Environmental Engineering Reference
In-Depth Information
mass spectrometry to get a semi-quantitative evaluation of the soluble salt
contamination of the stone. If we want to know the salts distribution
versus
the
depth, the method described above is not consistent. It is necessary to remove a
sample from the stone, if possible, and divide it into fractions, or to extract powder
in the last destructive way possible by drilling holes of small diameter (<5 mm).
Infrared spectrophotometry
UV and infrared absorption give a great deal of information on the nature of
molecules that form the matter contained in a sample. Nevertheless, within these
radiations, medium infrared (2.5-25 µm) gives more information on the composition
of matter [ROU 92].
Molecular spectra that are obtained are much more complex than those resulting
from atomic spectrophotometry. Indeed, the energetic transitions of molecules
corresponding to spectral transitions are much more numerous as they involve
quantified energies of rotation, vibration, and those due to the bonding of electrons.
Nevertheless, coupled to a computing system using a Fourier-transformed method,
infrared spectrophotometry is a powerful tool with high sensitivity. It is able
to provide an analysis of the whole spectrum (Fourier-transformed IR
spectrophotometry).
IR spectrophotometry is particularly well adapted to detect and analyze organic
compounds, sometimes present at the surface of stones and in the superficially
altered layers [LEM 96]. This analysis may be carried out directly, without any
preparation of the sample, or after the extraction of organic compounds with
solvents.
This technique does not give any indication of the way ions are assembled and is
of limited interest for salt identification. It is possible to confirm the presence of
various salts on the stone surface that have been detected with other techniques
(gypsum, nitrates, oxalates, etc.) [FAS 93b, VER 94b] except chlorides which are
not detectable.
The use of an IR spectrometer fitted with an optical microscope allows the
observation of a surface of a few square microns at the same time as analyzing its
microstructure. It is now possible to map surfaces and, with a very high resolution,
even get an image of the molecular composition of this surface.
IR spectrometry can be completed by Raman spectrometry. This technique is
based on the diffusion of a monochromatic light. It performs qualitative and
quantitative analyses of mineral, organic and biological phases. Due to the size of
the monochromatic laser beams used in Raman spectrometry, it is possible to
