Elsebeth Langholz Kenix (2009) Transmission and Reflection (ATR) Far-Infrared Spectroscopy Applied in the Analysis of
Cultural Heritage Materials University of Bologna (Italy), Dip. Chimica "G. Ciamician" Via Selmi 2, Bologna, Italy:
Abstract: Most infrared analysis is performed in the Mid-IR (MIR) range (4000-400 cm-1). In the case of inorganic components, the normal frequency of an organic stretching vibration decrease as the molecular weight increase, because the frequency of a vibration is inversely proportional to the square-root of the reduced mass. Thus, for high molecular-weight species such as metallic complexes or some halides, oxide and sulphides, this relationship shifts the vibrations into the Far-IR (FIR) region, roughly defined as below 400 cm-1. FIR spectroscopy is thus an alternative way of collecting spectra of many inorganic pigments and corrosion products found on art objects, which is not normally observed in the MIR region. Most FIR spectra are traditionally collected in transmission mode but as a real novelty it is now also possible to record FIR spectra in ATR (Attenuated Total Reflectance) mode.
In FIR transmission we employ polyethylene (PE) for preparation of pellets by embedding the sample in PE using much the same procedure as for preparing KBr pellets. Unfortunately, the preparation requires heating of the PE in order to produces at transparent pellet. This will affect compounds with low melting points, especially those with structurally incorporated water. Another option in FIR transmission is the use of thin films. We test the use of polyethylene thin film (PETF), both commercial and laboratory-made PETF. Here there is no heating of the sample and almost no preparation time, as the powdery sample is merely brushed onto the surface of the PETF and then the spectrum can be collected.
ATR collection of samples is possible in both the MIR and FIR region on solid, powdery or liquid samples. Changing from the MIR to the FIR region is easy as it simply requires the change of detector and beamsplitter (which can be performed within a few minutes). No preparation of the sample is necessary, which is a huge advantage over the PE transmission method. The only concern with ATR is the need for good contact between the ATR crystal and the sample. This can be a problem especially with solid samples as not all solid samples have a planar surface that can come into good contact with the ATR crystal. In this case micro-sampling from the sample surface is necessary.
In most cases comparison of standard spectra collected in FIR transmission and FIR ATR mode leads to only a few differences. The most obvious, when comparing transmission with ATR, is the distortion of band shape. The ATR band shape appears asymmetrical in the lower wavenumber region when compared to spectra collected in transmission mode. Also, intensity differences are noticed. However, the biggest difference can be the shift of strong absorbing bands moving to lower wavenumbers in ATR mode. In the majority of compounds, the shifts observed are small, approximately 1-10 cm-1, but for very strong absorbing compounds the shifts observed can be as big as 30-50 cm-1. These observed changes are in accordance with the established theory of ATR spectroscopy. It necessitates the collection of library standard samples in both FIR transmission and ATR modes, provided these two methods of collecting are to be employed for analyses of unknown samples.
Standard samples of 150 pigment and corrosion compounds are thus collected in both FIR transmission and ATR mode in order to build up a digital library of spectra for comparison with unknown samples. XRD, XRF and Raman spectroscopy assists us in confirming the purity or impurity of our standard samples. Focus of our attention are on the oxides and sulphides of metals, as they are not normally observed in the MIR region, but are visible in the FIR region. Many of these pigments and corrosion standards, particularly those originating from minerals, are available in FIR transmission libraries and literature. However, ATR spectra of pigments and minerals, are hardly available in the literature. This can complicate ATR identification of unknown samples in the FIR region.
24 didactic test tables, with known pigment and binder painted on the surface of a limestone tablet, are used for testing the established library and different ways of collecting in ATR and transmission mode. In ATR, micro samples are scratched from the surface and examined in both the MIR and FIR region. Additionally, direct surface contact of the didactic tablets with the ATR crystal are tested together with water enhanced surface contact. A drop of water on the ATR crystal before contact with the solid sample can enhance the signal intensity when there are problems achieving contact due to planarity issues. But the question is if the sample can tolerate the exposure to water. When the binding media on our test tablet is glue, it is not advisable to use water. Oil binding medium seem to be less affected by contact with the water drop. In FIR transmission we compare the powder from our test tablet on the laboratory PETF and embedded in PE. We also compare the PE pellets collected using a 4x beam condenser, focusing the IR beam area from 8 mm to 2 mm. In theory, this should make it possible to use less sample amount in our pellets. In practice we experience an increase in noise sensitivity in the FIR region compared to spectra of PE pellets collected without the beam condenser.
A few samples collected from a mural painting in a Nepalese temple, corrosion products collected from archaeological Chinese bronze objects and samples from a mural paintings in an Italian abbey, are examined by ATR or transmission spectroscopy.
Notes: Published articles:
â€¢ E. L. Kendix, S. Prati, E. Joseph, G. Sciutto & R. Mazzeo, ATR and transmission analysis of pigments by means of far infrared spectroscopy, Anal. Bioanal. Chem. 2009, 394(4), 1023-1032.
â€¢ E. L. Kendix, S. Prati, R. Mazzeo, E. Joseph, G. Sciutto, Far Infrared Spectroscopy of pigments in Art, Meddelelser om Konservering (Announcement about Conservation), 2009, 2, 3-10.
â€¢ E. Kendix, S. Prati, R. Mazzeo, E. Joseph, G. Sciutto, C. Fagnano, Far Infrared Spectroscopy in the Field of Cultural Heritage, Accepted for ePRESERVATIONScience (IRUG8 conference proceedings) (http://www.morana-rtd.com/e-preservationscience/index.html).
â€¢ R. Mazzeo, S. Prati, M. Quaranta, E. Joseph, E. Kendix, M. Galeotti, Attenuated total reflection micro FTIR characterisation of pigment-binder interaction in reconstructed paint films, Anal Bioanal Chem. 2008, 392, 65-76.
â€¢ E. Kendix, G. Moscardi, R. Mazzeo, P. Baraldi, S. Prati, E. Joseph, S. Capelli, Far Infrared and Raman spectroscopy analysis of inorganic pigments, J. Raman spectroscopy, 2008, 39(8), 1104-1112.
Scientific Presentations To Congresses (Invited Speaker):
ï‚§ 2009: E. L. Kendix, E. Joseph, G. Sciutto, D. Prandstaller, S. Prati, R. Mazzeo â€śThe Application of Far IR Spectroscopy Methods in the Characterization of Pigments and Corrosion Productsâ€ť CNR Workshop, Tecniche avanzate (PEM. IRRAS, VCD, TRS) di Spettroscopia FTIR e Microscopia IR/Raman per la caratterizzazione di materiali e superfici, Thermo Scientific, May 12 2009, Padova (Italy).
ï‚§ 2008: E. Kendix, E. Joseph, G. Sciutto, S. Prati, Vanna Minguzzi, R. Mazzeo, â€śApplication of Far Infrared Spectroscopy Methods in the Chemical Characterization of Pigments and Corrosion Productsâ€ť ISLS 2008 (XIII International Symposium on Luminescence Spectrometry), September 7-11, 2008, Bologna (Italy).
ï‚§ 2007: R. Mazzeo, E. L. Kendix, â€śFROM CURRIC â€¦â€¦.. TO EPISCON (European PhD in Science for Conservation)â€ť â€śScienza e Beni Culturali. La Diagnostica: Formatione e Professtioneâ€ť (AIEDBC), UniversitÃ¡ di Roma, Jan 31 2007, Rome (Italy).
Scientific Presentations To Congresses (Posters):
ï‚§ 2009: E. Kendix, G. Sciutto, S. Prati, R. Mazzeo, Far-IR Spectroscopy Applied for Identification of Pigments and Corrosion Products, presented at XXIII Congressi National della SocietÃ Chimica Italiana, July 5-11 2009, Sorronto (Italy).
ï‚§ 2008: E. Kendix, E. Joseph, S. Prati, R. Mazzeo, Macroscopic and microscopic ATR â€“ FTIR spectroscopy for the study of painting materials in FIR and MIR regions, presented at YOCOCU08 (Youth in Conservation of Culturel Heritage), November 24-25 2008, Rome (Italy).
ï‚§ 2008: E. Gurini, E. Joseph, G. Sciutto, A. Cagnini, E. Kendix, S. Porcinai, D. Prandstraller, S. Prati, Y. Junchang, R. Mazzeo, Compositional and Microstructural Study of Archaeological Bronzes Excavated from the Zhou Dynasty (1046 Bc-221 Bc) Aristocratic Cemetery in Liangdai Village, Shaanxi Province, China, presented at ISA08 (37th Int. Symposium on Archaeometry), May 12-16, 2008, Siena (Italy).
ï‚§ 2008: E. Kendix, S. Capelli, E. Joseph, G. Sciutto, S. Prati, R. Mazzeo, Far Infrared Spectroscopy Methods in the Field of Cultural Heritage, presented at IRUG8 (8th International conference on Infrared and Raman Usersâ€™ group), March 26-29 2008, Vienna (Austria).
ï‚§ 2007: E. Joseph, S. Capelli, E. Kendix, S. Prati, R. Mazzeo, Alternative sample preparation methods using ATR-MIR- and FIR-spectroscopy for the study of Cultural Heritage Materials, Presented at HSC5 (Hercules Specialised Course 5) at ESRF- European Synchrotron Radiation Facility, Oct. 2007, Grenoble (France).
ï‚§ 2007: P. Baraldi, E. Joseph, E. Kendix, R. Mazzeo, S. Prati, S. Capelli. Far Infrared And Raman scpectroscopy analysis of inorganic pigment, presented at RAA07 (the IV International Conference on the Application of Raman Spectroscopy in Art and Archaeology), September 5-11 2007, Modena (Italy).
ï‚§ 2007: R. Mazzeo, S. Prati, E. Joseph, E. Kendix, Mapping ATR-FTIR nella Caratteriz-Zazione E Localizzazione Stratigrafica Di Materiali Artistici ed Archeologici, Presented at the X Congresso Nazionale di Chimica dellâ€™Ambiente E dei Beni Culturali - CONOSCENZA E CREATIVITÃ€, June 12-15 2007, Lecce (Italy).