New Technique Helps Scientists Chemically Image Biological and Synthetic Materials

Scientists have long relied on infrared spectroscopy as an analytical tool to identify the chemical components of an unknown sample. Likewise, they have used a variety of imaging techniques to study spatial relationships between chemical components. Now scientists at the Commerce Department's National Institute of Standards and Technology have joined with a team at the National Institutes of Health to improve a system that combines infrared spectroscopy, an array detector and microscopy to rapidly obtain sensitive spectral images of complex samples.

The new spectral imaging technique is being used to study biological samples, such as brain tissue, as well as advanced polymer composites. The imaging technique has many potential applications in industry and biomedical studies.

"The novel combination of Fourier transform infrared spectroscopy for chemical analysis with the ability to visualize the composition of samples with array technology represents the future of infrared microscopy as an analytical tool," says E. Neil Lewis of the National Institute of Diabetes and Digestive and Kidney Diseases at NIH.

Edwin J. Heilweil of NIST's Optical Technology Division contributed expertise in infrared detector array technology to significantly improve the system Lewis' team had developed. The NIST/NIH work will be reported in the May 15 issue of the journal Optics Letters.

The advantages of infrared spectral imaging are its high data acquisition speed and spatial resolution, on the scale of 10 micrometers (millionths of a meter). Other approaches that couple single element infrared detectors and scanning infrared microscopes are slower and more cumbersome. The new system produces images in about an hour that otherwise would be impractical using a conventional scanning microscope and Fourier transform infrared spectrometer.

The new apparatus uses a mercury-cadmium-telluride infrared detector array, originally designed for use on board military missiles and adapted by NIST for time-resolved spectroscopic analyses. The detector array has 256 by 256 pixels, which can collect infrared spectra of reactive molecules or imaging data from over 65,000 points on a sample simultaneously. NIST holds a patent on the use of the detector for time- resolved chemical analysis. NIH holds the patent on the imaging approach.

The array is coupled to a step-scan, Fourier transform infrared spectrometer. In practice, when infrared light is directed through a sample onto the array, the resulting data are converted to detailed chemical images via Fourier transform, a mathematical technique for converting time-dependent intensities to wavelength data. The approach, as with conventional microscopy, requires the use of thinly sectioned samples but is otherwise non-destructive.

Molecules such as proteins, polymers or lipids absorb some of the infrared radiation at very specific frequencies that serve as "fingerprints" for spectral identification. NIH is using these chemical images in studies ranging from basic research to the analysis of diseased tissues. NIST is planning to apply this technique to gather transient spectroscopic images of intermediates formed during chemical reactions in polymer films.

As a non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. economic growth by working with industry to develop and apply technology, measurements and standards.