CLINICAL LAB SCIENEC

CHAPTER 8: PIPETTING AND USE OF GLASSWARE 203
pipette involves modifying the internal tensioning of a spring that governs
the piston’s travel distance. Springs subjected to changing tensioning behave
more smoothly and consistently when they are allowed to enjoy an interval
of rest as they settle into their new configuration. A pipette that is left idle
for at least 1 minute after a volume adjustment will perform more accurately
than one that is used too quickly. This is especially important when calibrating
a pipette.
Pipette Calibration
Pipette calibration, along with other maintenance, is periodically required for
pipettes used in diagnostic testing, such as that performed in a clinical laboratory,
to provide continued accurate, consistent, and repeatable operation and
measurements. The intervals between calibration vary depending on several factors,
which include but are not limited to:
• The skill and training of the operators. Skilled operators tend to operate
the instrument more correctly and make fewer accuracy-robbing mistakes.
• The liquid dispensed by the pipette. Corrosive and volatile liquids tend
to emit vapors, which ascend into the pipette shaft even under proper
operating conditions and may corrode the metal piston and springs, or
the seals and o-rings that provide an airtight seal between the piston and
the surrounding sleeve.
• Proper and careful handling. Pipettes that are frequently dropped, are
subjected to careless handling or horseplay, or are not properly stored in
a vertical position will tend to degrade in accuracy over time.
• The accuracy required by the instrument. Applications requiring maximum
accuracy also demand more frequent calibration. Instruments
used for purely research applications or in educational settings generally
require less frequent calibration.
Under average conditions, most pipettes can be calibrated semiannually
(every 6 months) to provide satisfactory performance. Institutions that are regulated
by the U.S. Food and Drug Administration generally benefit from quarterly
calibration, which is every 3 months. Critical applications may require monthly
service, while research and educational institutions may need only annual service.
These are general guidelines, and any decision on the appropriate calibration
intervals should be made based on knowledge and skill. Also, considerations
of the brand and type of pipette in question (some are more reliable than others)
should be included in the decision to calibrate pipettes. The conditions under
which the pipette is used and the operators themselves are also important factors
to consider.
There are two basic methods of calibration for pipettes. One type of calibration
is accomplished through means of gravimetric analysis. This requires
dispensing samples of distilled water into a receiving vessel placed on a precision
analytical balance. The density of water is a well-known constant, and therefore
the mass of the dispensed sample provides an accurate indication of the volume
that is delivered. Relative humidity, room temperature, and barometric pressure
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