Fourier transform infrared spectrophotometer (FTIR)
A Fourier transform infrared spectrophotometer (FTIR) is a sophisticated instrument used to analyze the infrared spectrum of a sample, which allows for the identification of its molecular components. The uniqueness of a molecule's infrared spectrum makes FTIR an invaluable tool in various fields, particularly forensic science, where it can be employed to detect controlled substances in drug samples or to analyze materials such as fibers and paints found at crime scenes.
The FTIR setup includes key components such as a source of infrared radiation, a beam splitter, mirrors, a sample chamber, and a detector. When infrared radiation is emitted, it is split into two beams, with one reflecting off a stationary mirror and the other off a moving mirror. This process generates an interferogram, which is then directed through the sample, often prepared in the form of a potassium bromide (KBr) disk.
The sample absorbs infrared radiation at specific frequencies, and the resulting interferogram is transformed into an IR spectrum through mathematical processing. This spectrum displays peaks corresponding to the characteristic wave numbers of the absorbed radiation, allowing for the identification of the sample's components. However, for accurate results, samples must be relatively pure to prevent contaminants from obscuring the absorption signals.
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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 or to identify the polymers present in fiber or paint evidence found at a crime scene.
A Fourier transform infrared (FTIR) spectrophotometer includes a source of infrared (IR) radiation, a beam splitter, two mirrors (one stationary and one moving), a sample chamber, and a detector. The sample to be analyzed is placed in the sample chamber, and IR radiation is emitted from the source. Before it interacts with the sample, however, the radiation hits the beam splitter, which divides the IR beam in 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 absorbancies are observed as peaks on the IR spectrum at the corresponding wave numbers.
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 absorbancies 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.
Bibliography
Bell, Suzanne. Forensic Chemistry. Upper Saddle River: Pearson, 2006. Print.
Houck, Max M., and Jay A. Siegel. Fundamentals of Forensic Science. Burlington: Elsevier, 2006. Print.
Rubinson, Kenneth A. and Judith F. Rubinson. Contemporary Instrumental Analysis. Upper Saddle River: Prentice, 2000. Print.
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 14 Aug 2024.