The classical way to carry out IR spectroscopy is to scan the frequency of the incoming light so that the detector can record changes in the light intensity for those frequencies at which the sample absorbs energy. A major disadvantage of this method is that the detector records meaningful information only while the scan is passing through absorption lines, while most of the time is spent scanning between lines when the detector has nothing to record. To over come this deficiency, modern IR spectrometers irradiate the sample with a broad band of frequencies simultaneously and then carry out a mathematicsl analysis of the resulting signal called a FOURIER TRANSFORMATION to convert the detected signal back into the classical form of the spectrum. The resulting signal is called a Fourier transform infrared FTIR spectrum.
FT-IR is a method of analysing the composition of organic materials based on the fact that every chemical bond has a characteristic energy level. In FT-IR an infrared laser beam is focused on a small sample from the object, which then absorbs energy. The energy that has not been absorbed is detected and displayed on a graph (spectrum) as a series of peaks. These peaks each represent particular chemical bond energies, enabling a conservation scientist to identify the chemical structure of the sample.
The Fourier transform technique is also widely used in nuclear magnetic resonance, to be discusseed below, öand in other branches of spectroscopy. Its initial use was in X-ray diffraction crystallography.
Water mark on a sheet of paper used by Rembrandt (Ashmolean Museum, Oxford Univ)
Ref: Frank Owens, Charles Poole, Phyics and Chemistry of Nanosolids, WILEY, pp 67, (539 pages) 2009