Page 108 - The Veterinary Laboratory and Field Manual 3rd Edition

The selection, use, maintenance and quality control of laboratory equipment and supplies 77

• direct contact of the piston with the sam-
ple (no air cushion)
• disposable piston (not a permanent part
of the pipette).

Automatic mechanical air-displacement
pipettes

Methodologies in modern laboratories, in par-
ticular in biochemistry and serology, demand
precise pipetting of volumes down to a few
microliter (µl). In district laboratories, it is com-
mon to use glass graduated pipettes for volumes
over 1 ml and for smaller volumes a micropipet-
tor. The mechanically operated micropippettors
use disposable plastic tips to hold liquids. In
district facilities, the most commonly used
automatic pipette is the hand-held mechanical
plunger type (air-displacement). These pipettes
work by means of a mechanical piston moving
up and down on manual pressure that displaces
a precise volume of air which, in turn, displaces
a known volume of liquid. These ‘displacement’
pipettors can be used for a fixed volume ranging
from 10 to 1000 µl or can be manually adjustable
between set volumes, for example 5 to 50 µl, 50 Figure 2.42 Micropipette. Photo: Dr Ziay Ghulam,
to 250 µl and 200 to1000 µl. These pipettes are Dr Wahidullah Bahaer, Central Veterinary Diagnostic
(usually available as single units or multi-chan- and Research Laboratory, Kabul, Afghanistan.
nel units of four, eight or twelve) designed for
work with microtiter plates. Manufacturers rec-
ommend that tips are not re-used. When not in • With the pipettor vertical and the tip 1–2 mm
use pipettes are stored upright in a stand (Figure below the surface of the liquid, release the
2.42). plunger smoothly and slowly to aspirate the
It is important to follow the manufacturer’s fluid. Wipe the outside of the tip, do not
directions otherwise large errors occur, espe- touch the open end of the tip.
cially when dispensing small volumes. • To dispense, touch the tip at an angle against
the side of the receiving vessel, press to the
HoW to uSE an ‘aIr-dISPLacEMEnt’ first stop and wait a few seconds and press
MIcroPIPEttor the plunger to the second stop to expel all
For aqueous solutions use forward pipetting. the fluid.

• Put the tip firmly on the pipettor. For high viscosity fluids (serum, plasma) and
• Press the plunger of the pipettor to the first solvents, use reverse pipetting (or use positive
stop. displacement pipettes).

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