Performing your own pipette calibration may sound scary, but in this article, we walk you through how to do it in simple, easy steps.
The Importance of a Well-Calibrated Pipette
A pipette is used to precisely measure and transfer small volumes of liquid in the laboratory in experiments that demand high accuracy.
However, the sources of error in pipette measurements could arise from device failure or due to misuse by the operator. Any discrepancy in volumes dispensed may affect the outcomes and reproducibility of an experiment such as qPCR results.
It is, therefore, necessary to check pipette calibration every few months to ensure accuracy by dispensing the right volumes. This is why pipette calibration is considered a fundamental part of good laboratory practice (GLP).
In a previous article, we discussed tips for preserving pipette accuracy—today we will learn how to perform pipette calibration.
Principles of Pipette Calibration
At a temperature of 20°C and one atmosphere of pressure, water density stays constant at 1 g/mL. This means that the volume of water can be determined by weighing dispensed water.
However, changes in temperature and barometric pressure can influence the accuracy of measurements. The effect of pressure is negligible in practice, but the temperature should be taken into consideration. This is done using the Z-factor or ‘Z correction factor’, which is applied in the calculation of the volume of water.
Comparing calculated volume with the theoretical volume allows you to determine the accuracy of the pipette.
Table 1 shows the Z-factors for distilled water at various temperatures
Pipette calibration table of Z-factors for distilled water at 1 atm pressure. Adapted from Wisconsin State Laboratory of Hygiene
Steps Involved in the Calibration of Your Pipette
Note: You should check the calibration of your pipette using two different volumes.
- Take distilled water in a beaker and record its temperature.
- Gather your pipette and the correct tips based on both the small and large volumes that the pipette can dispense.
- Place a weigh boat on a balance that can accurately weigh in the microgram range, and set it to zero after closing the balance door.
- Pre-rinse the tip by aspirating and dispensing the set volume three times and push fully to remove any remaining liquid.
- Aspirate the calibration volume without bubble formation and dispense the liquid slowly into the weigh boat. Then, record the weight on the balance and repeat the process ten times.
- Calculate the dispensed volume by using the equation:
V = W x Z
W = weight of the water
Z = Z-factor (see Table 1)
V = calculated volume of dispensed water.
- Next, determine the mean value from ten trials.
- Finally, calculate accuracy by using the equation:
A = 100 x Vavg/V0
A = accuracy of the pipette
Vavg = average calculated volume
V0 = theoretical volume you tried to dispense.
- If the accuracy value lies in the 99-101% range, the pipette is considered normal and calibrated.
Large changes in the testing environment, such as temperature, humidity, and atmospheric pressure, may impact evaporation during calibration, which could eventually lead to an unreliable test.
It is also important to adjust the pipettes to the calibration environment for more than an hour before starting the calibration.
What if My Pipette is Not Well-Calibrated?
If your pipette accuracy is outwith 99-101%, stop using the pipette and speak with your lab manager or supervisor. There may be existing arrangements available to get your pipette serviced and calibrated either in-house or with a company to ensure it’s performing correctly.
You may need to arrange to use an alternative pipette in the meantime. Ensure you check the accuracy of this pipette before embarking on new experiments.
How Often Should My Pipette be Calibrated?
As a minimum, it’s a good idea to check the calibration of your pipette every 3-6 months. If you are deep-cleaning your pipette, it’s a good idea to check after reassembly too.
The more regularly you check your pipette calibration, the sooner you can pick up issues that might affect your results.
Make sure you record when you last checked your pipette so that if you do have an issue in the future, you know the timescale of experiments that might have been affected.
For more tips, tricks, and hacks for getting your experiments done, check out the Bitesize Bio DIY in the Lab Hub.
Originally published May 2018. Reviewed and updated on October 2021.