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

Gravitational field-flow fractionation utilizes the Earth's gravitational f ield as an external force which causes settlement of particles towards chan nel accumulation wall. It is sometimes considered that gravitational field- flow fractionation is not a versatile technique because the force field can not 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 dif ferent densities. Hydrodynamic lift forces are also known to act on particl es in the carrier liquid flow and give some other means of modulating the f orce field in this technique by varying the flow-rate. This change can be r eached by using programmed pumps or channels with non-constant cross-sectio ns. 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 a nalytes and their dependence on the elution mode mechanisms are used to gai n insights into the different ways of field programming for each particular elution mode. (C) 1999 Elsevier Science B.V. All rights reserved.