Use of measurement uncertainty for determination of calibration frequency of the volumetric equipment

Analytical laboratories should confirm the traceability of volumetric measurements used in support of testing and calibration activities, by establishing procedures for calibration and check of the laboratory glassware used in volumetric measurements. Different national accreditation bodies establish different minimum requirements for the volumetric glassware used in a laboratory, regarding the validity of the calibration and the frequency for the verification of the suitability of the volumetric glassware. The intervals, methods, and specification criteria for qualification of volumetric glassware usually are established by each laboratory, taking into account the type of equipment following the relevant standard requirements. We have estimated the measurement uncertainty of the volumetric equipment that is mostly used in the laboratory for quality control of medicines (volumetric flasks, single-volume pipettes, and burettes), and identified that the temperature effect and meniscus reading (precision and accuracy) are the most contributing sources in the uncertainty. Additionally, we have performed statistical evaluation by comparing the obtained results with the calibration certificate values and with the experimental results from qualification in previous years. Based on the obtained results it was concluded that the defined frequency of calibration once a year can be extended, as long as the measurements are performed according to the standard operating procedures and in strictly controlled environmental conditions.