What Is Ultraviolet Spectroscopy?

Ultraviolet spectroscopy, often combined with visible spectroscopy, is a technique that is 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 performed 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 about 400-750 nanometers. The shorter, higher energy wavelengths lying just outside the visible spectrum, from about 10-400 nanometers, are classified as ultraviolet. Ultraviolet spectroscopy determines a chemical’s absorbance specifically in the ultraviolet part of the spectrum.

When light passes through a chemical solution, a certain amount of light is absorbed, and a certain amount is transmitted — it passes through without being absorbed. The amount of light that is absorbed by the compound can be measured by a spectrophotometer and can be used to determine the solution's concentration. 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, is transmitting green visible wavelengths and absorbing red and blue. Ultraviolet spectroscopy can measure absorbance and transmittance beyond the visible spectrum, in the ultraviolet range.

A spectrophotometer works by directing a beam of light through a cuvette, or clear tube, that contains the sample solution. In ultraviolet spectroscopy, the cuvette must be made of quartz glass rather than plastic, because plastics tend to absorb ultraviolet light. A detector on the other side of the cuvette converts the incoming light to electric current, which can be read by electronics in the device.

Double-beam ultraviolet-visible spectrophotometers measure in both the ultraviolet spectrum and the visible spectrum by using two cuvettes, one of which holds the sample, and the other of which holds a reference solution. There also are two light sources: one light source generates visible light, and the other generates ultraviolet light. A prism component splits the incoming light into two beams. One beam is fed through each cuvette, and a detector reads the results.

The information from the detector is used to generate a graph that plots wavelength against absorbance. Peaks in the graph indicate wavelengths that were heavily absorbed by the compound. They also quantitatively show the overall absorbance of the solution.