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 tempratures.
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.
a. b. 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 pump.
Figure 3. Thumb roller style pipette pump.
Figure 4. Proper handhold for using a Brinkmann style pipette pump.
Measuring 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.
Accuracy
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
Department of Biology, Bates College, Lewiston, ME 04240
