Analytical Techniques: Spectroscopy

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Author: Jessica Ford

Editors: Anne Schaffer, Kari Rayner

Spectroscopic analysis involves characterizing or identifying materials at the molecular level based on how they absorb, transmit, or scatter radiation from different regions of the electromagnetic spectrum. Data is presented as a spectrum, usually plotting radiation wavelength against the intensity of absorption or emission by the material component in question, whether atom, bond, or molecule (Skoog and Leary 1992).

X-Ray Fluorescence Spectroscopy

X-ray fluorescence spectroscopy (XRF) is a non-invasive technique in which x-ray radiation induces excitation of elements at the atomic level, producing characteristic fluorescence. Current detection limits typically prevent distinguishing between elements below the atomic weight of calcium on the Periodic Table; the use of accessories such as a vacuum attachment can increase the detection range down to sodium (Beckhoff et al. 2006). When used in analysis of paintings, XRF can assist in the identification of inorganic colorants present in paint, although detection of elements without molecular or other contextual information cannot specify exact colorants; mixtures of paint and multiple layers of paint further complicate interpretation. Reference standards are required for objective quantitative analysis, although proficient users can gain relative quantitative and stratigraphic information from comparison of numerous readings (McGlinchy et al. 2012). While this technique is usually performed as a spot analysis, recent development of custom-built XRF scanners allows for mapping of elements across an entire painting (Alfeld et al. 2013).

Fiber Optic Reflectance Spectroscopy

Fiber optic reflectance spectroscopy (FORS) is a non-invasive surface technique that measures reflected radiation in the ultraviolet, visible, and/or near-infrared range. The information results from the bonds between elements rather than the elements themselves, so qualitative information about both inorganic and some (non-transparent) organic compounds can be gained (Cavaleri 2013). Interpretation relies upon comparison to reference standards, ideally made in-house and customized to the pertinent analytical situation (Picollo et al. 2007), but online databases are also available, such as from IFAC-CNR. Ultraviolet/visible spectroscopy can also be performed with transmitted radiation, which requires the removal and solubilization of a sample.

Fourier Transform Infrared Spectroscopy

Fourier transform infrared spectroscopy (FTIR) yields qualitative molecular information based on how materials respond to infrared radiation, measuring characteristic changes in the vibration energy of functional groups. The infrared radiation can be either transmitted or reflected, and the two modes rely on separate reference libraries for interpretation. Transmission mode, currently more common, requires removal of a sample, which can be reused for future analysis; reflectance mode can be performed non-invasively, and its use is increasing as reference libraries improve. FTIR yields information about both organic and inorganic compounds, making it useful for analyzing colorants, binding media, coatings, and fillers. Organic compounds can be classified as gum, protein, oil, or resin, and many inorganic compounds can be identified more precisely, with notable exclusions being oxides and sulfides. Quantitative analysis is much less common, but it is possible to do so by creating a calibration curve. (Derrick et al. 1999)

Raman Spectroscopy

Raman spectroscopy uses a laser in either the ultraviolet or visible electromagnetic range to induce small, characteristic energy shifts in a molecule’s electric field, which are measurable upon re-emission of the energy from the sample. Most organic and inorganic pigments, as long as they are not excessively reflective or fluorescent, can be identified with this technique. This makes Raman useful for analyzing a wide variety of colorants and fillers. For example, it is one of the only ways to distinguish between the anatase and rutile phases of titanium dioxide (titanium white). Raman can be performed in-situ or on a removed sample; the technique may be destructive depending on the type and intensity of laser used. (Edwards 2005)

References

Alfeld, M., J. Vaz Pedroso, M. van Eikema Hommes, G. Van der Snickt, G. Tauber, J. Blaas, M. Haschke, K. Erler, J. Dik, and K. Janssens. 2013. "A mobile instrument for in situ scanning macro-XRF investigation of historical paintings." Journal of Analytical Atomic Spectrometry 28 (5): 760.

Beckhoff, B., B. Kanngießer, N. Langhoff, R. Wedell, and H. Wolff. 2006. Handbook of practical x-ray fluorescence analysis. Berlin: Springer.

Cavaleri, T., A. Giovagnoli, and M. Nervo. 2013. "Pigments and mixtures identification by visible reflectance spectroscopy." Procedia Chemistry 8: 45-54. DOI: 10.1016/j.proche.2013.03.007.

Derrick, M. R., D. Stulik, and J. M. Landry. 1999." Infrared spectroscopy in conservation science." Scientific tools for conservation. Los Angeles: Getty Conservation Institute.

Edwards, H. G. M., J. M. Chalmers, and Royal Society of Chemistry, eds. 2005. "Raman spectroscopy in archaeology and art history." RSC analytical spectroscopy monographs 9. Cambridge: Royal Society of Chemistry.

McGlinchy, C. 2012. "Handheld XRF for the examination of paintings: proper use and limitations." In Studies in archaeological sciences: handheld XRF for art and archaeology, edited by A. N. Shugar and J. L. Mass, 131-158. Leuven, Belgium: Leuven University Press.

Picollo, M., M. Bacci, D. Magrini, B. Radicati, G. Trumpy, M. Tsukada, and D. Kunzelman. 2007. "Modern white pigments: their identification by means of noninvasive ultraviolet, visible, and infrared fiber optic reflectance spectroscopy." Proceedings of Modern Paints Uncovered. London. 118-128.

Skoog, D. A, and J. J. Leary. 1992. "Properties of electromagnetic radiation." In Principles of instrumental analysis, 58-78. 4th ed. Fort Worth, Texas: Saunders College Pub.