Fourier transform infrared spectrophotometer (FTIR)

DEFINITION: Instrument used to collect the infrared spectrum of a sample to enable identification of its components.

SIGNIFICANCE: The infrared spectrum of a molecule is unique, and the technique of Fourier transform infrared spectroscopy can be used to identify molecules within a sample definitively. The instrument used to perform such spectroscopy has a variety of applications in forensic science; for example, it may be used to identify the controlled substances present in street samples of drugs,  to identify the polymers present in fiber or paint evidence found at a crime scene, to validate the authenticity of documents, or to determine counterfeit inks.

A Fourier transform infrared (FTIR) spectrophotometer includes a source of infrared (IR) radiation (usually globar), a beam splitter, two mirrors (one stationary and one moving), a sample chamber, and a detector (usually mercury cadmium telluride or deuterated triglycine sulfate). The sample to be analyzed is placed in the sample chamber, and IR radiation (mid-IR, 2.5–25 micrometers, 4000–400 cm^-1) is emitted from the source. Before it interacts with the sample, however, the radiation hits the beam splitter, which divides the IR beam into two.

Half of the IR beam is reflected off the beam splitter to one of the mirrors, while the other half of the beam is transmitted through the beam splitter to the second mirror, which is moved back and forth with respect to the beam splitter. Each half beam impinges on the respective mirror and is reflected back to the beam splitter. The two beams recombine at the beam splitter. Because the half beam that reflects off the moving mirror travels a distance different from that traveled by the beam that reflects off the stationary mirror, when the two beams recombine, the IR beam is in a form (known as an interferogram) that is different from the original beam from the source.

The interferogram is then directed through the sample. Solid samples are often analyzed in the form of a potassium bromide (KBr) disk. The forensic scientist prepares this disk by mixing some sample with KBr and then, using a special assembly, pressing the mixture into a disk by applying pressure. Ideally, the KBr disk should be transparent, containing particles of sample distributed throughout the disk. Only the sample particles will absorb IR radiation; KBr is transparent in this IR range and, hence, does not contribute to the absorbance.

Molecules in the sample will absorb IR radiation at certain frequencies according to the groups of atoms that are in the molecule. The pattern of the interferogram changes depending on the frequencies at which the IR radiation is absorbed. The sample interferogram passes to the detector, where the Fourier transform is performed. This is a mathematical procedure that is used to convert the interferogram into the IR spectrum, which is a plot of detector response versus wave number. Identification is based on the characteristic wave numbers at which the sample absorbs IR radiation. The absorbances are observed as peaks on the IR spectrum at the corresponding wave numbers. An extensive database of reference spectra in IR libraries speeds up the identification of the material based on the absorbances recorded on the IR spectrum.

FTIR spectroscopy offers a very powerful technique for identification purposes, but the sample must be in a relatively pure form, or contaminants in the sample may mask the IR absorbances of the sample. Steps may need to be taken to purify the sample before the technique can obtain a spectrum that is sufficiently detailed for identification to be possible. Attenuated total reflection (ATR) is an alternative option to the KBr method, which is preferred when liquids or surfaces are the specimens, when the time available is limited, or when a portable FTIR is required.

Bibliography

Bell, Suzanne. Forensic Chemistry. Pearson, 2006.

Gerard, Claire. “Infra-Red Spectroscopy (IR, Near, Mid, Fourier Transform, N-IR, M-IR, FT-IR).” University of Warwick, 24 Oct. 2024, warwick.ac.uk/services/ris/impactinnovation/impact/analyticalguide/ir/?utm_source=chatgpt.com. Accessed 9 Jan. 2026.

Houck, Max M., and Jay A. Siegel. Fundamentals of Forensic Science. Elsevier, 2006.

Kaur, H., et al. “Fundamentals of ATR-FTIR Spectroscopy and Its Role for Probing In-Situ Molecular-Level Interactions.” Modern Techniques of Spectroscopy: Progress in Optical Science and Photonics, edited by D. K. Singh, et al., Springer, 2021, doi:10.1007/978-981-33-6084-6_1. Accessed 9 Jan. 2026.

Li, Guiyang, et al. “Applications of FTIR in Identification of Foreign Materials for Biopharmaceutical Clinical Manufacturing.” Vibrational Spectroscopy, vol. 50, no. 1, 2009, pp. 152-159, doi:10.1016/j.vibspec.2008.10.016. Accessed 9 Jan. 2026.

Rubinson, Kenneth A., and Judith F. Rubinson. Contemporary Instrumental Analysis. Prentice, 2000.

Thain, Simon. “IR Spectroscopy and FTIR Spectroscopy: How an FTIR Spectrometer Works and FTIR Analysis.” Technology Networks, 16 Aug. 2022, www.technologynetworks.com/analysis/articles/ir-spectroscopy-and-ftir-spectroscopy-how-an-ftir-spectrometer-works-and-ftir-analysis-363938. Accessed 9 Jan. 2026.

Wei, Chun-Ta, et al. “Enhancing Forensic Investigations: Identifying Bloodstains on Various Substrates through ATR-FTIR Spectroscopy Combined with Machine Learning Algorithms.” Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2024, doi:10.1016/j.saa.2023.123755. Accessed 9 Jan. 2026