Precision - Accuracy - Performance
PIPETTE CALIBRATION
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.