By Rebecca Lampert, Product Manager at BRANDTECH Scientific
Learn how equipment and methods for liquid handling can directly impact lab data validity and repeatability.
Many applications in the lab require handling liquids in a variety of volume ranges. Selecting an instrument for a specific application is based the volume being dispensed, the type of liquid and personal preference on the function and feel of the instrument. When performing low to medium throughput liquid handling, a hand-held instrument is most often used. More and more automated pipetting systems are being used for repetitive tasks with increased throughput and accuracy. Two broad categories for these instruments are air displacement pipettes and positive displacement pipettes
Air Displacement Pipetting
An air displacement pipette, also known as an air cushion pipette, is a piston-operated instrument used to accurately aspirate and dispense precise liquid volumes. The piston and liquid in the plastic tip are separated by an air cushion allowing for contamination-free liquid handling. As the piston is moved up and down, a negative or positive pressure is created. This movement results in liquid being aspirated into or expelled from the tip.
Air displacement pipettes are the “workhorse” of the lab and ideal for liquids with similar density, viscosity, and vapor pressure (volatility) to water. When used with proper technique and a properly fitting tip, they provide very good accuracy. The BRAND Transferpette S is a quality pipette in this category, made from high grade materials and German workmanship to fit comfortably in the hand. However, the further the physical characteristics of the pipetted liquid differ from water, such as its density, viscosity, or vapor pressure, the less accurate the pipetted volume will be. For slightly volatile, viscous, or foaming liquids it is recommended to use a reverse pipetting technique. For highly volatile, viscous, or foaming liquids we recommend using a positive displacement instrument.
Positive Displacement Pipettes
Positive displacement pipettes, in contrast to air displacement pipettes, have the liquid in direct contact with the piston of the instrument. This difference allows for greater accuracy with liquids that have a wider range of densities, viscosities and vapor pressures than can be pipetted accurately with an air displacement pipette. Liquids do not have to be similar to water in their physical characteristics to be pipetted accurately with a positive displacement pipette. Positive displacement pipettes also provide accurate dispensing of partial volumes. If accuracy is required when multi-dispensing partial volumes, a positive displacement pipette is the correct tool for the application.
A specialized version of positive displacement pipette is the repeating or ‘stepper’ pipette. This is a category of positive displacement pipettes that is specifically designed to handle multi-dispensing of partial volumes accurately. Since they are a type of positive displacement pipette, the liquid comes into contact with the piston of the tip used with the pipette, providing accurate dispensing of a wide range of liquid types. BRAND offers the Handystep touch and touch S as versatile electronic repeating pipettes. With flexibility in volume and function, and touch screen programming, the Handystep is a lab favorite for auto dispensing, sequential dispensing, multi aspiration and even titration.
Automated Liquid Handling
When a labs’ throughput begins to increase, or for other considerations such as accuracy or data validity, an automated pipetting system may be the appropriate tool. Liquid handling automation differs from handheld pipetting in that it uses a robotic system to move liquid from a source to a destination. Automated liquid handling systems include a wide range of instrumentation from robots that take up very little space to those that can take up a whole room. Some systems, like the BRAND Liquid Handling Station, use air displacement liquid ends which are suitable for many types of liquids. They have the ability to reproducibly compensate for liquids that would be a challenge with manually operated air displacement pipettes. However, it is necessary to avoid chemicals that may be damaging to the materials and components of the instrument.
Traditionally, robotics in the lab have been reserved for highly repetitive and extremely high throughput, complex procedures. However, automated systems are now available in a wide range of size and capacity, making robotics accessible and sensible for even low to medium throughput procedures. Using automation for liquid handling removes user variation. This benefits the lab by reducing repetition of experiments due to different technique among laboratory staff. Also, by having a predefined process, experimental failure due to procedural error will be limited.
In summary, the manual air displacement pipette is the workhorse of the lab. This is the ‘go-to’ instrument for most people. With volume ranges from 0.1 μl to 10 mL it can handle a lot of tasks, and a lot of different liquid types. If user-to-user variability or repetitive strain are a concern, then an electronic air displacement pipette should be considered for accuracy and reproducibility. When pipetting viscous or volatile liquids, or performing accurate repeat pipetting, a positive displacement pipette is the instrument of choice. Finally, if the laboratory has increasing throughput in liquid handling, needs to improve repeatability or simply wants to optimize processes, it may be time to consider a switch to automation.
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