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Proceedings Paper

Recent Advances In FTIR Photoacoustic Spectroscopy
Author(s): John F. McClelland; Roger W. Jones; Jae Seung Oh; Linda M. Seaverson
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Paper Abstract

Advances in FTIR photoacoustic (PA) spectroscopy have significantly extended the scope and utility of the PA technique in the areas of: microparticle spectroscopy, compositional determinations via factor analysis, coal surface oxidation measurements, spectroscopy of highly opaque samples, and PA detector technology. A method is reported for measuring FTIR spectra of single particles in the tens of Am size range which uses a tungsten needle to pick up particles and hold them in the sample chamber of the PA detector. The tungsten needle is initially mounted on a micromanipulator and particle pick-up is performed under a microscope. The needle and sample are then transferred directly to the PA detector sample holder which positions the particle in the IR beam. No sample alignment or thinning are necessary. Compositional determinations of kaolinite and quartz in coal have been performed using the Perkin-Elmer CIRCOM factor analysis program. The IR spectra were collected by DRIFTS and PA methods using synthesized samples of known compositions for the learning set and unknown test samples. The PA spectra yielded slightly better correlations. Coal surface oxidation was studied using a calibrated UV irradiation of coal to generate carbonyl species, thereby gauging the freshness of coal surfaces by how much carbonyl is formed by the UV exposure. FT-IR-PA difference spectra are used to measure the increase in carbonyl. UV generated carbonyl is found to increase with surface freshness. This method avoids the need of a "fresh coal standard" which is difficult to reproduce. The method's probe depth is based on the decay length of UV rather than IR photons in coal resulting in an increase in surface specificity. The linearity of FT-IR-PA spectra as a function of absorbance has traditionally not been maintained at the peaks of strong bands in opaque samples. This leads to peak truncation and reduced spectral contrast. A method to extend linearity using the magnitude and phase information of the PA signal is reported based on the Rosencwaig-Gersho Theory of PA signal generations. Spectra of polymer slabs demonstrate the utility of this approach for enhancing spectral contrast. Developments are reported in PA detector technology which increase the scope of applications that commercial PA detectors can be used for. The new MTEC Photoacoustics Model 200 PA detector operates in diffuse reflectance, photoacoustic, and transmission measuring modes and handles both macro- and microsamples. The Model 200 has a simplified purging system and electronics designed for both fast scan and step scan FT-IR instruments. Spectra from a variety of applications are presented to demonstrate the Model 200 performance.

Paper Details

Date Published: 1 December 1989
PDF: 1 pages
Proc. SPIE 1145, 7th Intl Conf on Fourier Transform Spectroscopy, (1 December 1989); doi: 10.1117/12.969512
Show Author Affiliations
John F. McClelland, Iowa State University (United States)
Roger W. Jones, Iowa State University (United States)
Jae Seung Oh, Iowa State University (United States)
Linda M. Seaverson, Iowa State University (United States)

Published in SPIE Proceedings Vol. 1145:
7th Intl Conf on Fourier Transform Spectroscopy
David G. Cameron, Editor(s)

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