A source of intense excitation

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.