A source of intense excitation
Environmental concern is now supported by increasing legislation to limit the levels of mercury-laden effluents discharged into receiving waters, and manufacturers exceeding those limits face heavy financial penalties.
Atomic Fluorescence spectroscopy is, by its very nature, inherently sensitive. A typical atomic fluorescence arrangement consists of an intense excitation source focused on to an atom population in a flame. Fluorescence radiation, which is emitted in all directions, then passes to a detector,usually positioned at right angles to the incident light.
The source can be either an atomic line or a continuum and this serves to excite atoms by the absorption of radiation at specific wavelengths. The atoms are then deactivated, partly by collisional quenching with flame gas molecules and partly by emission of fluorescence radiation in all directions.
The wavelength of the fluorescence radiation is generally the same or longer than the incident radiation. The wavelength of the emitted radiation is characteristic of the absorbing atoms and the intensity of the emission can be used as a measure of their concentration.
There are five basic types of fluorescence that occur in flame measurements; (i) resonance fluorescence, (ii) direct-line fluorescence, (iii) stepwise-line fluorescence, (iv) thermally assisted direct-line fluorescence and (v) thermally assisted anti-Stokes fluorescence. Theoretically, increasing the intensity of the excitation source will increase the response and hence the sensitivity of measurement.
Despite its inherent advantages, atomic fluorescence has not been a big
success commercially. This has been due to
the matrix-interference effects that occur when real samples
are analysed. However, coupling a fluorescence measurement technique with a
vapour generation technique has the potential to overcome all of these
problems with an additional bonus: the pre-treatment required to generate
the vapour will in itself remove the great majority of the interfering
species, and the bonus is the increased transfer efficiency of the element
of interest to the measurement cell. Significant improvements to detection
limits can be achieved in this way for mercury, arsenic, selenium
and antimony, and it is comparable to ICP OES and ICP/MS