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206 Modern Analytical Chemistry
Figure 7.11
Illustration of a dialysis membrane in action.
In (a) the sample solution is placed in the
dialysis tube and submerged in the solvent.
(b) Smaller particles pass through the
membrane, but larger particles remain
within the dialysis tube. (a) (b)
dialysis Another example of a separation technique based on size is dialysis, in which a
A method of separation that uses a semi- semipermeable membrane is used to separate the analyte and interferent. Dialysis
permeable membrane.
membranes are usually constructed from cellulose, with pore sizes of 1–5 nm. The
sample is placed inside a bag or tube constructed from the membrane. The dialysis
membrane and sample are then placed in a container filled with a solution whose
composition differs from that of the sample. If the concentration of a particular
species is not the same on the two sides of the membrane, the resulting concentra-
tion gradient provides a driving force for its diffusion across the membrane. Al-
though small particles may freely pass through the membrane, larger particles are
unable to pass (Figure 7.11). Dialysis is frequently used to purify proteins, hormones,
and enzymes. During kidney dialysis, metabolic waste products, such as urea, uric
acid, and creatinine, are removed from blood by passing it over a dialysis membrane.
size-exclusion chromatography Size-exclusion chromatography, which also is called gel permeation or molecular-
A separation method in which a mixture exclusion chromatography, is a third example of a separation technique based on
passes through a bed of porous particles, size. In this technique a column is packed with small, approximately 10-mm, porous
with smaller particles taking longer to
pass through the bed due to their ability particles of cross-linked dextrin or polyacrylamide. The pore size of the particles is
to move into the porous structure. controlled by the degree of cross-linking, with greater cross-linking resulting in
smaller pore sizes. The sample to be separated is placed into a stream of solvent that
is pumped through the column at a fixed flow rate. Particles too large to enter the
pores are not retained and pass through the column at the same rate as the solvent.
Those particles capable of entering into the pore structure take longer to pass
through the column. Smaller particles, which penetrate more deeply into the pore
structure, take the longest time to pass through the column. Size-exclusion chro-
matography is widely used in the analysis of polymers and in biochemistry, where it
is used for the separation of proteins.
7 F.2 Separations Based on Mass or Density
If there is a difference in the mass or density of the analyte and interferent, then a
separation using centrifugation may be possible. The sample, as a suspension, is
placed in a centrifuge tube and spun at a high angular velocity (high numbers of
revolutions per minute, rpm). Particles experiencing a greater centrifugal force have
faster sedimentation rates and are preferentially pulled toward the bottom of the