Retention (steric) inversion in field-flow fractionation: practical implications in particle size, density and shape analysis
The purpose of this paper is to develop a generalized treatment for the retention (steric) inversion phenomenon in field-flow fractionation (FFF) and to examine the implications of this inversion on particle analysis by FFF. The retention inversion represents one of the most serious complications in the theoretical treatment and practical implementation of the conceptually simple FFF family of techniques. The inversion phenomenon can complicate particle size analysis for particle populations with diameters that span the inversion diameter, di(≈ 1 µm). One alternative is to shift di to one end of the particle diameter range through intentional variations in operating parameters. In order to understand and control these shifts, an approximate but general theoretical treatment of retention, inversion and inversion shifts is given. The theory is based on retention ratio additivity and a power law dependency of the retention ratio on relevant variables in both normal and steric FFF. This approach yields explicit equations for the inversion diameter and the inversion retention ratio or retention time. The theory is supported and made more understandable through logarithmic plots. Other FFF strategies for analysing particle populations that overlap di are noted. The possibility that the steric inversion can become an experimental tool for the measurement of particle densities and shapes, along with their distributions, is discussed.