Separation techniques field-flow fractionation

Biomolecule Separations. Advances in chemical separation techniques such as capillary zone electrophoresis (cze) and sedimentation field flow fractionation (sfff) allow for the isolation of nanogram quantities of amino acids and proteins, as weU as the characterization of large biomolecules (63—68) (see Biopolymers, analytical techniques). The two aforementioned techniques, as weU as chromatography and centrifugation, ate all based upon the differential migration of materials. Trends in the area of separations are toward the manipulation of smaller sample volumes, more rapid purification and analysis of materials, higher resolution of complex mixtures, milder conditions, and higher recovery (69). 

Field-Flow Fractionation. Field-flow fractionation is a general name for a class of separation techniques that fractionate a particle population into groups according to size. The work in this area has been reviewed (59). 

Currently, there are several molecular weight separation techniques, such as OTHdC, PCHdC, SEC, thermal field flow fractionation (ThFFF), and sedimentation field flow fractionation (SdFFF). The molecular weight separation range... 

Techniques which seem less suitable for routine size analysis are (1) analytical ultracentrifugation combined with a Schlieren optical system (Mason and Huang, 1978 Weder and Zumbuehl, 1984) (2) the sedimentation field flow fractionation (SFFF) technique to separate heterogeneous dispersions (e.g., Kirkland et al., 1982). 

There are many combinations of separations techniques and methods of coupling these techniques currently employed in MDLC systems. Giddings (1984) has discussed a number of the possible combinations of techniques that can be coupled to form two-dimensional systems in matrix form. This matrix includes column chromatography, field-flow fractionation (FFF), various types of electrophoresis experiments, and more. However, many of these matrix elements would be difficult if not impossible to reduce to practice. 

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...

Field-flow fractionation, commonly designated as FFF, is a versatile family of separation techniques able to separate and characterize an enormous assortment of colloidal-supramolecular species in a wide range of dimensions/molecular weights. Giddings is considered the inventor of this technique since he contributed to the development of theory, different techniques, instrumentation, methodology, and applications [1], even if studies on the theoretical fundamentals of fractionation under force and flow fields had appeared before and/or independently [2]. 

The first section of the book explores emerging novel aspects of HPLC and related separation methods based on the differential velocity of analytes in a liquid medium under the action of either an electric field (capillary electromigration techniques) or a gravitational field (field-flow fractionation). The section focusing on applications highlights four significant areas in which HPLC is successfully employed chiral pharmaceutical, environmental analysis, food analysis, and forensic science. 

Field flow fractionation (FFF), as a gentle size fractionation coupled to ICP-MS, offers the capability to determine trace metals bound to various size fractions of colloidial and particulate materials.112 On line coupling of FFF with ICP-MS was first proposed by Beckett in 1991 -113 Separation is achieved by the balance between the field force and macromolecular diffusion in the FFF channel. Depending on the field force used, FFF is classified into different techniques such as sedimentation, gravitational, electrical, thermal and flow FFF.112... 

One such consequence is their use in the physical characterization of colloidal dispersions and macromolecular solutions. Let us highlight one such application through one element of a class of analytical separation techniques known as field flow fractionation (FFF). 

Because the chapter is about DOM, detailed information about the role of colloids and the analytical techniques are given elsewhere (e.g., Buffle and Leppard, 1995 Kretzschmar et al., 1999 Frimmel et al., 2007). Different separation techniques, like ultrafiltration, size exclusion chromatography, and flow field-flow fractionation can be coupled with UV-vis absorption and ICP-MS to show the interaction of metals and colloids. Elements like Ni, Cu, Cr, and Co are associated mainly with smaller-size DOM fractions whereas Al, Fe, lanthanides, Sn, and Th are associated with larger-size DOM fractions (Bolea et al., 2006). The laser-induced breakdown detection (LIBD) is a new, sensitive method for the quantification of aquatic colloids of lower-range nanometer size in very low concentration, which cannot be... 

The follow-up section will deal with separation methods based upon (a) molecular size and related to it hydrodynamic volume (size-exclusion chromatography and ultrafiltration), (b) molecular size and related to it molecular diffusivity (field-flow fractionation), and (c) charge/size ratio and related to it molecular polarity (electrophoresis and mass spectrometry). Also reviewed will be hyphenated techniques or those that combine separation by chromatography or electrophoresis with spectral detection. 

Another technique widely used for size separation of humic materials is field-flow fractionation (FFF) (e.g., Baalousha et al., 2006 Boehme and Wells, 2006 Geckeis et al., 2003 Hassil ov et al., 2007 Siripinyanond et al., 2005 Suteerapataranon et al., 2006 Zanardi-Lamardo et al., 2002). This technique was developed and introduced in 1966 by Giddings (1966) as a method for the separation and characterization of materials ranging in size from macromolecules to particulates. Similar to SEC, FFF... 

Some of these fractionation problems can be ameliorated by the use of the relatively new technique of field-flow-fractionation (FFF). Its advantages include high-resolution separation and sizing of particulate, colloidal and macromolecu-lar materials covering 105-fold range from about 10 3 to 1()2/rm (see Chapter 8). 

Clearly, sedimentation FFF is a separation technique. It is an important member of the field-flow fractionation (FFF) family of techniques. Although other members of the FFF family (especially thermal FFF) are more effective for polymer analysis, sedimentation FFF is advantageous for the separation of a wide assortment of colloidal particles. Sedimentation FFF not only yields higher resolution than nearly all other particle separation techniques, but its simple theoretical basis allows a straightforward connection between observed particle migration rates and particle size. Thus size distribution curves are readily obtained on the basis of theoretical analysis without the need for (and uncertainties of) calibration. 

By applying a steric sub technique (Giddings et al., 1980), field-flow fractionation was shown to have the potential to separate stable polysaccharide suprastructures in greater than hydrocolloidal diameters. Moon and Giddings (1993) used the procedure to size starch granules into a bimodal distribution of mass greater and less than 10 pm. 

The coverage of separation techniques in many important textbooks in chemical analysis is largely limited to chromatography (e.g., refs. 9, 12, 13). While chromatography is of central importance in analysis, the omission of modern electrophoretic (both gel and capillary) and field-flow fractionation techniques leaves a large void in the description of separative capabilities, particularly in the biochemical and macromolecular realm. 

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. 

In the filtration-type methods (the first three techniques listed above), components accumulate as a steady-state (polarization) layer at a barrier or membrane [4] this occurs in much the same way as in field-flow fractionation or equilibrium sedimentation. However, there are several complications. First, fresh solute is constantly brought into the layer by the flow of liquid toward and through the filter. This steady influx of solute components can be described by a finite flux density term J0. Second, components can be removed from the outer reaches of the layer by stirring. Third, the membrane or barrier may be leaky and thus allow the transmission of a portion of the solute, profoundly affecting the attempted separation. In fact, one reason for our interest in layer structure is that leakiness depends on the magnitude of the solute buildup at the membrane surface. As solute concentration at the surface increases, more solute partitions into the membrane and is carried on through by flow. 

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. 

Principal separation techniques, if they exist, are listed for each held. 5 FFF = field-flow fractionation clnertial transport term important... 

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