Don't Compromise on Accuracy & Precision in Your Pipetting Applications

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.

Quality Pipettes

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.

ISO/EN 8655

The ISO/EN 8655 standard gives the technical specifications that must be respected by a pipette at the level of the nominal volume only. RAININ and Gilson impose specifications much more stringent than the norm, in order to guarantee optimum performance from your precision instrument. For example RAININ Classic 1000 (working range 200-1000µl) has a systematic error of 3µl at its minimum volume and 8µl at its maximum volume. ISO/EN 8655 specifies a systematic error of 8µl for a 1000µl pipette regardless of the selected volume. Thus, when pipetting a sample of 200µl, the maximum permissible error according to RAININ specifications will be only 1.5% but 4% according to ISO/EN 8655.

Thus, a pipette may be judged to conform to the norm ISO/EN 8655, but not to conform to manufacturer's specifications. This is an important factor to consider when selecting a service agency. Does it calibrate your pipettes to ISO/EN 8655 specifications or to the manufacturers specifications which may be higher and thus give your pipette a better performance?