Quadra 4 Operator Manual
Tomtec
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Another point of consideration is, how many weighing's of each value is required for
the desired accuracy. This is a reflection of the repeatability of the pipetting system.
Tomtec's testing has been to measure 5 values at each point. From 5µL to 450µL, the
CV of 5 values has consistently been better than 0.5%. This implies fewer reading could
be used provided they are consistent without "flyers". The "flyers" are outlayers that
are due to other factors i.e., faulty weighing, non-wetting, etc.
On the 450µL backfill head, Tomtec has included a generic calibration curve. Generic
meaning it is an average calibration curve, applied to all 450µL backfill heads. This
generic curve will provide accuracy within 2%, for volumes from 450µL, down to 100µL.
From below 100µL to 25µL, the accuracy will be within 3%. From 25µL to10µL, the accu-
racy will be within 5%.
The user may create a specific calibration curve for the pipettor head in the Quadra 4.
This will improve the accuracy. With specific calibration, accuracy of less than 1% can
be obtained from 100µL to 450µL. Improvements can also be made in the lower
volume ranges.
To achieve pipetting accuracy within 2%, for volumes from 100µL to 450µL, Tomtec
recommends taking 3 readings at the following volumes: 100µL,150µL, 200µL, 250µL,
300µL, and 450µL. For accuracy within 3%, for volumes from 25µL to 100µL, readings
at the following volumes are recommended: 10µL, 15µL, 20µL, 25µL, 30µL, 40µL, 50µL,
70µL, 80µL, 90µL,100µL.
The mean value of the data at each calibration point is entered into the pipettor
calibration table. Tomtec has included an Excel program for calculating mean and CV
values, for the collected data. After entering the calibration data, the various points
should be rechecked for verification. 2 to 3 values at selected points are
recommended.
Since the Quadra pipettor head is calibrated empirically, it may be calibrated for any
pipettable liquid. A specific name is assigned to that calibration curve, allowing it to be
called on demand.
The distinct advantage of Tomtec's 450µL pipettor head is its uniquely wide volume
range from 450µL down to 10µL with disposable tips. This, combined with our tip blow
through feature, has made it the standard for BioAanalytical applications. 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.
Stepper motors are used for pipettor drives due to their precision and reliability. Within
its range a stepper motor will move the pipettor piston a precise distance for each
software pulse it receives. Calibration simply means equating how many software steps
are required to dispense a specific volume.
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 10µL
may require 18 to 19 steps per microliter. At 450µL, 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.
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