Good pipetting: further guidance on the use and maintenance of liquid-handling devices

Precise and accurate delivery of samples and reagents is fundamental to the sensitive techniques employed in today’s laboratories, particularly with the use of such small volumes and the increase in molecular biology methods. The introduction of automatic pipetting devices has revolutionised liquid-handling procedures in the laboratory and offers many benefits (eg ease of use, speed of action, improved reproducibility and safety) over the more traditional methods using glass pipettes.
There is a considerable variety of pipettes on the market, ranging from the fixed-volume, single-channel, manually operated pipette to the very latest range of electronic multichannel instruments with fixed or variable volume settings. However, such pipettes fall into two basic groups (air displacement and positive displacement), each one utilising a different principle of operation.

Air-displacement pipettes
Air-displacement pipetting is used for standard applications. This type of pipette relies on a piston to create the suction necessary to draw the sample into a disposable tip. A wide range of volumes can be achieved and single or multistep options are available. When using the adjustable type, the volume required is achieved by turning the adjusting knob and the delivery volume is displayed on a digital readout on the handle.

Three techniques are in common use (forward, reverse and repetitive) and there is also a method for whole blood. Following attachment of a disposable tip to the pipette and selection of the desired volume, the correct pipetting sequence consists of three actions: aspirate to draw up the sample, dispense to deliver the sample and blow out to empty the tip.