| Proteinaceous binding media,as the major part of the painting layers of the polychromy cultural relics,is vital for the preservation of the colors.However,due to the long-term underground and overground aging,there would be great losses in the amount and intrinsic cohesion,resulting in kinds of diseases such as the shedding of pigments.Therefore,the accurate analysis of the composition and content of the proteinaceous binders is the focusing analytical work as well as the foundation for choosing proper restoration materials,fulfilling the principle“no change of the originality ”of cultural relics conservation.But the analysis of the cultural relics’ proteinaceous binders differs from that of the fresh binders due to the two great challenges.Firstly,the interference problem.The soluble salts from the groundwater,the pigments in the painting layer would greatly interfere the accuracy of the GC-MS analysis.Secondly,the aging problem.The composition and amino acid contents of the polychromy pottery,mural paintings and colored paintings would change and get complicated after underground soil burial or overground photothermal aging.As a result,how to eliminate the interference factors(purification)and accurately analyze the aged cultural relic samples are the bottlenecks in the GC-MS analysis.Regarding the above problems,this paper firstly introduced the SPE-C4 technique into the analysis system of the proteinaceous binders of the ancient Chinese polychromy relics with GC-MS,the methodology of the optimized method was also systematicallycarried out.On this basis,GC-MS and FTIR techniques were applied to investigate the changes in amino acids and secondary structures’ contents of the collagen,eggs and milk after the burial and photothermal aging.Based on the optimized system and the amino acids’ changing rules,the specific composition of the proteinaceous binders in polychromy relics samples could be identified,laying scientific foundations for archaeology,historical research and future restoration.The research contents mainlyinclude:(1)Thepurification with SPE C4 in the GC-MS analysis.Ion chromatography(ICS)was used to quantitatively evaluate the salt removal effect with SPE C4.The optimized GC-MS analysis system for proteinaceous binders was thus established.The applicability of this technique in GC-MS anlysis of relics samples was discussed by exploring the effect of the SPE C4 method on amino acid quantification results,the actual protection efficiency of column and the test results of cultural relic samples.Results showed that the SPE C4 could adsorb polar soluble salt ions with the 99.99% average elution rate.The chromatograms of the samples were good and the column was properly protected.Therefore,SPE C4 technology is suitable for the pretreatment of GC-MS analysis of ancient Chinese proteinaceous binders,which was of high practical valus.(2)The methodological study on proteinaceous binders with GC-MS.Regarding the optimized GC-MS analysis method,the quantitative standard curves,linear ranges,precision,accuracy,detection limit and quantification limit of the 11 amino acids(Ala,Gly,Val,Leu,Ile,Ser,Pro,Phe,Asp,Glu and Hyp)were systematically explored.Results showed that the GC-MS quantitative calibration curves of the 11 amino acids were of good linearity,most amino acids’ correlation was more than 0.99,RSD was less than 9% and relative errors was no more than 6%.The detection limit of the 11 kinds of amino acids was about 0.202μgwhile the quantification limit was 0.673μg.The established database of amino acid contents of the fresh proteinacecous binders was the scientific basis for the aging research and the identification of the proteinacecous binders in the relics samples.(3)The effect of soil burial on amino acid analysis of the common proteinaceous binders.The one-year anaerobic,low temperature and dark soil aging experiment of pig skin,whole egg,egg white,egg yolk and milk was conducted.The changes of the amino acid and secondary structure contents were analyzed by GC-MS and FTIR.The descriptions and reasons of the PCA positions changes were analyzed by principal component analysis.The changes in amino acids and secondary structure were discussed from the perspective of amino acid character and protein structure.Experiment illustrated that the contents of Ser、Gly and Hyp decreased while Pro、Asp and Glu increased,different binders were with obvious various amino acids distribution features.Moreover,although the five binders still showed typical protein FTIR characteristics,their structure had already changed from ordered to disordered states.(4)Influence of photothermal aging on amino acid analysis of the common proteinaceous binders.Accelerated Xenon lamp and ten years’ natural photothermal aging were combined to age the pig glue,egg white,egg yolk,egg and casein.GC-MS and FTIR were applied in studying the changes of amino acid and secondary structure contents.With principal component analysis,the PCA position changes and the corresponding reasons were analyzed.The light,heat aging mechanism helped to explain how the amino acid and secondary structure contents changed.The GC-MS experiment showed that the contents of Ile、Ser、Asp、Glu and Hyp,especially aromatic Phe and Tyr decreased,while the contents of Ala,Gly,Val,Leu and Pro increased.In spite of this,the five binders were still of great difference in the amino acid composition.FTIR results indicated that the five pure binders were still of proteins’ absorbance features,but they had turned into the disordered states.(5)Analysis of the proteinaceous binders in the polychromy relics with GC-MS.Based on the optimized GC-MS analysis system and the common proteinaceous binders’ amino acid content features after soil burial or photothermal aging,typical samples from the mural paintings and colored paintings in the Summer Palace were qualitatively and quantitatively analyzed with GC-MS to identify the category of the protein.Experiments manifested that the mural samples’ binders were composed of collagen and egg while pig’s blood and collagen made up the proteinaceous binding media in the painting layers of the colored paintings in the Summer Palace.Generally,the residual binders were extremely low in amount,which presumably was the key factor resulting in diseases such as the powdering and shedding of pigment.It is thus advised that the original or similar binding materials should be used in the restoration. |
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