Photoacoustic Detection of Rapid-Scan Fourier Transform Infrared Spectra from Low-Surface-Area Solid Samples

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Abstract
We have demonstrated that the PAS cell volume, and the state of the background material as well as the interferometer mirror velocity and the cell gas composition, must be controlled when one is recording spectra of solid samples. An optically thick, totally absorbing material with a volume matching that of the sample is needed in order to properly normalize spectra of solid samples. With the proper detection bandwidth, mirror speeds of up to 0.181 cm/s can be used with helium as a transfer medium, for the cell specified. The improvement attained at high frequency by using helium can be as much as 10-fold over that obtained with air. The Helmholtz design of the cell produces a resonance at 1.4 kHz with air and 2.65 kHz with helium. These resonances are also affected by the volume and composition of the gas in the sample chamber. Thus, it is essential to select an appropriate background sample for the purpose of normalizing spectra.