Pipet accuracy

klier at iscsvax.uni.edu klier at iscsvax.uni.edu
Mon Jul 12 13:33:31 EST 1993

There are a couple of basic techniques to get accurate and repeatable
air displacement pipetter dispensing...

I was taught:
  1) keep the pipetter near vertical in use (no more than 15-20o off vertical)
  2) release the plunger slowly
  3) wait a second so that the full volume is drawn up
  4) place the tip against the side wall of the receiving tube, and depress
     plunger slowly.  Wait at least a second for volumes under 0.5 ml, 2
     seconds for 0.5-1 ml, 3-4 seconds for 2-5 ml.  THEN depress plunger
     to second stop, and withdraw with plunger depressed and tip dragging
     along tube wall.  Then allow the plunger to fully return to the up

If you're pipetting viscous solutions, allow more time to dispense the liquid.

If reproducibility is a major concern, pre-rinse the tip with the liquid
by taking liquid up, then releasing it before taking your measured sample.
This is important with materials like blood serum, protein solutions, and
some of the organic solvents.

You can also do "reverse mode pipetting" to minimize film retention on the
  1) depress plunger to second stop
  2) withdraw sample by releasing plunger slowly
  3) wipe excess liquid off the tip
  4) dispense sample by touching tip to tube, depressing to first stop, and
     waiting a couple of seconds.
  5) remove the tip from the tube
  6) blow out excess in tip.

The relative accuracy and reproducibility of most pipetters are worst at the
lower end of their calibrations, and better at larger volumes.  For instance,
a P-2 Pipetman (0.1-2 ul range) has about a +/- 12% accuracy at 0.2ul, and
a precision* of 6%.  At 2ul, it has a +/- 1.5% accuracy, and a 0.7% precision.
      *precision = repeatability

A P-200 (50-200ul) has a +/-1% accuracy at 50 ul, precision of 0.4%; while at
200, it has a 0.8% accuracy, 0.15% precision.

Oh yes, please don't lay a pipetter flat that still has liquid in its tip...
it's a good way to get liquid inside the body.  If I had $0.05 for each time
I've warned a student about that, I'd be rich, rich, rich!  ;-)

Kay Klier    Biology Dept  UNI

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