Ammonium sulfate precipitation

A generally employed first separation step is ammonium sulfate precipitation. This method exploits the fact in which the solubility of most proteins is lowered at high salt concentrations.  As the salt concentration is rise a point is reached where the protein comes out of precipitates and solution.  The concentration of salt needs for this salting-out effect varies from protein to protein, and therefore this process can be used to fractionate a mixture of proteins. For instance, 0.8 M ammonium sulfate precipitates out the clotting protein fibrinogen from blood serum, whereas 2.4 M ammonium sulfate is needs to precipitate albumin. Moreover, several other proteins will also precipitate out at these concentrations of ammonium sulfate. Thus this is a relatively crude separation method, thus it often gives a convenient concentration step. Salting  out is also many times  used  at  later  stages  in  a  purification  process  to  concentrate  a dilute solution of the protein since the protein precipitates and can then be redissolved in a smaller volume of buffer.