Hoefer SG500 - Manual (Page 8)

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2. Generating convex and
concave exponential gradients
By holding the volume in the mixing chamber
constant (as opposed to declining for linear
gradients), the gradient generated will have an
exponential concentration curve. The mixing
volume in the SG500 unit can be held constant
by sealing the top of the mixing chamber with a
rubber stopper and tubing assembly (Fig 4). The
total volume of the gradient can be greater than
the maximum volume of the gradient former
because additional solution can be repeatedly
added to the reservoir chamber. Using Eq 1,
examples of the gradients generated by various
combinations of volumes and concentrations
listed in Table 1 are plotted in Fig 3. Concave
gradients are generated high concentration first,
and thus containers such as gel cassettes must
be filled from the top. Convex gradients must
be filled from the bottom. Most frequently used
is the concave pore gradient for increased reso-
lution in the high molecular weight region of
acrylamide gels.
Eq 1.
Table 1. Examples of concave and convex
exponential gradients
A B C D E
C
res
* 0 1 1 1 0.5
V
res
500 500 100 500 500
C
mix
1 0 0 0.5 1
V
mix
100 100 500 100 100
*Volumes are relative.
C: concentration, V: volume, res: reservoir chamber,
mix: mixing chamber