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In the case of cubic particles, these may pack wholly regularly if they are of equal size, as shown in Fig.2a, and there is then no free space. However, if the packing is irregular as in Fig.2b, there will be some and this is likely to be rather greater if the cubic particles are not of equal sizes.
It is clear therefore that it is highly desirable that solid samples for examination by infrared spectroscopy should not contain a significant proportion of particles whose sizes are comparable to the wavelength range of the spectrum to be measured. In analytical infrared spectroscopy the wavelength range is huge, 2.75-25 microns, so we have to be careful. Fortunately, there is normally no great difficulty in reducing the average particle size below 2.75 ? so that, on this count, the problem is much reduced. There is, however, another factor which produces scattering when the particle size is below ? and this will be considered below.
So, we plan to coat the particles of a solid sample, preferably reduced in size to about 1 ?, with a transparent liquid of comparable refractive index. How may this be done in practice? Apart from meeting the refractive index criterion, the liquid of choice must be as transparent as possible over the range 2.75-25 ?. Inevitably, compromise is necessary, as is the case with solvents used for solution studies. Paraffinic hydrocarbons provide one such compromise, although they absorb rather strongly at the frequencies specific for the various types of C-H vibrations. 2b1af7f3a8