The average total path length traveled by photons in mid-infrared diffuse reflection spectrometry of powdered potassium chloride matrices coated with a thin layer of silicone oil has been estimated. The average path length of the photons that had passed through the silicone oil layer was first calculated by the application of the Beer–Lambert law. The total path length through the coated particles was then estimated by dividing the path length of the silicone oil by its concentration. The average path length of the photons that had been diffusely reflected from KCl particles with an average diameter of ∼5 μm that had been coated with a very thin layer of the silicone oil (corresponding to a concentration of 0.01% w:w) was estimated to be about 8 mm. This path length was significantly reduced either as the silicone oil concentration or the absorption of the matrix was increased. The result helps to explain why diffuse reflection spectrometry is a far poorer sampling technique for trace analysis when the matrix has strong absorption and why diffuse reflection is not a very promising technique for the standoff detection of nonvolatile chemical warfare agents.

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