Transmission IR works by passing the infrared beam through a sample and comparing the light reaching the detector to a background recorded without a sample. Transmission IR is ideal for recording gases using a gas cell containing a sample (often at reduced pressure to avoid off-scale absorbances).

Liquid samples can be recorded as a thin film between salt discs using transmission IR as an alternative to ATR-IR. Salt discs need to be selected that are transparent to IR light at the wavelengths of interest and a variety of salt discs are available. Most commonly NaCl or KBr are used.

Solid samples need to be recorded as a suspension between salt discs. The suspension is usually prepared using Nujol, which is a mineral oil. Compounds are carefully ground to a fine powder then a suspension is prepared using a few drops of Nujol and this is smeared between salt discs.