Fourier Transform Infrared Spectroscopy (FTIR) is another important test that should be conducted to ensure the potency and consistent quality of a batch of essential oil. This testing method examines the structural components that make up compounds found within an essential oil.

In an FTIR scan, infrared light of different frequencies is shined through a sample of essential oil and the amount of light absorbed by the sample is measured. As light energy passes through a molecule, bonds connecting the atoms will move (twist, vibrate, stretch, contract, bend, etc.) at varying amounts based on the frequency of the light. The frequency where the molecule is moving most rapidly is said to be the frequency of light that a particular molecule absorbs. For example, smaller molecules with lower molecular weights tend to absorb light at higher frequencies, while molecules with a higher molecular weight tend to absorb light at a lower frequency. Molecules with double or triple bonds usually absorb light at higher frequencies than molecules with only single bonds. Specific functional groups will absorb frequencies within a particular range. Looking at the FTIR reading allows you to determine if certain desirable structural components, and thus compounds, are present in an essential oil. The quality of the sample is determined by comparing the results from an FTIR reading to a historical database with absorption patterns of many other high quality samples.