Environmental Engineering Reference
In-Depth Information
Atomic absorption spectrophotometry, flame absorption spectrophotometry and
atomic emission spectrophotometry
In these three types of spectrometry, the stone sample is heated to a few thousand
degrees (in a burner or an electro-thermic oven), in order to vaporize and reduce it to
the state of free atoms. There are very sensitive methods and it is possible to detect
ppm. They cannot provide information on atomic bonding in the stone sample in its
natural state [ROU 92]. Quantification of the anions and cations of soluble salts is
often made with these methods.
Atomic absorption spectrophotometry
In this case, atomized elements absorb the radiation emitted by a (hollow
cathode) lamp at a specific wavelength characteristic of the atom we want to
quantify. The beam is changed for each new element to be quantified, and the
resulting absorption spectrum identifies every atom present in the gas. The
quantification of an element is linked to the intensity of absorption. Sensitivity is
around the ppm and a few mg of matter is sufficient for the analysis. It is a very
useful technique for the quantification of metallic ions.
Flame absorption spectrophotometry
In this case, the material to be quantified is sprayed and atomized in a flame. The
intensity of rays emitted by atoms while they go from an excited state to a lower
energy level is measured. Alkaline cations (Ca 2+ , Na + ) are easily quantifiable by
flame emission. The amounts of matter needed and the sensitivity are close to those
of atomic absorption spectrophotometry.
Atomic emission spectrophotometry
The principle of atomic emission spectrophotometry is the same as for flame
emission but the temperature of nebulization is much higher (5,000 to 8,000°C) due
to using an argon plasma. This high temperature considerably improves performance
compared to flame emission (rapidity and sensitivity). It is possible to quantify
many elements simultaneously or successively at lower concentration rates than
flame emission, to a level of 10 ppb. Cations, such as Ca 2+ , Na + , K + , Mg 2+ , are often
quantified using this method.
The chromatographic and spectroscopic methods described above are destructive
for the sample analyzed. Nevertheless, for analyzing salts present underneath the
surface, it is possible to collect a sample without damaging a piece of art. The
method usually used for this purpose is poulticing, which consists of applying
cellulose (foil or powder) to the stone wetted with distilled water for few tens of
minutes to permit the migration of soluble salts towards the poultice [SCH 96b, VIE
96]. The solution obtained can be analyzed using chromatography, spectroscopy and
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