CHEMICAL IMAGING OF HAIR BY IMS 77 Table I. Vibration of the Hair (1-4,6) Region (cm -1) Assignment 3400 OH bonded water 3290 vNH, amide A 3200 vNH, amide B 3063 vNH, amide B 2962 1,'asCH 3 2918 VasCH 2 2875 VsCH 3 2850 l•sCH 2 1650 vCO, amide I 1540 vCH and 8NH, amide II 1450 8asCH 3 and 8CH 2 1390 85CH 3 1238 Amide III and nucleic acid 1219 Cisteic acid moiety 1196 va•S=O thiosulfate ions (Bunte salt) 1188 VasS=O sulphonate 1121 Cystine dioxide 1080 Nucleic acid 1071 Cystine monoxyde 1040 v•_S = O sulphonate 1022 v•S=O thiosulfate ions (Bunte salt) IR active modes exhibit the different structural domains of the hair. Images obtained using the vNH and vCH bands, though, gave the best contrast, probably because smaller wavelengths suffer from less diffraction and pass more efficiency through the instrument. We began with the chemical image obtained from the wide amide A band (3290 cm-•), using the internal globar source (see Figure 4A.) Notice that it is not possible in this image to distinguish either the medulla (about 12 microns in diameter) or the cuticle (annulus having a thickness of about 5 microns). The graded appearance of the image at the hair surface is also due to the 24-pm apertures and not indicative of a particular structure. Concerning the synchrotron radiation experiments, the IR beam was confined by aper- tures to an 11-pm x 11-pm area, within our noise criteria. The other experimental conditions were identical to those set up with the globar source. In this case the lateral resolution was still determined by geometrical considerations rather than by diffraction. The chemical image obtained is shown Figure 4B. The cortex and medulla are now clearly seen with the contrast coming from the different concentrations of the amide A band. We show that the concentrations of the stretching amide groups are less important in the medulla than in the cortex. This is not surprising if we consider that the structure of the medulla is different from that of the cortex (19), wherein there are airspaces (20) that give hair its important thermoregulatory properties. The graded appearance at the hair edge is still a consequence of the 11-pm aperturing. The high brightness of the synchrotron source allows the apertures to be reduced even more, improving the lateral resolution of the chemical images. A good signal-to-noise ratio can be achieved even when the size of the apertures is at the diffraction limit or below. In this case, the lateral resolution approaches the wavelength, i.e., a few microns

78 JOURNAL OF COSMETIC SCIENCE 2.0 - 1.5 0.5 Amide I , I • I , I • I 3500 3000 25(X) 2000 Amide III 0.0 I • I Frequency (off •) Figure 2. IR spectrum of a hair cross section. Spectrum was recorded with 12-1am x 12-1am apertures at 4 cm-• resolution, and 64 scans were co-added. 15 Hm Figure 3. Optical image of hair cross section. presenting the possibility of imaging even thinner structures such as the cuticle. In Figure 5, we present chemical images of the medulla and the cuticle of the hair acquired through a square area of 3 lam x 3 iam, stepping every 2 microns. Co-added for each