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Sedimentation Field-Flow FractionationSdFFF is a set of high resolution liquid chromatography-like elution methods used for sizing and separating colloidal matter into size fractions. SdFFF separations are performed within a flat open channel, usually having a rectangular cross-section and triangular end pieces where the sample and carrier fluid enters and leaves. SdFFF has excellent resolution but can only process small quantities (<1 mg) of sample in a single run1, 3, 4, 7, 9. The mechanism for particle separation involves only physical interactions4. The sample is introduced into the channel through a septum or injection valve, and then the flow is turned off. A centrifugal field is then applied at right angles to the flat face of the ribbon-like channel. This flat channel sits within a centrifuge basket and the centrifugal field drives the particles towards the accumulation wall. There they form equilibrium clouds whose average thickness or elevation above the accumulation wall (l) depends on how strongly the particles interact with the field and also their diffusivity1, 3, 4, 9. When the carrier liquid flow is turned on at the end of the stop flow (relaxation) period, the run begins. The carrier flow in the thin flat channel is laminar with the linear fluid velocity being zero at the channel walls and increasing with distance away from each wall, thus approaching a maximum at the centre of the channel. The particles with a larger effective mass will have more compressed sample clouds (i.e. a smaller l) and will consequently be swept down the channel by the flow at a lower average velocity than the smaller particles. In this normal mode of SdFFF the smallest particles will elute first7. The retention ratio R, for a constant field normal mode FFF run is obtained from the measured elution volume Vr and channel void volume V0 according to the expression: where , with l being the cloud thickness and w being the channel thickness. The equivalent spherical particle diameter, d, can be calculated from l provided the density difference between the particle and the carrier liquid Dr is known.7 The expression used is: where k is the Boltzmann constant, T the absolute temperature, w the centrifuge speed (radians s-1) and r the centrifuge radius. For samples which contain a broad size distribution the field decay program strategy of Williams and Giddings can be used5, 6 This program uses an initial constant speed wo for a period t1 , after which the centrifuge speed decays according to the power equation: where ta is a constant that controls the rate of decay and t is the run time. This enables the smaller particles to be adequately resolved from the void peak utilizing the higher field strength period while avoiding excessively long retention times for the larger particles. The run can be optimised to achieve a desired level of resolving power across the size range if suitable values of wo, t1, ta and flow rate are used7. Applications of SdFFF:
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