Precision pipetting is standard

The careful dispensing of precise volumes of liquids is an essential daily component of many scientists’ lives.

In laboratories of practically every discipline, almost all this liquid handling is carried out using handheld pipettes which, despite their simple lines, hide a complex mechanism that can achieve accuracy and precision even in the hands of novices.

However, every sophisticated mechanism needs maintaining with care and one of the problems facing the handheld pipette is that, as it is so ubiquitous, and hides its complexity so completely, we can all be inclined to keep on pushing the button automatically.

And, there are a lot of buttons to press.

Anachem recently surveyed over 8,000 laboratory scientists and found that over 90% used variable volume single channel pipettes routinely in their work. A further 60% used 4 - 6 different pipettes, while 20% used more than 10 different pipettes.

The protection offered by an international standard

The sheer volume and variety of pipettes in use means that it is essential to safeguard the accuracy and precision of your work through a unified approach to routine checking procedures and calibration.

All responsible pipette manufacturers recognise the importance of calibration and make their specification sheets available.

However, the maximum permissible errors of individual pipettes and volume ranges quoted for different makes of pipettes can be bewildering. Moreover, individual manufacturers’ calibration certificates supplied at the time of purchase carry on the complexity.

Fortunately, there are norms and regulations that describe the calibration and operating parameters to which precision measuring instruments should conform.

For pipettes, this is the seven-part international standard EN ISO 8655.

Part of the standard identifies and details the design requirements which are valid for pipettes manufacturers, as well as the maximal permissible errors which manufacturers can use as norms for their quality assurance programme, pipette operators can use as acceptance criteria and regular in-use testing criteria, and testing institutions can use as a basis for independent certification.

Vitally, adopting EN ISO 8655 in your laboratory means a common set of standardised specifications for all pipettes of any type and nominal volume - regardless of make or model.

You can use the standard to compare and contrast between rival performance claims when shopping for pipettes - comparing “apples to apples” in your purchasing decision.

Furthermore, you can ensure that testing and calibration throughout the pipette’s lifetime is carried out in a way that continues to guarantee its accuracy and precision in your experiments.

Importantly, you can also adopt this standard and write it into any standard operating procedures (SOPs) for your laboratory.

ISO 8655 in practice

At Anachem, we have configured and equipped our new calibration laboratory to comply with the requirements of ISO 8655.

For instance, the standard is stringent when it comes to the minimum requirements for the weighing equipment to be used, which, for practical purposes, depends on the nominal volume of the pipette being tested.

As part of the Mettler Toledo group, we are fortunate to be able to access the most suitable technology for each of our balances.

However, if your laboratory does not have access to precision balances, you would need to adjust for the uncertainty this brings to your own calibration measurements if you choose to perform regular checks on your pipettes.

Three things you must get right

ISO 8655 is clear that the total pipetting system consists of three elements - the pipette, the pipette tip and the operator.

And, it means that in your work, just as much as in your calibration procedures, it is vital to think in terms of getting “the right environment, the right equipment, and the right technique”.

The standard recognises that the combination of pipette and pipette tip, even when not sourced from a single manufacturer, should conform to the maximum permissible errors set out in the standard.

You can decide whether to have your pipettes calibrated either to the manufacturers specification (with the manufacturers pipette tip) or to the ISO standard using the tip of your choice.

And, ISO 8655 allows you to choose a pipette tip for your individual applications based on design, quality and fit rather than brand.

ISO 8655 also recognises that the source of greatest variability is the operator. It addresses this by prescribing exactly how the pipetting of the test volume into the weighing vessel shall be carried out and when and how you must record the data.

Common sense

It is common sense to look after your pipettes and keep them in good working condition. And, it is good practice to test and calibrate them regularly.

As a general rule, a 20% annual failure rate can be expected if a pipette is not maintained or serviced regularly. Over 95% of these failures are attributable to preventable sealing system defects.

Therefore, daily pipette checks and routine basic servicing will reduce failures and lower the risk of experimental failure.

Alongside your in-house preventative maintenance, making sure your pipettes are professionally serviced and calibrated by an ISO 17025 compliant service centre is essential.

I do not advocate specialist training in ISO 8655 procedures for every person that picks up a pipette. But, I do encourage training in pipetting techniques for every person who is expected to use a pipette in the course of their work.

And, I support pipette users that critically examine their work to see if their pipetting technique is influencing the results they achieve and how this may be corrected.