Environmental Engineering Reference
In-Depth Information
Absorbed wavelengths of a ray correspond to those emitted when the
components of the material leave a given energy level for a lower energy one. It is
said that this incident beam produces a resonance relating to the absorption.
Each spectroscopic method can be described in a simple way:
- an incident beam directed towards the sample (the material being analyzed);
- a system chosen is that best able to separate the different rays of the beam due
to absorption, diffusion or emission (diffraction network or interferometry);
- one or several appropriate detectors that lead to the spectrum are chosen.
This spectrum is then compared to standard spectra and/or analyzed ones through
computing. In most of cases, the analysis is also quantitative [CER 96, ROU 92].
X fluorescence spectroscopy
X fluorescence is a phenomena resulting from bombarding a material with X-ray
radiation or particles, very often electrons as in the X microprobe (see section
10.1.1.3). In order to observe this X fluorescence, the energy of the incident
X photons must be high enough (tens of keV, as for SEM). The spectrum emitted,
which is typical of the excited elements, depends very little on the incident beam
(X-rays, particles) or bonding between atoms and molecules [ROU 92].
Different kinds of fluorescence are available depending on the environment: air,
under vacuum or helium, and they allow the detection and quantification of all
elements from Z = 5 [BEN 93]. Some portable instruments are useful to perform
quick
in situ
analyses. The detection of fluorescence occurring in the air, even under
helium flux, was unable to detect elements with Z lower than 13 (Al) and sodium
was excluded [LOR 95]. In recent years, portable devices fitted with new (silicon)
detectors have appeared on the market allowing the detection of magnesium (Z=12)
in rough atmospheric conditions.
X fluorescence may be also used for the chemical characterization of salts. The
X-ray detector can be an energy dispersive probe or wavelength dispersive one. One
of the main advantages of quantitative analysis by X fluorescence is that it does not
require special preparation of samples and it does not destroy the surface analyzed.
For quantification, a special preparation is necessary to create a homogenous
sample, but it is not used in our field of interest.
As fluorescence comes from the first layers of matter, it is well adapted for the
characterization of superficial alterations [SCI 87]. The amount of matter required is
very small (a few mg), and it is possible to detect ppm, or even ppb in the case of
fluorescence due to proton illumination [LOR 95].
