Micro-fourier Transform Infrared Spectrometry

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 work—comparing 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.