Pipette linearity, Gravimetric method, ISO8655 all explained
Why is it important to check pipette linearity?
For mechanical reasons, pipettes can only deliver samples accurately over a limited range of volumes for each particular model. Ideally, the volume of liquid delivered in that range follows a precise linear relationship to that selected. It is essential to check this linearity when the pipette is serviced to ensure correct functionality at both the upper and lower limits of the pipettes capacity.
To increase their throughput some service agents only perform checks at a single mid-point volume with three or five weighings, and even issue calibration certificates based on these minimal readings. As the majority of pipettes are variable volume how can you ensure that a pipette is linear across it's specification range by testing only one volume?
Even with Anachem’s most basic level of service, volumes are checked at both extremes of a pipettes range. Our standard service level is 2.2, meaning two volumes are checked with two weighings to ensure both accuracy and linearity. Always ask what level of service you are getting for your money.
What is ISO8655?
ISO/EN8655 is an international standard for volumetric calibration of Piston Operated Volumetric Apparatus (POVA).
The standard, which is in seven parts has become an ISO standard and a European Norm and all European National Standards such as BS6018, BS7532 and DIN 12 650 have been superseded in its favour. This standard recommends that all POVA devices are calibrated at least once every three months. This entails scheduling and record keeping for all apparatus.
• Calibration methods are detailed in the standard and the gravimetric test is the reference method
• Maximum errors for trueness (Inaccuracy) and precision (CV%) are listed for all types of POVA device
• Apparatus shall be calibrated with noted tips (A pipette calibrated with a specific tip shall only be deemed within calibration with that tip)
ISO8655 recognises pipettes, tips & service as a complete system.
What is the Gravimetric Method?
The Gravimetric test is the only recommended method for pipette calibration by pipette manufacturers and international standard organisations. It is based on the determination of the weight of water samples delivered by the pipette. Implementation of this method requires the strict monitoring of environmental conditions and the systematic use of adequate and controlled equipment. Ambient temperature, barometric pressure and relative humidity must all be checked and regulated.
Conversion of weight to volume must take into account the density of the liquid as well as evaporation during the cycle time. For each measurement, the corresponding volume (Vi) can be calculated as follows:
Vi = (Wi + e) Z
Wi is the weight as read on the balance
e is the mean evaporation loss during the cycle time
Z expressed in µl/mg, is a conversion factor incorporating density of water buoyed in air, at test temperature and barometric pressure.
Anachem has been servicing pipettes since 1980 and has held UKAS accreditation since 1998.
For more information on Anachem's Pipette Calibration & Service call 01582 456256 or email pipetteservice@anachem.co.uk.
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