Field-flow fractionation profile

By coupling flow field-flow fractionation (flow FFF) to ICP-MS it is possible to investigate trace metals bound to various size fractions of colloidal and particulate materials.55 This technique is employed for environmental applications,55-57 for example to study trace metals associated with sediments. FFF-ICP-MS is an ideal technique for obtaining information on particle size distribution and depth profiles in sediment cores in addition to the metal concentrations (e.g., of Cu, Fe, Mn, Pb, Sr, Ti and Zn with core depths ranging from 0-40 cm).55 Contaminated river sediments at various depths have been investigated by a combination of selective extraction and FFF-ICP-MS as described by Siripinyanond et al,55... 

FIG. 2.1 Sedimentation field flow fractionation (SdFFF) (a) an illustration of the concentration profile and elutant velocity profile in an FFF chamber and (b) a schematic representation of an SdFFF apparatus and of the separation of particles in the flow channel. A typical fractionation obtained through SdFFF using a polydispersed suspension of polystyrene latex spheres is also shown. (Adapted from Giddings 1991.)... 

Fig. 26. Schematic design of field flow fractionation (FFF) analysis. A sample is transported along the flow channels by a carrier stream after injection and focusing into the injector zone. Depending on the type and strength of the perpendicular field, a separation of molecules or particles takes place the field drives the sample components towards the so-called accumulation wall. Diffusive forces counteract this field resulting in discrete layers of analyte components while the parabolic flow profile in the flow channels elutes the various analyte components according to their mean distance from the accumulation wall. This is called normal mode . Particles larger than approximately 1 pm elute in inverse order hydrodynamic lift forces induce steric effects the larger particles cannot get sufficiently close to the accumulation wall and, therefore, elute quicker than smaller ones this is called steric mode . In asymmetrical-flow FFF, the accumulation wall is a mechanically supported frit or filter which lets the solvent pass the carrier stream separates asymmetrically into the eluting flow and the permeate flow which creates the (asymmetrical) flow field...

Another use of cell disruption as a step in the analytical process is for obtaining a suspension of single cells — that can be used under optimal fermentation conditions — by ultrasonic disruption of cells manufactured in active dry wine yeast. Their potential was confirmed by comparing the elution profiles of non-sonicated and sonicated yeast sample dispersions obtained using two different field flow fractionation techniques [88]. 

Field-flow fractionation is, in principle, based on the coupled action of a nonuniform flow velocity profile of a carrier liquid with a nonuniform transverse concentration profile of the analyte caused by an external field applied perpendicularly to the direction of the flow. Based on the magnitude of the acting field, on the properties of the analyte, and, in some cases, on the flow rate of the carrier liquid, different elution modes are observed. They basically differ in the type of the concentration profiles of the analyte. Three types of the concentration profile can be derived by the same procedure from the general transport equation. The differences among them arise from the course and magnitude of the resulting force acting on the analyte (in comparison to the effect of diffusion of the analyte). Based on these concentration profiles, three elution modes are described. 

Field-flow fractionation (FFF) represents a family of versatile elution techniques suited for the separation and characterization of macromolecules and particles. Separation results from the combination of a nonuniform flow velocity profile of a carrier liquid and a nonuniform transverse concentration profile of an analyte caused by the action of a force field. The field, oriented perpendicularly to the direction of the flow, forms a specific concentration distribution of the analyte inside the channel. Because of the flow velocity profile, different analytes are displaced along the channel with different mean velocities, and, thus, their separation is achieved. 

Field-flow fractionation experiments are mainly performed in a thin ribbonlike channel with tapered inlet and outlet ends (see Fig. 1). This simple geometry is advantageous for the exact and simple calculation of separation characteristics in FFF Theories of infinite parallel plates are often used to describe the behavior of analytes because the cross-sectional aspect ratio of the channel is usually large and, thus, the end effects can be neglected. This means that the flow velocity and concentration profiles are not dependent on the coordinate y. It has been shown that, under suitable conditions, the analytes move along the channel as steady-state zones. Then, equilibrium concentration profiles of analytes can be easily calculated. 

Another interesting approach to particle separation is called field-flow fractionation, and this technique can be used in combination with dielectrophoresis (Davis and Giddings, 1986). Particles are injected into a carrier flow and another force, for example, by means of dielectrophoresis, is applied perpendicular to the flow. Dielectric and other properties of the particle will then influence the particles distance from the chamber wall and hence its position in the parabolic velocity profile of the flow. Particles with different properties will consequently be released from the chamber at different rates and separation hence achieved. Washizu et al. (1994) used this technique for separating different sizes of plasmid DNA. 

Fig. 1 Principle of field-flow fractionation. 1—Solvent reservoir, 2-carrier liquid pump, 3—injection of the sample, 4— separation channel, 5—detector, 6—computer for data acquisition, 7—transversal effective field forces, 8—longitudinal flow of the carrier liquid. A—Section of the channel demonstrating the principle of polarization FFF with two distinct zones compressed differently at the accumulation wall and the parabolic flow velocity profile. B—Section of the channel demonstrating the principle of focusing FFF with two distinct zones focused at different positions and the parabolic flow velocity profile. C—Section of the channel demonstrating the principle of steric ITF with two zones eluting at different velocities according to the distance of their centers from the accumulation wall.

It is well known that the essence of field-flow fractionation (FFF) is in the interaction between the distribution of the sample particles in the transversal field and the non-uniformity of the longitudinal flow profile. The classical FFF is realized in the channel with the flow driven by the pressme drop. The flow, in this case, is called PoiseuiUe flow and its profile is parabolic. 

The study of the interfacial phenomena between the channel wall and the colloidal suspension under study in sedimentation field-flow fractionation (SdFFF) is of great significance in investigating the resolution of the SdFFF separation method and its accuracy in determining particles physicochemical quantities. The particle-wall interactions in SdFFF affect the exponential transversal distribution of the analyte and the parabolic flow profile, leading to deviations from the classical retention theory, thus influencing the accuracy of analyte quantities measured by SdFFF. Among the various particle-wall interactions, our discussion focuses on the van der Waals attractive and electrostatic repulsion forces, which play dominant roles in SdFFF surface phenomena. 

Gimderson, J.J. Caldwell, K.D. Giddings, J.C. Influence of temperature gradient on velocity profiles and separation parameters in thermal field-flow fractionation. Sep. Sci. Technol. 1984,19, 667. 

Martin, M. Giddings, J.C. Retention and nonequiUbrium peak broadening for a generalized flow profile in field-flow fractionation. J. Phys. Chem. 1981, 85, 727. 

In field flow fractionation (FFF), retention can be related to the applied field through a well-defined equation governing physicochemical parameters of the analyte. Therefore, in principle, FFF is a primary measurement technique that does not require calibration, but only if the governing physiochemical parameters are the analyte parameters of interest, or their relationship to the parameter of interest (such a molecular weight) is well defined. This entry outlines the procedures for obtaining molecular weights and molecular weight distributions (MWDs) from ThFFF elution profiles in various experimental situations. 

Fig. 2 Flow field-flow fractionation fractogram and MALLS profile of polymeric protein fraction for the variety Katepwa.

Fig. 3 Flow field-flow fractionation (flow FFF) analysis of size-exclusion HPLC sublractions from Katepwa wheat flour, a, SE-HPLC subfractions Ci g b, flow FFF profiles of SE-HPLC subfractions ci. ...

Asymmetrical flow field-flow fractionation was used for size determinations of SLN in comparison to an emulsion and oil-loaded SLN. The differences found in the sizes and elution profiles were attributed to differences in the particle shapes. Due to their anisometric, platelet-like shape it is likely that SLN are more retained by the cross flow applied compared to spherical emulsion droplets. This method appears very promising as additional size determination method particularly with regard to separation and detection of different colloidal structures. 

Giddings et have examined a new method for polymer analysis, termed Field Flow Fractionation (FFF). In this technique a flow profile of parabolic shape is set up in a thin flow channel by pumping solvent through it. At the same time some form of field (usually a thermal gradient) is applied across the flow channel. If a polymer is injected into the field with no flow then the field causes it to concentrate at one side of the flow channel. This concentration is opposed by diffusion so that a concentration profile is set up across the field, in which the molecules are resolved according to their thermal diffusion coefficients and hence their molar masses. When the flow is applied the polymer is eluted with molar mass resolution. As yet the method has not shown anything like its theoretical resolution but preliminary results are very interesting. 

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