- Cold vapour atomic fluorescence spectroscopy
Cold vapour atomic fluorescence spectroscopy, sometimes referred to by the acronym CVAFS, is a subset of the analytical technique known as atomic fluorescence spectroscopy (AFS).
Use for mercury detection
Used in the measurement of trace amounts of volatile heavy metals such as mercury, cold vapour AFS makes use of the unique characteristic of mercury that allows vapour measurement at room temperature. Free mercury atoms in a carrier gas are excited by a collimated ultraviolet light source at a wavelength of 253.7 nanometres. The excited atoms re-radiate their absorbed energy (fluoresce) at this same wavelength. Unlike the directional excitation source, the fluorescence is omnidirectional and may thus be detected using a photomultiplier tube or UV photodiode. The technique differs from the more conventional atomic absorption (AA) technique in that it is more sensitive, more selective, and is linear over a wide range of concentrations. However, any molecular species present in the carrier gas will quench the fluorescence signal and for this reason, 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 media. The traps are then heated, releasing the mercury from the gold while passing argon through the cartridge. This preconcentrates the mercury, increasing sensitivity, and also transfers the mercury into an inert gas.
A number of companies have commercialized mercury detection via CVAFS and produced transportable analysers capable of measuring mercury in ambient air. These devices can measure levels in the low parts per quadrillion range (10-15).
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 CVAFS.
Other analytical techniques suitable for analyzing heavy metals in air or water:
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