Ion Chromatography - The BasicTheory of Ion Exchange
The BasicTheory of Ion Exchange
An ion exchange stationary phase may contain negatively charged groups attached to the surface (e.g. -SO3- ) and, thus, acts as a cation exchanger or it may contain positively charged groups attached to the surface (e.g. -N(CH)3+) and, thus, can act as an anion exchanger. To ensure electrical neutrality an equivalent number of oppositely charged counter ions must also be available. The actual ion exchange process requires a competition between the counter ions and the 'sample' ions for the charged sites on the stationary phase. Thus, when the sample ion interacts with the stationary phase it displaces a like counter ion from the surface into the mobile phase and is held on the stationary phase surface resulting in chromatographic retention. If both the solute (sample) ions and the counter ions are monovalent then the equilibrium involved can be represented by the following equations.
B (S) +Y(S)_ + X_ B (S) +X(S)_ + Y_
A (S) _ M(S) + + NH+ A(S) _ NH(S) + + M+
Where the subscript (S) represents the stationary phase and KXY and KNNM are the equilibrium constants for the ion exchange process. X- and NH+ represent the solute ions and Y- and M+ the counter ions respectively whereas B+ and A- are the interacting groups on the stationary phase matrix.
The magnitude of the constants KXY and KNHM depends on a number of variables but other chemical equilibria also play a part and can be employed to control the distribution coefficients and, thus, the chromatographic retention. It is clear that solutes can only be retained by ionic interaction if the molecules are dissacociated. Thus, for acids and bases the charge will be related to the pKa of the solute and, consequently, the pH of the mobile phase.
Thus, for acids XH H+ + X_
and for bases -NOH OH_ + -N+
Now it can be assumed for simple ions (excluding the macromolecules involved in biochemistry) that only the charged form of the solute exists on the stationary phase. (Note, larger molecules can be held on the stationary phase by polar and dispersive interactions, as already discussed, and consequently, they can be held on the stationary phase in an unionized form). Under those circumstances where simple ions are being eluted an expression for the distribution coefficient (Kx) can be uniquely defined as the ratio of the concentration of the solute on the stationary phase to the sum of the associated and unassociated solute in the mobile phas of the eluting ion. An equation for (Kx) can be developed that takes the following form.
It is clear that, to the first order of magnitude, the distribution coefficient as defined by (Kx) is proportional to the equilibrium coefficient (KXY, KNM), the availability of the interactive groups on the exchange matrix as actually measured by (B+(S)Y-(S), A-(S)M+(S)) and the concentration of the counter ion by (Y-(S), M+(S)). (Kx) will also depend on the (pKa) value of the acid or base and on the (pH) of the mobile phase. The (pH) and the concentration of the counter ion provide a practical means of adjusting the retention and selectivity of the chromatographic system. In addition to the (pH) of the mobile phase and counter ion concentration, other techniques can be used to change solute retention and selectivity such as the introduction of complexing reagents in the mobile phase to modify the elution behavior of metal ions.