Field flow fractionation principles

Fig. 9. Principles of field-flow fractionation (a) sample equilibrium position before flow is initiated, (b) fractionated sample after flow initiation, and (c) a...

In principle, all powerful element-specific methods that are able to monitor continuously the effluents of separation processes commonly in the range of a few mimin-1 and in element concentrations of some Klpg liter-1. A well-suited method is based on modern element-specific quadrupole mass spectrometry (MS) with an inductively coupled plasma (ICP) interface to the separation unit [e.g., liquid chromatography (LC) or field-flow fractionation (FFF)].Tlie ICP-MS detection can also be used for continuously characterizing the effluent of any kind of packed column (Metreveli and Frimmel, 2007). By this, the transport and elution properties of... 

Figure 13.6. Separation principle of field-flow fractionation (FFF) is based on physical interactions of particles within an applied field and subsequent field-induced migration to the FFF channel wall ( accumulation wall ). Molecules, depending on their size and diffusion coefficient, are distributed over different velocity lines of axial flow, and they separate accordingly. Larger particles possess less diffusional motion and higher interaction with the applied field hence, they will be caught up in slower-moving streams near the channel wall and elute later than smaller particles.

This paper outlines the basic principles and theory of sedimentation field-flow fractionation (FFF) and shows how the method is used for various particle size measurements. For context, we compare sedimentation FFF with other fractionation methods using four criteria to judge effective particle characterization. The application of sedimentation FFF to monodisperse particle samples is then described, followed by a discussion of polydisperse populations and techniques for obtaining particle size distribution curves and particle densities. We then report on preliminary work with complex colloids which have particles of different chemical composition and density. It is shown, with the help of an example, that sedimentation FFF is sufficiently versatile to unscramble complex colloids, which should eventually provide not only particle size distributions, but simultaneous particle density distributions. 

Although F(+) separation methods are powerful, they are relatively complicated in physicochemical detail. In this chapter we will provide a general framework for F(+) methodology, outline the principles and applications of field-flow fractionation, and introduce the theoretical basis of chromatography. Chromatography will be treated in greater detail in Chapters 10-12. 

In the final five chapters, the general principles of the first part are utilized to better understand families of techniques and specific methods. The coverage highlights electrophoretic and sedimentation techniques, field-flow fractionation, and chromatography. Future editions are expected to cover extraction and membrane methods in more detail. 

Ion Chromatography, edited by James G. Tarter 38. Chromatographic Theory and Basic Principles, edited by Jan Ake Jonsson 39. Field-Flow Fractionation Analysis of Macromolecules and Particles, Josef Janca 40. Chromatographic Chiral Separations, edited by Morris Zief and Laura J. Crane 41. Quantitative Analysis by Gas Chromatography, Second Edition, Revised and Expanded, Josef... 

Separation Principle of Asymmetric Flow Field-Flow Fractionation... 

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. 

Fig. 1 Principle of field-flow fractionation (1) solvent reservoir, (2) carrier liquid pump, (3) injection of the sample,...

In field-flow fractionation (FFF), retention can be related through a well-defined equation to the applied held and 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 either the analyte parameters of interest or their relationship to the parameter of interest (such a molecular weight) is well deflned. 

In their fundamental principle, field-flow fractionation (FFF) methods exploit the cell physical characteristics by means of their selective elution in a paraUelepipedic channel laminarUy flowed by a carrier phase under the effect of an external field apphed perpendicularly to the great surface of the channel and, by consequence, perpendicularly to the flow direction. In sedimentation FFF, the external field is gravitational (G-FFF) or multigravi-tational (Sd-FFF). 

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. 

Figure 11. Diagram showing the principle of dielectrophoretic field-flow fractionation (DEP-FFF).

Figure 32-1. Principle of field flow fractionation. (Courtesy - Giddings, J.C., Graff, K.A., Myers, M.N., and Caldwell, K.D., Sep. Sci. TechnoL, 15, 615, 1980)...

Figure 32-12. The principle of flow field flow fractionation.

What is the basic principle of how large molecules are separated in field flow fractionation ... 

Fig. 7.1. Principle of field-flow fractionation. Reprinted from Ref. 78, by courtesy of Marcel Dekker, Inc.

Thermal field-flow fractionation (TFFF) belongs to the historically oldest subtechniques of FFF. It is based on the principle of thermal diffusion. In early works... 

Magnetic field-flow fractionation (MFFF) has been the youngest subtechnique of FFF. So far the only work [66] dealing with MFFF defined elementary theoretical principles of the separation, and demonstrated in practice retentions of bovine serum albumin in the presence of nickel(II) ions in a magnetic field of 400 G. A coiled Teflon capillary with an inside diameter of 0.15 cm and length of 304 cm was used as a channel. In the absence of nickel(II) ions no retention was observed. 

A very recent volume edited by Berthed (2002) is on countercurrent chromatography - the support-free liquid stationary phase. Ebdon et al. (1987) review directly coupled liquid chromatogramphy-atomic spectroscopy. The review by Uden (1995) on element-specific chromatographic detection by atomic absorption, plasma atomic emission and plasma mass spectrometry covers the principles and applications of contemporary methods of element selective chromatographic detection utilizing AA, AES and MS. Flame and furnace are considered for GC and HPLC, while MIP emission is considered for GC and ICPAES for HPLC. Combinations of GC and HPLC with both MIPAES and ICPAES are covered and supercritical fluid chromatographic (SFC) and field flow fractionation (FFF) are also considered. 

Sedimentation Field Flow Fractionator. The chromatography-related principle of this particle size and size distribution analyzer is based upon the interaction of the particle suspension under centrifugal field motion in a thin channel. The elution time of the particles is a function of particle size, particle density, flow rate of mobile phase, density of mobile phase, and the centrifugal force applied. After the size separation has occurred, the particles are detected in the mobile phase using a turbidity detection system. The dynamic range of the instrument is dependent on particle density and operating conditions and is typically within 0.03 /rm— 1 /rm range. 

Particle Size and PSD. According to the basic principles that they are based on, the techniques for measuring these important characteristics of the latexes are classified into four major groups [196] (i) microscopy, (ii) light scattering, (iii) particle movement (e.g., capillary hydrodynamic chromatography and field flow fractionation methods), and... 

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