Quadra 4 Operator Manual
Tomtec
183
Each Quadra 4 instrument will have certain mechanical dimensions. To enable the
use of the common software dimensions, they must be related to the specific
mechanical dimensions, the stage must move on each instrument. This is
accomplished with the "Z" offset factor.
The caveat is if the Quadra 4 is used for aliquoting different volumes from one
aspirate. The calibration curve does not handle this situation as well. Assume the
Quadra 4 aspirated 100µL and then dispensed it in 10µL aliquots. For the 100µL, the
calibration curve dictated 16.532 steps, per microliter. The stepper motor moved the
piston 1653 steps, which aspirated 100.3µL. To dispense 10uL, the calibration curve
shows 18.678 steps, per microliter or 187 steps for each 100µL aliquot. After 8
dispenses, the stepper motor had moved 1496, leaving only 157 steps left, which is
insufficient for the last two aliquots.
This simply confirms the standard practice when aliquoting. The last 1 or 2 volume
should be dispensed back to the source. Otherwise, cumulative error will appear in
the last volume. The Quadra 4 calibration software, recognizes this condition, by
comparing the asked for aspirated volume to the dispensed volume to prevent an out
of range error from occurring.
In summary, the calibration curve is most efficient at improving accuracy of pipetting,
when aspirating a single volume from the source and dispensing that volume to the
destination. The Quadra's ability to make a timed dispense, which
blows pressurized air through the tip after the dispense, negates the necessity of an
initial air gap to serve that purpose. The timed dispense is far more effective at
clearing the tip of capillary residual than the initial blow out air gap. It simply has a
higher velocity. The timed dispense function also enables non-contact dispensing.
The example of 10 dispenses from 1 aspirate, is the extreme condition chosen to
demonstrate a point. The Quadra 4 can be used for aliquoting. The user simply needs
to be aware of potential loss of accuracy, when various pipetting methods are used.
The software database defines the distance from the top of the stacker nest, resting
on the shuttle, to the tips as 1455 steps. Using the utility function, the stage is indexed
up to the point the tips just touch the top of the bare stacker nest. Assume this
reading is 1465 steps. The "Z" offset (under factory settings) is then reset from its
existing value to -20 steps or less. When retried, the stage height should now read
1455. This change in the "Z" offset is used to align the mechanical "Zero" of the
Quadra 4 with the "Zero" of the command control software.
Making a new entry in the database is intuitive. Open to the Quadra 4 and software
initialize the instrument. Click on the database on the tool bar, follow the description.
Prior to entering the plate dimensional data, you must specify the nest that will be
used to carry the plate or reservoir on the shuttle.
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