Absorption Spectroscopy
Absorption spectroscopy is a "workhorse" technology widely used in industry and in chemistry, biology, medicine, and other fields of scientific research. Numerous sciences, including chemistry and astronomy, have reached their current state of advancement only though use of absorption spectroscopic instruments.
Spectroscopy studies the way electromagnetic radiation interacts with matter. When radiation meets matter, the radiation is either scattered, emitted, or absorbed. This gives rise to three principal branches of spectroscopy. Emission spectroscopy observes light emitted by atoms excited by radiation/matter interactions. Raman spectroscopy monitors light scattered from molecules. Absorption spectroscopy studies radiation absorbed at various frequencies (wavelengths).
The roots of absorption spectroscopy can be traced to 1802, when the English chemist William Hyde Wollaston (1766-1828) observed what later became known as the Fraunhofer lines--the many dark lines seen in the spectrum of sunlight. In 1859, the German physicist Gustav Kirchhoff (1824-1887) determined that these lines resulted from absorption of specific frequencies of solar radiation by vapors in the Sun's atmosphere. Subsequent study of these lines taught scientists many things about the chemical composition of the Sun and other astronomical objects.
In its early years, spectroscopy was largely confined to the study of emitted radiation. By the mid-1950s, however, several technical difficulties had been overcome, and manufacturers began producing absorption spectroscopy equipment for routine laboratory analysis.
In absorption spectroscopy, light illuminates a sample of material to be analyzed. The sample, which can be liquid, solid, or gas, is usually enclosed in an absorption cell, which in turn may be enclosed in a oven to vaporize and atomize the material. Each element or compound in the sample absorbs particular wavelengths of light, resulting in one or more dark lines on its spectrum. These lines are a "fingerprint" identifying what chemical substances are present in the sample and their quantities, as well as other information about detailed structure and activity.
The technology is capable of identifying both basic elements and highly complex molecules in concentrations as small as parts per billion. It is used in a wide range of applications, including analysis of body fluids and tissues, municipal water supplies, workplace atmospheres, metal alloys, and geological samples. In forensic science, absorption spectroscopy detects arsenic and other poisons in the bodies of crime victims, analyses gunshot residues, and even allows investigators to detect counterfeit whisky. In industrial settings it can monitor the amount of fat in peanut butter or carbon dioxide in soda, as well as the quality of cosmetics, detergents, paints, drugs, and thousands of other manufactured products.
One common use of absorption spectroscopy is called a chem 20. This is a widely-used series of 20 or 30 tests on a patient's blood, prescribed by doctors to help determine the cause of illness and prescribe appropriate treatment.
Another example of the usefulness of this technology is in determining potential environmental impacts of using methyl alcohol instead of gasoline in internal combustion engines. Samples of exhaust gas produced by this fuel were placed in a chamber about 13 ft (4 m) long, through which infrared light was passed. The resulting spectra indicated the presence of methyl nitrite--a potentially dangerous substance whose presence was not indicated by other analytical methods.
As the millennium drew to a close, Western scientists were expressing great interest in a technique known as intracavity laser absorption spectroscopy that was quietly developed in the Soviet Union during the 1970s.Lasers amplify light by bouncing small numbers of photons back and forth in a cavity with mirrors at either end. By placing the sample to be analyzed inside a laser's light amplification cavity, the Soviet scientists found they could amplify faint signals from just a few molecules of gases that would otherwise go undetected. Because of the Cold War, news of this development took some time to reach researchers in other parts of the world, and was largely ignored when it did arrive. However, by the late 1990s, Western scientists acknowledged that the laser technique was far more sensitive than traditional absorption spectroscopy, and were using it for many applications, including research aimed at understanding the rapid chemical changes that occur during combustion.
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