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Encyclopedia of Physical Science and Technology EN008B-382 June 30, 2001 18:58

692 Liquid Chromatography

FIGURE 20 Three-dimensional presentation of the geometries of cyclodextrin. [From Braithwaite, A., and Smith, F.
J. (1996). “Chromatographic Methods, 5th Ed.” Chapman & Hall, London.]

log k =−(y/x) log[E x ] + (log B)/x, fore, buffered solutions are almost always the major com-
ponent of a mobile phase for ion-exchange LC. For weak
where y = charge of the solute, x = charge of the eluent, acidic or basic solutes, the mobile phase pH controls their
and B = the product of the capacity of the packing and ionized state and ability to interact with the resin. The
the equilibrium constant for the ion-exchange process. capacity of weak ion-exchange resins is in addition in-
Although silica has been used, the most common ion- fluenced by pH. All other factors considered equal, the
exchange supports are PS–DVB resins because of their greater the capacity of the resin, the greater the ion reten-
stability at pH extremes. The non-cross-linked benzene tion. Finally, the pH as well as the buffer salt can contribute
rings are available for functionalization. Sulfonation of significantly to the overall ionic strength of the mobile
PS–DVB resin yields the strong cation exchanger, Res– phase. Ionic strength is calculated by taking one-half of the
+
SO X , while chloromethylation and subsequent ami- sum of the ion concentration times their charges squared.
−
3
nation forms the strong anion exchanger, Res–CH 2 –N + As the ionic strength increases, the amount of counter ion
−
(CH 3 ) 3 X . The capacity of these resins, the number of inthemobilephaseincreases,drivingtheequilibriumback
exchangeable groups per gram of resin, can range from to the left. This competition of the counter ions for the sta-
0.1 to 2 meq/g, depending on reaction conditions. Surface tionary ionic sites results in a reduced retention of the so-
agglomerization is a convenient method to prepare low lute ions.The lowerthe resin capacity,the smallertheionic
capacity ion exchange packings for ion chromatography. strength that is required to elute the solute ions from the
For example, sulfonated PS–DVB microspheres (5–40 column. The ionic strength is often intentionally increased
µm) are contacted with colloidal anion exchange particles graduallytoimprovetheseparationofweaklyandstrongly
˚
(100–1000 A) to electrostatically form a surface agglom- retained ions in a mixture (see Section IV, Fig. 26).
erated anion exchange resin. The ion-exchange capacity The nature of the ionic solutes often affects their ion
of the resin can be controlled by changing either the size of exchange retention. As expected, polyvalent ions are held
the microspheres or of the colloidal particles, as well as the more tightly than monovalent ions. Within a given charge
degree of functionalization of the latter particles. The cor- group, retention generally increases with the size of the
responding weak anion exchanger Res–NH (CH 3 ) 2 X − ion but decreases with the size of the hydrated radius. Sol-
+
and cation exchanger Res–COO X have also been de- vated ionic radii limit coulometric interactions between
−
+
veloped for use in the separation of labile molecules such ions and energy must be put into the system to strip the
as proteins. water away. The retention order for the alkali metals is
+
+
+
+
+
+
The mobile-phase factors of pH and ionic strength pri- Cs > Rb > K > NH > Na > H > Li . Because of
+
4
+
+
marily control the retention of ion exchange resins. There- its greater hydration, Li is retained less than H .