Metrohm 732 IC Detector - Manual (Page 142)

5.1 Practical notes on ion chromatography
732 IC Detector / 733 IC Separation Center
135
5.1.3 Eluents
Treatment
For the preparation of the eluents only chemicals of a purity degree of
at least "p.a." should be used. For diluting please use only high purity
water.
Fresh eluents should always be microfiltered (0.45
µ m filter) and de-
gassed (with N
2
, He or vacuum). For high sensitive measurements, the
eluent should be continuously stirred with a magnetic stirrer, particu-
larly when the recycling procedure is employed or when alkaline eluents
are used. For alkaline eluents and eluents with low buffering capacity
one should preferably use a CO
2
absorber (e.g. the absorber supplied
with the optional 6.5324.000 Bottle rack).
The supply vessel containing the eluent must be closed as tightly as
possible to avoid excessive evaporation. This is primarily important with
eluents containing organic solvents (e.g. acetone), the evaporation of
which can lead to drifts in the long term. If work is performed in a very
sensitive range, even if one drop of condensate falls back in the eluent
this can cause a noticeable change in the background conductivity.
Influence of various parameters on anion columns
• Concentration: An increase in the concentration usually leads
to shorter retention times and quicker
separation, but also to a higher background
conductivity.
• pH: pH alterations lead to shifts in the dissociation
equilibrium and thus to changes in the
retention times.
• Organic modifiers: Addition of an organic solvent (e.g. methanol,
acetone, acetonitrile) to aqueous eluents
generally accelerates lipophilic ions.
Eluent change
When the eluent is changed, it must be ensured that no precipitates
can be formed. Solutions used in direct succession must therefore be
miscible. If the system has to be rinsed with an organic solution, several
solvents with increasing or decreasing lipophilic character may possibly
have to be used (e.g. water ↔ acetone ↔ chloroform).