The Effect of High Humidity on Pipette Performance

ARTEL found that in the cool and very humid environment at Olympic National Park, pipettes performed extremely well. The data shown Figure 1 summarize pipette accuracy taken at two locations at Olympic, with 74% and 60% relative humidity, and also in laboratories with 40% and 22% relative humidity, more typical of working laboratories. All accuracy results are compared the calibration laboratory conditions (green dot in Figure 1).

In Figure 1, each environment is ranked according to evaporation potential which is a function of the ambient air temperature and relative humidity. Low temperatures and high humidity combine to severely limit evaporation rates (i.e., low evaporation potential). The results show that pipettes performed remarkably well in environments with low evaporation potential yet under-delivered significantly in drier facilities mimicking typical laboratory conditions.

Figure 2 shows pipetting imprecision results in the four environments studied. Pipetting imprecision is expressed as a squared ratio compared to the pipette manufacturer’s specification. Ratios less than one, indicate that the pipette performed better than the manufacturer’s specification, and ratios greater than 2.41 are significantly worse than manufacturer’s spec (F statistic at 99% confidence). Like the accuracy data, this imprecision data shows larger pipetting errors in the laboratory with the driest laboratory with the greatest evaporation potential. In the laboratory with the greatest evaporation potential (21°C and 22% relative humidity), five out of six pipettes tested showed CV performance that was significantly worse than manufacturers specifications, while those same pipettes almost always performed near manufacturer’s specification in the environments with less evaporation potential (higher humidity and lower temperature environments).

CONCLUSIONS

Since pipette calibration facilities are often operated under low evaporation conditions (20°C and 60% relative humidity) the data indicate that pipettes calibrated at under calibration lab conditions are prone to under-deliver once back in their usual environments.

Drier and warmer environments cause pipettes to under-deliver aqueous solutions due to evaporation. When pipetting aqueous solutions, evaporation occurs within the pipette tip with the first aspiration and less liquid is dispensed. Over time, as the pipette is continually exposed to the aqueous liquid being pipetted, the pipette tip becomes humidified, causing the delivery volume to increase with each dispense. This upwards trending causes variability in data and larger imprecision (i.e., larger CV).

Conversely, in humid and cooler environments, the evaporation potential is minimal. When pipetting, the instrument is consistently humidified, resulting in little to no evaporation. Without evaporation, volume dispenses are more consistent and accurate. In high humidity environments, pipetting becomes a more stable, repeatable process.