Gradient Master - Wolf Laboratories

to establish the sharpness of your bands with gentle acceleration and braking and then seeing if
faster starts and stops hurt the resolution.
Section 4. Fractionating gradients
4.0 The essential problem: Laminar capillary flow; getting a flat band into a thin column.
There is a round hole/square peg problem that has plagued gradient work from its
inception: the bands are thin horizontal discs and the devices used to retrieve them are invariably
thin tubes at some point. Thus the problem is one of conversion between the two geometries. In
addition to this, the tubing exhibits laminar capillary flow, where the central lamina has the highest
velocity and the zero boundary adhered to the wall is stationary. This makes the conversion even
more difficult than it already is and is a little-appreciated part of gradient fractionation because the
high velocity central lamina tends to pull gradient solution from directly in front of itself, reaching far
beyond the zone that is supposed to be entering the tubing system. This degrades resolution because
different layers are being sampled at the same time.
4.1 Needles; straight and side hole:
As shown by the figures, these are the poorest ways to fractionate a gradient. The central
lamina is offered unfettered access to layers far beyond the one next to the tip of the needle and the
resultant loss of resolution is severe.
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