Dialysis is a technique used to separate molecules in solution based on their molecular weight. It involves placing a sample inside a semi-permeable membrane bag and submerging it in a larger volume of buffer solution. Only small molecules can diffuse through the membrane, leaving larger macromolecules concentrated inside the bag. Dialysis is useful for desalting samples, buffer exchange, and purification of biomolecules, though it is a slow process. Variations like pressure dialysis and ultrafiltration allow for faster concentration of samples.

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DIALYSIS AND
ULTRAFILTRATION

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3.  Dialysis is an
operation to separate
dissolved molecules
based on molecular
weight.
◦ In practice, a biological
sample is placed inside
a tube of semi
permeable membrane,
and placed inside a
much bigger container.
Dialysis
bag
Concentrated
solution
Buffer
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4. 1. Only small molecules diffuse through the collodion
membrane.
2. At equilibrium, the concentration of small molecules is
the same inside and outside the membrane.
3. Macromolecules remain in the bag.
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5.  The only two variables in this method are:
1. The type of membrane (most common are
cellophane & cellulose)
2. The size of pores or the molecular weight cut
off.
Only molecules or ions smaller than MWCO
will move out of the dialysis bag.
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6. Advantage of dialysis
1. Dialysis is still in use today for it is very simple and is still
the only way to deal with large-volume samples.
2. characterization of a candidate drug in serum binding
assays or detailed study of antigen-antibody interactions
3. proves to be the most accurate method available.
4. inexpensive and easy to perform
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7. Disadvantage of dialysis
 Slow process several hours for completion, and thus, has
been replaced by gel filtration for most applications.
Other forms of dialysis includes flow-dialysis and
pressure-dialysis
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10mm = 6.4mm Dia; 0.32 ml/cm Vol/L
16mm = 10mm Dia; 0.79 ml/cm Vol/L
24mm = 16mm Dia; 1.8 ml/cm Vol/L
31mm = 20mm Dia; 3.1 ml/cm

9. 1. Removal of salts and low molecular weight compounds
2. Buffer exchange
3. Concentration of macromolecules
4. Purification of biotechnological products
5. Medical applications: kidney dialysis and Haemodialysis
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10. Dialysis tubing
Bed of powdered
polyethylene glycol
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11. concentrate the material inside the dialysis tubing.
The filled bag is packed in a dry, water-soluble polymer (which
can't enter the membrane) such as polyethylene glycol.
Water then leaves the bag to equilibrate which the dry external
phase.
polyethylene glycol and macromolecules can’t bath through the
membrane solution is concentrated
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12. We must be careful when using reverse dialysis, this is
because:
1. Equilibrium is never reached.
2. Water and salts are continually removed until the sample
is totally dry.
3. Most macromolecules become irreversibly bound to the
dialysis tubing and hence, for all practical purposes they
are lost.
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13.  An important modification of dialysis
tubing is the Diaflo or Pellicon
membrane
 Pressure dialysis is a common
technique for concentrating
samples.
 Other applications of pressure
dialysis include: desalting,
buffer exchange, and
purification of macromolecules.
Air
Sample
Support Media
outlet
Pressure
Membrane
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14.  The basic design is ultra filtration cell.
 There are a wide variety of filters to choose from
(materials and cut off limits).
 The applied pressure can be gas (N2) pressure,
centrifugation, or mechanical forces.
 They have very thin polymer membranes. (0.1-1.0
m).
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16.  Their pore size range from 2A-100A
 The flow rate through these membranes is
very low, so they are operated under
pressure.
 Either small or large molecules can be
purified in this way
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17. It is an example for flow
dialysis, tubes containing
polypropylene filter,
◦ Tubes comes in variety of sizes
suitable for samples
◦ (Tubes+ samples) are centrifuged
to concentrate the samples.
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18. ◦ Simple, easy, and rapid
◦ No stirring or foaming by N2
◦ High quality materials to minimize non-specific
binding
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19. ◦ Concentrating and desalting of biological
samples, especially small-volume
samples
◦ Buffer exchange
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20. Semi permeable glass fibers are valuable devices
for both :
 Dialysis
 And concentration.
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21.  They are fibers whose glass walls contain pores of
controlled size
 Molecules smaller than the pores pass freely
through the wall of the fiber .
 These fibers are usually used in bundles, thus
providing a very large surface area.
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23.  A solvent flows through the fibers, a small
molecules enter the fibers
 Thus reducing the concentration of small
molecules in the sample (purification)
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24.  Vacuum is applied to the filter bundle and the
solvent and small molecules enter the fibers
 Thus, concentrating any macromolecules in the
sample
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