Metrohm 840 PC Control 5.0 / Touch Control - Manual (Page 329)

6 Appendix
PC Control / Touch Control
317
6.5.3 Pipetting sequences
Principle
Measuring out the liquid to be pipetted takes place in the pipetting tub-
ing, which is filled with a carrier liquid (preferably water). In order to
avoid carryover or mixing an air bubble (separating bubble) must be in-
serted between the sample and the carrier liquid. The tubing tip should
be drawn out by hand to form a fine point.
A Dosino drive is used to transfer the liquids to the tubing. The pipetting
tubing is attached to Port 1 of the dosing unit. The dosing unit is
mounted on a storage bottle containing the carrier liquid. This can be
refilled via filling Port 2 of the dosing unit. The carrier liquid can also be
used for rinsing and diluting the sample.
Numerous pipetting sequences are possible. The following conditions
must always be observed:
• The sample must always be measured out in the pipetting tubing; it
must never enter the dosing unit cylinder.
• The sample should always be aspirated after the dosing cylinder
has been ejected.
• The difference in level between the surface of the sample liquid and
that of the carrier liquid in the dosing cylinder should be as small as
possible.
• Aqueous sample solutions can be pipetted without having to rinse
the tubing. Non-aqueous samples require the pipetting tubing to be
rinsed with carrier liquid (same solvent as the sample) to avoid
carryover.
Only the most necessary commands are mentioned in the following
command sequence. A complete command sequence is defined in the
method template "SP Pipetting Ext. Position" (see Section 3.15.2).
Preparing the dosing unit
The dosing unit must first be prepared, i.e. the filling tubing and pipet-
ting tubing must be rinsed and completely filled. The preparation step is
best carried out at an external position. The cylinder contents must then
be ejected. This command sequence is best combined to a subse-
quence. This preparation step is only necessary at the beginning of a
sample series. Therefore the subsequence is defined as Start se-
quence (see Section 4.9.6).
Command sequence:
MOVE move to external position
LIFT move to work position
PREP prepare dosing device
LQH eject cylinder
These steps should be carried out before every sample series involving
pipetting. Before each pipetting process the dosing unit cylinder con-
tents must be completely ejected. This is the only way to achieve re-
producible, accurate pipetting.