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 effluent 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 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.

Flame measurements
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 systems.



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