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  • Tracking Where the Chips Fall

Photonics Spectra
Feb 2006
Hank Hogan

Where others see a work of art, Thomas J. Tague Jr. sees an opportunity — and a challenge. Art conservators, museum directors, interested individuals and sometimes the general public want to know if a painting or an antiquity is real or fake. Authentication involves making sure that the components of the work, such as the pigments and the canvas, are consistent with its presumed age and origin.

Researchers, therefore, analyze paint chips and other small fragments. This is easier said than done, however. Paint chips, for example, usually cannot be sectioned because they are very fragile. They must be tested as is, and a broad-brush characterization will not do, according to Tague, a senior applications scientist with Bruker Optics Inc. of Billerica, Mass.

“These paint chips are, in my opinion, as difficult as it gets,” he said. “There are small particles and layers of interest embedded in other materials of interest.”

Tracking Where the Chips Fall
A variation of attenuated total reflection IR microanalysis enables the study of paint chips and other small fragments for art conservation and authentication. Courtesy of Bruker Optics Inc.

When analyzing artwork and other objects, attenuated total reflection infrared microanalysis techniques offer a powerful advantage in that they do not require sample preparation. In these setups, a crystal is pressed against the sample, and a laser beam is sent into the crystal. If the angle of incidence at the crystal/sample boundary is below a critical value determined by the refractive indices of the materials, the beam bounces around inside the crystal. That produces an evanescent wave in the sample, which interrogates it to some depth and reveals its infrared spectral fingerprint.
There is a drawback, however. Because the crystal must be pressed against the sample, it gets in the way of visible polarization, fluorescence illumination, dark-field illumination and other optical contrast-enhancement methods.

A solution, Tague said, is found in Bruker’s new twist on attenuated total reflection infrared microanalysis. It uses a vertical slide that moves the crystal up and down on command and enables positioning of the crystal in the field of view with an accuracy of 1 to 2 μm. With the crystal moved up and out of the way, the device functions like a microscope, and standard microscopy viewing and techniques are possible. Attenuated total reflection measurements are made with the crystal down and pressed against the sample. When touching the sample, the crystal becomes part of the optical path, boosting the numerical aperture of the device and helping the user resolve smaller features.

Typically, crystals for attenuated total reflection are made of zinc selenide, diamond, silicon or germanium. The last has an index of refraction of 4, the highest of these materials and an important consideration in its selection for use in the internal reflection element of the Bruker instrument.

Because the crystal acts as a numerical-aperture-increasing lens and because some samples can have a high index of refraction themselves, Tague said, high-index materials are needed in the instrument.

He added that the use of germanium also increases the signal level by 16 times. The material provides a magnification of 4×, which means that the aperture can be four times bigger and the light-collecting area 16 times as large as that of a material with a magnification of 1×. That translates into faster data collection, with better signal-to-noise characteristics.

Tague said that the new attenuated total reflection microanalysis of paint chips has enabled the careful characterization of each component in several test cases.

A solid with a structure that exhibits a basically symmetrical and geometrical arrangement. A crystal may already possess this structure, or it may acquire it through mechanical means. More than 50 chemical substances are important to the optical industry in crystal form. Large single crystals often are used because of their transparency in different spectral regions. However, as some single crystals are very brittle and liable to split under strain, attempts have been made to grind them very...
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