Spectrophotometry - From Visible To Invisible Light

A man named Dobson changed the way for analytical chemistry forever when he developed a light measuring device called a spectrophotometer

. He used this complex device in the 1920's to measure the atmosphere of earth, which led to the discovery of the ozone layer. This in turn led to the discovery of the ozone layer's depletion, and what was causing it.

This milestone in analytical chemistry became known as Spectrophotometry. A spectrophotometer is a device that measures wavelengths of light from a substance. Visible light transmitted through a substance gets partially absorbed by the material, and the wavelengths that make it through are analyzed and computed. This gives the user an indication of the molecular or even atomic structure of the analyzed substance.

Visible light has all the colors of the rainbow, and when transmitted through a glass or crystal prism, they can be seen. Sir Isaac Newton discovered the different colorimetric wavelengths of light using such a prism. A rainbow itself is nature's spectrophotometer, in a matter of speaking. Light refracted through moisture in the atmosphere interfering with sunlight causes this.

In a spectrophotometer, these colors of light are known as "wavelengths" of light. They are colored and seen by the human eye due to the waves of their light particles. White light contains all of the known spectra, and sunlight has even more, such as ultraviolet and infrared wavelengths. Most spectrophotometers use visible light to analyze materials, but there are some that use the ultraviolet wavelengths.

Ultraviolet wavelengths are invisible to human eyes. This wavelength abounds in nature, and many insects use it to find food. Most flowers are beautiful when viewed in visible light, but look at them from a bee's point of view and you will see a whole different pattern. Spectrophotometers that use ultraviolet see much the same as the insects do; they see all the wavelengths of ultraviolet light.

Some Ultraviolet spectrophotometers use the fluorescent action of substances to measure the absorption of the ultraviolet light. This is a very sensitive approach, and can reveal trace amount of elements in a substance. Most, however, are strictly for use at near-Ultraviolet range, which is still in the visible wavelength of light.

The Beer-Lambert Law is an absorption logarithm that is used in calculating the absorption of substances in solution. It is a generally used logarithm and can have limitations. The law indicates that the given wavelength of analytical light, in conjunction with the absorption of the known solvent, will eventually yield results of what is dissolved in the solution by the absorption of the particular wavelength by the dissolved material.

That being said, the resulting number given by the logarithm after absorption is measured, is then fed into a computer, which matches the results to a large database, thus giving the exact element names and molecular makeup. Quantitatively, the spectrophotometer can give the amount measurement of a given substance, for example, blood CO2 content. This leads to advances in biochemical and medical analytical chemistry. Spectrophotometers have been in medical laboratories for decades, and their uses are taken for granted. Laboratory personnel should be ever-thankful for this ingenious and useful tool.

by: Andrew Long

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