Raman spectroscopy

Numerous analytical techniques have been used to examine the nature of skin, and in particular the barrier structure of human stratum corneum, including thermal methods (1,2), electron spin resonance (3), and x-ray diffractometry (4,5). One approach has been to use vibrational spectroscopy to probe the molecular nature of the skin, with early work concentrating on the use of Fourier transform infrared (FTIR) spectroscopy (6). While the infrared technique has provided valuable insights into the concentration of water in the tissue (7) and has been used successfully to probe the tissue in vivo using attenuated total reflection methods (8,9), infrared is not ideally suited to examine the vibrational modes of biological samples that are naturally hydrated. This is because such materials show interference from water vibrational modes, while the water absorbs strongly the infrared radiation. An alternative vibrational spectroscopic technique is Raman spectroscopy. Raman spectroscopy is both a qualitative and quantitative technique for characterizing materials at the molecular level in terms of molecular normal vibrational frequencies. If a sample is irradiated with an intense beam of monochromatic radiation (usually from a laser) operating at a wavenumber n0, most of the radiation is transmitted by the sample. However, a small portion of the exciting radiation (about 1 photon in 10) scatters elastically with a wavenumber equal to that of the incident radiation. An even smaller fraction (around 1 photon in 10) scatters inelastically with wavenumbers different to the incident radiation. The elastically scattered radiation is known as Rayleigh scattering, whereas the