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What Is an Absorption Spectrum?

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  • Written By: M.J. Casey
  • Edited By: Daniel Lindley
  • Last Modified Date: 06 November 2016
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An absorption spectrum is obtained by exposing a sample of a pure compound to light. The amount of energy taken up by the molecules in the sample is plotted as a spectroscope scans the wavelengths from red to ultraviolet. Chemists use an absorption spectrum to identify unknown organic and transition-metal compounds. Absorption spectra are used by biologists to relate the wavelengths of light absorbed during photosynthesis to various plant pigments.

Visible light, or light that can be detected by the human eye, ranges in wavelength from about 400 to 700 nm (1.5 x 10-5 to 2.8 x 10-5 inches ). For an object to appear colored, it must absorb energy within this band. The atomic structures that do so are called chromophores and are of two main types: transition-metal ions and conjugated organic bonds, such as occur in double and triple carbon-carbon bonds.

The energy absorbed by the transition metals is related to the quantum energy jump as an outer shell electron is boosted into a more energetic orbital. These excited states are not stable, and the energy is quickly released again. Transition metals appear in the middle of the periodic table.

Conjugated organic molecules often consist of a series of double bond-single bond pairs in a long chain. Lycopene, with 12 double-single pairs, is the red pigment of tomatoes, and beta-carotene, with 11 pairs, is the orange pigment of carrots. The molecules absorb the energy of photons of a single wavelength over the length of the molecule.

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An absorption spectrum exhibits broad responses rather than the single sharp peaks that would be expected from the absorption of single wavelengths of light. This is due to the non-quantum absorption of energy by other parts of the molecule. The spectra are characteristic enough to be used for qualitative identification of compounds. Organic labs have reference books of absorption spectra.

Flame atomic absorption instruments measure the concentration of metal solutions by vaporizing the metal ion. By ridding the sample of other components, the metal atoms will be in their ground state. When the metal gas is exposed to light, a sharp response will be recorded as an outer shell electron absorbs energy of a particular wavelength. Quantitative analysis of metals is possible with this technique.

Biologists use an absorption spectrum study to identify the wavelengths absorbed in the photosynthetic process. By correlating photosynthetic output with wavelength and a known absorption spectrum for each plant pigment, the activity of each pigment can be examined. Similar techniques are used for other light-induced reactions.

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