particles using differential inertial focusing

Rapid separation and filtration of particles in solution has a wide range of applications including blood cell separation, ultrasound contrast agent preparation, and purification of fermentation products. However, current techniques that provide quick processing rates are high in complexity. We present a rapid microfluidic filtration technology capable of separating particles based on size, with purities from 90 to 100% and high-volume throughputs of 1 mL/min. Data for separation of rigid particles, deformable emulsions, and platelets from whole blood are presented. The system is based upon differential inertial focusing of particles of varying sizes and allows continuous separation based only on intrinsic hydrodynamic forces developed in a flow through an asymmetrically curved channel. A theoretical description of the underlying forces is developed, and in combination with data determining a size cutoff for separation, a semiempirical relationship describing how channel geometry is related to this cutoff is shown. Cascading separations in series is shown to be useful for increasing purity and yield. This type of microfluidic system can filter deformable particles, is largely independent of particle density, and can provide throughputs typical of macroscale filtration in a compact format, enabling applications in blood filtration and particle concentration.

Onset of non-linearity in zonal elution separators: the concept of effective analyte concentration

When an analyte injected in a zonal separation method (chromatography, capillary zone electrophoresis, field-flow fractionation) is not highly diluted in the carrier fluid, the retention ratio, R--or ratio of the cross-sectional average migration velocity of the analyte to that of the carrier fluid--depends on the local concentration, c, of the center of mass of the analyte zone, and the zone migration occurs in non-linear conditions. Because the zone broadens as it moves along the separator, R varies continuously from the inlet to the outlet of the separator. That concentration, c(eff), for which R(c(eff)) is equal to the length-averaged apparent retention ratio, R(app), is called effective concentration, and that distance, z(eff), from the separator inlet, for which c(z(eff)) is equal to c(eff), i.e. for which R(z(eff)) is equal to R(app), is called effective position. Assuming that near the onset the non-linear behavior, R(c), is a linear function, values of R(app), c(eff) and z(eff) have been computed in a wide range of operating conditions which are typical of situations encountered in capillary zone electrophoresis, liquid chromatography, or field-flow fractionation. Computations have been performed both in presence and in absence of the dispersion arising from the concentration dependence of the analyte migration rate (called thermodynamic dispersion in chromatography or electromigration dispersion in capillary zone electrophoresis). It is found that, whatever the range of analyte concentration covered from inlet to outlet of the separator, c(eff) is always close to two times the analyte concentration, c(out), at the outlet of the separator, and z(eff) between one-fourth and one-third of the separator length. As c(out) is easily determined from the peak recorded by a concentration-sensitive detector, a simple pragmatic expression is given for the estimation of c(eff). This effective concentration is the appropriate concentration to be used for comparing predictions of theoretical models of R(c) with experimental retention data. This is of particular interest for validating such models in field-flow fractionation.

Increasing the Sensitivity of Asymmetrical Flow Field-Flow Fractionation: Slot Outlet Technique

A new enrichment approach is described to improve the sensitivity of the asymmetrical flow field-flow fractionation, with and without ICPMS online coupling for elemental detection. For the slot outlet technique, a part of the laminar carrier stream is removed through an additional pump. This allowed an enrichment of the colloidal particles in the separation channel up to a factor of 14. Additional improvements of the separation efficiency permitted us to separate colloids with differences in their molecular masses of only 2 kDa. Different polymer standards, proteins, and synthesized tracer colloids, as well as real samples (wastewater, liquid manure, serums) were used to assess the performance of the new technique.

The use of asymmetrical flow field-flow fractionation in pharmaceutics and biopharmaceutics

Field-flow fractionation (FFF) is a family of flexible analytical fractionating techniques which have the advantage that the separation of analytes is achieved, solely through the interaction of the sample with an external, perpendicular physical field, rather than by the interaction with a stationary phase. The rapid progress in pharmaceutical biotechnology goes along with an increasing demand in potent, high-efficient analytical methods. Thus, FFF techniques are gaining increasing attention for their ability to separate and characterize populations of polymers, colloids and particles of up to about 100 microm in size. It is the intention of this review to provide an overview on common FFF techniques, to summarize inherent advantages and limitations and to introduce both established and challenging applications in the (bio)pharmaceutical field. Thereby, asymmetrical flow FFF is addressed predominantly, since it is the most versatile applicable FFF technique.