Use of Raman Spectroscopy and Multivariate Classification Techniques for the Differentiation of Fingernails and Toenails

For many years, nails have been used for forensic purposes, especially for the detection of drug abuse and arsenic intoxication. In addition, nails have also been used as biomarkers for fluoride and tobacco smoke exposure and as a diagnostic tool for various disease states. However, most of these nail analyses were carried out in destructive ways and involved many sample preparations. The well-established techniques for these nail analyses include gas chromatography-mass spectrometry (GC-MS), high-performance liquid chromatography (HPLC), and atomic absorption spectrophotometry (AAS). In contrast to these analytical techniques, Raman spectroscopy offers a totally different approach, one which is non-contact, nondestructive, and needs only very little or no sample preparation. The use of Raman spectroscopy to characterize the molecular structure of human nails has been reported repeatedly. William et al. used Raman spectroscopy to study the molecular structure of human keratotic biopolymers, such as skin, callus, hair, and nails. They found that the principal structural dissimilarities were in the sulfur content of tissues containing hard keratin (hair and nail) and soft keratin (stratum corneum and callus). Gniadecka et al. used Fourier transform (FT)-Raman spectroscopy to investigate the structure of water, proteins, and lipids in intact human skin, hair, and nail. They found that the internal water of nail was mainly present in the bound water. De Faria and de Souza have also characterized human skin and nail by using 632.8 nm visible laser light as the Raman excitation source. After 30 min of irradiation on the sample, the strong fluorescent background was photobleached and good Raman spectra could be obtained. Hydration of human nail was investigated by Wessel et al. Their results suggested that the softening of the nail following hydration may be due to changes in the matrix protein molecular structures induced by water. A comparison study on Raman spectra of different keratotic biopolymers (stratum corneum, human nail, feather, and bull’s horn) was carried out by Akhtar and Edwards. This study revealed that the structurally important m(SS) and a(CS) bands, near 500 cm 1 and 630 cm , were different for human, mammalian, and avian samples. In their most recent study, Pillay et al. performed a Raman spectroscopy study on fingernail to explore the relationship between nail and bone health. As has been shown, Raman spectroscopy is able to detect subtle differences in various keratotic biopolymers. Therefore, in the present work, we aimed to detect the subtle differences between two very similar biomaterials, human fingernails and toenails, using Raman spectroscopy. The distal growth rate for fingernail is about three times faster than that of toenail (growth rate for fingernail is about 0.1 mm/day, and growth rate for toenail is about 0.03 to 0.04 mm/day). This phenomenon can be attributed to different growth rate mechanisms, which may be reflected in different biochemical intensities and compositions. Principal component analysis (PCA) and linear discriminant analysis (LDA) were used to perform the data classification.