Pipette Calibration international standards state pipette specifications, testing methods,conditions and sources of error that need accounting for. In addition the trend toward accreditation outlines the specifications and testing methods for multichannel pipettes and how they are calibrated or validated. In these situations and more importantly where work results are critical per well, in non duplicated samples for example, each channel shall be regarded as a single channel and tested and reported this way.
To comply with these standards therefore means a protocol must be chosen
which tests each and every channel. The vast majority
of multichannel pipettes use a central plunger mechanism
to activate all channels at the same time in a consistent
manner. This not only makes the mechanics easier to manufacture
and design but ensures all pistons act under the same
direction and force. The piston assemblies however amongst
varying makes and models show substantial differences.
There are a number of companies who specialise in supplying equipment for multichannel pipette calibration. These are standalone or multi-workstation modules.
The issue with this type of multi-channel calibration is that testing every channel can be very time consuming. A 2.2 point calibration on a 12-channel pipette requires 48 pipetting cycles and weighing's. Depending on the speed of an operator and various other practical factors this could take up to 1.5 hours to do. This can become even more laborious and time consuming of you consider a UKAS calibration! This would result in 360 weighing's (3 volumes x 10 repetitions x 12 channels).
Undertaking these procedures in house can be completely prohibitive and it may be best to farm them out to companies who specialise in these types of calibration. Some of thee companies have invested in expensive automated equipment, such as that available from Mettler or Sartorius which is designed for automating the process.
Some laboratories will take a "view" on the calibration protocols depending on their work and needs, to reduce this level of work. Some laboratories will only test selected channels and may also use gravimetric analysis across an entire microplate and take averages. Visual interpretation and "drip" tests also reveal malfunctioning channels, but the only reliable method is to test all channels individually.