Sensitive IR narrow band optimized microspectrometer

Abstract

Customization of a standard model confocally masked FT-IR microspectrometer to maximize the signal for a particular narrow band of the spectrum and minimize noise is described. In this case the motivation was to detect minor concentrations of deuterated species in a matrix of tissue. However, the instrumental modifications used for this particular task are applicable to narrow band sensitization in general. A confocal single mask double pass FT-IR microspectrometer with 32× infinity corrected Swartzschild objective and condenser was used. The stock configuration included a cooled 50 μm sized MCT detector that had a peak response at a wavelength of approximately 12 μm which coupled with 32× mirror lenses was suited to operation with small mask sizes. Consideration of customization included a different “special order” MCT detector with a peak response at the desired wavelength of approximately 5 μm, reduction of noise across the interferogram by use of a notch filter, and possibly the use of a source having a greater emissivity in the desired range than the stock globar. Single beam spectra show the enhanced sensitivity of the custom detector and the wavelength selectivity resulting from its narrow response profile. The improved signal to noise response for an absorption band in the 2200–2000 cm −1 range of interest is demonstrated with transmission spectra from metabolically deuterated cerebellum white matter in addition to reflection spectra of films containing deuterated compounds that were deposited on mirror surfaces. The transmission single beam spectrum of the notch filter when compared using both detectors favored the custom detector, but the benefit of noise elimination in the overall spectrum was accompanied by signal reduction in the narrow band of interest. A tungsten source equipped with a sapphire window was directed to the interferometer but emissivity as commercially configured did not offer the advantage predicted.