Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)[1] is an infrared spectroscopy sampling technique used on powder samples without prior preparation.
The infrared light on a sample is reflected and transmitted at different amounts depending on the bulk properties of the material.
Shape, compactness, refractive index, reflectivity and absorption of the particles are all characteristic of the material being analyzed.
The particle size should be smaller than the wavelength of the incident light in order to minimize Mie scattering, so this would infer that it should be less than 5 μm for mid-infrared spectroscopy.
Alternative plots of Kubelka-Munk units can be used, which relate reflectance to concentration using a scaling factor.