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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
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