Applications of Raman spectroscopy in forensic science. I: Principles, comparison to infrared spectroscopy, and instrumentation

There have been several significant advances in Raman spectroscopy instrument technology during the past few decades, including the introduction of several new laser sources, the development of holographic gratings, efficient Rayleigh line rejection filters, and CCD array detectors, and the advent of FT-Raman spectrometers. In view of these developments, Raman spectroscopy is now a fully mature analytical technique on par with its counterpart, infrared spectroscopy. The latter technique experienced a quantum leap in use in the forensic science laboratory following the introduction of inexpensive FT-IR spectrometers in the 1980s, but forensic scientists have been slower to embrace Raman spectroscopy. This may stem in part from the perception that fluorescence prevents its use for many samples. However, a more significant factor may be insufficient understanding of the unique capabilities of Raman spectroscopy, including how it can provide information not accessible using other methods. This promising technique is finally making some inroads into the forensic science laboratory, and this will continue as forensic scientists gain a greater appreciation of its features and merits. To facilitate this process, this article presents a comprehensive review of Raman spectroscopy, with an emphasis on how and why this underutilized cousin to infrared spectroscopy can be a very valuable tool for the analysis of a wide variety of evidentiary materials. Owing to the wide scope of this review, it is presented in two parts. Most forensic scientists are not very familiar with inelastic scattering and Part I of this article describes the principles and instrumentation of Raman spectroscopy. Forensic scientists, however, are generally more knowledgeable about infrared spectroscopy, and a comparison of the spectral data produced by these two related vibrational methods for various categories of analytes is also presented and discussed.