Metrohm 761 Compact IC - Manual (Page 181)

5.1 Practical notes on ion chromatography
761 Compact IC
171
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 alkaline eluents and eluents with
low buffering capacity one should preferably use a CO
2
absorber (see
section 2.6.3).
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).
5.1.4 Peristaltic pump
The pump tubings used by the peristaltic pump are consumable mate-
rial with a limited lifetime and should be exchanged at regular intervals
(approx. every 4 weeks under continuous use; see section 5.2.10).
The working life of pump tubing depends to a considerable extent on
the contact pressure. This is why the contact pressure must be correctly
set as described in section 2.9.2. If the pump is to remain switched off
for a lengthy period of time the tubing cartridges 48 should be raised
completely by loosening the snap-action lever 51 on the right-hand side
(the set contact pressure remains unchanged).