Experimental observation of steric transition phenomena in sedimentation field-flow fractionation

Abstract

The steric transition region of field-flow (FFF) is described as that part of a fractogram, found at very high retention volumes, in which normal FFF undergoes a transition to steric FFF by virtue of increasing particle diameter. The steric transition region is treated theoretically by assuming first that the steric factor ..gamma.. is constant and second that it is related by a simple power law to particle diameter. The authors then report the first experiments in which fractograms display the characteristic signature predicted by theory for the steric transition: a narrow terminal peak followed by a rapid dropoff to base line. It is shown that the steric transition point, which coincides with the dropoff, is displaced to higher retention volumes with increasing field strength, approximately as expected. The expected steric transition phenomena are further confirmed by collecting narrow fractions of a polydisperse poly(vinyl chloride) sample near the steric transition point and subjecting them to electron microscopy. The particle size distribution of the fractions is found to be sharply bimodal, in accordance with steric transition theory. However, satisfactory agreement between the measured particle diameters and the theoretical expressions is found only by application of the more complicated equations in which ..gamma..more » is assumed to be size dependent.« less