Do I expel the last drop when I use an eppendorf pippet?

[moonchips]

Thanks a lot

[juddc]

That depends upon how you pipette.

If you only pull from the first stop, as I believe most do, then yes. I would also recomment pre-rinsing your pipette tip, too.

If you pull from the second (full) stop, then push to the first stop, then no.

If you have any doubts as to which is more accurate or precise, check your pipetting on an analytical balance.

[HW Mueller]

We did extensive tests with this type of pipet. First off, I recommend to get rid of all air piston pipettes and get positive displacement types instead.
The reason we did this testing is that there were person specific results in analyses in this lab. It turned out to be due to the pipetting technique of each person. This was eliminated by having everybody slide the pipet tip along the vessel wall (above the solution level, and before lifting the pipette out) from which the sample was taken and then touch the receiving device with the pipette tip during transfer.
The pipettes I have used eject the pipet tip on the second "stop". The instructions that come with pipettes elucidate the pipetting mechanics.

[juddc]

I find that as long as your technique is consistent and the pipettes are checked and calibrated routinely, there is no issue with using eppendorf type pipettes. I also write my methods in a way that pipetting precision counts more than absolute accuracy - e.g. sample and standard stocks both are pipetted similarly.

I suppose for larger labs or those with a significant tunrover of staff this is a greater concern, however there are only 2 people in my lab, I trained the other one, and our data are indistinguishable.

With that said, I'd be interested in seeing some numbers. Could you post some to illustrate?

[HW Mueller]

We did that a long time ago, I only remember that the interperson variations came uncomfortably close to the German Ringversuch range (+ or - 10% SD). These were 10µL pipets, so if one person has a drop still hanging there, another does not, one can easily have a 20% diff.

I think this was discussed before: The reason for shunning the air piston pipets is that the hand temperature can cause an expansion of the air above the liquid, thus part expulsion. With water this is not much of a problem as a possibly emerging drop will fall off less easily as with organic liquids.

[juddc]

I see where you're coming from now.

I don't make a practice of pipetting anything less than about 100-200ul for LC analysis using an eppendorf type pipettor. In my experience they're sufficiently accurate from about 100-5000ul and quite precise for routine quantitative LC work when properly used. Furthermore, I don't pipette highly volatile materials that way. Solvents are usually, water, methanol, acetonitrile, or some mixture thereof and it's a simple matter to determine whether there's a temperature related volatility issue with anything else at those volumes.

[HW Mueller]

Not the volatility but the expansion of the air between the measuring piston and the tip (liquid) is the problem here (also with water, but here the pushed out liquid doesn´t drop off as easily....).
Certainly I agree that one can muster an adequate accuracy if one invests a good amount of discipline. I am just not willing to invest this discipline any longer with positive displacement pipets being available.

If one looses liquid on transfer the question of the "last drop" becomes mute.

[JGK]

http://www.biosciencetechnology.com/Sho ... monCount=0

Has some useful hints on technique

[mbicking]

An excellent reference that every pippettor user should read. There are 9 - 10 separate technique issues that affect accuracy and precision, but most users are only aware of a few. Some of these issues can produce errors of 30%!

A few years ago I checked the accuracy specs for positive displacement vs. air displacement, and was surprised to see that they were not different. So, the manufacturers do not seem to recognize a difference. I think either will work well with properly trained staff, but I guess that's always the case.

Also, pipettor specs are not as good as an old fashioned Class A glass pipet. So if it is accuracy you want, the old method is best; not faster though.

[HW Mueller]

I never had solutions pushed out of the pipet tip by a warm hand in positive displacement pipets. Certainly, under ideal testing conditions (weighing of water pipetting by an experienced person) will give similar results to that of a positive displacement pipet.
To "old fashioned" glass pipets: I am simply not willing any more to invest the time and discipline to handle a 1µL, 2µL, 5µL, or 10µL glass pipet (capillary), with its interchangeable rubber bulp, to pipet 3µL, or 7, or . . . . (To be honest, we must have been clumpsy as ~50% off was not unusual).