Micro-Fourier transform infrared spectrometry (micro-FTIR)
Micro-Fourier transform infrared spectrometry (micro-FTIR) is a specialized analytical technique that combines Fourier transform infrared (FTIR) spectroscopy with microscopy to identify molecules in small or trace samples. This nondestructive method is particularly valuable in forensic science, where it assists in analyzing unknown substances, such as fibers, inks, or paint chips. The micro-FTIR spectrometer directs infrared light onto a focused area of a sample, causing molecular bonds to vibrate and absorb specific wavelengths. This absorption creates an infrared spectrum that reveals the types of chemical bonds and functional groups present, enabling the identification of the substance.
The process starts by positioning the sample beneath a microscope lens, allowing the analyst to focus on the area of interest. Once aligned, data acquisition generates the infrared spectrum, which can be compared against a digital library of known spectra using specialized software. Micro-FTIR is adept at distinguishing even closely related substances, making it possible to differentiate between various grades of materials like nylon or analyze individual layers within complex paint chips. This technique proves essential for forensic investigations, providing precise identification critical for evidence analysis.
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Subject Terms
Micro-Fourier transform infrared spectrometry (micro-FTIR)
DEFINITION: Type of absorption spectroscopy used in the identification of molecules.
SIGNIFICANCE: Forensic scientists use micro-Fourier transform infrared spectrometry to identify the substances present in unknown microscopic or trace samples based on the substances’ absorbance of infrared light.
Micro-Fourier transform infrared (micro-FTIR) spectrometry is a nondestructive technique used to identify unknown substances present within trace samples. In micro-FTIR spectrometry, Fourier transform infrared (FTIR) spectrometry is coupled with microscopy; the technique uses a microscope to focus infrared light onto a particular part of a sample in order to create an infrared spectrum. When it is necessary to obtain the infrared spectrum of a microscopic sample—such as a single fiber, a bit of ink, or a small paint chip—a micro-FTIR spectrometer is used.
Micro-FTIR spectrometry uses infrared light to excite and detect bonds within molecules. The wavelengths of light that make the bonds in the molecules vibrate are absorbed by the bonds, and this information is used to form the infrared spectrum. The infrared spectrum of an unknown molecule gives information about the kinds of bonds and chemical functional groups present, which helps to identify the molecule. Even very similar molecules can have very different infrared spectra, which is why micro-FTIR spectrometry can be used to identify unknown substances in evidence samples.
During micro-FTIR spectrometry analysis, the sample is first placed beneath the lens of the microscope and brought into focus. While looking through the microscope, the forensic scientist can see a circle within the vision field that indicates exactly where the infrared light beam will pass during data acquisition. The sample must be positioned so that the circle is centered above the portion of the sample of interest. After the sample is aligned, data acquisition is completed and the infrared spectrum is created. The analyst can then interpret the infrared spectrum by using specialized computer software to compare the spectrum with the infrared spectra of known substances stored in a digital library.
The analysis of a single fiber typically requires the use of a micro-FTIR spectrometer because the sample can be aligned using the microscope to ensure that the infrared light beam will pass through the tiny fiber. The technique can usually determine not only the broad class the fiber belongs to but also the subclass as well. For example, different grades of nylon can be differentiated from one another using micro-FTIR spectrometry.
Paint chips from automobiles contain several layers of paint that are deposited during the manufacturing process. Using micro-FTIR spectrometry, individual paint layers may be analyzed independently from surrounding layers in order to get an infrared spectrum of a component layer. The micro-FTIR spectrometry microscope is able to focus on an individual layer on a thin cross-section of a paint chip sample so that the layers do not need to be separated manually prior to analysis.
Bibliography
Bell, Suzanne. Forensic Chemistry. Upper Saddle River, N.J.: Pearson Prentice Hall, 2006.
Rathore, Chayanika, et al. "Standardization of Micro-FTIR Methods and Applicability for the Detection and Identification of Microplastics in Environmental Matrices." Science of the Total Environment, vol. 888, 25 Aug. 2023, doi.org/10.1016/j.scitotenv.2023.164157. Accessed 15 Aug. 2024.
Rubinson, Kenneth A. and Judith F. Rubinson. Contemporary Instrumental Analysis. Upper Saddle River, N.J.: Prentice Hall, 2000.
Siegel, Jay A. Forensic Science: The Basics. Boca Raton, Fla.: CRC Press, 2007.