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.
A European waste incineration company approached PS Analytical to configure
an on-line effluent monitoring system to its wastewater treatment process.
The company processes its stack gas scrubber liquors in
a wastewater treatment plant
on-site. The treatment involves pH correction of the gas filtration
through to removal
of the majority of heavy metals as sludge. The resulting wastewater is
further treated by
passing through sand beds and finally through activated carbon filter beds
before discharge to the local river. Previously, the company analysed its
effluent for mercury manually, approximately three times a day. The
collection, transportation, sample preparation and calibration, followed by
subsequent determination, could take as long as five hours.
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
The wavelength of the fluorescence radiation is generally the same or
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
(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
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
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