<P> Samples for UV / Vis spectrophotometry are most often liquids, although the absorbance of gases and even of solids can also be measured . Samples are typically placed in a transparent cell, known as a cuvette . Cuvettes are typically rectangular in shape, commonly with an internal width of 1 cm . (This width becomes the path length, L (\ displaystyle L), in the Beer - Lambert law .) Test tubes can also be used as cuvettes in some instruments . The type of sample container used must allow radiation to pass over the spectral region of interest . The most widely applicable cuvettes are made of high quality fused silica or quartz glass because these are transparent throughout the UV, visible and near infrared regions . Glass and plastic cuvettes are also common, although glass and most plastics absorb in the UV, which limits their usefulness to visible wavelengths . </P> <P> Specialized instruments have also been made . These include attaching spectrophotometers to telescopes to measure the spectra of astronomical features . UV - visible microspectrophotometers consist of a UV - visible microscope integrated with a UV - visible spectrophotometer . </P> <P> A complete spectrum of the absorption at all wavelengths of interest can often be produced directly by a more sophisticated spectrophotometer . In simpler instruments the absorption is determined one wavelength at a time and then compiled into a spectrum by the operator . By removing the concentration dependence, the extinction coefficient (ε) can be determined as a function of wavelength . </P> <P> UV - visible spectroscopy of microscopic samples is done by integrating an optical microscope with UV - visible optics, white light sources, a monochromator, and a sensitive detector such as a charge - coupled device (CCD) or photomultiplier tube (PMT). As only a single optical path is available, these are single beam instruments . Modern instruments are capable of measuring UV - visible spectra in both reflectance and transmission of micron - scale sampling areas . The advantages of using such instruments is that they are able to measure microscopic samples but are also able to measure the spectra of larger samples with high spatial resolution . As such, they are used in the forensic laboratory to analyze the dyes and pigments in individual textile fibers, microscopic paint chips and the color of glass fragments . They are also used in materials science and biological research and for determining the energy content of coal and petroleum source rock by measuring the vitrinite reflectance . Microspectrophotometers are used in the semiconductor and micro-optics industries for monitoring the thickness of thin films after they have been deposited . In the semiconductor industry, they are used because the critical dimensions of circuitry is microscopic . A typical test of a semiconductor wafer would entail the acquisition of spectra from many points on a patterned or unpatterned wafer . The thickness of the deposited films may be calculated from the interference pattern of the spectra . In addition, ultraviolet - visible spectrophotometry can be used to determine the thickness, along with the refractive index and extinction coefficient of thin films as described in Refractive index and extinction coefficient of thin film materials . A map of the film thickness across the entire wafer can then be generated and used for quality control purposes . </P>

Uv visible absorption spectroscopy in quantitative organic analysis