Experimental Techniques for Characterizing Alterations - Environmental Geomechanics - page 282

perform microanalyses (particles of 1 µm in diameter) under optical microscopy
[CER 96].
10.1.1.7. Mass spectrometry
This method is able to determine the nature of compounds present in a sample of
matter and to then quantify them. Overall it allows us to identify and analyze
molecular compounds and their structure, organic phases in particular. It is useful to
look for traces of organic compounds that have been left from previous stone
treatments. These are often present in the alteration layers.
The material to be analyzed is first ionized under high vacuum. Ions are then
accelerated under the effects of electric fields, separated according their mass/charge
ratio (by means of their trajectory), and then detected by appropriate detectors
whose outlet signal leads to a mass spectrum. This spectrum is linked to the nature
of each element and its quantity. For this purpose, libraries of spectra are used [CER
96, ROU 92].
Many mass spectrometries are available that can be distinguished by their type of
ionization and detection of ions. This method requires a very small amount of matter
and shows a high sensitivity (around the ppb) that theoretically permits us to detect
all elements. It is used to identify organic dyes and certain organic processes. Its use
for the quantification of salts is scarce because it is very expensive.
10.1.1.8. Other methods
Within the numerous other methods used, it is worth mentioning:
- The analysis by neutron activation : an intense flux of neutrons falling down on
a material generates radioisotopes. The energy due to this radioactivity is analyzed
with a suitable detector. This makes it possible to detect all the elements between
atomic numbers 11 and 81 with a high sensitivity. This is the major benefit of this
technique, which can go far below ppb. It may be useful for the restoration of
monuments by detecting trace elements that are the signature of the quarry [HOL
94] from where the stones used for the construction were extracted.
- Thermogravimetric methods are based on the mass variation of the sample.
Increases in temperature allow us to detect, in particular, unstable organic
compounds [DEL 95]. Combined with differential thermal analysis, the methods are
able to quantify and recognize water and CO 2 within a mortar due to decomposition
of the calcitic binder.
- Electric capacitive methods aim to measure the water content (see Chapter 11,
section 11.1.2). These methods are based on the in situ measurement of the electric
conductivity (or electric resistivity) of stone using the four electrodes principle. The