InfraRed Spectroscopy provides structural infomation on the functional groups present within the compound being analysed.

Attenuated Total Reflectance (ATR)

Attenuated Total Reflectance InfraRed spectroscopy (ATR-IR) is an infrared spectrascopic method which requires no special sample preparation for analysis. A sample (solid or liquid) is placed onto the ATR crystal (usually diamond) and the beam passes through the crystal bouncing off the surface with absorbances occurring at the interface between the crystal and compound. The ease and speed of ATR-IR spectroscopy makes it a suitable technique for widespread use. However some functionalities are better analysed using alternative IR methods and gas-phase compounds are unsuitable for ATR-IR spectroscopy.

Transmission

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.