TomTec QUADRA 4 - Section X: Calibration of the Quadra 4 450µL; Calibration Curve Principles and Efficiency

Quadra 4 Operator Manual
Tomtec
190
Section X – Calibration of the Quadra 4
®
450µL
96 Position Pipetting Head
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 450uL pipettor head has wide volume range from 450uL down to 10uL with
disposable tips. This wide range for an air displacement pipettor presents specific
physical requirements that must be overcome. The calibration curve over this volume
range is not a linear relationship, due to the compressibility of the air column within the
disposable tip.
Due to the compressibility of air, the number of steps to pipette a microliter of volume, is
higher at the low volumes that it is at higher volumes. To aspirate or dispense 10uL,
may require 18 to 19 steps per microliter. At 450uL, it requires 15 to 16 steps per
microliter. If the Quadra 4
®
is asked to aspirate a volume and then dispense the same
volume, i.e. empty the tip, the calibration curve handles this well.
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 100uL and then dispensed it in 10uL aliquots. For the 100uL, the
calibration curve dictated 16.532 steps per microliter. The stepper motor moved the
piston 1653 steps, which aspirated 100.3uL. To dispense 10uL, the calibration curve
shows 18.678 steps per microliter or 187 steps for each 10uL 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 volumes
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.
A calibration curve for the Quadra 4
®
may be created for any pipetttable liquid. The
procedure is the same. Aspirate and dispense the desired volume with a setting of 16
steps per microliter. Gravimetrically measure the actual volume dispensed. Divide the
actual volume by the number of steps used to define the calibration point. The
calibration curve is not linear on an air displacement pipettor. Therefore it is necessary
to define a sufficient number of points on the curve to achieve the desired accuracy,
over the desired pipetting range of volumes.