G. Semisolids, Gels, and Liquids

FTIR-PAS can be applied to semisolid, gel, and liquid samples when considerations of light scattering or contact with infrared window materials makes transmission or ATR methods impractical. In these instances only enough sample should be used to cover roughly a 5 mm diameter area centered in a stainless steel cup or other vessel. If the sample is aqueous a desiccant should be used under the sample cup as discussed in Section III.A. Lower mirror velocities will help in reducing the intensity of vapor spectra which may be superimposed on the sample spectrum. If necessary, spectral subtraction can be used to remove the vapor spectrum. A spectrum of the vapor alone can be obtained by covering the sample with a small disk of aluminum foil and thereby masking the sample from the infrared beam. The vapor will still be present above the foil and will produce only a vapor spectrum.
Figure 36 shows a spectrum of an aqueous collagen gel which is primarily water obtained with a desiccant but without spectral subtraction to remove the small amount of moisture and CO₂ spectra that are present. The spectrum clearly shows the absorbance bands of the non-water species.
Care should be taken when vapor species are associated with samples to avoid vapors that might damage or contaminate the photoacoustic detector.

Fig. 36. FTIR-PAS spectrum of an aqueous collagen gel.