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Asymmetric feild-flow-fractionation
The asymmetric field-flow-fractionation (AFFF) is a method for separation of proteins, water soluble polymers and colloids without a stationary phase. The separation is achieved by applying an external field, which acts perpendicular to an axial flow. Dependent on the nature of the field, one can distinguish between thermal, sedimentation, electric or flow field-flow-fractionation.
The functionality of the AFFF is explained in the following pictures:
| The bimodal sample (sketched by the red and blue spheres) is injected in a thin channel with a height h (100-300 mm), due to a spacer. The lower wall consists of a water permeable ultrafiltration membrane, the upper wall of Perspex. The external force pushes the colloids onto the membrane. Diffusion processes act against the external field so the colloids move towards the middle of the channel. The smaller colloids can move faster than the larger ones so the larger colloids will remain closer to the lower wall than the smaller ones (relaxation). In the elution mode an axial flow is switched on in addition to the external force. The axial flow effects a parabolic flow profile with the highest velocity in the middle of the channel. The smaller particles, which are nearer at the middle of channel will be eluted faster as the larger particles at the bottom of the channel. Due to this mechanism we obtain a separation of the sample. With a detection system (e.g. UV detector) we can measure the elution times and further we can calculate the radii of the particles. |
The asymmetric geometry is shown in the following picture. The flow velocities are labeled by vin (entry of eluent), vinj (entry of sample), voutc (cross flow). vout and vc are aligned with a valve and cannot be diversify independently (a higher cross flow results in a lower vout). Due to the asymmetric geometry of the channel we have a constant axial flow velocity along the channel and we obtain shorter elution times.
By the separate inlet for the carrier liquid and sample it is possible to focus the sample at the sample inlet. In the focusing mode the carrier liquid flows into the channel through the channel inlet and outlet.
This picture shows the measurement of a trimodale sample and demonstrates the power of separation. The sample consists of colloids with radii R=80, 100 und 130 nm, and the elution time was 20 minutes. (outcoming of eluent) und v