Absorption spectroscopy

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Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a function of frequency, and this variation is the absorption spectrum. Absorption spectroscopy is performed across the electromagnetic spectrum.

Absorption spectroscopy is employed as a analytical chemistry tool to determine the presence or absence of a particular substance and, in many cases, to quantify the amount of the substance present. Infrared and ultraviolet-visible spectroscopy are particularly common in analytical applications. Absorption spectroscopy is also employed in studies of molecular and atomic physics, astronomical spectroscopy and remote sensing.

There are a wide range of experimental approaches to measuring absorption spectra. The most common arrangement is to direct a generated beam of radiation at a sample and detect the intensity of the radiation that passes through it. The transmitted energy can be used to calculate the absorption. The source, sample arrangement and detection technique vary significantly depending on the frequency range and the purpose of the experiment.

Contents

[edit] Basic Theory

More technically,[1][2] absorption spectroscopy is based on the absorption of photons by one or more substances present in a sample, which can be a solid, liquid, or gas, and subsequent promotion of electron(s) from one energy level to another in that substance. Note that the sample can be a pure, homogeneous substance or a complex mixture. The frequency at which the incident photon is absorbed is determined by the difference in the available energy levels of the different substances present in the sample; it is the selectivity of absorbance spectroscopy - the ability to generate photon (light) sources that are absorbed by only some of the components in a sample - that gives absorbance spectroscopy much of its utility. Typically, X-rays are used to reveal chemical composition, and near ultraviolet to near infrared wavelengths are used to distinguish the configurations of various isomers in detail. In absorption spectroscopy the absorbed photons are not re-emitted (as in fluorescence) rather, the energy that is transferred to the chemical compound upon absorbance of a photon is lost by non-radiative means, such as transfer of energy as heat to other molecules.

While the relative intensity of the absorption lines do not vary with concentration, at any given frequency the measured absorbance ( − log(I / I0)) has been shown to be proportional to the molar concentration of the absorbing species and the thickness of the sample the light passes through. This is known as the Beer-Lambert law. The plot of amount of radiation absorbed versus frequency for a particular compound is referred to as the absorption spectrum. The normalized absorption spectrum is characteristic for a particular compound, does not change with varying concentration and is like the chemical "fingerprint" of the compound. At frequencies corresponding to the resonant energy levels of the sample, some of the incident photons are absorbed, resulting in a drop in the measured transmission intensity and a corresponding dip in the spectrum. The absorption spectrum can be measured using a spectrometer and by knowing the shape of the spectrum ,the optical path length and the amount of radiation absorbed, one can determine the structure and concentration of the compound.

[edit] Relation To Emission Spectroscopy

Emission is a process by which a substance releases energy in the form of electromagnetic radiation. Emission can occur at any frequency at which absorption can occur, and this allows the absorption lines to be determined from an emission spectrum. The emission spectrum will typically have a quite different intensity pattern from the absorption spectrum, though, so the two are not equivalent. The absorption spectrum can be calculated from the emission spectrum using appropriate theoretical models and additional information about the quantum mechanical states of the substance.

[edit] Relation To Scattering and Reflection Spectroscopy

The scattering and reflection spectra of a material are influenced by both its index of refraction and its absorption spectrum. In an optical context, the absorption spectrum is typically quantified by the extinction coefficient, and the extinction and index coefficients are quantitatively related through the Kramers-Kronig relation. Therefore, the absorption spectrum can be derived from a scattering or reflection spectrum. This typically requires simplifying assumptions or models, and so the derived absorption spectrum is an approximation.

[edit] Application

[edit] Analytical Chemistry

Absorption spectroscopy is useful in chemical analysis because of its specificity and its quantitative nature. The specificity of absorption spectra allows compounds to be distinguished from one another in a mixture. For example, absorption spectroscopy is used to identify the presence of pollutants in the air, distinguishing the pollutant from the nitrogen, oxygen, water and the other expected constituents.[3] The specificity also allows unknown samples to be identified by comparing a measured spectrum with a library of reference spectra. In many cases, it is possible to determine qualitative information about a sample even if it is not in a library. Infrared spectra, for instance, have characteristics absorption bands that indicate if carbon-hydrogen or carbon-oxygen bonds are present.

An absorption spectrum can be quantitatively related to the amount of material present using the Beer-Lambert law. Determining the absolute concentration of a compound requires knowledge of the compound's absorption coefficient. The absorption coefficient for some compounds is available from reference sources, and it can also be determined by measuring the spectrum of a calibration standard with a known concentration of the target.

[edit] Atomic and Molecular Physics

[edit] Remote Sensing

[edit] Astronomy

[edit] Experimental Methods

[edit] Basic Approach

Visible light absorption spectra can be taken in anything that is visibly clear. Polystyrene, quartz glass, and borosilicate (Pyrex) cells, often called cuvettes, are the most commonly used. UV light is absorbed by most glasses and plastics, so quartz cells are used. The Si-O moieties in glasses and quartz, and the C-C moieties in plastics absorb infrared light. Therefore, infrared absorption spectra are typically carried out with a thin film of the sample held in place between sodium chloride sample plates. Other methods involve suspending the compound in a substance that does not absorb in the region of study. Mineral oil (Nujol) emulsions and potassium bromide glasses are perhaps the most common. NaCl and KBr, being ionic, do not have significant IR absorptions, and Nujol has a relatively uncomplicated IR spectrum.

[edit] Specific Approaches

[edit] References

  1. ^ Modern Spectroscopy (Paperback) by J. Michael Hollas ISBN 0470844167
  2. ^ Symmetry and Spectroscopy: An Introduction to Vibrational and Electronic Spectroscopy (Paperback) by Daniel C. Harris, Michael D. Bertolucci ISBN 048666144X
  3. ^ "Gaseous Pollutants - Fourier Transform Infrared Spectroscopy". http://www.epa.gov/apti/course422/ce4b4.html. Retrieved 2009-09-30. 

[edit] See also

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