CHEMICAL IMAGING OF HAIR BY IMS 83 1.5 1.0 0.5 0.0 1 ! , : ? I :I : I : ! ! ............ Co flex -- Cuticle i -0.5 1800 1700 1600 1500 1400 Frequency (cm -1) Figure 8. Infrared spectra between 1850 cm • and 900 cm • of cuticle and cortex of the untreated hair. Spectra were recorded with 3-pm x 3-pm apertures, at 4 cm -• resolution, and 128 scans were co-added. keratin oxidation. Hydrogen peroxide used for bleaching leads to oxidative cleavage of disulfide bonds and finally to the formation of cisteic acid. The effect of bleaching and weathering on hair can be studied by looking at the symmetric and asymmetric sul- phonate S--O stretching vibrations at 1040 cm • and 1188 cm -• (2). The band at 1219 -1 ß 1 -1 cm lS assigned to cysteic acid moiety the bands at 1196 cm and 1022 cm are assigned to the assymetric and symmetric S--O stretch vibrations of thiosulphate ions (Bunte salt) (2) (Table I). Other characteristic bands are available below 900 cm i but were not studied here because of increased diffraction effects and the refractive index variation of the BaF 2 substrate (22). Figure 9 shows the spectra obtained between 1850 -1 cm and 900 cm • before and after bleaching. The typical sulphonate stretching vibration band at 1040 cm -1 is shown. At higher frequencies, the other vibrations all overlap to form a large band between 1138 cm -1 and 1286 cm -• Although bleaching is a well-known chemical process, the spatial distribution of the induced chemical changes have never been studied. For this purpose, we use the sym- metrical sulphonate vibration, where the band intensity is directly related to the cysteic acid content of the hair (4). Figure 10B shows the contour map obtained from the integrated absorbance of the symmetric stretching vibrations S=O (1040 cm -1) as a function of position. The mapping was acquired through a square area of 11 pm x 11 pm in 6-pm steps, and 64 scans were co-added for each spectrum. The sulphonate signature is present everywhere within the hair. We noticed a heterogeneous distribution of the sulphonate band through the cortex in this region, and the contribution of the sulphonate is between 20% and 30% higher than elsewhere in the hair. The sulphonate lateral distribution can be compared with the distribution of the amide A band shown

84 JOURNAL OF COSMETIC SCIENCE 2.0 1.5 o.5 0.0 • Bleaching ............ U ntrea ted I q40 cm '1 I • I • I • I • I 1800 1600 1400 1200 1000 Frequency (cm Figure 9. Infrared spectra between 1850 cm • and 900 cm -• of the hair untreated and after bleaching. Recording conditions are the same as in Figure 2. in Figure 10A. No correlation between the two images has been found. In addition, aliphatic and other peptide distributions (amides I, II) have also been studied, and no correlation has been found. Thus, the distribution of those specific functional groups of the hair structure appears to be homogeneous throughout the cortex. We conclude, therefore, that the heterogeneity of sulphonate distribution is due to variations in the concentration of sulphonate and not to a variation of thickness of the sample. Optical microscopy experiments show that bleaching physically deteriorates the cuticle, but this process is not spatially homogeneous. Therefore, we suggest that the non-homogeneity of the sulphonate in the cortex is due to the non-homogeneous structural alteration of the cuticle, which could lead to a non-homogeneous enhancement of the porosity of the cuticle. No specific interaction with the medulla has been shown. It can also be noticed that, as expected from diffraction theory, the edge of the hair is better resolved in Figure 10A than in Figure 10B due to the smaller wavelength (3 pm compared to 10 pm). This result has been verified on five cross sections from the same hair fiber. To study the interaction of the cuticle with the bleaching agent, chemical mapping was carried out using a 3-prn x 3-pm square aperture and rastering the sample at 2-pm steps, with 128 scans co-added for each spectrum. Figure 11 shows the signatures of the cuticle, cortex, and resin in the amide A region before and after bleaching. The signature of the cortex before and after treatment is identical, but the shape of the wide amide A band of the cuticle is modified after bleaching. A contribution centered at 3400 cm -• appears and is due to bonded water (1). To quantify the water in the region of the cuticle, the ratio of the amide A to the broad water band at 3400 cm -• was made after