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Infrared spectroscopy

The basic principle:

We exposed materials to infrared radiation produced by a thermal source ( heated at 1500K) and the infrared energy may be absorbed and the rest will be transmitted or reflected. In placing a detector responding in the infrared region in the path of the transmitted beam, one can determine how the beam was changed. This is the basic principle of an infrared spectrometer.

Radiation is absorbed by a molecule only if the frequency of the radiation provides energy of the precise amount required by one of the bonds in the molecules.

Depending of the number of ways it can move, each kind of bands may absorb infrared region at one or more frequencies. In the above example, the double bond between C and O in the ethyl acetate molecule absorbs at around 1750 cm-1 (or 5.7 microns).


Technical aspects:

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The infrared spectrometer contains a thermal source which provides the infrared radiation. The beam, collected from the source, is collimated onto an interferometer, which modulated the phase of all the frequencies in the infrared region. The resulting signal is sent to the sample and, after passing through it, detected. The resulting signal has the form of an interferogram, as shown in the figure. To demodulate the signal and obtain the true signal, a Fourier Transformation is processed by the computer, and the spectra appear as it is displayed in the above figure.

ATR( Attenuated Total Reflection) spectroscopy is a powerful technique for infrared analysis of a wide range of sample types. ATR sampling is fast and easy because little or no sample preparation is required. The infrared beam in focused onto a high reflective index material ( infrared transparent) , and upon reflection, an evanescent wave probes the material deposited and pressed on its surface. We used Zinc Selenide ( ZnSe) ATR substrate.


In our case, we used about 20 nanograms ( 20 billionth of gram)  of sample for each analysis. The following  picture displays the whole set up, which has been moved inside the I.19 Tomb in Saqqara, inside the Bubasteion area.


Initial results:

We have mainly analyzed samples in the I.19 tomb. Data for comparison studies have been collected also in I.1 , I.20, and I.27 tombs. We also moved the instrument to the storeroom to crosslink results  with those obtained in the Aper-El artefacts.

At first glance, we have identified several organic compounds inside various samples, one of which being more probably originating from beeswax. The recorded spectrum of this wax, together with the reference spectrum, is displayed in the following figure; the sample originates from the shirt of the artist.

The complete interpretation of the recorded spectrum required adding other components, and this is illustrated in the figure: Clay minerals and quartz are present and may explain the additional features.

© P. Dumas, Ph. Walter et A. Zivie (2006)