Fourier Transform Infrared Spectrophotometer (FTIR) (World of Forensic Science)
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 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.
Micro-fourier Transform Infrared Spectrometry (World of Forensic Science)
Spectrometry of various kinds is used in the laboratory analysis of trace evidence, because it can produce a chemical "fingerprint," which helps in identification and comparison. Fourier transform infrared spectrometry (FTIR) is a particularly useful tool for the forensic scientist because it allows the analysis of such a wide variety of trace evidence including paint, drugs, lubricants, cosmetics, and adhesives. Micro-fourier transform infrared spectrometry combines a microscope with an FTIR instrument, providing even more information because microscopic examination is always the first step in the examination of trace evidence.
The basic technique of micro-FTIR is infrared spectrometry. Fourier transformation is a mathematical process that improves the quality of the signal at the detector. Infrared spectrometry can provide chemical fingerprints for both organic and inorganic compounds that are components in trace evidence. It works on the principle of chemical bonds absorbing energy in the infrared region of the electromagnetic spectrum. The frequency at which a bond absorbs energy depends upon its polarity, that is, the nature of the constituent atoms making up a bond. A carbon-hydrogen bond absorbs energy at a different frequency from a carbon-carbon bond, for instance.
The sample is inserted into the FTIR machine and then exposed to a scan of different infrared frequencies over the whole of the infrared range. As each bond absorbs energy, a peak appears on the detector. The scan produces a fingerprint, or spectrum, that is characteristic of that compound. Mixtures of compounds also give characteristic fingerprints. Research has produced huge libraries of reference infrared fingerprints for known compounds and products. Therefore, the spectrum of the trace evidence can be compared, by rapid computer analysis, with reference samples that should provide an identification match. Micro-FTIR can also be used in comparison workomparing a flake of paint from the scene of a crime to a reference sample taken from a suspect's car, for instance, which could be helpful in investigating a hit and run accident. The technique has also been found particularly useful in the analysis and comparison of hairs and fibers.
SEE ALSO Infrared detection devices; Microspectrophotometry.