Field-flow fractionation mechanism

Figure 1. Schematic of an FFF channel with the separation mechanism for normal FFF shown in detail. Reprinted from [7] Beckett, R. and Hart, B. T. Use of field flow fractionation techniques to characterize aquatic particles, colloids and macromolecules . In Environmental Particles. Vol. 2, IUPAC Series on Analytical and Physical Chemistry of Environmental Systems. Series eds. Buffle, J. and van Leeuwen, H. P., pp. 165-205. Copyright 1993 IUPAC. Reproduced with permission...

The word flow implies fluid moving through (or across) a rigid framework or conduit (a container, tube, or packed bed) and not being carried with it as in the case of mechanical transfer. Flow is an integral part of many separation techniques, including chromatography, field-flow fractionation, ultrafiltration, and elutriation. The flow process is not itself selective, but it enables one to multiply by many times the benefits of separations attempted without flow. This point is explained in Chapter 7. 

The 2D approach to separation offers not only a great increase in separation power over one-dimensional (ID) methods, but also greater versatility. We have noted that 2D separation requires the use of pairs of ID displacements. If N kinds of ID displacements can be employed, then N2 different pairwise combinations can be found for 2D use. For example, dozens of 2D methods can be envisioned that use a field-flow fractionation (FFF) mechanism these methods fall in four categories in which a given FFF mechanism can be combined with (1) another FFF subtechnique, (2) a form of chromatography, (3) an applied field (e.g., electrical), and (4) bulk flow displacement [20]. For separations generally, literally thousands of kinds of 2D separation systems are possible, although only a handful have been developed [8]. 

Perpendicular flow occurs in chromatography, countercurrent distribution, field-flow fractionation, and related methods. Below we explain the basic mechanism by which flow assumes its vital role in these separation techniques. 

The above band-broadening mechanism assumes a more concrete form when it is described specifically for chromatography in Section 10.6. When this mechanism is expressed mathematically it becomes the nonequilibrium theory, an important tool describing zone evolution in chromatography (Section 10.6) and field-flow fractionation [2, 3J. 

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...

In two papers, Palkar and Schure studied the mechanism of electrical field-flow fractionation in detail [269,270]. An electrical circuit of the channel was... 

Sedimentation field-flow fractionation was used also for the kinetic study of hydroxyapatite (HAP) particles aggregation in the presence of various electrolytes to determine the rate constants for the bimol-ecular process of aggregation and to investigate the possible aggregation mechanisms describing the experimental data. The HAP sample contained polydis-perse, irregular colloidal particles with number-average diameter dys = 0.262 + 0.046 /rm. 

Flow field-flow fractionation (flow FFF) is a separation method that is applicable to macromolecules and particles [1], Sample species possessing hydrodynamic diameters from several nanometers to tens of microns can be analyzed using the same FFF channel, albeit by different separation mechanisms. For macromolecules and submicron particles, the normal-mode mechanism dominates and separation occurs according to differences in diffusion coefficients. Flow FFF s wide range of applicability has made it the most extensively used technique of the FFF family. 

In all of the many forms of chromatography, detection is an inherentiy important final step. The type of detection can aid in the analysis by gathering information that can be used to identify the peaks seen. There can be many peaks that elute from the column of a gas, hquid, or supercritical-fluid chromatograph. Certain detectors are in fact spectrometers that examine each peak for specific information on its identity. This chapter deals with this use of spectrometers as the tail-end detector in chromatography. Other separation techniques, such as field-flow fractionation or capillary electrophoresis, differ in their separation mechanisms, but as far as coupling to spectrometers behave like one of these three types of chromatography. 

This entry provides an overview of thermal field-flow fractionation (ThFFF) and its application to polymer and particle analysis. The separation mechanism is described and contrasted with that in size exclusion chromatography (SEC). The two techniques are somewhat complementary. Thus, SEC typically provides superior resolution of low molecular weight (M) polymers (M <100,000 Da), while ThFFF excels in the separation of high-molecular weight polymers (M >100,000 Da), gels, and particles. 

Copolymers are becoming increasingly important for high performance and new materials with specific mechanical, optical, and electrical properties. The bivariate composition and mass distribution controls many aspects of the materials behavior, such as tensile strength, processability, surface, phase stabihty, and so on. Nonetheless, determining the bivariate distribution can be time consuming and costly, and usually requires the use of complementary techniques, such as thermal field-flow fractionation [8], temperature rising elution fractionation (TREE) [9], Fourier transform infrared (FTIR) spectroscopy [10], and other methods [11] for... 

Diffusion and dispersion are important mechanisms for the transport of chemicals. This chapter first addresses diffusion and dispersion in the single phase flow. Then it discusses the fractional flow curve analysis in the water/oil two-phase flow. Fractional flow curve analysis may not provide an accurate estimate of actual field flood performance, but it is a good tool for mechanism analysis. 

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