The determination of moderately weak and weak acids by ion exclusion chromatography and conductivity detection is a well-known method whose potential is not fully used
The determination of moderately weak and weak acids by ion exclusion chromatography and conductivity detection is a well-known method whose potential is, however, still not being used to the full extent.
The separation of the (usually) organic acids takes place on a strongly acidic cation exchanger; dilute strong acids (eg sulphuric or perchloric acid) are used as eluents.
The water molecules of the mobile phase form a hydration sphere around the sulphonic acid groups of the stationary phase. This results in the formation of a partially negatively charged layer, the so-called Donnan membrane, which is impermeable to anions.
Under the given conditions the organic acids to be determined are predominantly present in undissociated form.
They can therefore diffuse through the Donnan membrane and enter into interactions with the stationary phase.
This results in the retardation and separation of the organic acids according to their size and dissociation constants.
With direct conductivity detection it is essentially the deprotonated fraction of the organic acids that is measured, ie, the acid anions.
These are measured against the background conductivity of the eluent, which is relatively high.
As already mentioned, the acids to be determined are only weakly dissociated owing to their low acidic strength and the low pH value of the eluent.
This means that relatively small peaks are obtained.
The solution: chemical suppression.
To put it simply, chemical suppression is a method of reducing the background conductivity of the eluent and converting the components to be determined into a more conductive form.
For ion exclusion chromatography this means the conductivity of the eluent is reduced by using ion exchange to replace the highly conductive protons by cations with a relatively low conductivity (eg Li+).
With organic acids there are two opposing effects.
On the one hand, as a result of the cation exchange described above the pH value of the eluent is increased.
This boosts the dissociation of the organic acids and thus increases the concentration of the ions.
On the other hand, the highly conductive protons are again replaced by the respective cation.
