Introduction

SPLITT fractionation (SF) is a relatively new family of separation techniques primarily - but not exclusively - applicable to macromolecules and particles. The SF techniques utilize a thin ribbon-shaped flow cell and achieve fractionation by differential transport across the thin (transverse) axis of the cell. Since the cell is only a few hundred micrometres thick, the separation path - which may be less than the nominal channel thickness - is extremely short and the separative transport is correspondingly rapid. Separation is typically accomplished in only a few minutes. This is a particularly valuable feature for example for fragile species that must be fractionated rapidly to avoid degradation (e.g. biological samples). The fluid that carries dissolved or suspended components through the SPLITT cell is divided at both ends by thin flow splitter elements (see Figure 1). The inlet splitter element allows for the smooth merging of two incoming laminae, one carrying the suspended feed material and the other generally containing only the pure car rier liquid. Differential transport of feed components between the two laminae (after they are brought into contact) then occurs as a result of a transverse driving force or gradient. At the outlet end, the flowing liquid volume is divided at a predetermined position by a second splitter element, thus producing two sub-streams that are enriched or depleted in the desired components as a result of the differential transport.

Both preparative and analytical fractionation process can occur in the SPLITT cells, depending on the injection procedure. A continuous (CSF) process of feeding the cell is advantageous for preparative fractionation (gram, kilogram), offering rapid throughput, minimum holdup volumes, and a sharp separative cut off; examples of continuous fractionation can be found in the separation of mineralogical, industrial and food samples.

The analytical version of SPLITT fractionation (ASF) is often more practical to operate. The injection of discrete pulses can be made, if desired, to follow one another in close sequence. In this use, separation is performed for the measurement of quantitative properties of sample components and the fractiona-tion is termed 'analytical SPLITT fractionation';

Figure 1 Side view of a generic SPLITT cell.

examples of quantitative determinations include diffusion coefficients of proteins, settling velocity and the relative content of oversized particles above a cutoff diameter in a particulate material. Moreover because of its ease of theoretical interpretation, the SPLITT cell can be used for the rapid measurement of transport-related properties such as particle size and particle size distribution. The throughput of SF is proportional to many variables such as the sample concentration in the feed stream, the volumetric flow rate of the sample stream, the applied field strength and the SPLITT cell area. For preparative applications there is obviously a trade-off between the resolution and throughput in the operation of SF: maximizing the throughput and maintaining an acceptable resolution is the common choice.

The effectiveness of the SPLITT process can be modulated by simply varying the flow rates of the inlet and outlet substreams, which determine the position of the inlet splitting plane (ISP) and the outlet splitting plane (OSP) and controlling the thickness of the transport region; sometimes, unwanted displacements of a few tens of micrometres may be difficult to discern but they are sufficient to interfere with effective separation. In some cases, the efficiency of the SPLITT process can be controlled by altering the strength of the field or gradient driving the separative transport.

The efficacy of SF separation depends instead, on the hydrodynamic integrity of the SPLITT cell. Effective separation is based in fact, on two central requirements:

• there must be no hydrodynamic mixing across stream planes; and

• the splitters must be absolutely perfectly aligned so that they are capable of splitting the film of flowing liquid evenly along the stream plane.

Success in fulfilling these requirements is not easy to judge because of the thinness of the cell and the shortness of the transport path.

The selectivity of SPLITT fractionation comes from the applied force. The principal transverse driving forces used include gravity, centrifugation, diffusion, electrical potential gradients, magnetic gradients and hydrodynamic lift forces. The geometry of all the different cells is similar to that depicted in Figure 1, except for the curvature characteristic of the centrifugal SPLITT cell.

The simplicity of the SPLITT cell leads to rather rigorous theoretical guidelines on the conditions necessary to achieve a given level of separation.

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