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I’d bet that your trusty pipette is probably one of the most often used tools in the lab. But do you have any idea how to check the accuracy of your pipette? It’s one of those things that can get brushed aside in the heat of experimentation.
Why Should You Check the Accuracy of Your Pipette
Pipetting accuracy is critical to successful experiments, especially for 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 the accuracy of your pipette. We’ll take you through how to do this in just 7 easy steps.
How Often Should You Check for Your Pipette for Accuracy?
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.
How to Check the Accuracy of Your Pipette: 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 exactly 0.001 g using a high-precision balance. 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.
What Can Affect The Accuracy of Your Pipette?
Before we dive into the process of measuring the accuracy of your pipettes, there are some things you need to know that can impact accuracy (and therefore, also impact how you measure accuracy).
How Temperature and Pressure Affect Pipetting
Temperature and air pressure can affect the density of liquids, which you should consider when checking your pipettes. Pure water has a density of 1 g/mL at 20°C and 1 atmosphere of pressure, so ideally, measure the accuracy of your pipettes under these conditions.
Notice I said pure water? If you use tap water, which contains dissolved ions the density may vary. Is this enough to impact your results? There’s no need to risk it—just use distilled water.
The effect of pressure is negligible, and therefore, variations in this are unlikely to alter your results dramatically. However, the ambient temperature can have more of an effect, with higher temperatures resulting in lower density because of thermal expansion.
Luckily, there are ways to adjust for this, so ensure you measure the ambient temperature when measuring the accuracy of your pipette.
Read our article on Performing Pipette Calibration to find out how to compensate for temperature differences when pipetting.
Note that you also need to make sure you are using a balance that has been properly calibrated too.
How Pipetting Technique Affects Accuracy
Before you check the accuracy of your pipette, you need to ensure you are using the proper pipetting technique. If not, you’ll be measuring the inaccuracy of your pipetting skills, rather than the 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 the 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!
7 Steps to Checking Your Pipette’s Accuracy
Now that you’ve read the background, here’s how to check the accuracy of your pipette in 7 easy steps. This works whether you are using a positive displacement pipette or an air displacement pipette.
- 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, along with 50% of the maximum volume.
So What is Considered Accurate for a Pipette?
What is considered an acceptable level of error may differ between labs and applications (so check to see if your lab has set tolerances). However, a good general guide is for the measured amount to be within ±1%.
For example, 0.099 g for 100 µL would be ok, but 0.097 would be inaccurate.
If your pipette is inaccurate, it’s time to get it calibrated.
How to Maintain the Accuracy of Your Pipettes
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. This handy guide explains how to clean specific contaminants from your pipette.
- 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.
How to Check The Accuracy of Your Pipette Summarized
So in short, if your pipette is not accurate, it can have large and damaging effects on your experiments. To minimize inaccuracy, check the accuracy of your pipette every three months. If the pipette is inaccurate, get it calibrated.
For more information on accuracy and how it differs from precision, see our article How to Measure and Improve Lab Accuracy and Precision, which includes a handy example of checking both the accuracy and precision of pipettes.
We hope this article has helped you understand how to easily check the accuracy of your pipette and whatever you use your pipette for, take care of it, and it will take care of your experiments in the long run!
This article was originally published July 21, 2014 and reviewed and updated June 2022.