Thermal field-flow fractionation ThFFF

Currently, there are several molecular weight separation techniques, such as OTHdC, PCHdC, SEC, thermal field flow fractionation (ThFFF), and sedimentation field flow fractionation (SdFFF). The molecular weight separation range... 

The early research of Myers et al. [1,2] shows that polymer thermal field-flow fractionation (ThFFF) retention and thermal diffusion are solvent dependent. Recently, Sisson and Giddings [3] indicated that polymer ThFFF retention could be increased by mixing solvents. Rue and Schimpf [4] extended the molecular-weight range that can be retained by ThFFF to much lower molecular weights (<10 kDa) by using solvent mixtures without using extreme experimental conditions. There are several other reports on the effect of solvents on polymer retention, selectivity, and the universal calibration in FFF in last few years [5]. 

Thermal field-flow fractionation (ThFFF) can he applied to the analysis of virtually any polymer or copolymer that can be dissolved in an organic solvent, subject to low-molecular-weight limitations discussed in this entry. Water-soluble polymers are more difficult to separate because thermal diffusion, and therefore retention, is weak in water and other protic solvents. Still, certain non-ionic polymers can be separated, and with the use of mobile phase additives, even charged materials have been retained. Proteins, on the other hand, have not been successfully separated by ThFFF. 

This entry provides an overview of thermal field-flow fractionation (ThFFF) and its application to polymer and particle analysis. The separation mechanism is described and contrasted with that in size exclusion chromatography (SEC). The two techniques are somewhat complementary. Thus, SEC typically provides superior resolution of low molecular weight (M) polymers (M <100,000 Da), while ThFFF excels in the separation of high-molecular weight polymers (M >100,000 Da), gels, and particles. 

Controlling the temperature of the cold wall is important for obtaining the greatest possible precision in thermal field-flow fractionation (ThFFF). This entry describes the effect of temperature on the transport process, which underhes separation by ThFFF, as well as the practical imphcations of temperature when ThFFF is apphed to pol3mier analysis. 

Kassalainen and Williams [135] coupled thermal field flow fractionation (ThFFF) and matrix-assisted laser desorption/ionisation time-of-flight mass spectroscopy (MALDI-ToF-MS) to yield a powerful combination of techniques for the analysis of polydisperse PS. ThFFF high selectivity and sensitivity to chemical composition were used to separate polydisperse polymers and polymer mixtures into the narrow polydispersity and homogeneous chemical composition fractions essential for MAT.DT-ToF-MS analyses. On the other hand, because it is possible to measure directly using MALDI-ToF-MS, it alleviates the need for polymer standards for ThFFF. Kassalainen and Williams [135] address the coupling of ThFFF and MALDI-ToF-MS and identify compatibility issues. Optimum conditions were determined and developed to maximise the capabilities of the combined technique. Depending on the polymer and the method of matrix-assisted laser desorption/ionisation (MALDI) sample deposition, fractions from 1-10 ThFFF runs were combined for MALDI-ToF-MS analysis. Binary solvents are used to enhance ThFFF retention and resolution of low (<15 kDa) polymers, and methods developed to allow routine MALDI-ToF-MS analyses of PS polymers up to 575 kDa. Overall, the compatibility of the two techniques was extended from several kilodaltons to several hundred kDa. Polymer... 

SEC, size exclusion chromatography OTHdC, open tubular hydrodynamic chromatography PCHdC, packed column hydrodynamic chromatography ThFFF, thermal field flow fractionation. 

X HERMAL FIELD-FLOW FRACTIONATION (ThFFF) separates polymers according to their molecular weight and chemical composition. The molecular weight dependence is well understood and is routinely used to characterize molecular weight distributions (1-4). However, the dependence of retention on composition is tied to differences in the thermal diffusion of polymers, which is poorly understood. As a result, the compositional selectivity of ThFFF has not realized its full potential. How-... 

Fig. 1 Plots of log XAT vs. log TJ 298 for polystyrene in tetrahydrofuran. The data were gathered using a variety of different ThFFF channels. Values of AT ranged from 30 to 70 K. Source Reproduced with permission from Thermal field-flow fractionation universal calibration Extension for consideration of variation of cold wall temperature, in Anal. Chem. ...

Universal calibration of ThFFF channels can be achieved, but one additional piece of information is required for each polymer-solvent system under consideration, which is the thermal diffusion coefficient. Fortunately, this coefficient is independent of molecular weight, so only one such coefficient is required for each system. Such coefficients are available in the literamre for many systems, or they can be measured using ThFFF. Once the thermal diffusion coefficients are obtained, universal calibration plots are based on the same principles as those used in SEC, and utilize intrinsic viscosity measurements. A thorough discussion of ThFFF and its applications can be found in the Field-Flow Fractionation HandbooL ... 

The understanding of the effects of sample concentration (sample mass) in field-flow fractionation (FFF) has being obtained gradually with the improvement of the sensitivity (detection limit) of high-performance liquid chromatography (HPLC) detectors. Overloading, which was used in earlier publications, emphasizes that there is an upper limit of sample amount (or concentration) below which sample retention will not be dependent on sample mass injected into the FFF channels.Recent studies show that such limits may not exist for thermal FFF (ThFFF) (may be true for all the FFF techniques in polymer separation), although some of the most sensitive detectors on the market were used. ... 

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