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RAMAN SPECTROSCOPY

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  p112-113

2026-07-01

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RAMAN SPECTROSCOPY

Recall that the absorption of IR radiation by a molecule causes the bonds to absorb energy and vibrate. A plot of the amount of energy absorbed versus the wavelengths of radiation absorbed comprises the IR spectrum. There are rules that these absorptions must obey in order for them to show up in the IR spectrum (called IR active transitions). Some vibrations are IR inactive. These vibrations may however, be active in Raman spectroscopy, which is a companion technique to IR spectrophotometry. The Raman effect involves radiation in the UV/visible range. Instead of absorbing this radiation, the molecule causes it to be scattered. The vast majority of the scatter ing incidents are elastic; that is the energy of the scattered photon is the same as that of the incident or absorbed photon. Approximately one out of a million of the scatterings are inelastic; the energy of the scattered photon can be greater (anti-Stokes line) or less (Stokes line) than that of the incident one. The Raman spectrum is measured as the chemical shifts of the emitted photons, which is the difference in energy between the incident photon and the inelastically scattered emitted photon. The scattering of the radiation causes molecular vibrations in the IR region that are called Raman active. In general, Raman active vibrations are IR inactive and vice versa. One of the significant advantages of Raman over IR is in sampling. Many types of glass and plastic polymers are Raman inactive. So, for example, a pure drug sample inside a plastic bag can be analyzed by Raman spectroscopy without interference from the bag. This can be useful if it is necessary to obtain fingerprints from the bag. A few years ago, one of the authors of this book was involved in a somewhat bizarre case that was solved with Raman spectroscopy. Personnel from a police bomb squad were dismantling a safe when they found a steel cage containing a sealed glass tube that had about 75 ml of a clear liquid inside. There was a metal button on the cage that, if pressed, would break the glass and release the liquid. The bomb squad suspected that this might be a “booby trap”. They brought the glass vessel to the forensic science laboratory at Michigan State University. The liquid was tested using Raman spectroscopy inside the sealed glass tube and the liquid was identified as CS gas (a type of tear gas), with the help of the FBI. CS is a liquid at room temperature but is very volatile. The glass vessel was indeed a trap. If a safe cracker hit that but ton, CS gas would have spread all over the place, driving away the burglar without damaging the contents of the safe! Since Raman spectroscopy is a light scattering method, materials can be analyzed as they are. Inks and dyes can be tested while still on paper. A paint chip can be cross sectioned and laid on a piece of aluminum foil (transparent in the Raman region) and each layer can be sampled. As with IR and UV/visible spectrophotometry, Raman spectroscopy can be used in conjunction with a microscope. In this case, a laser is used as the light source and laser light passes through the microscope and the sample. Raman spectroscopy is routinely used in the analysis of drugs, paints, inks and dyes, and fibers.