2012 Corporate Capabilities - Spectroscopy

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98 Molecular Spectroscopy APPLICATION NOTES – DECEMBER 2011 Mid-Infrared Refiectivity Measurements of Diffuse Materials Jenni L. Briggs, PIKE Technologies The approach to infrared measurements of diffusely scattering materials often is dictated by the objective of the analysis. Spectral data from three different mid-infrared refiectance sampling accessories are contrasted. 100 95 90 85 80 75 70 Infrared analysis of highly scattering or textured samples is often accomplished via diffuse refiectance sampling accessories. Among two of the most popular diffuse refiectance accessories are the confocal ellipsoidal mirror design, which results in only a fraction of the refiected light being collected, and the integrating sphere design (1). Te speciflc accessory used for the measurement will be determined by the information desired. Te aim of this application note is to compare mid-infrared spectral data of a diffuse sample obtained by using various refiection accessories. Experimental Conditions A powdered coated metal panel was analyzed using the PIKE UpIR , a diffuse refiection accessory with an ellipsoidal collection mirror, or the PIKE mid-infrared IntegratIR , a gold-coated integrating sphere equipped with a liquid nitrogen cooled MCT detector. Te sampling surface of the UpIR is positioned above the FT-IR making it ideal for analyzing large samples. Additionally, a spectrum was collected using a 10 o specular refiection accessory. Te background and sample collection time was 20 s, and the resolution was set to 4 cm -1 . Results All spectra are shown in Figure 1. Te spectrum of the powdered coated sample collected with the 10° specular refiection accessory resulted in refiectivity values near 1% and minimal chemical information is discernable due to the diffuse characteristic of this sample. Only the specular component refiecting at the angle equivalent to the angle of incidence, 10° in this case, is collected. Specular refiection accessories are appropriate for specular samples such as mirrors, optical windows, and coatings on refiective surfaces. Chemical information from a high quality spectrum is obtained from the ellipsoidal collection mirror (UpIR) and the integrating sphere accessory. Terefore, either accessory is suitable for this purpose. Te refiectivity measurements were different; at 4000 cm -1 65% refiectivity was measured using the integrating sphere while the refiectivity of the sample collected using the UpIR was 93%. However, only the integrating sphere accessory produces re- % Reflectance 65 60 55 50 45 40 35 30 25 20 15 10 5 0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm -1 ) Figure 1: Spectrum of painted panel obtained using an elliptical collection type accessory (shown in purple), an integrating sphere ( shown in blue), and a 10 o specular refiection accessory (shown in red). liable refiectivity data because it is able to collect close to the entire available hemispherical (2π steradians) scattered photons. Te integrating sphere has a highly refiective, close to a Lambertian surface, such that the light enters the sphere, bounces around the highly refiective diffuse surface of the sphere wall, and flnally impinges upon the detector. In addition to refiectivity measurements as described here, the PIKE IntegratIR may be used for diffuse transmission measurements. Despite the 100 year history of the sphere, new applications are continually developed. Conclusions For refiectivity measurements of diffuse samples, using an integrating sphere is preferred, whereas for obtaining only chemical information a ellipsoidal mirror design diffuse refiection accessory is adequate. References (1) L.M. Hanssen and K.A. Snail, Handbook of Vibrational Spectroscopy, Chalmers and Griffiths, Eds. (2002). PIKE Technologies 6125 Cottonwood Drive, Madison, WI 53719 tel. (608) 274-2721; fax (608) 274-0103 Website: www.PIKEtech.com
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