Spectroscopy

Another instrument used to aide in identification of compounds is the Fourier Transform infrared spectrophotometer (FTIR). The sample is bombarded with infrared radiation. Polar bonds found in organic compounds have a natural frequency of vibration similar to the frequency of infrared radiation. When the frequency of the infrared radiation matches the natural frequency of the bond, the amplitude of the vibration increases, and the infrared is absorbed. The output of an infrared spectrophotometer charts the amount of light absorbed vs. the wavelength, typically with units of percent transmission and wavenumbers(cm−1). Because both the frequency and the intensity of absorption are dependent on the type of bond, a skilled chemist can determine the functional groups present by examining the infrared spectrum. As with the GCMS the FTIR spectrum can be compared to that of a known sample, thus providing evidence for the identification of a compound. Spectroscopy can also help to identify materials used in failed products, especially polymers, additives and fillers. Samples can be taken by dissolution, or by cutting a thin slice using a microtome from the specimen under examination. Surfaces can be examined using Attenuated total reflectance spectroscopy, and the method has also been adapted to the optical microscope with infra-red microspectroscopy. Ultraviolet-visible-near infrared spectroscopy is used to test for certain drugs of abuse. UV-visible-NIR microspectrophotometers are instruments able to measure the spectra of microscopic samples. The UV-visible-NIR microspectrophotometer is used to compare known and questioned samples of trace evidence such as fibers and paint chips. They are also used in the analysis of inks and papers of questioned documents and to measure the color of microscopic glass fragments. As these samples are not altered, UV-visible-NIR microspectroscopy is considered a non-destructive technique. Thermoplastics can be analysed using characterization techniques such as infra-red spectroscopy, ultraviolet–visible spectroscopy, nuclear magnetic resonance spectroscopy, and an environmental scanning electron microscope. Failed samples can either be dissolved in a suitable solvent and examined directly (UV, IR and NMR spectroscopy) or be a thin film cast from solvent or cut using microtomy from the solid product. Infra-red spectroscopy is especially useful for assessing oxidation of polymers, such as the polymer degradation caused by faulty injection moulding. The spectrum shows the characteristic carbonyl group produced by oxidation of polypropylene, which made the product brittle. It was a critical part of a crutch, and when it failed, the user fell and injured herself very seriously. The spectrum was obtained from a thin film cast from a solution of a sample of the plastic taken from the failed forearm crutch.