Field-flow fractionation instrumentation

Bernard, A. Paulet, B. Colin, V. Cardot, P.J.P. Red blood cell separations by gravitational field-flow fractionation Instrumentation and applications. TrAC, Trends Anal. Chem. 1995,14 (6), 266-273. 

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]. 

Data from a number of different particle size analysis instrumental methods including light scattering, field flow fractionation, hydrodynamic chromatography and microscopy were obtained for a series of polymethylmethacrylate latexes and were compared to DCP results (2). These and other comparative results have demonstrated the accuracy of the instrument and method. The reproducibility and precision of the instrument also were studied and are reported elsewhere ( 1 ). 

The first volume concentrates on separation techniques. H. Pasch summarizes the recent successes of multi-dimensional chromatography in the characterization of copolymers. Both, chain length distribution and the compositional heterogeneity of copolymers are accessible. Capillary electrophoresis is widely and successfully utilized for the characterization of biopolymers, particular of DNA. It is only recently that the technique has been applied to the characterization of water soluble synthetic macromolecules. This contribution of Grosche and Engelhardt focuses on the analysis of polyelectrolytes by capillary electophore-sis. The last contribution of the first volume by Coelfen and Antonietti summarizes the achievements and pitfalls of field flow fractionation techniques. The major drawbacks in the instrumentation have been overcome in recentyears and the triple F techniques are currently advancing to a powerful competitor to size exclusion chromatography. 

Koehler and Provder [317] sized monodisperse PMMA latexes with a range of instruments Disc centrifugal sedimentation (DCP), sedimentation field flow fractionation (SFFF), hydrodynamic chromatography (HDC), photon correlation spectroscopy (PCS), turbidimetry and transmission electron microscopy (TEM). TEM gave the smallest sizes, DCP and SFFF were in fair agreement in the center and PCS the highest sizes. 

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. 

Field-flow fractionation was developed about the same time as SEC the first FFF paper was published in 1966 [1]. However, major instrumentation problems were not solved as quickly for FFF as they were for SEC as instrumentation became available for SEC, there was then less incentive for the development and application of FFF instrumentation. For almost twenty years, the only active group involved in the development of FFF for polymer analysis was the University of Utah group from which the FFF concept originated. However, as the demands placed on polymer characterization tools become more stringent and the applications more exotic, the FFF approach is gaining increased attention from a widening circle of users. 

Figure 8.1 Diagram of instrumental system for field-flow fractionation.

Fig. 1 Instrumental schematics of FLFFF with on-channel preconcentration showing the three different steps. The first involves emptying of the sample loop into either the forward or backward flows and subsequent focusing of the sample material at the focusing point. In the next step, the samples are allowed to relax at the equilibrium position by applying cross-flow only, and then the channel flows are switched on and elution is commenced. Source From Optimisation of on-channel preconcentration in flow field-flow fractionation for the determination of size distributions of low molecular weight colloidal material in natural waters, in Anal. Chim. Acta. ...

In field-flow fractionation (FFF), like chromatography, retention and resolution are affected by temperature. For calibration curves to be as precise as possible, or in the case of ThFFF, to universally apply a calibration curve to channels in different laboratories, it is important to closely control the temperature. In ThFFF, the analyte is typically compressed into a layer very close to the cold wall. Therefore, the temperature of the analyte is usually within a few degrees of the cold wall temperature. Consequently, the retention of a given component from one run to the next, or from one instrument to another, will be identical only if the cold wall temperatures are identical. 

Continued improvements in FFF instrumentation, refinements in technique and hyphenation with detectors such as MALLS have broadened the application of this method and effectively elevated it from the status of possibly useful technique to a reliable applied research tool for assessing the size and shape of wheat proteins. For additional reading concerning FFF theory, principles, and applications, the reader is referred to other sections of this encyclopedia as well as to Field-Flow Fractionation Handbook edited by Schimpf et al. A review of the application of FFF to wheat protein analysis can also be found in Preston and Stevenson. 

Fieid—Fiow Fractionation. Field-flow fractionation (FFF) employs a one-phase chromatographic system (251,252). Commercial instrumentation is available from Postnova Analytics and Tecan. Separation occurs in a thin channel containing a single moving fluid. The field applied across the channel may be selected on the basis of the solute. Possible fields include sedimentation, cross-flow, concentration, dielectric, thermal, and magnetic. A book (253) and a review (254) of this technique and its comparison with GPC for the characterization of polymer molecular weights have been published. 

Williams SKR, Benincasa MA (2000) Field-flow fractionation analysis of polymers and rubbers. In Meyers RA (ed) Encyclopedia of analytical chemistry instrumentation and applications. Wiley, Chichester, pp 7582-7608... 

Whether DLS, DWS, Mie scattering, or other applications in which unfractionated samples are analyzed, the resulting distributions produced by modern instruments, while frequently facile to obtain and neat in appearance, must be treated with caution, as there is usually a large amount of data smoothing, fitting, and assumptions applied in using inverse Laplace transform and several other commonly employed methods. The best means of finding distributions of size and mass continue to be fractionation methods, such as SEC [32-34], field flow fractionation (FEE) [35-37], capillary electrophoresis [38], capillary hydrodynamic fractionation [39], and so on. 

FIFFF flow field flow fractionation INAA instrumental neutron activation an2ilysis... 

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