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Ultraviolet spectroscopy is used to determine which wavelengths of light a chemical solution absorbs, allowing for analysis of the sample. A spectrophotometer directs light through a cuvette and measures the amount of light absorbed. Dual-beam spectrophotometers measure both the ultraviolet and visible spectrum. The results are plotted on a graph to show the wavelengths absorbed and the overall absorbance of the solution.
Ultraviolet spectroscopy, often combined with visible spectroscopy, is a technique used in scientific and industrial laboratories to determine which wavelengths of light a chemical solution absorbs. This information allows the researcher to analyze the contents of the sample solution. Ultraviolet spectroscopy is done with a special device known as an ultraviolet-visible spectrophotometer.
Light is a form of electromagnetic radiation, energy that travels in waves. This type of radiation can be characterized based on its wavelength. The total range of these wavelengths forms a scale known as the electromagnetic spectrum.
The wavelengths of visible light range from approximately 400-750 nanometers. The shortest and highest energy wavelengths that are just outside the visible spectrum, from about 10-400 nanometers, are classified as ultraviolet. Ultraviolet spectroscopy determines the absorbance of a chemical specifically in the ultraviolet part of the spectrum.
When light passes through a chemical solution, some light is absorbed and some light is transmitted: it passes without being absorbed. The amount of light absorbed by the compound can be measured with a spectrophotometer and can be used to determine the concentration of the solution. Higher levels of absorption indicate a more concentrated solution.
Chemical compounds not only absorb light, they also absorb light at specific wavelengths. A solution that appears green, for example, transmits green visible wavelengths and absorbs red and blue. Ultraviolet spectroscopy can measure absorbance and transmittance beyond the visible spectrum, into the ultraviolet range.
A spectrophotometer works by directing a beam of light through a cuvette, or clear tube, that holds the sample solution. In ultraviolet spectroscopy, the cuvette should be made of quartz glass rather than plastic, as plastic tends to absorb ultraviolet light. A detector on the other side of the cuvette converts the incoming light into an electric current, which can be read by the electronics in the device.
Dual-beam ultraviolet-visible spectrophotometers measure both the ultraviolet spectrum and the visible spectrum using two cuvettes, one of which contains the sample and the other contains a reference solution. There are also two light sources: one light source generates visible light and the other generates ultraviolet light. A prismatic component splits the incoming light into two beams. A beam is fed through each cuvette and a detector reads the results.
Information from the detector is used to generate a graph that plots wavelength versus absorbance. The peaks in the graph indicate wavelengths that have been heavily absorbed by the compound. They also quantitatively show the overall absorbance of the solution.
