Common laboratory absorption studies involve liquids – either as pure solvents or solutions of light absorbing compounds in transparent solvents. Such studies are made possible by making use of containers of precise dimensions, usually 10 mm path length, which are transparent to the wavelength of light required for the purpose. Such absorbance cells are also commonly called cuvettes.
Cuvettes are commonly made from different transparent materials such as optical glass, quartz or transparent plastic. At first sight all such materials appear to be perfectly transparent and fit for all types of absorbance studies.However, each material has unique light absorbing properties and it is important to know about such optical characteristic properties before making your selection of the cuvette material.
The article examines the optical characteristics of such materials that will help you make the right choice before proceeding with absorbance measurements. Irrespective of the material selected it must be borne in mind that cuvettes, except for transparent plastic cuvettes, are fragile and require careful handling. The earlier article titled Proper care and handling of UV – Vis absorption measurement cells highlights the precautions required in careful handling of cells and on ensuring a high degree of accuracy of your measurements.
Glass cells are most common in school and college undergraduate laboratories because of their lower cost. Optical glass shows absorbance throughout the visible and IR regions extending from nearly 340 nm to 2500nm covering majority of organic and in inorganic species. However, glass absorbs strongly in UV region and its application is not recommended for wavelengths below 340 nm.
Like glass transparent plastic cuvettes find application in absorbance measurements in visible region. The additional benefit is that such cuvettes are unbreakable but on the other hand they cannot be used in UV absorbance studies as well as with certain organic solvent compounds
Quartz is expensive in comparison with glass and transparent plastics but has the additional benefit of covering both UV and visible regions right from 190 nm. However, it is even more fragile than glass and the cuvettes need to be handled with greater care.
It is important to bear in mind that oxygen present in the atmosphere absorbs significantly below 200 nm and nitrogen absorbance assumes significance below 190 nm. Absorbance measurements made below 200 nm should be conducted with oxygen free nitrogen purging of sample compartment. However, for measurements below 185 nm it is best to maintain vacuum in the sample chamber.
A blank reference measurement provides highly reliable results but for the purpose cell pair with optically matched windows should be used to prevent any absorbance mismatches.