FTIR is a form of vibrational spectroscopy, which is known to exhibit excellent selectivity for the purposes of material identification in applications such as hazmat unknown chemical identification, pharmaceutical raw material identification, explosives precursor identification and many others. The sections below are designed to provide a general overview on how FTIR works as well as provide application-specific information.

FTIR Overview

In FTIR spectroscopy, the unknown substance is illuminated with a broad-spectrum (many wavelengths or frequencies) of infrared light, which can be absorbed, transmitted, reflected or scattered by the sample of interest. The illumination is typically in the wavelength range of 2.5 to 14 microns. The light intensity as a function of wavelength is measured before and after interacting with the sample, and the absorbance caused by the sample is calculated.

Light is absorbed in varying amounts by the sample at particular frequencies corresponding to the vibrational frequencies of the bonds of the molecules in the sample. Since the bonds for every molecule are different, the FTIR absorption spectrum for every molecule is also different. Thus, a spectral "fingerprint" can be generated by recording the absorbance of light as a function of wavelength.

Most substances absorb this infrared light very readily, and if all of the light is absorbed, there will be no light reaching the detector to be analyzed.  Therefore, care must be taken in how the light is delivered to and collected from the sample.  The most user-friendly method for field use is called attenuated total reflection (ATR), and uses a diamond to send light into just the first few microns of the sample before returning back out.