An air displacement pipette may work with high accuracy when measuring small volumes of aqueous and non-viscous liquids in the laboratory but when it comes to liquids that are difficult to handle (e.g., viscous, dense, volatile, corrosive, high pressure and/or radioactive liquids), your best bet would be a positive displacement pipette. But what makes these pipettes ideal for this purpose and how do they work?
Positive Displacement Pipettes: Getting Down to the Basics
While an air displacement pipette has an air cushion between the piston and the sample to help dispense liquids, a positive displacement pipette has none. In fact, the piston (which is not a permanent part of the pipette) is in direct contact with the aspirated sample. Positive displacement pipettes are also known to provide highly reproducible results since the tight-fitting piston literally gets the last drops of the aspirated liquid off the walls of the tips.
Since there is no air cushion between the piston and the aspirated sample, the physical properties of the samples do not affect the aspiration force. This makes these pipettes perfect for use with viscous liquids (e.g., glycerol) which draw air bubbles when aspirated too quickly and stick to the pipette tip wall when dispensed. They are also ideal for use when working with volatile liquids (e.g., acetone, methanol) which tend to evaporate quickly and/or leak out when aspirated with an air displacement pipette.
The absence of an air cushion also makes positive displacement pipettes perfect for use with hot or cold samples. As you are probably aware, cold liquids cause the air cushion to shrink while hot liquids cause it to expand. In both cases, the volume of the aspirated liquid will be affected and cause the pipette either to over-deliver or under-deliver. However, if you are using a positive displacement pipette, you won’t encounter such problems. You can expect to achieve greater accuracy regardless of the temperature of your liquid samples.
In addition, positive displacement pipettes are fitted with a disposable capillary/piston or CP tip, so there is little risk of contaminating the pipette or the other samples in the lab. This means you can safely use them when working with corrosive and/or radioactive samples.
How Does a Displacement Pipette Work?
While the working principles are almost the same, one of the main differences between air displacement pipettes and positive displacement pipettes is the absence of an air interface between the piston and the aspirated liquid in the latter. Without an air cushion, the accuracy of measurement is greatly enhanced, and even liquid samples that are highly viscous, extremely volatile or hazardous can be pipetted with ease. Here’s a step-by-step guide on operating a positive displacement pipette.
- Set the required volume. This will move the piston to the appropriate start position.
- Prepare to aspirate. Hold the pipette vertically and press the plunger button to the first stop position. This will make the piston go down to the end of the capillary.
- Aspirate the sample. Immerse the CP tip below the surface of the liquid sample and release the plunger slowly, so it moves up to the home position. The piston will then move up and the desired volume of liquid will be forced through the orifice and into the capillary.
- Dispense the sample. To dispense the liquid out of the capillary, press the plunger button to the first stop position.
- The CP tip can be ejected hands-free by pressing the plunger all the way to the second stop position. Unlike in air displacement pipettes, a blow-out step is not necessary for removing the residual sample from the tip.
- Replace the piston and capillary frequently to avoid cross-contamination.