Different elution modes and field programming in gravitational field-flow fractionation I.: A theoretical approach

Gravitational field-flow fractionation utilizes the Earth's gravitational field as an external force which causes settlement of particles towards channel accumulation wall. It is sometimes considered that gravitational field-flow fractionation is not a versatile technique because the force field cannot be changed as in the other field-flow fractionation techniques. However, there are several possibilities to change the force acting on particles (e.g., by changing the angle between the Earth's gravitational field and the longitudinal axis of the channel, or the use of carrier liquids having different densities. Hydrodynamic lift forces are also known to act on particles in the carrier liquid flow and give some other means of modulating the force field in this technique by varying the flow-rate. This change can be reached by using programmed pumps or channels with non-constant cross-sections. Some of these possibilities can be exploited for field programming if a unified approach to control the retention variables in gravitational field-flow fractionation can be worked out. Several different elution modes can be indeed observed in gravitational field-flow fractionation, even mixed in some experiments. The equations describing the concentration profiles of analytes and their dependence on the elution mode mechanisms are used to gain insights into the different ways of field programming for each particular elution mode. (C) 1999 Elsevier Science B.V. All rights reserved.