Even small variations in pipetting procedures may seriously impact assay results. This is of particular significance if the findings may be relevant for clinical or manufacturing decisions, such as in the case of the Limulus amebocyte lysate (LAL) assay used to assess samples for endotoxin contamination. However, imprecisions have been detected in a substantial proportion of individuals performing manual pipetting, and especially when pipetting small volumes. In one study, the mean intra-individual pipetting imprecision was found to be 5.7%, 0.8%, and 0.2% for pipetting 10 μL, 100 μL, and 1 mL, respectively. Moreover, the mean inter-individual imprecision was 8.1%, 1.1%, and 0.4% for pipetting 10 μL, 100 μL, and 1 mL, respectively.1
All steps of the pipetting procedure, including preparation, liquid uptake (aspiration), and liquid discharge should be considered to ensure the use of a proper technique.2 Excessive handling of the pipette and tip should be minimized to prevent contamination. It is recommended to pre-wet the pipette tip by pipetting the selected volume up and down. For liquid uptake, the pipette tip should be immersed a few millimeters below the liquid’s surface, and the liquid should be aspirated slowly and evenly. After aspiration, the experimenter should pause for 1–3 s to let the liquid rise up in the tip. Consistent speed and plunger pressure should be applied throughout the liquid discharge. Finally, the tip should be examined before and after each liquid discharge to ensure that the full volume has been dispensed.
In addition to the described characteristics of a proper pipetting technique, adhering to the following tips would aid the delivery of consistent and reliable results: