How much time do you spend thinking about the accuracy of your pipette? Probably not much.
It’s one of those things that get brushed aside in the heat of experimentation. Pipetting accuracy though, is critical to successful experiments–especially in sensitive experiments. For example, qPCR relies upon accurate pipetting—calculations depend on having the same amount of template DNA (cDNA or otherwise) in each reaction. A pipetting inaccuracy of as little as 5% can result in a 2 ng variation in template DNA. This 5% error is geometrically amplified throughout the reaction and leads to hard-to-interpret results.
In a previous article we showed you the basics of cleaning a pipette—now, let’s talk about how to check that your pipette is still accurate.
Check for Pipette Accuracy Often
Aside from sending your pipette off for calibration every year, your pipettes should be checked for accuracy on a regular basis. How often you do it depends on the frequency you use them. The faithful pipette you use every day? Check it every three months. The beloved but not used so often pipette? An annual check should suffice.
But, just how exactly do you do it?
The Weight of Water
The most common way to check your pipette accuracy is by weighing water.
The density of water is 1 g/mL. This means that every microliter (µL) should weigh 0.001 g. In other words, if your pipette is accurate, the amount of water you dispense will equal the amount the water weighs. So, if your pipette is set to 100 µL, then the scale should read 0.1 g.
Pipetting Technique Will Affect Accuracy
One caveat before beginning, be sure to use proper pipetting technique when measuring the accuracy of your pipette. Proper technique involves drawing up (aspirating) and dispensing (ejecting) the liquid in a smooth motion. Putting the pipette tip in the water and quickly releasing the plunger will give you inaccurate results—even if your pipette is properly calibrated. In fact, using proper pipetting technique will not only increase the robustness (and reproducibility) of your results but will also decrease the wear and tear on your pipette. A win-win!
Suck It Up, Eject, and Repeat
- Get some ddH2O ready that has been at room temperature a while, along with your pipette.
- Place a weigh boat on the microbalance (or analytical balance). Make sure to close the doors and then zero the balance.
- Set your pipette to the desired volume within the pipette’s range.
- It’s a good idea to “prime” the pipette tip a few times by pipetting up and down with water before you take your first measurement.
- When everything is ready, aspirate (suck up) the water into the tip again and eject it slowly into the weight boat. Then, shut the door and record the weight.
- Zero the balance, repeat steps 3–5 around 10 times, and average your findings.
- Do this for each volume you want to check. Accuracy is often lost at the end of the pipette’s range, so it’s a good idea to test both the minimum and maximum your pipette is designed to do.
Take Care of Your Valuables
The best way to keep your pipettes accurate is to treat them as the valuable tools they are. Be sure you:
- Check your pipette on a daily basis for dirt and/or contaminants
- Store your pipette correctly (i.e., don’t just throw it on the bench)
- Reset your pipette to the maximum when not in use so there is less pressure on the spring
- Try to adjust your pipette as little as possible to avoid wear and tear
Whatever you use your pipette for, take care of it, and it will take care of your experiments in the long run.