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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 (e.g. sulfuric
or perchloric acid) are used as eluents. The water molecules
of the mobile phase form a hydration sphere around the
sulfonic 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, i.e. 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
(e.g. 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.
Read more about the implementation and interesting applications
in the article "The use of inverse suppression for
the determination of organic acids".
Get more information: Metrohm
Information, Issue 3/2002 (Article in English and German available
for download)
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