Particle size distribution by sedimentation/steric field-flow fractionation: development of a calibration procedure based on density compensation

Because of the important but mathematically complex role played by hydrodynamic lift forces in sedimentation/steric FFF, applied generally to particles greater than 1 micron in diameter, retention cannot readily be related to particle diameter on the basis of simple theory. Consequently, empirical calibration is needed. Unfortunately, retention is based on particle density as well as size so that a purely size-based calibration (e.g., with polystyrene latex standards) is not generally valid. By examining the balance between driving and lift forces, it is concluded that equal retention will be observed for equal size particles subject to equal driving forces irrespective of particle density. Therefore by adjusting the rotation rate to exactly compensate for density, retention can be brought in line with that of standards, a conclusion verified by microscopy. Linear calibration plots of log (retention time) versus log (diameter) can then be used. This approach is applied to two glass bead samples (5-30 and 5-50 microns) using both a conventional and a pinched inlet channel. The resulting size distribution curves are self consistent and in good agreement with results obtained independently.