Atomic Fluorescence Spectroscopy

Posted by: chrisball | Posted on: October 11th, 2011 | 1 Comments

The energy stored in atoms can be released in a variety of ways. When it is released as light, this is known as fluorescence. Atomic fluorescence spectroscopy (AFS) measures this emitted light. The wavelength of the emitted light tells you the identity of the atoms. The intensity of the fluoresced light is directly proportional to the concentration of atoms and the intensity of the excitation source. Atomic fluorescence is generally much more sensitive than atomic absorption.

Some atomic structures, when irradiated with light of a certain wavelength, absorb this radiation. A reaction occurs within the atomic structure causing emission of radiation at a different wavelength.

Used in the measurement of trace amounts of volatile heavy metals such as mercury, cold vapour atomic fluorescence spectroscopy makes use of the unique characteristic of mercury that allows vapour measurement at room temperature. Mercury atoms in an inert carrier gas are excited by a collimated ultraviolet light source at a wavelength of 253.7 nanometres. As the atoms return to their non-excited state they re-radiate their absorbed energy at the same wavelength.

Unlike the excitation source, the fluorescence is emitted omni-directionally and may be detected using a photomultiplier tube or UV photodiode. Fluorescence is generally measured at a 90° angle from the excitation source to minimize collection of scattered light from the excitation source. The technique differs from atomic absorption in that it is more sensitive, more selective, and is linear over a wider range of concentrations. However, any molecular species present in the carrier gas will tend to quench the fluorescence signal, so the technique is most commonly used with an inert carrier gas such as argon.

Gold coated traps may be used to collect mercury in ambient air or other gases. The traps are then heated, releasing the mercury from the gold while passing argon through the cartridge. This pre-concentrates the mercury, greatly increasing the sensitivity of the analysis, and also facilitates transfer of the mercury into the argon gas stream. This is an extremely fast and effective method of allowing atomic fluorescence spectroscopy to be used in environmental air sampling.

Various analytical methods approved by the United States Environmental Protection Agency are in common use. US EPA Methods 245.7 and EPA Method 1631, are commonly used methods for mercury measurement in waters and industrial effluents using cold vapour atomic fluorescence spectroscopy (CVAFS).