Absorption spectra are used to identify organic compounds and transition metals. Chromophores, such as transition metal ions and conjugated organic bonds, absorb energy within the visible light range. Flame atomic absorption instruments measure metal concentration, while biologists use absorption spectra to study photosynthesis.
An absorption spectrum is obtained by exposing a sample of a pure compound to light. The amount of energy absorbed by the molecules in the sample is plotted as a spectroscope scans wavelengths from red to ultraviolet. Chemists use an absorption spectrum to identify unknown organic compounds and transition metals. 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 this are called chromophores and are of two main types: transition metal ions and conjugated organic bonds, such as occur in carbon-carbon double and triple bonds.
The energy absorbed by transition metals is related to the quantum energy jump when an outer shell electron is boosted into a more energetic orbital. These excited states are not stable and energy is released again rapidly. Transition metals appear in the center 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 single double pairs, is the red pigment in tomatoes, and beta-carotene, with 11 pairs, is the orange pigment in carrots. Molecules absorb the energy of photons of a single wavelength throughout the length of the molecule.
An absorption spectrum shows broad responses rather than the single sharp peaks one would expect from the absorption of single wavelengths of light. This is due to the non-quantum absorption of energy by other parts of the molecule. Spectra are characteristic enough to be used for qualitative identification of compounds. Organic labs have reference books on absorption spectra.
Flame atomic absorption instruments measure the concentration of metal solutions by vaporizing the metal ion. By freeing the sample from 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 external electron absorbs energy of a particular wavelength. Quantitative analysis of metals is possible with this technique.
Biologists use a study of the absorption spectrum to identify the wavelengths absorbed in the photosynthetic process. By correlating the photosynthetic output with the 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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