CLINICAL LAB SCIENEC

CHAPTER 8: PIPETTING AND USE OF GLASSWARE 193
considered qualitative would most often measure in drops or an approximate
value (usually marked on a disposable dropper pipette). Quantitative pipettes
range in size from those that measure and dispense extremely small amounts to
those that are fairly large that measure and dispense from 1 mL up to 50 mL,
based on the particular need for accuracy, sufficient volume, and preservation
of samples necessary for the required procedures. The most common are those
used to measure 20 mL or less. They may be used in conjunction with automated
devices or jar-style measure and delivery methods. As are flasks, pipettes are also
classified as Class A or other class, depending on the accuracy of volume needed.
The following section describes some of the common types of pipettes and their
respective uses.
There are two basic types of macropipettes, which are designed to measure
amounts greater than 1 mL. One type, the volumetric pipette, has a large
bulb and is calibrated for a single volume. Typical volumes are 10, 25, and
50 mL. Alternatively, Mohr pipettes and standard serological pipettes are
straight-walled and are graduated for different volumes such as 5 mL in 0.5-mL
increments. The single-volume pipette is usually more accurate, with a standard
error of ±0.1 or ±0.2 mL.
The pipette is filled by dipping the tip in the solution to be measured and
drawing up the liquid with a pipette filler past the inscribed mark. Several
pipetting aids will be presented later in this section, which are used for safely
aspirating fluid into either of these macropipettes. The volume is then set by
releasing the vacuum using the pipette filler or a damp finger until the solution
reaches the desired point. While moving the pipette to the receiving vessel,
care must be taken not to shake the pipette because the column of fluid may
“bounce,” causing the loss of a drop of liquid, which would affect the accuracy
of the procedure.
Volumetric and Serological Pipettes
Volumetric pipettes are more accurate than serological (also known as graduated)
pipettes and contain a bulb that is filled along with a portion of the stem of
the pipette, from the meniscus line to the tip (Figure 8-8). The volumetric pipette
is intended for delivering only one volume of solution, which may range from
0.5 mL up to 50 mL. The tip is normally touched against the side of the container
(not the liquid) to remove a small amount of the residual left in the tip when a
solution is being transferred.
Serological pipettes are also categorized with respect to the design and accuracy
of the particular type (Figure 8-9). Both serological and Mohr pipettes are
used to deliver varying amounts of solution. The serological pipette may be
used to measure from point to point and from point to top, as it is graduated
to the tip. Mohr pipettes are used for point to point measurements as the last
1
1ml TD 20C 0.006
FIGURE 8-8 Volumetric pipette with bulb.
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