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A cuvette is a kind of laboratory glassware, usually a small tube of circular or square cross section, sealed at one end, made of plastic, glass, or optical grade quartz and designed to hold samples for spectroscopic experiments. The best cuvettes are as clear as possible, without impurities that might affect a spectroscopic reading. Like a test tube, a cuvette may be open to the atmosphere on top or have a glass or Teflon cap to seal it shut. Parafilm can also be used to seal it.
Inexpensive cuvettes are round and look similar to test tubes. Disposable plastic cuvettes, while not as pure as glass or quartz versions, are often used in fast spectroscopic assays, where speed is more important than high accuracy.
Some cuvettes will be clear only on opposite sides, so that they pass a single beam of light through that pair of sides; often the unclear sides have ridges or are rough to allow easy handling. Cuvettes to be used in fluorescence spectroscopy are clear on all four sides. Some cuvettes, known as tandem cuvettes, have a glass barrier that extends 2/3 up inside, so that measurements can be taken with two solutions separated, and again when they are mixed. Typically, cuvettes are one cm across, to allow for easy calculations of coefficients of absorption.