Vacuum Assisted Pipette
Non-piston-driven vacuum assisted pipettes are hollow narrow cylinders which work like a straw and require the use of some kind of additional suction device. Originally pipettes were made of soda-lime glass, but currently many are made of borosilicate glass which is tougher and more chemically resistant. Disposable and single use pipettes are often made of polystyrene. All of these are commonly used in chemistry, mainly with aqueous solutions. There are two types. One type, the volumetric(or bulb)pipette, has generally a large bulge with a long narrow portion above with a single graduation mark as it is calibrated for a single volume. Typical volumes are 10, 25, and 50 mL. The second type, the graduated pipette, is straight-walled and as the name implies has graduation marks along most of its length.
The pipette is filled by dipping the tip in the fluid, then drawing up the liquid by using a pipette filler to create a partial vacuum above the fluid. The surface of the fluid inside the pipette is generally concave and this is called the fluid meniscus. The fluid level is read at the center of the meniscus and by aligning it with the graduation marks while holding the pipette at eye level. Liquid is dispensed by releasing the vacuum created by the pipette filler ; slow release for gradual dispensing and complete removal for fast dispensing. With safe fluids the finger may be used on the open end of the pipette to control the vacuum. While moving the pipette to the receiving vessel, care must be taken not to shake the pipette because the column of fluid may "bounce". Complete emptying of the pipette is accomplished by either blowing the pipette out with air from the filler (blow-out pipettes), or touching the tip against the side of the receiving vessel, according to pipette type.
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