Identification of dyes on ancient Chinese paper samples using the subtracted shifted Raman spectroscopy method

The Stein Collection in the British Library contains the Diamond Sutra, the world's oldest, dated, printed document. The paper of the Diamond Sutra an d other documents from the Stein collection is believed to be dyed yellow b y a natural extract, called huangbo, from the bark of Phellodendron amurens e, which contains three major yellow chromophores: berberine, palmatine, an d jatrorrhizine, Conservation of these documents requires definite informat ion on the chemical composition of the dyes but no suitable, completely non invasive analytical method is known. Here we report resonance Raman studies of a series of prate dyes, of plant materials and extracts, and of dyed an cient and modern paper samples. Resonance Raman spectroscopy is used to enh ance the spectra of the dyes over the signals from the paper matrixes in wh ich they are held. The samples an give resonance Raman spectra which are do minated by intense fluorescence, but by using SSRS (subtracted shifted Rama n spectroscopy) we have obtained reliable spectra of the pure dyes, native bark from the Phellodendron amurense, modern paper dyed with huangbo extrac ted from this bark, and ancient paper samples. For both ancient paper sampl es whose pigment bands were detected, the relative intensities of the bands due to berberine and palmatine suggest that the ancient paper is richer in berberine than its modern counterpart, This is the first nondestructive in situ method for detection of these pigments in manuscripts, and as such ha s considerable potential benefit for the treatment of irreplaceable documen ts that are believed to be dyed with huangbo but documents on which conserv ation work cannot proceed without definite identification of the chemical c ompounds that they contain.