Field-flow fractionation advantages

In conclusion one can say that SEC is a very powerful method for polymer characterization, especially in combination with other composition sensitive or absolute calibration methods. A big advantage is also that the sample amount is fairly small, typically 10 mg. For more complex polymers, such as polyelectrolytes, enthalpic effects often become dominant and also for rather high molecular weight polymers chromatographic methods such as field-flow fraction (FFF) techniques might be more suitable. For fast routine measurements linear columns are often used. 

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. 

The capacity factor k to be discussed shortly, is an alternate measure of retention. While k is used more often than R in chromatography, the use of R is advantageous because (i) it is directly proportional to peak migration velocity and is thus a more direct measure of retention than k (ii) most equations describing chromatography are simpler when expressed in terms of R rather than k and (iii) R is a more universal measure of retention R but not k applies to other perpendicular flow methods such as field-flow fractionation. 

Commercially available fractionation methods include hydrodynamic chromatography (HDC), field flow fractionation (FFF) and disc centrifugation (DSC). One advantage of fractionation methods over nonfractionation methods is that the particles are separated physically according to size, prior to detection, which allows much higher resolution in determining the size distribution [40]. 

In thermal field flow fractionation (TFFF), a temperature gradient is applied. The primary potential advantage of this technique is that it can be used to size particles in the range 0.01 pm to 0.001 pm, an order of magnitude smaller than SFFF. Fffractionation market a TFFF polymer fractionator channel module with 286/16 MHz IBM compatible PC, super VGA color monitor workstation to include data acquisition software, hardware and data analysis software. A linear UV detector and single channel high performance pump are optional. 

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. 

Field-flow fractionation (FFF) describes a group of analytical techniques that are becoming quite useful in the separation and characterization of dissolved or suspended materials such as polymers, large particles, and colloids. Although the FFF concept was first described by Giddings in 1966, only recently have practical applications and advantages over other methods been shown. 

E-3 Advantages of Field-Flow Fractionation over Chromatographic Methods... 

Field-flow fractionation appears to have several advantages over ordinary chromatographic methods for some applications. First, no packing material or stationary phase is needed for separation to occur. In some chromatographic systems, there may be undesirable interactions between the packing material or stationary phase and the sample constituents. Some solvents or sample materials adsorb or react with the stationary phase or its support. Macromolecules and particles are particularly prone to such adverse interactions. 

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. 

Thermophoresis is applied in industry for the separation of (large) molecules or small particles from their solvent in the so-called thermal field-flow fractioning (TFFF) [13]. A downscaled version of this process to microscopic scales demonstrated thermophoretic separation on microscopic scales [14, 15]. The advantage of using the very small confinement in a microfluidic device is that the separation times can be reduced, so that thermophoretic separation can be used. Braun and Libchaber [16] use a combination of thermophoresis and convection to concentrate DNA samples. The disadvantage of this approach is that it is a batch process. 

Figure 2 gives some characteristics of the size separation techniques that can be used to study the distribution of trace elements associated with various constituents of natural waters. It is obvious that the dimensions given in the figure are tentative as various factors influence the association/dissociation and aggregation/dispersion processes. However, preservation of real equilibria and labile species of elements, especially at concentrations of less than 10 g 1 prior to analysis is a much more serious problem encountered with methods that are not based on a direct physical separation. From this point of view, membrane filtration as well as some variants of field-flow fractionation (FFF) have advantages, although some uncertainties connected with equilibria shifts always exist. 

Thermal field-flow fractionation was invented by Giddings. The universal applicability of thermal FFF for the analysis of various polymers had been already demonstrated in 1979. Several applications of TFFF to the analysis of polymers and colloidal particles were published (see Refs. for a review) but the contemporary TFFF channels have practically the same dimensions (roughly 50 X 2 X 0.01 cm) as those constructed at the very beginning. Giddings concluded, in 1993, that the miniaturization of the FFF channels could provide only some limited advantages. The experimental studydealt only with the effect of the reduced channel thickness on the performance of TFFF. 

In the opinion of this author it is unlikely that the fundamental resolution and distribution capabilities of PCS measurements will be dramatically increased, though it has been proposed that simultaneous data reduction of multiangle ACFs will do just that. To date, multiangle measurements have yielded incremental advantages in special situations without pushing the resolution of the technique into the realm of its competitors disc centrimgation, column hydrodynamic fractionation, sedimentation field flow fractionation, and electron microscopy. 

Field-flow fractionation (FFF) is a relatively new analytical technique applicable to the separation of fine particles, polymers and macromolecules in solutions. Recent efforts concerned with Sedimentation field-flow fractionation (SdFFF) is to separate a wide variety of particulate species and to apply it to the particle size measurement. That is because SdFFF has advantages that it employs the fractional collection sorted by the particle mass, and has a high resolution over a wide range of particle size compared to other methods of sub-micrometer particle size determination. 

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