To avoid total absorbance, the sample is diluted which allows the IR light to pass through the sample without being absorbed too strongly. The substance that is used to dilute the sample should not absorb IR light in the same range as the sample. Otherwise, the substance used to dilute the sample would also appear on the resulting spectrum.

To analyze a liquid sample, the liquid is diluted with a solvent. The usual choice is carbon tetrachloride (CCl4). To analyze a solid sample, the solid must be ground and mixed with another solid, typically potassium bromide (KBr) which does not absorb light in the MIR range. The resulting mixture is pressed into a pellet to be analyzed. Alternatively, the sample can be very thinly sliced and placed on a KBr window. This sample preparation can only be skipped if the sample is extremely thin (< 15 µm).

The sample preparation process for transmission detection is very time consuming and requires a lot of effort. Additionally, preparing the samples this way destroys the original sample. As a result, transmission detection is only used for specific spectroscopic applications such as examining polymer films, proteins, and samples containing oil in water. However, transmission is widely used in FT-IR microscopy in the field of forensics, as well as while analyzing tissue samples and microplastics.