Absorption spectroscopy

A white beam of light strikes a sample in 1992. Photons matching the energy gap of the molecules present are absorbed to excite them. Other photons transmit unaffected through the material. The intensity of this absorption varies as a function of frequency. This variation creates an absorption spectrum that reveals the atomic and molecular composition of the sample. Absorption lines occur when radiation matches the energy difference between two quantum mechanical states. Rotational lines appear in the microwave spectral region while vibrational lines show up in the infrared region. Electronic lines correspond to changes in electronic state and typically fall within the visible and ultraviolet regions. X-ray absorptions involve the excitation of inner shell electrons in atoms.

Spectral Categories Defined

Scientists categorize techniques by electromagnetic radiation range including X-ray, infrared, ultraviolet, and microwave methods. A table lists five major types: X-ray absorption spectroscopy, UV, vis absorption spectroscopy, IR absorption spectroscopy, Microwave absorption spectroscopy, and Electron spin resonance spectroscopy. Nuclear magnetic resonance spectroscopy also appears on the list. Infrared and ultraviolet, visible spectroscopy are particularly common in analytical applications. Some sources inherently emit a broad spectrum like globars or mercury lamps. Others generate a narrow spectrum but can be tuned to cover a spectral range. Synchrotron radiation covers all these spectral regions as a recently developed source. Lasers across the infrared, visible, and ultraviolet region provide another option for generating light.

Measuring The Invisible

Infrared gas analyzers identify pollutants in air by distinguishing them from nitrogen, oxygen, water, and other expected constituents. An absorption spectrum relates quantitatively to the amount of material present using the Beer, Lambert law. Determining absolute concentration requires knowledge of the compound's absorption coefficient. Reference sources often contain this data for known compounds. Calibration standards with known concentrations allow scientists to determine coefficients for unknown substances. Specificity allows compounds to be distinguished from one another in a mixture. Carbon-hydrogen bonds show characteristic absorption bands in infrared spectra. Carbon-oxygen bonds appear as distinct features in the same spectral region. This quantitative nature makes absorption spectroscopy useful for wide variety of applications.

Common questions

What is absorption spectroscopy and how does it work?

Absorption spectroscopy measures the absorption of radiation by a sample to reveal its atomic and molecular composition. Photons matching the energy gap of molecules are absorbed while other photons transmit unaffected through the material. This variation in intensity creates an absorption spectrum that identifies specific substances.

When was the first direct detection and chemical analysis of an exoplanet atmosphere performed using absorption spectroscopy?

The first direct detection and chemical analysis of the atmosphere of an exoplanet occurred in 2001. Sodium in the atmosphere filters the starlight of HD 209458 as the giant planet passes in front of the star. Astronomical spectra contain both absorption and emission spectral information used for this discovery.

Where do rotational lines appear in the electromagnetic spectrum during absorption spectroscopy measurements?

Rotational lines appear in the microwave spectral region when radiation matches the energy difference between two quantum mechanical states. Vibrational lines show up in the infrared region while electronic lines fall within the visible and ultraviolet regions. X-ray absorptions involve the excitation of inner shell electrons in atoms.

How does Beer Lambert law relate to determining concentration in absorption spectroscopy applications?

An absorption spectrum relates quantitatively to the amount of material present using the Beer, Lambert law. Determining absolute concentration requires knowledge of the compound's absorption coefficient found in reference sources. Calibration standards with known concentrations allow scientists to determine coefficients for unknown substances.

Why is remote analysis possible without bringing instruments into contact with samples in absorption spectroscopy?

Measurements can be made without bringing the instrument and sample into contact because radiation traveling between them contains the necessary spectral information. Toxic or hazardous environments become safe zones when operators stay far away from the danger. Sample material does not have to touch the instrument preventing possible cross contamination.