Pipettors often fail to meet the manufacturers stated specifications1-5 and pipetting is a major source of laboratory error even when the pipettor is operating properly. Accuracy and precision specifications typically provided by pipettor manufacturers are obtained under ideal conditions. In the laboratory, performance may be very different due to the varying technical skills of the operators, environmental factors and the nature of the solutions being dispensed.1,6 - 8 In order to reduce pipetting errors, an integral part of the QC system in every laboratory should be verification of pipettor calibration. Pipettor manufacturers do not generally specify the frequency of calibration because it depends on usage. The College of American Pathologists, the National Committee for Clinical Laboratory Standards (NCCLS) and the American Society for Testing and Materials (ASTM) suggest a quarterly check and monthly quick checks, or more frequently as indicated by the physical condition or extent of use of the apparatus. 6,7,9
Two common methods for checking precision and accuracy of pipettors are gravimetric and spectrophotometric.1,6-8 The gravimetric method is based on the weight of water dispensed from the pipettor and is considered the primary method.6-9 The water is weighed using a balance calibrated with NIST-traceable weights. The actual dispensed volume is calculated from the measured weight and the density, taking into account temperature and evaporation rate. The gravimetric method is useful for dispense volumes of 10 L and above.
The spectrophotometric method employs a solution containing a known concentration of a highly-colored chromogen. Aliquots of the sample solution are dispensed into a known volume of diluent and the absorbance measured. The actual dispensed volume is th