Observing IR spectroscopic absorptions practical problems

15) where two of the three IR active vibrational modes are below 200 cm a specialized far-infrared spectrometer may be used to observe such absorptions. 

Samples for IR spectroscopy are often prepared in cells with optical windows which themselves absorb within the 4000 and 200cm range common materials are NaCl and KBr and these materials cut off at 650 and 385 cm respectively with the effect that absorptions (due to the sample) below these values are masked by the absorption due to the optical window. Solution cells are used, not only for neat hquid samples but for solutions of the sample in a suitable solvent. This adds a further problem, since absorptions due to the solvent may mask those of the sample in regions of strong solvent absorption, the transmitted radiation is essentially zero and so no absorptions at frequencies due to the sample may be detected. 

We have just described how to establish the number of vibrational degrees of freedom for a simple molecule with H atoms, how to deduce the total number of normal modes of vibration, and so determine the number of absorptions 

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