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Synchrotron Is Ideal Source for IR Microscopy

Photonics Spectra
May 2001
Kevin Robinson

Mid-infrared light is immensely useful to biological studies because it offers a direct probe into the vibrational modes of the chemical bonds that hold atoms together. While intense sources of mid-IR radiation improve the resolution, they can produce a great deal of heat, which can kill living cells. Researchers at Lawrence Berkeley National Laboratory have shown that the mid-IR radiation from a synchrotron does not overheat biological samples, opening the door to using it in mid-IR Fourier transform infrared microspectroscopy. In addition, the source can be focused into a very bright spot.

Researchers have demonstrated that synchrotrons are exceptional infrared light sources for biological research. Their mid-IR radiation negligibly heats samples, such as these lung fibroblasts, and can be focused into much brighter spots than conventional sources. Courtesy of Lawrence Berkeley National Laboratory.

"Conventional infrared sources are physically large and cannot be focused to sizes much below 50 to 100 µm," explained Michael C. Martin, a researcher at the laboratory, who reported the work with his colleagues in Applied Spectroscopy, Vol. 55, No. 2. The synchrotron, in contrast, is essentially a point source, and the researchers can focus the beam into a diffraction-limited, 3- to 10-µm spot 200 times brighter than conventional thermal sources.

To determine if sample heating is a problem with synchrotron light, the team measured the heat gained in a mixture of water and a lipid system that is sensitive to temperature changes at around 315 K. "[It] has a phase transition a little above room temperature," Martin said. "This phase transition causes the shift of some infrared vibrational modes, meaning we can directly monitor the temperature by simply monitoring the infrared spectrum."

Stay cool

The researchers calibrated the temperature measurements by using a lower-intensity source. When they switched on the synchrotron beam, they found that the temperature rose by approximately 0.5 °C, too little to cause damage in most samples.

Synchrotrons are large and expensive, but they can produce the full range of the electromagnetic spectrum all at once, making them useful for a variety of research projects simultaneously. Martin said that the facility hopes to add a beamline dedicated to infrared biomedical research.

Martin predicts that despite the expense, the synchrotron will be useful for fundamental research. "Building a synchrotron at a hospital is out of the question," he said. "But once marker frequencies [for a disease process, for example] are known, one could then build a diagnostic device consisting of small sets of lasers to measure only these marker frequencies.

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