Pipets are used to measure and transfer small volumes (10 mL or less) of liquids. Pipets are long graduated tubes that allow one to accurately measure and transfer small volumes. There are several different types and sizes of pipets, which are used for slightly different purposes. Be sure that you know how to identify the different types of pipets and that you can determine the total volume and the gradations on each.
Volumetric or transfer pipettes are designed to deliver a single volume precisely (the volume will be indicated near the top of the pipet (i.e., 2 mL).
Above the bulb in the tube is an engraved ring. Fluid must be drawn up the pipet to above the ring and then released slowly until the bottom of the meniscus is exactly at the ring (the tip of the pipet should be touching the wall of the sample vessel as fluid is released). To transfer this volume to a second container, touch the pipette tip to the inside of the new container and allow the liquid to drain out.
Mohr or measuring pipets are graduated but stop at a baseline before the pipet begins to narrow.
To accurately transfer fluid with this type of pipet, the meniscus must be precisely on a calibration mark both at the beginning and at the end of a transfer.
Near the top of this type of pipet you will find the total volume indicated and the size of the smallest gradations (i.e., 5 mL in 1/10, the total volume of the pipet is 5 mL, and it is graduated in 1/10 mL increments). In this type of pipet the marking near the tip of the pipet should be the same as the total volume listed near the top of the pipet.
Serological pipets are graduated to deliver (there is no base mark).
The appropriate amount of fluid is drawn into the pipet (with the meniscus precisely on the correct mark) and the entire amount is transferred.
You will find the total volume indicated and the size of the smallest gradations (i.e., 5 mL in 1/10, the total volume of the pipet is 5 mL, and it is graduated in 1/10 mL increments). There are two types of serological pipettes. Those with a single painted or frosted ring at the top should be allowed to simply drain with the tip placed against the side of the receiving vessel. Those with double rings are designed to be "blown out" by pushing a small amount of air out of the pipet, completely emptying it.
Since serological pipets are labeled with the zero mark at the top of the pipet you will need to subtract the amount you are going to pipet from the total volume of the pipet to determine the exact mark to fill the pipet to. For example if you are going to pipet 6.5 mL using a 10 mL pipet you would fill the pipet to the 3.5 mL mark. To determine the total volume of the pipet look near the top of this type of pipet.
Most mechanical pipets can be set to draw and dispense different volumes. They are usually set by turn the knurled nob near the top. The volume is read in the window. Mechanical pipets are operated by depressing the plunger. On the downward stroke of the plunger there are two stops. The first offers firm resistance, and the second is a hard stop. To take up a volume in the pipet, place a tip on the end of the pipet. Depress the plunger to the first stop and insert into the sample to be transferred. Draw the liquid into the pipet by slowly releasing the plunger. To dispense the liquid from the pipet, place the tip of the pipet into the opening of the well and slowly depress the plunger all the way to the second stop. When the liquid has been dispensed withdraw the pipet tip from the well before releasing the plunger.
A. After selecting the appropriate sized pipet choose the proper pi-pump. The small (blue) pi-pumps are to be used with pipets up to 2.0 mL in volume. The large (green) pi-pumps are to be used with pipets greater than 3.0 mL in volume.
B. Gently push the top (the end cut off flat) of the pipet into the pi-pump.
C. Rotate the wheeled knob with your thumb moving the pi-pump plunger up. This should pull the solution up into the pipet.
D. Once the correct volume has been pulled up into the pipet place the tip of the pipet against the side of the new container and depress the plunger. This should empty the contents of the pipet into the new container.
E. If you are using a blow-out pipet you will need to move the plunger up slightly before filling the pipet. This will allow you to completely blow out the pipet.
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