Serological pipettes (or volumetric transfer
pipettes) are temperature calibrated devices used to transfer
a desired volume of solution from one container to another. The
pipette volume markings are calibrated to be accurate at temperatures
between about 20-25 C (68 - 77 F), more or less standard room
The serological pipettes we are likely
to use in the biology courses are of two types: TC ("to
contain") or TD ("to deliver"). Both types of
pipettes deliver all the volume including the tip which is calibrated
to be 1 ml. If the top of the pipette has double rings (Fig.
1b), it is a "blow out" pipette - it must be "blown
out" to get the last drop. Blowing out is done using the
associated pipette pump, NEVER your mouth. Non-blow out pipettes
are calibrated to allow the entire tip to drain (Fig. 1a), leaving
a tiny bit in the tip that should not be delivered. Before using
a given pipette, be sure to check the label on the body near
the top to ascertain which type it is (see Fig. 1b and Fig. 5).
Mohr pipettes do not have the last milliliter
calibrated to the tip and should only be used to deliver specific
volumes with draining the pipette. We seldom use Mohr stuyle
pipettes in our curriculum.
Figure 1. (a) TD pipette tip and
(b) double rings indicating a "blow out" pipette.
Serological Pipettes Are Used with Pipette Pumps
Serological pipettes are usually provided
as a disposable item. They will be prepackaged either singly
or in bulk packs of 25-50, and almost always come pre-sterilized.
These pipettes are designed to be used with a hand pump of which
there are many varieties, including the "Brinkmann"
style shown in Fig. 2 and the thumb roller type shown in Fig.
3. Note that the top of the pipette has a cotton plug - do not
remove this - it functions as a barrier to overfilling. The pipette
inserts firmly into the bottom of the pump. With the Brinkmann
style pumps, squeezing the bulb provides suction for drawing
liquid into the pipette; loading and discharging is regulated
by the little lever ("Load / Discharge" in Fig. 2).
When using TC pipettes, a small rubber button ("final spritz"
in Fig. 2) is used to blow out the tip. Within the pump are a
standard 2 micron syringe filter and a valve system - both of
which are replaceable. If the pump will no longer discharge,
it usually means that the filter has been contaiminated by overfilling
and it must be replaced. Proper hand hold on the Brinkmann style
pump is shown in Fig. 4.
We also have available thumb roller pumps
of capacities 5, 10, and 25 ml. These are held similarly to the
Brinkmann style pump, but the thumb is used on the roller to
raise the piston and thereby draw solution into the pipette.
The solution is expelled by reversing the rolling action or by
pushing the piston down.
Figure 5. Explanatory label
of a TD 10 ml pipette.
Figure 2. Brinkmann style pipette
Figure 3. Thumb roller style pipette pump.
Figure 4. Proper handhold for
using a Brinkmann style pipette pump.
and Reading Volumes
We typically have 1, 5, 10, and 25 ml
pipettes available for use in labs. The pipettes are clearly
labelled as to the volume markings and subdivisions. For example,
5 and 10 ml pipettes are numbered on the milliliter marks, but
have minor interval marks on 0.1 ml increments. It is imperative
that you read the label on the pipette body (Fig. 5) before using
it to know precisely how to read it. When loading the pipette,
the volume is read at the bottom of the meniscus that forms on
top of the column of liquid (Fig. 6). Pay attention to the sequence
of numbers when determining volume - they may be printed tip
to top, or vice versa, or often times both directions. When reading
the volume, ALWAYS view the pipette dead-on at eye level (Fig.
7) with the pipette held vertically, perpendicular to the ground.
Volumes may be measured such that the
entire volume is delivered and the pipette drained completely,
or, you may do point-to-point delivery (one volume marking to
another to achieve delivery of a specific volume ). Arguably,
the latter is the more accurate method.
Figure 6. Volume is most accurately
read at the bottom of the meniscus.
The pipettes are no more accurate than
the smallest marked increments - usually 0.1 ml for pipettes
of 2 ml or larger. If greater precision is needed, you can use
the serological pipet in combination with a micropipette.
Figure 7. Read pipette volume
dead-on at eyelevel with pipette held vertically.
Modified 9-26-14 gja
of Biology, Bates College,
Lewiston, ME 04240