Pipettes and liquid handling in the laboratory
— Equipo JP Selecta
Handling liquids in laboratories is a critical operation because it determines the actual concentration of solutions, sample traceability and assay repeatability.
Dispensing a reagent, preparing a dilution or transferring a sample requires selecting an instrument whose operating principle is compatible with the volume, viscosity, volatility and risk level of the liquid.
Positive displacement pipettes are particularly useful when an air displacement micropipette loses precision because of the physical characteristics of the sample. However, they are not the only solution: an efficient laboratory combines pipettes, dispensers, volumetric glassware and pumping systems according to the type of operation.
What is a positive displacement pipette?
A positive displacement pipette is a laboratory instrument for measuring and transferring liquids. It uses a piston that comes into direct contact with the liquid inside a sealed capillary tip. As it moves, the piston aspirates and dispenses a defined volume without leaving an air cushion between the mechanism and the sample. This detail distinguishes how it works from a conventional air displacement micropipette.
The absence of an air cushion reduces the effect of evaporation, density, viscosity and vapour pressure on the volume transferred. It also helps to limit the carry-over of aerosols into the body of the pipette, which makes it useful when the sample requires stricter physical separation between the fluid and the actuating mechanism.
Which types of liquid is it recommended for?
It is recommended for:
- Viscous liquids, such as glycerol, oils or gels.
- Volatile liquids, such as certain organic solvents.
- Dense or foaming samples.
- Hazardous, radioactive or potentially infectious liquids.
In all these cases, the disposable capillary tip and direct displacement improve control of the transfer. The final choice must take into account the chemical compatibility of all materials in contact with the fluid.
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Advantages over the conventional air displacement pipette
The main advantage is dispensing stability with liquids that do not behave like a standard aqueous solution. In addition, because the liquid only comes into contact with the disposable capillary tip and its piston, the risk of cross-contamination is reduced and difficult matrices are easier to handle.
In biological applications, reducing the generation and carry-over of aerosols contributes to safer operation. These advantages are no substitute for correct technique: the user must check the volume range, use compatible consumables and keep the instrument calibrated.
Other alternatives for handling liquids in the laboratory
Which instrument to use for measuring liquids depends on the accuracy required, the volume and the frequency of use. It is also important to distinguish between measuring, containing, dispensing repetitively or pumping a fluid continuously.
Air displacement micropipettes are the usual choice for small volumes of aqueous samples. Burettes allow variable volumes to be dispensed precisely in titrations. Graduated cylinders are used to measure and contain liquids quickly when the accuracy of a pipette is not required, while volumetric flasks make it possible to prepare solutions to a fixed volume.
For repetitive dispensing, multi-dispensers reduce the number of manual cycles and improve consistency between aliquots. Bottle-top dispensers allow dispensing directly from the container with an adjustable volume, and fixed- or variable-volume pipettes cover both the repeated transfer of the same volume and the transfer of different volumes.
When liquid handling is continuous or requires higher flow rates, a peristaltic pump is a suitable alternative. The liquid flows inside the tubing and does not come into contact with the mechanical parts of the pump, an advantage in repetitive transfers and dosing systems. The actual flow rate depends on the diameter and material of the tubing, the speed and the pump head configuration.
| Instrument | Suitable for | When to choose it? |
|---|---|---|
| Positive displacement pipette | Viscous, volatile, dense or foaming liquids | When an air displacement pipette may lose precision |
| Air displacement micropipette | Small volumes of aqueous liquids | For standard samples and precision operations |
| Burette | Dispensing variable volumes | In titrations and controlled additions |
| Graduated cylinder | Quick measurement of liquids | When high accuracy is not needed |
| Volumetric flask | Preparing solutions | When an exact final volume is needed |
| Multi-dispenser | Multiple identical aliquots | For repetitive dispensing |
| Bottle-top dispenser | Repeated dispensing from a container | When working directly from a bottle |
| Fixed- or variable-volume pipette | Transferring liquids | To repeat a specific volume or work with different volumes |
| Peristaltic pump | Continuous transfer of liquids | When a constant flow rate needs to be maintained |
How do you choose the right instrument for each liquid?
The process is based on three steps:
- Define the precision required.
For small aliquots and aqueous samples, an air displacement micropipette may be sufficient; for viscous, volatile or critical liquids, positive displacement provides greater control.
Next, it is worth assessing the total volume, the frequency of the operation and whether the work is occasional or repetitive.
- Analyse the nature of the liquid.
Solvents, corrosive liquids, samples containing particles or biological fluids require checking chemical compatibility, the type of consumable and the protective measures.
- Assess the workflow.
The workflow determines whether a manual pipette, a bottle-top dispenser, a multi-dispenser or a peristaltic pump is the best option.
For aspirating and dispensing with pipettes, J.P. Selecta offers COM-PIP pipette aspirators in 0-2, 0-10 and 0-25 ml versions and, for the continuous transfer of liquids, peristaltic dosing pumps, with the technical support needed to choose the right solution for each procedure.