Pipette Calibration is a fundamental part of Good Laboratory Practice (GLP) and must be considered a vital part of any laboratory regime where precise volumes of fluid need transferring or diluting. Pipettes are a ubiquitous tool in laboratories and come in many shape and sizes, from many different manufacturers, but all perform the same function. This function is the transfer, through aspirate and dispense actions of precise amounts of fluid.
To look at all types of pipettes would entail many
hundreds of thousands of words, so we have covered a number of the pipette
types, the actions involved, the mechanics or electronics of standard
units and the physics of pipette calibration.
There are many suppliers of manual pipettes and the vast majority are of the same design, with a thumb plunger action which depresses a plunger and a spring mechanism to return the piston.
There are some however that have slightly different designs such as those made by Brand or the Ovation pipette from Vista labs, that have unusual designs.
Inaccuracy & Imprecision: Prior to discussing pipette calibration it is important to understand accuracy and precision or to be more precise inaccuracy and imprecision. Inaccuracy can be expressed as the deviation of the mean of a number of sample replicates from a set point volume and is expressed in either absolute units such as microliters, or relative units such as percent. Imprecision is expressed as the standard deviation (STD) of the number sample replicates and is expressed as the coefficient of variation (CV) of samples volume replicates. ISO standards recommend 10 readings.
This concept can be much more easily understood through use of an image. The first image shows the average is accurate (not precise), the second is precise (not accurate) and the third, both accurate and precise..
Nominal Volume: This is the greatest possible user-selectable volume which is specified by the manufacturer. For maintaining accuracy and precision even through repeated pipetting regimes, pipettes should be calibrated at periodic intervals.