Ultimately the only reason for using variable volume precision pipettors is to enable exact dispensing of liquids, reproducibly. A droplet of reagent or sample so small that it can hardly be seen can have major implications in research, quality control and clinical laboratories. Thus, whilst ergonomic factors must be considered, pipettes must first and foremost, reliably offer the highest accuracy, precision and robustness over many years of use.
The new ISO/EN 8655 standard is generating increasing discussion regarding accuracy and precision when using volumetric apparatus, particularly manual pipetting systems. Error-free pipetting requires both accuracy and precision.
- A pipette is accurate to the degree that the volume delivered is equal to the specified volume.
- Precision, on the other hand, is concerned with the closeness of several measurements to each other, rather than to a standard value, that is the reproducibility of the pipetting samples.
A pipette can be consistently inaccurate but this inaccuracy could be very precise, for example if a pipette reads consistently low.
Many scientific studies involve either dilutions made with pipettes, or the addition of very small volumes of liquid to an experiment. Any undesired variations in these quantities can jeopardise results and so it is essential to take all steps necessary to maximise pipetting accuracy and precision and reduce the level of uncertainty.
A Complete System for Maximum Accuracy & Precision
The only way to minimise risk of error in manual liquid handling is to choose a total pipetting system as specified in ISO/EN 8655. This should consist of the highest quality pipettes that are professionally maintained and calibrated and used with the recommended quality matched tips. Whilst this will ensure best accuracy, precision is also affected by laboratory practice. The correct pipetting angle and speed, and a smooth, consistent pipetting rhythm contributes to precision.
It is important to recognise that from new not all pipette specifications are equal [Table 1] and to obtain the most consistent and exact research results equipment that offers the minimal source of error should be used. It is also important to be aware that even with the most popular brands there is significant variation in specifications.