CLINICAL LAB SCIENEC

CHAPTER 8: PIPETTING AND USE OF GLASSWARE 199
and are used to transfer or dispense small quantities of liquids. They are usually
glass tubes tapered to a narrow point and fitted with a rubber bulb at the top.
Pasteur pipettes come in various lengths. They are sold in boxes of hundreds
and are generally considered cheap enough to be disposable. However, so long
as the glass point is not chipped, the Pasteur pipette can potentially be washed
and reused indefinitely. The term eye dropper also may refer to early models of
fountain pens, which had to be refilled with some kind of Pasteur pipette before
the invention of rubber sacks, piston devices, or the modern ink cartridge.
Plastic Beral pipettes are used in a similar manner to Pasteur pipettes and
are also referred to as transfer pipettes. They have their stems and bulbs in the
form of a single piece made of plastic and are of different sizes depending on
the purpose for which they are being used. The volumes are usually marked on
the stem, although the markings are usually simple etched lines and are not particularly
accurate.
Plastic Pasteur pipettes are often used in biology where most media are
aqueous, and solvent resistance is not important. Most organic solvents, such as
hexane and acetone, cannot be used in plastic Pasteur pipettes. They dissolve the
plastic in these instruments, rendering them inadequate for many types of applications.
The pipettes are also hard to wash and are usually discarded with other
biohazard or toxic waste after each use. Plastic bulb pipettes are considered not
precise enough to be used for even reasonably accurate measurements, whereas
their glass counterparts can be extremely precise in some instances. The longer
and thinner the tip of a glass pipette, the more accurate is the measurement. Usually
these pipettes will be used in conjunction with a scale to accurately measure
a volume of fluid.
Piston-Driven Air Displacement Pipettes
Gilson micropipettes are the most accurate and precise pipettes. They are more
commonly used in biology, although they are frequently used by chemists as
well. These pipettes are designed and manufactured for accurate and precise liquid
sample measurements, and are accurate for volumes from 0.2 μL to 10 μL.
The plastic pipette tips are designed for aqueous solutions and are not recommended
for use with organic solvents which may dissolve the plastic. These
pipettes operate by piston-driven air displacement. A vacuum is generated by the
vertical movement of a metal or ceramic piston within an airtight sleeve. As the
piston moves upward, driven by the depression of the plunger, a vacuum is created
in the space left vacant by the piston. Air from the tip rises to fill the space
left vacant, and the air is then replaced by the liquid, which is drawn up into the
tip and thus available for transport and dispensing elsewhere.
Pipette Accessories
Pipette accessories come in a variety of designs (see Figure 8-10). They are used
to facilitate the safe and efficient filling of pipettes.
• Pipette fillers are used to fill the pipette easily, avoiding the need for
mouth pipetting.
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