Quadra 4 Operator Manual
Tomtec
166
3.0 Quadra 4 Positive Displacement Pipettor Heads
Tomtec offers 4 positive displacement pipettor heads in addition to the air displacement
design. The 96 SV and 384 SV heads offer 60µL capacity. The 96 LV and 384 LV heads
offer 200µL capacity. These positive displacement pipettors use a fixed stainless steel
dispensing needle that is titanium nitrite coated and extends the full length of the tip bore.
This eliminates the dead air volume, common to an air displacement pipettor design.
Why is this important? On an air displacement pipettor, the dead air volume, at a
minimum, is equal to the tip volume. A 200µL tip volume has 200µL of air between the
orifice and the piston. The 200µL SV positive displacement design, has a 2µL air gap
between the end of the piston and the tip orifice.
Both are governed by the general gas law, which is PV=nRT. With air at the same
temperature, this becomes P
1
V
1
=P
2
V
2
, where 1 is at the start of an aspirate and 2 is after
the piston has moved. For illustration, assume your finger has blocked the orifice. Aspirate
200µL with both pistons. For the air displacement pipettor V
1
= 200µL the dead volume of
the tip and P
1
= 14.7psi atmospheric. V
2
becomes 400µL, thus P
2
becomes 7.35psi, and
which is the negative pressure created to aspirate liquid through the orifice.
Using the same scenario for the positive displacement design, VI = 2µL and Pi = 14.7.
However, V
2
now is 202µL. P
2
= P
1
V
1
/V
2
or 0.14psi, which is a much higher vacuum to
aspirate the liquid into the tip. In summary, the air displacement pipettor, moved the full
tip volume of 200µL to reduce the pressure by half. The positive displacement design only
moves 2µL to reduce the internal pressure by the same amount. This same logic, applies to
the dispense motion. Thus, the positive displacement design, more equally matches the
piston motion to the fluid motion in the tip. Piston motion is what the software logic controls.
Tomtec has developed an ultrasonic tip washing system for positive displacement
pipettor heads. It is also very effective with the polypropylene disposable tips. A peristaltic
pump provides fresh water, entering the bottom of the wash station and overflowing the
weir at the top. The water in the station is excited with an ultrasonic transducer, which
cleans by cavitation at the tip surface. The tips aspirate fresh water from the bottom portion
of the station, and then dispense it above the liquid level, allowing the effluent to wash over
the top and be carried to the drain.
In all of the testing by Tomtec and others, we have not been able to measure any carry-
over with Fluorescence, Colorimetric, and Scintillation detection. In all cases, the carry
over wells were equivalent to the blank wells. This means if there was
carry over, it was below the detection limits of the measuring instrument. The last test was
with a Micro Mass Spectrometer. There was no detectable carry-over. The mass spec
detection limits are very low.
On all of Tomtec positive displacement designs, the tip and seal unit is a removable
assembly. It may be removed from the Quadra if more rigorous cleaning is required. The
tip or seals may be replaced as required. This may be accomplished by the user or the tip
and seal assembly may be returned to Tomtec for reconditioning.
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