Identification of Cellulosic Fibres by FTIR Spectroscopy DIFFERENTIATION OF FLAX AND HEMP BY POLARIZED ATR FTIR

Abstract The ability to identify accurately natural fibres is of importance to conservators, but this can prove problematic when dealing with fibres which possess similar chemistry and microstructure – the bast fibres flax and hemp are particularly challenging in this regard. Conventional techniques, such as microscopic examination, the measurement of fibre widths and the use of the twist test, will often prove unreliable, especially when dealing with degraded materials. This research has concentrated on the development of an attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic technique employing polarized radiation to differentiate these two fibre types on the basis of the characteristic nature of the winding of their secondary cell walls: while the angle of fibrillar wind is roughly the same for both fibres (∼7°), the senses of wind are opposite – S (anti-clockwise) in the case of flax; Z (clockwise) for hemp. A crystallinity index, derived from the spectra, was found to vary in a systematic manner as the fibres were rotated with respect to the electric vector of the incident radiation. A good correlation between this variation and theoretical models was observed. It was found that the characteristic sense and angle of wind of the fibres could be determined, and thus hemp and flax distinguished.