FTIR investigation of the damage produced on human hair by weathering and bleaching processes: implementation of different sampling techniques and data processing.
Untreated and bleached hair samples were analysed by FTIR spectroscopy. Cysteic acid and cysteine-S-thiosulphate were analysed by different data manipulations and various sampling techniques. Single-fibre analysis was achieved using the IR-microscope attachment to the FTIR instrument (transmission mode). An ATR diamond crystal was also used for both single fibres and tresses; this sampling technique was compared to a contact sample ATR cell based on a ZnSe crystal. The microscope was found to be very useful for the analysis of small portions of hair fibres; the effect of weathering along the length of the fibres and the extent of variability among the single fibres was assessed by this technique. The results clearly show a trend in the cysteic acid content increasing from the mid-point to the tip; it was found that hair samples are very heterogeneous. A standard variation of up to 11% is reported for the mean value of the intensity of absorption of cysteic acid in untreated fibres belonging to the same tress sample. The value of the variation decreases when the tress is bleached -- the longer the bleaching process the lower the value of the standard variation (4.5% for hair fibres bleached for 60 min using hydrogen peroxide). The contact sample ATR cell gave poor reproducibility as only poor sample-crystal contact was achieved and the level of noise recorded was too high to allow accurate detection. However the ATR diamond crystal was found to give sensitive and reproducible spectra since much higher levels of transmittance were achieved (better sample-crystal contact); this technique allowed measurements of groups of seven to nine hair fibres at once, but did not provide reproducible spectra of single fibres. Different data manipulations have been assessed; these include derivatization and calculation of peak heights and areas (ratioed by the intensity of absorption of the peaks chosen as internal standards to the amide III 1232--1242 cm-1 and CH2 stretching 1451 cm-1), normalization, averaging spectra and spectral subtraction.
Department of Colour Chemistry and Dyeing, University of Leeds Leeds LS2 9JT UK.
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Lewis