Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Pipette

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For high-accuracy chemical analysis and research work,

a volumetric transfer pipette is preferred. Volumetric transfer

pipettes are calibrated to deliver a fixed liquid volume with

free drainage, and are available in sizes ranging from 0.5–200

mL. Class A pipettes with volumes greater than 5 mL have a

tolerance of +/-0.2% or better. The accuracy and precision of

the smaller Class A pipettes and of the Class B pipettes are

less. The Ostwald-Folin pipette is similar to the volumetric

transfer pipette, except that the last drop should be blown out.

Mohr and serological pipettes have graduated volumetric

markings, and are designed to deliver various volumes with an

accuracy of +/- 0.5-1.0%. The volume of liquid transferred is

the difference between the volumes indicated before and after

delivery. Serological pipettes are calibrated all the way to the

tip, and the last drop should be blown out. The calibration

markings on Mohr pipettes, on the other hand, begins well

above the tip. Lambda pipettes are used to transfer very small

liquid volumes down to 1 microliter. Dropping pipettes (i.e.,

medicine droppers) and Pasteur pipettes are usually uncali-

brated, and are used to transfer liquids only when accurate

quantification is not necessary.

Automatic dispensing pipettes and micropipettes are

available commercially. Automatic dispensing pipettes, in

sizes ranging from 1–2,000 mL, permit fast, repetitive deliv-

ery of a given volume of solution from a dispensing bottle.

Micropipettes consist of a cylinder with a thumb-operated air-

tight plunger. A disposable plastic tip attaches to the end of

the cylinder, the plunger is depressed, and the plastic tip is

immersed in the sample solution. The liquid enters the tip

when the plunger is released. The solution never touches the

plunger. Micropipettes generally have fixed volumes, how-

ever, some models have provisions for adjustable volume set-

tings. Micropipettes are extremely useful in clinical and

biochemical applications where errors of +/- 1% are accept-

able, and where problems of contaminationmake disposable

tips desirable.

Researcher dispensing sample into an analysis tray.

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