A cuvette is a small tube of circular or square cross section, sealed at one end, made of plastic, glass, or fused quartz (for UV light) 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 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 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[1] must be clear on all four sides because fluorescence is measured at a right-angle to the beam path to limit contributions from beam itself. 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 1 cm (0.39 in) across, to allow for easy calculations of coefficients of absorption.
Cuvettes to be used in circular dichroism[2] experiments should never be mechanically stressed, as the stress will induce birefringence[3] in the quartz and affect the measurements made.
There are three different types of cuvettes commonly used, with different usable wavelengths:
and a matching tolerance of 1% at 220 nm
and a matching tolerance of 1% at 200 nm
and a matching tolerance of 1% at 2,730 nm
The Lambert-Beer-Law[5] empirically relates the absorption of light to the properties of the sample. The law states that there is a logarithmic dependence between the transmission of light through a specific sample (T = I/Io with I = outgoing light and Io = incoming light), the molar extinction coefficient for a specific compound (ε), the concentration of the absorbing species in the material (c) and the distance the light travels (d).
log (Io/I) = ε x c x d
By leaving all conditions constant but changing the pathlength (d, distance the light travels), you can bend the Beer-Law to get an automatic dilution of your sample. This principle is realized within disposable plastic cuvettes like the DiluCell [6]. Reduction of the pathlength from 10 mm (standard cuvette) to 0.5 mm (DiluCell 20) or 1.0 mm (DiluCell 20) results in an automatic dilution of the sample by factor 20 (DiluCell 20) or factor 10 (DiluCell 10). Manual dilution steps can be avoided. This saves time, excludes dilution errors and cross contaminations making DiluCell ideal for GLP. In addition, reduction of the pathlength enables spectrophotometeric measurements with minimal volumes of 100 µl (DiluCell 20) and 200 µl (DiluCell 10).
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