Pipetting
A shockingly large amount of modern science involves moving small quantities of liquids from one container to another container. We do this by using precise suction/pressure systems, pipettes. A common discipline training for young scientists is to pipette a precise amount of liquid as measured on a scale in a reproducible manner. For instance moving 0.5 \(\mu L\) in a reproducible manner as verified on a scale can be quite challenging.
I’m not a huge fan of that, young scientists will learn the skills of discipline and precision over the course of their research as is demanded by their research. But this is field specific, and as you drift closer to biochemistry, formal training of the discipline becomes more important. For those that are not getting trained in this manner, here are some tips about pipetting that I’ve learned over the years.
Ensuring proper liquid pull, and liquid deposit
- Pull liquid from near the top of the liquid, but below the surface.
- The pipette is calibrated to create a certain pressure vaccum, which should pull up a certain amount of liquid. If the tip is far underneath the liquid, the pressure of the liquid above the tip changes the force balance equation. Similarly if it is right on the surface, the surface tension will change the force balance equation
- Too close to the surface, and you also risk pulling up air. Too far down, and you are coating the outside of the pipette with liquid.
- Watch the liquid enter and leave the pipette.
- Frequently a small air bubble, or a quick pull, will simply prevent liquid from being aspirated correctly. This is extremly common in the 0.1 to 0.5 \(\mu L\) range. If you are working in this range, learn what 0.5 and 1 \(\mu L\) of liquid looks like by eye in your particular pipette tips, so you can tell when the aspiration did not work. You can practice this with food coloring if you are having trouble seeing small volumes.
- When working with unusually viscous or non-viscous fluids consider the time scales
- When pipetting up glycerol for instance, wait a long time before removing the pipette
- Consider “reverse” pipetting. Taking up a large amount of liquid, then dispensing just the right amount. To perform a reverse pipette, press the plunger down all the way to the second stop and aspirate the liquid slowly. Yhen dispense by pressing only to the first stop.
Storage of Pipettes and Pipette Tips
Make sure to leave pipette tip boxes closed. I cannot emphasize this enough. Dust falling into your pipette tips is a major problem!
You can autoclave pipette tips to steralize them, but you cannot reuse pipette tips. They will have solid contamination, like salt residue. This was a big conversation that we had with many people on the internet from a LabOnTheCheap post!
Controversial, but it does not matter if you lay the pipettes down on their sides when they are not attached to a pipette tip.
- The pipette works via a spring system that isn’t modified over time by gravity. This was a big ongoing conversation we had while I was a graduate student.
- When they are attached to a pipette tip, if you pull something up too quickly, or lay the pipette on their side, the pipette can get liquid into the base of the pipette, which is a disaster.