In the lab pipetting is a basic function used for many important procedures. These days, there is a pipette for nearly every requirement you could possibly think of, so how do you know where to start when looking for the right tool for your needs? Well we’re here to advise you on some of the factors to think about when making this important decision.
When choosing a pipette for your application you need to consider the physical properties of your sample, along with the type of analysis you are performing and the volume range required. These factors will determine which pipette and tip you should use.
Air-Displacement Pipettes
Aqueous DNA
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For Aqueous samples or general lab work, air-displacement pipettes are the recommended tool. Most commonly used in biology and biochemistry, Air-displacement pipettes always have a cushion of air (dead volume) between the pipette piston and the liquid sample.
How do air-displacement pipettes work?
The piston inside an air-displacement pipette is moved by pressing the push-button. When the piston moves it draws the liquid from the sample into the pipette tip, pushing out the air from the tip and moving it into the vacant space left by the piston.
Positive-Displacement Pipettes
Viscous Radioactive Corrosive DNA
When dealing with problem samples (viscous, dense, volatile, radioactive, corrosive), positive-displacement pipettes are recommended for the job. A Positive-displacement pipette does not have an air cushion between the piston and the sample. Because the piston is in direct contact with the sample, the piston in a positive-displacement pipette is disposable.
How do positive-displacement pipettes work?
Positive-displacement pipettes work like a syringe. As the disposable piston is in direct contact with the sample, there is no air cushion to expand or contract meaning that the aspiration force remains constant and unaffected by the physical properties of the sample. This ensures accurate pipetting with very viscous or high density samples, such as mercury or toothpaste.
Positive-displacement pipettes are recommended for forensic or diagnostic assays, where it is essential to eliminate any potential of cross contamination between samples.
The Importance of Both Accuracy and Precision
Whatever task you are doing, ensuring that your tool is delivering accurate and precise results is absolutely crucial. A droplet of reagent or sample, no matter how small can have major implications in research, quality control and clinical laboratories. Although ergonomic factors are important that must be considered, ultimately, pipettes must reliably offer the highest accuracy, precision and robustness over a number of years of use.
So, what’s the difference between accuracy and precision? Accuracy is when the volume delivered by the pipette is equal to the volume specified. Whereas precision is the closeness of several measurements to each other, rather than to a standard value that is the reproducibility of the pipetting samples. Therefore, a pipette can be consistently inaccurate, however this inaccuracy could be very precise, e.g. if a pipette reads consistently low.
Precise but Accurate but Accurate and
not accurate not precise Precise
As many scientific studies involve either dilutions made with pipettes or the addition of extremely small volumes of liquid to an experiment, the slightest undesirable variation on these quantities could jeopardise the results. Consequently, it is absolutely vital that pipettes are accurate and precise, if one or the other is out, your results could be compromised.
There’s only one way to minimise the risk of error in manual liquid handling, and that is to choose a total pipetting system, as specified in ISO/EN 8655, which is the standard that gives the technical specifications that must be respected by a pipette at the level of the nominal volume only. A total pipetting system should consist of the highest quality pipettes that are professionally maintained and calibrated and used with the recommended quality matched tips. Now this will ensure best accuracy, however when it comes to precision, this is also affected by good lab practice, such as correct pipetting angle, speed and a smooth consistent pipetting rhythm.
It is important to keep in mind that, even with the most popular brands, there is significant variation in specifications. Even from new, not all pipette specifications are equal, and to ensure the most consistent and exact results, scientists should use equipment that offers the minimal source of error.
