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Calibration

Calibration of pipettes (both glass and automatic) is normally carried out by a suitable accredited third-party. In addition to class A and class B glassware, it is also possible to purchase ‘certified’ class A glassware which is supplied with a calibration certificate detailing the actual volume delivered by that item (plus an estimate of the uncertainty).

Analysts will need to check the calibration of pipettes from time to time to ensure that the performance of the equipment is within specification. The points listed below are some of the calibration issues that the analyst should be aware of:

• the difference between calibrating an item (usually carried out by an accredited third-party) and checking the calibration;
• the contents of a calibration certificate and how to interpret them;
• how to check that automatic pipettes are performing satisfactorily, e.g. by ‘successive check deliveries’ and measuring the weight of liquid discharged;
• how to check the calibration of a glass pipette (by repeat fill and weigh experiments using water);
• the effect of temperature fluctuations on the volume of the liquid delivered (due to expansion and contraction)
• the coefficient of expansion of liquids can be used to estimate effects of temperature variation;
• calibration checks should be conducted at the correct temperature;
• the required calibration frequency and expiry date (if appropriate).