[Identification of polysaccharide binding materials used in cultural relics by pyrolysis-gas chromatography/mass spectrometry]

Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) is suitable for the qualitative analysis of natural organic materials in cultural relics owing to its ability of accurately and rapidly identifying trace quantities of organic components in samples. In the present study, pyrolytic components of starch and peach gum, which are commonly used in ancient Chinese cultural relics such as calligraphic works, paintings, architecture, and objects, as well as gum Arabic, which is commonly used in western cultural relics, were systematically characterized using Py-GC/MS. As polysaccharide materials are often found mixed with other organic materials in cultural relics, an online methylation Py-GC/MS procedure previously established by the authors' research group, which is suitable for the qualitative analysis of drying oils, proteins, and waxes, was performed to analyze the starch, peach gum, and gum Arabic reference samples. The established experimental procedure can be used to comprehensively analyze the polysaccharides and other organic materials in cultural relics owing to its slow heating rate, long running time, wide mass spectrometry detection range, and high chromatographic separation efficiency. The experimental results were examined by dividing the pyrolytic products into three sections depending on the retention time. In the first section (2.5-10 min), pyrolytic products from the three sample types were similar and mostly included small molecule furans and ketones. However, the peach gum and gum Arabic contain a wider variety of pyrolytic products than starch, possibly because starch contains only glucose, whereas peach gum and gum Arabic contain a variety of monosaccharides. In the middle section (10-19 min), the pyrolytic products of starch, peach gum, and gum Arabic mostly consisted of furanones and other components. However, a unique profile was produced for each material type. Nevertheless, it should be noted that 2-methoxy-phenol and maltol were detected in all three materials with high chromatographic peak intensities. Therefore, these components are suitable markers for the identification of polysaccharides. In the last section (19-60 min), a variety of monosaccharide derivatives and monosaccharide oligomer derivatives were detected in the reference samples. The pyrolytic products of peach gum and gum Arabic were similar but completely different from those of starch. Therefore, starch, peach gum, and gum Arabic could be distinguished according to their pyrolytic products. In particular, 1,6-anhydro-β-D-glucopyranose was detected in starch with an extremely high intensity and was undetected in either of the other reference materials. Thus, this compound could act as a characteristic pyrolytic component for the identification of starch. Peach gum and gum Arabic could be rapidly distinguished according to the extracted ion chromatograms for m/z 60 and m/z 101, which represented major fragments of the pyrolytic products of peach gum and gum Arabic in the last section of the chromatogram. The established Py-GC/MS method was successfully applied to the identification of starch in binding materials from the rim of a globular carved red lacquer vase with dragon and cloud motifs, as well as in paper collected from a Tieluo with Bingdihanfang written by Min-ning. Both objects are part of the Palace Museum collection and originate from the Qing Dynasty. Research results of the present work are easy to be popularized. This study provides a method suitable for the accurate and rapid identification of polysaccharide materials in cultural relics, as well as a scientific basis for the research, conservation, and restoration of similar cultural relics. However, it should be noted that aging and the presence of other organic or inorganic materials in cultural relics may interfere with the detection of polysaccharide materials. Therefore, a further investigation on the aging behavior of polysaccharides and the effects of other materials on the identification of polysaccharides is required.