Metrohm 840 PC Control 5.0 / Touch Control - Manual (Page 330)

6.5 Liquid handling – schematic process
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PC Control / Touch Control
The titration cell can then be sucked off, rinsed and prepared with fresh
solvent.
Pipetting
The pipetting procedure takes place in five steps:
• Eject cylinder contents (see above) and form separating bubble
• Move to sample
• Aspirate / Measure out sample
• Move to target
• Eject sample
These five Steps can be combined to a subsequence defined as Sam-
ple sequence (see Section 4.9.6).
Forming a separating bubble
In order to prevent the carrier liquid and sample solution from mixing, a
separating bubble must be formed that occupies a length of at least 5
mm in the pipetting tubing. However, the separating bubble must also
be measured out with sufficient accuracy. This is why a dosing unit with
max. 20 mL cylinder capacity should be used for pipetting.
Command sequence:
LIFT move to shift position
LQH aspirate air
Move to sample
The (dissolved or liquid) samples can be contained in open beakers on
the rack or in sealed vials. In the latter case instead of the pipetting tub-
ing a suitable injection needle should be connected to Dosino Port 1
with a normal piece of tubing (∅ 2 mm).
Command sequence:
MOVE move to sample
LIFT move to work position
Aspirate sample
Aspiration of the sample should be carried out at a reduced filling rate
(< 10 mL/min). The appropriate setting can be made in the liquid han-
dling command under Rate.
With non-aqueous samples in particular an additional small air bubble
should be drawn into the pipetting tip in order to prevent the sample so-
lution from dripping.
Command sequence:
LQH aspirate sample
WAIT pause
LIFT move to shift position
LQH aspirate air
Move to target
Before the pipetting tubing containing the measured-out sample is
moved, the corresponding Dosino port must be closed. Ideally a
change should be made to filling Port 2 as this certainly contains no air.