Potassium bromide (KBr) is transparent in the mid-IR. KBr powder can be pressed into a window easily using the right tools. The KBr powder can be mixed with an unknown or known powdered sample and pressed into a KBr window. Passing an IR beam through this window made up of KBr and the unknown allows one to collect an infrared spectrum of the mixture.
The requirements are to make the particle size of the unknown small compared to the wavelengh of the IR. The standard range of an FTIR spectrometer is 4000 to 400 cm-1 using a KBr beamsplitter and DTGS detector and that corresponds to 2.5 to 25 microns in wavelength). The normal fingerprint range is considered to be 2000 - 400 cm-1 or 5 to 25 microns. Particles that are larger than the IR wavelength will produce scattering and noise in the IR spectrum. This is usually a consideration, but not serious unless one is doing quantitative analysis on the components in the spectrum.
One needs KBr powder. A method of grinding the sample into a powder. A press of some kind.
There are two common ways to grind a sample. The old tried and proven is a motr and seple
Presses - hand presses, hydrologic presses
There are three types of KBr presses