Field flow fractionation theory

Three-Dimensional Effects in Field-Flow Fractionation Theory Victor P. Andreev... 

Kowalkowski T, Buszewski B, Cantado C, Dondi F (2006) Field-flow fractionation theory, techniques, applications and the challenges. Cirt Rev Anal Chem 36 129-135... 

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]. 

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. 

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. 

Dense-Gas Chromatography of Nonvolatile Substances of High Molecular Weight, L. McLaren, M. N. Myers, and J. C. Giddings, Science, 159, 197 (1968). Nonequilibrium Theory of Field-Flow Fractionation, J. C. Giddings, J. Chem. Phys., 49, 81 (1968). 

Simplified Nonequilibrium Theory of Secondary Relaxation Effects in Programmed Field-Flow Fractionation, J. C. Giddings, Anal. Chem., 58, 735 (1986). Feasibility Study of Dielectrical Field-Flow Fractionation, J. M. Davis and J. C. 

Martin M (1998) Theory of field-flow fractionation. In Brown PR, Grushka E (eds) Advances in chromatography. Marcel Dekker, New York, pp 1-138... 

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... 

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. 

A field-flow fractionation (FFF) channel is normally ribbonlike. The ratio of its breadth b to width w is usually larger than 40. This was the reason to consider the 2D models adequate for the description of hydrodynamic and mass-transfer processes in FFF channels. The longitudinal flow was approximated by the equation for the flow between infinite parallel plates, and the influence of the side walls on mass-transfer of solute was neglected in the most of FFF models, starting with standard theory of Giddings and more complicated models based on the generalized dispersion theory [1]. The authors of Ref. 1 were probably the first to assume that the difference in the experimental peak widths and predictions of the theory may be due to the influence of the side walls. 

Data Analysis. The computer program used for data analysis was developed at the Field-Flow Fractionation Research Center. The underlying theory is similar to that discussed by Giddings et al. (4). For normal mode characterizations, the fractograms are converted to particle size distributions by using developed theory. However, for steric mode analyses, calibration curves are required (15, 20). 

Ftg- 2 Elemental size distributions of the colloidal material in a freshwater sample as given from an FLFFF coupled to ICPMS. A UV detector is placed on line prior to the ICPMS and the UV size distribution is included. The signals are plotted as a function of retention time, hydrodynamic diameter (from FFF theory), and molecular weight (from standardization with PSS standards). Source From Determination of continuous size and trace element distribution of colloidal material in natural water by on-line coupling of flow field-flow fractionation with ICMPS, in Anal. Chem. J... 

Janca, J. Chmelik, J. Jahnova, V. Novakova, N. Urbankova, E. Principle, theory and applications of focusing field-flow fractionation. Chem. Anal. 1991, 36, 657. 

Giddings, J.C. Nonequilibrium theory of field-flow fractionation. J. Chem. Phys. 1968, 49, 81. 

Hoyos, M. Martin, M. Retention theory of sedimentation field-flow fractionation at finite concentration. Anal. Chem. 

Giddings, J.C. Simplified nonequiUbrium theory of secondary relaxation effects in programmed field flow fractionation. Anal. Chem. 1986, 58, 735-740. 

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. 

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