Absorption spectroscopy

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Absorption spectroscopy is an analytical tool used by chemists and physicists. The absorption spectrum is characteristic for a particular element or compound, and does change with varying concentration. It is based on the absorption of quanta of light by a chemical substance, due to the promotion of electrons from one atomic orbital or molecular orbital to another in that substance. The wavelength of the photon will determine the energy level of excitation according to Planck's law. 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.

While the relative intensity of the several absorption lines does not vary, at a given wavelength the measured absorbance 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 absorption versus wavelength for a particular compound is referred to as the absorption spectrum. The absorption spectrum is measured using a spectrophotometer, which disperses the transmitted light using a diffraction grating and subtracts from the known incident spectrum to determine opacity at each wavelength in the measured range. At wavelengths 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.

Visible light absorption spectra can be taken in anything that is visibly clear. Polystyrene, quartz, 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 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.

Spectroscopy as an analytical tool

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Often it is of interest to know not only the chemical composition of a given sample, but also the relative concentrations of the several compositing compounds. To do this, a scale, or calibration curve, must be constructed using known concentrations, in much the same way as a spring scale must be calibrated using known weights. For instance, if you happened to have on hand a 50 kg mass, a 100 kg mass, and a spring which is suitably stiff (i.e. it remains elastic and continues to obey Hooke's law in the regime of interest), it would be possible to determine my mass by noting that I stretch or compress the spring 1.44 times as far from rest as does the lighter and much lighter masses.

Example: a cyanide standard at 200 parts per million gives an absorbance with an arbitrary value of 1540. An unknown sample gives a value of 834. The math could be stated as: "if 200 gives you 1540, what gives you 834?" Since this is a linear relation and goes through the origin, the unknown is easily calculated to be 108 parts per million. Note the beauty of the method in that it is not necessary to know anything about the governing coefficients, or chromophores, or the experimental cell length, or even the underlying theory - it all must divide out.

A somewhat more complicated method which will give superior results is the use of a calibration curve: initially an estimate of the order of magnitude of the concentration of the sample is made by the method given above, then a number of standards within the appropriate order of magnitude (so chosen that they will be convenient to fit) are measured and fitted either by hand or by mathematical software. The resulting formula can be used to determine concentration of the sample more accurate then with only one standard, because this method excludes random interference (noise) which is often a problem in this type of experiment.

Absorption spectroscopy

Spectroscopy as an analytical tool

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