Calibration assure a pipette's accuracy by evaluating the quality of liquid dispensed at specific volume settings. A variety of factors, including the equipment used, qualifications of the personnel, laboratory environment and specific processes can affect the certainty of the measurements used when calibrating a pipette.
ISO 17025 accredited labs deliver quality service. The requirements vary slightly from country to country, but all focus on a tightly controlled laboratory environment:
Other requirements include published measurement uncertainties, a documented technician proficiency test program and documented processes.
Pipette technicians calibrate pipettes by:
The consistency of results or precision is calculated via a standard deviation. Expert technicians are able to maintain a very low standard deviation, therefore assuring their measurements are close to the expected value.
According to the internationally recognized ISO 8655 standard, small volume pipettes like 2µL or even 10µL require a 6 or even 7 digit balance. These balances are extremely sensitive and more often than not fail to work properly in an uncontrolled laboratory environment.
ISO 8655-6 recommends that every channel on a multichannel pipette be calibrated as if it were an individual pipette. We use METTLER TOLEDO MCP balances, which are specifically designed for multichannel pipettes and calibrate all channels simultaneously.
ISO 8655 defines the useful volume range of a pipette as being from 10% of nominal to 100% of nominal. The most commonly used volume settings take after the ISO standard; the nominal volume (100%), 50% of nominal and the lower limit or 10% of nominal volume (ISO 8655-6:2002 section 7.1.1).
Volume settings can sometimes vary depending on the calibration standard applied. For example, certain service providers have adopted 25% for minimum, 75% for medium and 100% for maximum as their volume settings. This unfortunately negates the use of the pipette for volumes settings below 25% of nominal volume.
The time and hassles associated with internal and government audits means documentation is the only proof of compliance. The best type of documentation is a calibration certificate generated
by software, which is directly linked to the balance.
This software balance link is important as it restricts or even eliminates potential human intervention on reported results. Calibration certificates should specify the serial number of the pipette, date of the service, balance used, technician and more.
