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Chemical Analysis Using Light


Many analytical measurement techniques make use of spectroscopy, the study of the interaction of radiation with matter. Before we delve into various analytical techniques it will be helpful to briefly introduce some important terms and concepts with some simple diagrams.

Absorption and Emission Processes


All atoms and molecules possess quantized energy levels. Spectroscopists study the "transitions" or jumps between these energy levels. Two fundamental (and related) processes are absorption and emission.

Absorption


In this process, energy in the form of electromagnetic radiation is taken up by an atom or molecule (LHS of diagram) to place it in a higher energy state (RHS).

In a sample undergoing this process there will be a reduction in light intensity as the sample is traversed. This can be exploited analytically, since the measured change in light intensity can be used to determine the concentration of the absorbing sample.
A spectrophotometer is an instrument that measures the amount of light absorbed by a sample at each incident wavelength. It has a means to sequentially change the colour of the light passing through the sample (i.e. to scan the wavelength) allowing the sample's "absorption spectrum" to be recorded.

Lower cost instruments called colorimeters employ light emitting diodes (LED's) to measure absorption in a narrow band of wavelengths.

Emission


In atoms and molecules, a valence electron can be excited from a ground state to an excited state as a result of the absorption of UV or visible light (LHS of diagram). The reverse process, in which the electron drops back from an excited state to a lower energy state and the molecule re-emits light (RHS) is often called "fluorescence".
Instruments that are used to measure fluorescence (spectrofluorimeters) are manufactured with separate excitation and emission monochromators that allow the selection of both the wavelengths of excitation (absorption) and emission. Often, the sample is irradiated with light from a xenon flash lamp, and the emergent fluorescence is detected by a photomultiplier oriented at 900 to the angle of irradiation.

A fluorometer is a simpler, lower cost fluorescence instrument that typically employs a light emitting diode of a particular colour as the excitation source, and measures the fluorescence emission using a cut-off filter and a photodiode.

Fluorescence can be an extremely sensitive method for quantitation of certain molecules; however not all molecules fluoresce. Dyes such as those used in highlighter pens (and even certain minerals) fluoresce strongly when illuminated by a suitable light source, such as a black light.

Another form of emission is chemiluminescence. In this case the excited state species is formed in a chemical reaction. Chemiluminescence is another very useful and extremely sensitive analytical technique.