Air-displacement pipettes always have a cushion of air between the pipette piston and the liquid. The simpler of the two instrument types, air-displacement pipettes are easily influenced by factors such as temperature, atmospheric pressure, specific gravity, and the liquid’s viscosity. Keeping the liquid away from the piston or barrel is an advantage, but also provides many limitations in terms of maintaining accuracy. For instance, working with liquids that may easily evaporate would require the use of the other type of pipette–positive-displacement pipettes.

How is it used? Air-displacement and positive-displacement micropipettes involve a quicker and more efficient procedure for transferring liquids of specific volumes. The first step to using these automatic micropipettes is the push-button volume adjustment. Hold the body of the micropipette in one hand and use the other hand to rotate the push-button. Do not attempt to force the volume setting beyond the limits of your micropipette, at the risk of internal damage. Also, make sure that the numbers of your volume setting are in proper alignment. Afterwards, a disposable pipette tip is attached.

After setting the volume and attaching the disposable tip, the next step is the process of pre-rinsing. Pre-rinsing, also called pre-wetting, is a highly-advised procedure for achieving greater uniformity and precision, as it provides identical contact surfaces for the liquid to be aspirated. To pre-rinse, simply aspirate with the tip and then dispense back into the original reservoir or to a waste receptacle. Pre-rinsing should be performed each time you change a pipette tip, and when you increase the volume setting.

The operation of automatic micropipettes involves two points of resistance — the first stop, the point at which the piston reaches the calibrated volume on the digital volumeter, and the second stop, at which the remaining liquid is fully expelled or purged. When aspirating, two factors can influence the accuracy of your measurement — the aspiration angle and immersion depth. Prepare the micropipette by holding it in a nearly vertical position and depressing the plunger or push-button to the first stop position using your thumb. Once the push-button is in the first stop, immerse the pipette tip just below the surface of the liquid, to avoid droplets sticking on the outside of the pipette tip and slowly and smoothly release the plunger or push-button back to the rest position as you wait for the liquid to move up and into the tip. Do not let the plunger snap back up as this will result in air bubbles.

Once the liquid is collected, place the pipette tip at an angle of 10 to 45 degrees against the inside wall of the receiving vessel, and depress the plunger smoothly back to the first stop position to dispense the liquid. Wait one second, and then depress the plunger once more into the second stop position to “purge” or “blow-out” the tip of any residual liquid. Finally, remove the pipette tip by sliding it off the sidewall. The procedure is the same for positive-displacement micropipettes, except both the disposable pipette tip and piston are ejected and replaced after each transfer.