CLINICAL LAB SCIENEC

194
ESSENTIALS OF CLINICAL LABORATORY SCIENCE
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KIMAX
10ml TD 20C
FIGURE 8-9 Serological pipette that is graduated with a frosted ring.
Source: Delmar/Cengage Learning.
milliliter of the pipette is not graduated. When a frosted band is present at the
suction end of the stem, this indicates that the contents are to be “blown out”
with a pipetting aid for accuracy rather than just draining the solution from the
pipette. Suctioning devices or pipetting aids are used with this type of pipette
(Figure 8-10).
Mohr Pipettes
FIGURE 8-10 Pipetting aids for aspirating and dispelling solutions.
Source: Delmar/Cengage Learning.
Mohr pipettes (Figure 8-11) are graduated pipettes that also use a series of
marked lines (as on a graduated cylinder) to indicate different calibrated volumes.
However, the graduated lines for a Mohr pipette do not extend completely
to the end of the pipette but are designed for measuring from point to
point, that is, a 2-mL amount might be measured from the 5.0-mL mark to the
3.0-mL mark. As a rule, measurements performed from one point of the graduation
marks to the tip are less accurate than those from “point to point.” Mohr
pipettes also come in a variety of sizes. These are used much like a buret in that
the volume is found by calculating the difference of the liquid level before and
after liquid is dispensed. Typically, the precision of a graduated pipette is not as
great as that of a volumetric pipette. The vacuum for all of these types of pipettes
was originally provided by mouth pipetting of
solutions, most often for the measurement of
only a few milliliters of material. During the era
where most tests were performed manually and
as individual tests, it was quite time consuming
to pipette all of these reagents and samples
accurately. It is inappropriate and unsafe to use
the mouth as a source of vacuum to draw the
solution into any pipette, as some reagents are
quite toxic, and there are many biohazards that
could be contracted through mouth pipetting.
Several different devices are available for providing
a vacuum to aspirate fluids. Most often,
a bulb with valves or other device will be used
with this type of pipette. Automated instruments
will often employ a piston-driven type
of aspirating system for measuring both agents
and patient samples.
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FIGURE 8-11 Mohr pipette without graduation markings near tip.
Source: Delmar/Cengage Learning.
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Techniques for Using Quantitative Pipettes
• To deliver pipette (TD)
A to deliver pipette is indicated with a “TD”
inscription near the large open end of the pipette.
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