Electrical field flow fractionation

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... 

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]. 

Electrical Field-Flow Fractionation of Proteins, K. D. Caldwell, L. F. Kesner, M. N. 

Field flow fractionation (FFF) is an elution technique suitable for molecules with a molecular weight > 1000 up to a particle size of some 100 pm. Separating, driving, external field forces are applied perpendicular to a liquid carrier flow, causing different species to be placed in different stream lines (Fig. 26). Useful fields are gravity, temperature, cross flow, electrical charge, and others [128-131]. 

Electrical field-flow fractionation belongs to the most sophisticated experimental techniques of FFF although the schematical setup looks quite simple (see Fig. 21). However, the electrical fields are difficult to implement in practice and extensive problems, discussed below, can occur. Therefore, relatively few papers on El-FFF exist even though the first publication appeared as early as 1972 [35], and the high intensity and manipulability (such as through pH changes) of electrical forces principally promise a high potential of El-FFF [35,255]. 

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... 

Another separation technique of particular application for proteins, high-molar-mass molecules, and particles is the general class known as field-flow fractionation (FFF) in its various forms (cross-flow, sedimentation, thermal, and electrical). Once again, MALS detection permits mass and size determinations in an absolute sense without calibration. For homogeneous particles of relatively simple structure, a concentration detector is not required to calculate size and differential size and mass fraction distributions. Capillary hydrodynamic fractionation (CHDF) is another particle separation technique that may be used successfully with MALS detection. 

Magnetic field-flow fractionation (FFF) employs static or quasi-static magnetic fields and excludes electromagnetic fields. Electromagnetic fields having frequencies in the kilohertz to megahertz range are used in dielectrophoretic FFF. Static electric fields are used in electrical FFF (see the entry Field-Flow Fractionation Fundamentals). 

The particle sizing by field flow fractionation (FFF) is based on the different effect of a perpendicular applied field on particles in a laminar flow [63-66], The separation principle corresponds to the nature of the perpendicular field and may, for example, be based on different mass (sedimentation FFF), size (cross-flow FFF), or charge (electric-field FFF). Cross-flow FFF has been applied recently to investigate nanoemulsions, SLN, and nanostructured lipid carriers (NLC, particles composed of liquid and solid lipids) [58], Although all samples had comparable particle sizes in PCS, their retention in the FFF was very different. Compared to the spherical droplets of the nanoemulsion, SLN and NLC were pushed more efficiently to the bottom of the channel because of their anisotropic shape. Their very different shapes have been confirmed by electron microscopy. 

G. H. Markx, J. Rousselet and R. Pethig, DEP-FFF field-flow fractionation using non-uniform electric fields, J. Liquid Chromatography Relat. Technol., 20, 2857-2872 (1997). 

Figure 32-14. Illustration of electrical field flow fractionation.

EFFF Electric field flow fractionation RFLP Restriction fragment length... 

In capillary electnrphoresis and eleciroclironialocra-phy, separations occur in a bulfer-lillcd capillary tube under the influence of an electric field as seen in the schematic of Figure 30-1, Separations in field-flow fractionation, on the otherhand, occurin athin ribbon-like flow channel under the influence of a sedimentation, electrical, or thermal field applied perpendicular to the flow direction. 

The specific, particle sizing method chosen depends on the type of. size information needed and the chemical and physical properties of the sample. In addition to the three techniques discussed here, molecular sieving, electrical conductance, microscopy, capillary hydrodynamic chromatography, light obscuration counting, field-flow fractionation, Doppler anemometry, and ultrasonic spectrometry-are commonly applied. Huch of the particle sizing methods has its advantages and drawbacks for particular samples and analyses. 

Griffiths, S.K. and Nilson, R.H., Charged species transport, separation, and dispersion in nanoscale channels Autogenous electric field-flow fractionation. AnaZ. Chem., 2006, 78 8134—8141. 

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