Thermal field-flow fractionation of polystyrene samples

Abstract

desirable spectral properties. On the other hand, spectrophotometric measurements can frequently be used at lower concentrations and for the determination of a particular species in a mixture.High frequency titration methods (35, 36) employ comparable radio frequencies and have the advantages that the electrodes are not in contact with the solution and that conducting solutions can be measured. However, the equivalent circuit of the necessary cell is more complicated and e or conductance of the solution is not related in any simple way to the measurement. The measurements are usually strictly empirical, titration curves vary in shape with frequency, and (35) W. observed variations are usually due mainly to conductance changes in solution (36) rather than changes in e.Other techniques applicable to low dielectric constant solvents have both advantages and disadvantages. Thermometric titrations are limited to fast reactions with large heats of reaction. Cryoscopic and ebullioscopic methods can only provide information about chemical behavior at the freezing and boiling points. Vapor pressure techniques are restricted to non-volatile solutes dissolved in volatile solvents. Conductance methods are limited because the principal reactions in low E solvents do not produce ions.

Parameters for Optimum Separations in Field-Flow Fractionation

“…It can be shown that direct thermal diffusive separations are virtually impossible because the largest practical temperature drops (around 100°C) are incapable of providing adequate fractionation. By contrast, thermal FFF is capable of producing multicomponent polymer separations (10).…”

“…This would not only keep the system closer to optimum resolution throughout the run, but it would also speed up the highly retained peaks which might otherwise require an inordinate time for elution. Downloaded by [Columbia University] at 10:24 08 December 2014 570 GIDDINGS Another way to express VOp, is attained by using the basic formula ( 2 ) for I I = DjU (10) where U is the drift velocity-the mean molecular velocity induced by the field. When this is substituted for I in Eq.…”

Unraveling ultrafiltration of polysaccharides with flow field flow fractionation

“…In this analyses method, mixtures are analyzed using external force fields and strictly controlled flows. Commonly applied fields are centrifugal, thermal [15], electrical, and flow fields [16]. Flow field fractionation is most commonly used for separation in an aqueous environment.…”

Polymer characterization by thermal field flow fractionation with a continuous viscosity detector

“…Since the sample concentration C of the TFFF effluent also is monitored by the differential refractometer, the polymer [9] value a t every TFFF elutionvolume "slice" can be directly determined by the ratio of the excess viscometer signal A S to the refractometer signal C, according to eq. (10).…”

“…(10). By this means, the [9] distribution curves for the polymer samples can be obtained. The sample [ 91 distribution curves so acquired are absolute, without the need of calibration.…”