Improving data quality and reproducibility (Part 4): Heat and dry air reduce pipette accuracy
In this, the last of our four-part series on how the environment affects pipettes, a team of Artel scientists set out to visit one of the hottest and driest places on earth to see how bad it can get. Death Valley National Park is an ideal place to investigate the effect of dry heat on pipetting accuracy. Here the team found that the combination of high temperature and low humidity dramatically alters pipette performance, and thus data quality. Based on these Death Valley experiments, the effect of seasonal humidity and temperature variations are discussed.
This blog post summarizes the findings; be sure to read the full Extreme Pipetting Expedition article for complete details on the magnitude of pipetting performance variability in dry heat.
When it’s hot and dry, pipettes under-deliver
Under the extreme conditions the team encountered in Death Valley, 44 °C (111 °F) and 7% relative humidity, pipettes delivered much lower than their calibrated volume. Pre-wetting is promoted as a way to mitigate this effect. While pre-wetting did improve pipetting performance, it could not completely eliminate inaccuracies under these extreme conditions (Figure 1).
Figure 1. The effects of heat, humidity and pre-wetting on liquid transfer accuracy
using a 20 µL pipette.
Why so much under-delivery of sample?
The effect measured at Death Valley is due to evaporation inside the pipette tip during sample aspiration. When an empty pipette tip is immersed into a sample, an amount of air is isolated inside the tip. During aspiration, some of the sample fluid evaporates within the tip. This evaporation increases the total volume of the gas inside the pipette tip and reduces the amount of liquid which is aspirated.
Pre-wetting raises the humidity level inside the tip and improves accuracy, but does not completely mitigate the evaporation effect. Since pre-wetting does not completely eliminate under-delivery errors, it is important to calibrate pipettes in the same conditions as the laboratory where they are used.
What does this mean for you and your lab?
Many labs experience seasonal humidity and temperature variations which can affect pipetting performance. Timing the calibration of your pipettes can minimize error due to changes in temperature and humidity. The key factors to consider:
- when your pipettes are calibrated with respect to seasonal humidity levels, and
- what the environmental conditions are in the calibrating lab.
For example, if your pipettes are calibrated in the winter when relative humidity is low, you may want to re-calibrate in the summer, when humidity is higher. In addition, if your pipettes are sent for off-site calibration, you should compare the humidity levels (and temperature) in the calibrating lab to your own laboratory conditions. Be sure that calibrations are performed under conditions that closely resemble the environmental conditions in your own lab.
Additionally, these studies highlight the importance of pre-wetting to minimize the effects of temperature and relative humidity. Pre-wetting clearly helped decrease variability, which may be sufficient to keep your assay data within acceptable performance metrics.
The complete picture
The data presented here are only a subset of all the conditions tested during the Death Valley research trip. To see all the data and conditions, read the full Extreme Pipetting Expedition article: The effect of dry heat on laboratory data.
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