Fourier Transform Infrared Spectrophotometer (FTIR)

A Fourier transform infrared spectrophotometer (FTIR) is an instrument used to examine specimens, both to detect the presence of target compounds and to measure the quantities of the compounds (quantification). FTIR can be an important analytical instrument in a forensic investigation.

A FTIR can be useful in detecting both organic chemicals (i.e., those that contain carbon) and inorganic chemicals. As with other forms of spectrophotometry, FTIR utilizes light. In this case, the wavelength of the light (the distance between a point of one light wave and the corresponding point of an adjacent wave) is in the infrared range. Infrared light lies in between the visible light and microwave portions of the electromagnetic spectrum . The infrared light that is nearest to visible light ("near infrared") has a wavelength of approximately 770 nanometers (nm; 10^–9 meter). At the other end of the range, infrared light that is nearest to microwave radiation ("far infrared") has a wavelength of approximately 1,000,000 nm (1.0 millimeter).

The basis of FTIR is the absorption of the infrared light by various molecules in a sample. Depending on their chemical structure and three-dimensional orientation, the different sample molecules will absorb different portions of the infrared spectrum.

Depending on the nature of the chemical bond that absorbs the infrared light, a chemical bond will vibrate in varying ways. Reflecting the different types of bonds, a number of events can occur. For example, the input of vibrational energy can stretch the bonds between the carbon atom and the surrounding hydrogen atoms in CH₃. Also, the carbon-hydrogen linkages of CH₃ can remain the same length while the linked atoms are moved back and forth laterally to one another (rocking). Other chemical linkages, such as that between a silicon atom and CH₃ group, can be altered asymmetrically along their lengths, with some regions of the bond stretching and other regions contracting (asymmetric deformation).

The absorption of light by the sample will decrease the energy of the infrared light that exits the sample chamber or produce a wave that is "out of synch" with light that has not passed through the sample. A computational comparison of the frequency patterns of the incoming and exiting infrared light can be made as described subsequently and displayed as a series of peaks rising above the background baseline. The height of the peaks corresponds to the degree of absorption and/or to the nature of the chemical bond change (i.e., stretching, rocking, deformation).

Within the spectrophotometer, the incoming infrared light beam is split in two by a mirror. Half of the beam is directed through the sample. The aforementioned chemical interactions within the samples will produce an emerging light beam that is different in optical character from the portion of the light that has been directed away from the sample.

The two light beams will be out of phase will one another. Since light consists of waves, the out of phase waves can cancel one another or lessen the overall wave intensity through interference. The pattern that results from the interaction of the two beams is known as an interferogram.

The end result of the Fourier transform is the spectrum of peaks and valleys that is displayed to the analyst. The resulting absorption pattern can be compared to the millions of patterns that are stored in computer databases, both on-site and remotely via the Internet. If a matching spectrum is obtained, then the identity of the sample compound can be determined.

FTIR is a valuable forensic technique because of its detection sensitivity and versatility. Chemicals from a variety of sample types including blood , paints, polymer coatings, drugs and both organic and inorganic contaminants can be identified.

Liquid samples such as blood can be prepared for FTIR examination by placing a drop between two plates made of sodium chloride (salt). The salt molecules are transparent to the infrared light and so form convenient sandwiching layers to produce a thin layer of sample. Solid samples can be converted to a fine powder in combination with a carrier material like potassium bromide (KBr, which is also infrared transparent). Alternatively, solids such as polymers can be dissolved in a solvent such as methylene chloride and added to a salt plate. When the solvent evaporates, the sample forms a thin layer on the salt plate.

Solids as complex as soil have been successfully analyzed using FTIR in forensic studies.

FTIR is not a technique that can be done at the scene of a crime or accident. The spectrophotometer and ancillary computer equipment are too bulky and heavy for transport. Rather, samples need to be carefully collected and transported to a specialized laboratory that has the necessary FTIR equipment.

see also Analytical instrumentation; Breathalyzer®; Gas chromatograph-mass spectrometer; Infrared detection devices; Micro-fourier transform infrared spectrometry; Spectroscopy.