Table 1. Sampling depths in for typical thermal diffusivities from 0.1 to 0.0001 cm²/s and modulation frequencies from 10^-1 to 10⁴ Hz. D is given by where and are thermal conductivity, density and specific heat of the material, respectively

Nonhomogeneous samples are more difficult to analyze when the thermal diffusivity varies significantly with depth and when the absorption coefficient is not small relative to the thermal-wave decay coefficient. Variations in the thermal diffusivity between thin layers of a material result in variations in sampling depth within the sample and in thermal-wave reflection effects⁹ at interfaces, both of which make interpretation of results more complicated. Analyses based on stronger absorption bands also can lead to ambiguities since the sampling depth is now determined by the decay of both the infrared beam and the thermal-waves within the sample. Furthermore, when the sampling depth is reduced by increasing the modulation frequency, reduction occurs in the saturation induced truncation of strong bands. This phenomenon can be incorrectly interpreted as an increase in species concentration near the surface. (See Section V. D.).
Very useful practical information can be obtained from varying the sampling depth of FTIR-PAS measurements despite the above cautionary considerations and the frequent lack of known infrared and thermal-wave absorption coefficients. Such coefficients, when available, are used to estimate sampling depth as the reciprocal of whichever absorption coefficient (infrared or thermal-wave) is larger. In practice, it is best to use spectral regions where absorption coefficients are known to be low by comparison with other spectral bands and to estimate sampling depth from the expression . The thermal diffusivity is given by where and are the thermal conductivity, density, and specific heat, respectively. These properties can usually be found in the literature for general classes of materials, if not for a specific sample.


III. INSTRUMENTATION


A. Photoacoustic Detector

FTIR photoacoustic detector accessories are currently manufactured by Bio-Rad (Cambridge, MA USA) and MTEC Photoacoustics (Ames, IA USA) for use with FTIR spectrometers. This discussion will be limited to the MTEC model 200 unit because it is available from distributors and all major FTIR companies throughout the world. Figure 3 is