A method for identifying organic pigments in murals based on thermal cracking fingerprint and application thereof

Abstract

This invention relates to the field of cultural relic research and restoration technology. It discloses a method for identifying organic pigments in murals based on pyrolysis fingerprint spectroscopy and its application. The identification method includes the following steps: S1, establishing a pyrolysis fingerprint spectroscopy library of standard organic pigment samples; S2, obtaining pyrolysis fingerprint spectra of organic pigments in the mural sample to be tested; S3, organic pigment identification: comparing the spectra obtained in step S2 with the spectra in the pyrolysis fingerprint spectroscopy library established in step S1, and determining the type of organic pigment in the mural sample based on the comparison results. This invention, by studying the pyrolysis and methylation behavior of samples under specific pyrolysis conditions, clarifies characteristic pyrolysis pathways and their corresponding fingerprint pyrolysis products and combinations of characteristic pyrolysis products. Applying the fingerprint spectroscopy library obtained by this method to the analysis of actual mural samples can accurately analyze the types of pigments used in the murals, demonstrating good feasibility and reliability.

Description

Technical Field

[0001] This invention relates to the field of cultural relic research and restoration technology, and in particular to a method for identifying organic pigments in murals based on pyrolysis fingerprinting and its application. Background Technology

[0002] From a materials analysis perspective, organic pigments typically exhibit diverse molecular structures, complex chemical compositions, and relatively limited environmental stability, and are often present in murals in trace amounts. In actual mural systems, organic pigments frequently coexist with inorganic mineral pigments, protein-based or oil-based binders, and their aging products. The superposition of multiple components and strong matrix interference significantly increase the difficulty of reliable identification. Therefore, accurate interpretation of organic pigments in complex systems has always been a technical challenge in mural materials research.

[0003] In recent years, various spectroscopic and chromatographic analysis techniques have been applied to the study of organic dyes. Raman spectroscopy (RS) and fiber optic reflectance spectroscopy (FORS) possess the potential for non-destructive analysis; however, when the organic pigment content is low or affected by matrix interference, the reliability of signal intensity and characteristic peak identification remains limited. Chromatographic techniques such as gas chromatography-mass spectrometry (GC / MS) and high-performance liquid chromatography-mass spectrometry (HPLC / MS) have advantages in the separation and characterization of organic dyes, but they typically rely on extraction steps and require relatively large sample volumes. In complex aging systems, extraction efficiency and component integrity may also be affected. Therefore, achieving stable identification of organic pigments in mural systems with extremely small sample volumes and complex compositions still faces methodological challenges.

[0004] Thermally assisted methylation pyrolysis-gas chromatography / mass spectrometry (THM-Py-GC / MS) offers another possibility for studying organic components in trace solid samples. This method introduces methylating agents during the pyrolysis process, achieving in-situ derivatization of carboxyl and hydroxyl functional groups, improving the volatility and detection stability of the pyrolysis products, and allowing direct analysis of solid samples without complex extraction. Furthermore, THM-Py-GC / MS can obtain information on multiple organic compounds in a single injection, thus offering practical advantages in mural systems with multiple components and limited sample volumes. In recent years, this method has been applied in the study of lacquer, oil, and protein materials.

[0005] However, it should be noted that thermal decomposition and derivatization processes may introduce structural rearrangement or fragment reconstruction, and there may be overlap in the sources of decomposition products. In complex mural systems, relying solely on a single decomposition product for identification often introduces uncertainty. For certain structurally similar organic pigment categories, THM-Py-GC / MS may only provide category-level classification information, making it difficult to achieve more refined differentiation. Therefore, the key issue for this technology in the identification of organic pigments in murals is not simply the application of the method, but rather how to establish a decomposition product interpretation system based on structural correlation to improve the reliability and reproducibility of the interpretation. Although existing research has involved thermal decomposition analysis of some organic pigments, systematic research on organic pigments in ancient Chinese mural systems remains relatively insufficient, especially lacking a systematic discussion of the structural correlation and classification criteria of decomposition products under complex multi-component environments. Summary of the Invention

[0006] To address the above technical problems, this invention provides a method for constructing a thermal decomposition fingerprint library of organic pigments, comprising the following steps: Organic pigment standard samples and aqueous solutions of methylation derivatization reagents were placed together in an inert atmosphere for thermal pyrolysis. The pyrolysis products were then introduced into a gas chromatography column with a carrier gas for decomposition. The fingerprint spectrum of the organic pigment standard samples was obtained by gas chromatography-mass spectrometry. Based on the thermal pyrolysis fingerprint spectrum of the standard samples, a fingerprint spectrum database and a characteristic peak database were established. The thermal pyrolysis included the following conditions: pyrolysis temperature of 450-650℃ and pyrolysis time of 45-55 min.

[0007] In some embodiments of the present invention, the organic pigment standard sample includes at least one of indigo organic pigments, anthraquinone organic pigments, flavonoid organic pigments, or benzopyran organic pigments.

[0008] In some embodiments of the present invention, the organic pigment standard sample includes at least one of indigo, madder, cochineal, shellac, turmeric, or gamboge.

[0009] In some embodiments of the present invention, if the organic pigment standard sample includes indigo, the fingerprint database contains the following fingerprint cleavage product peaks: anthranilic acid methyl ester, N-Methyl anthranilic acid methyl ester, indirubin tetramethylated, indigotin trimethylated, and tryptathrin. The characteristic peak database contains the following characteristic cleavage product peaks: aniline, 3-methylbenzenamine, methyl benzoate, indole, 1,3-dimethyl-1H-indole, and 2-(dimethylamino)-benzoic acid methyl ester.

[0010] In some embodiments of the present invention, the structural formula of tryptophan ketone is as follows: .

[0011] In some embodiments of the present invention, if the organic pigment standard sample includes madder, then the fingerprint database contains the following fingerprint cleavage product peaks: 1,2-Dimethoxybenzene, 1,2-Dimethyl phthalate, Xanthopurpurin 3-O-methyl, Alizarin 2-O-methyl, Alizarin 1,2-di-O-methyl, and Xanthopurpurin 1,3-di-O-methyl. The characteristic peak database contains the following characteristic cleavage product peaks: 1,4-dimethoxy-benzene, 2,3-dimethoxytoluene, 1,2,3-trimethoxybenzene, methyl 3-methoxy-benzaldehyde, methyl 3,5-dimethoxy-benzoic acid methyl ester, methyl 3,4-dimethoxy-benzoic acid methyl ester, and methyl 3,4,5-trimethoxy-benzaldehyde methyl ester.

[0012] In some embodiments of the present invention, if the organic pigment standard sample includes carmine, the fingerprint database contains the following fingerprint cleavage product peaks: methyl benzoate, methyl 4-methoxy-benzoic acid methyl ester, galactose marker 1, and galactose marker 2. The combined characteristics of anthraquinone-related benzoate compounds, polysaccharide derivatives, and long-chain lipid cleavage products can accurately identify carmine pigment in ancient murals. The structural formula of galactose marker 1 is as follows: It is 5,6-dimethoxy-2-methyltetrahydro-2H-pyran-3-ylacetate.

[0013] The structural formula of galactose marker 2 is as follows: That is, 2,3-dimethoxy-6-methyltetrahydro-2H-pyran-4-ylacetic acid ester.

[0014] In some embodiments of the present invention, if the organic pigment standard sample includes shellac, then the fingerprint database contains the following fingerprint cleavage product peaks: methyl 9,10-dimethoxyhexadecanoic acid methylester, lacijalaric acid trimethyl isomer 1 (isomer 1), jalaric acid tetramethyl (also known as Ken), laksholic acid tetramethyl derivatives, shellac acid tetramethyl derivatives, jalaricacid tetramethyl (also known as Henk), and aleuritic acid trimethylisomers.

[0015] In some embodiments of the present invention, the structural formula of isomer 1 of the shellac trimethyl derivative is as follows: .

[0016] In some embodiments of the present invention, if the organic pigment standard sample includes gamboge, then the fingerprint database contains the following fingerprint cleavage product peaks: Bornylene and isoprene-dimer, and the characteristic peak database contains the following characteristic cleavage product peaks: 3-methylenecyclopentene, 2-heptene, 4-methyl-Cyclohexene, 1-methyl-Cyclohexene, 2-methyl-1-heptene, and 5,6-dimethyl-1,3-cyclohexadiene. yl-1,3-Cyclohexadiene), 3-methyl-2-cyclopenten-1-one, 1-dodecene, 1-tridecene, 1-tetradecene, 1-pentadecene, 1-heptadecene, 1-octadecene, 1-nonadecene, hexacosane, heptacosane, octacosane, and nonacosane.

[0017] In some embodiments of the present invention, if the organic pigment standard sample includes turmeric, then the fingerprint database contains the following fingerprint cleavage product peaks: beta-Eudesmene, gamma-Cadinene, ar-tumerone, cis-methyl p-methoxycinnamate, and 2-(3,4-dimethoxyphenyl)-2-acrylate. The characteristic peak database contains the following characteristic cleavage product peaks: 2-methylanisole, 2-methoxy-Phenol, 1,2-dimethoxybenzene, p-methoxystyrene, 1,2,3-trimethoxybenzene, 1,2,4-trimethoxybenzene, and 3,4-dimethoxybenzoic acid methyl ester.

[0018] In some embodiments of the present invention, the pyrolysis temperature is 550°C and the time is 50 min.

[0019] In some embodiments of the present invention, the methylation derivatizing agent includes methylammonium hydroxide.

[0020] In some embodiments of the present invention, the concentration of the methylation derivatizing reagent in the aqueous solution is 20-30%, for example, 25%.

[0021] In some embodiments of the present invention, the organic pigment standard sample is mixed with an aqueous solution of methylation derivatization reagent at the following mass-volume ratio: 0.05~0.15mg:3μL.

[0022] In some embodiments of the present invention, the pyrolysis is performed using a Frontier Lab PY-3030D dual-gun pyrolysis apparatus.

[0023] In some embodiments of the present invention, the pyrolysis interface temperature is 300°C.

[0024] In some embodiments of the present invention, the gas chromatography-mass spectrometry includes a device that couples a pyrolyzer to an Agilent Technologies 7890B gas chromatograph / 5977B inert mass selective detector (MSD).

[0025] In some embodiments of the present invention, the injection temperature of the organic pigment sample solution is 285~300°C. The temperature of the split / non-split injector is approximately 290°C.

[0026] In some embodiments of the present invention, the carrier gas is an inert gas, such as helium.

[0027] In some embodiments of the present invention, the carrier gas flow rate is 0.8~1.2 mL / min, such as 1.0 mL / min.

[0028] In some embodiments of the present invention, the gas chromatography column is an Agilent J&W Ultra-Inert DB-5MS capillary column.

[0029] In some embodiments of the present invention, the gas chromatography column has dimensions of 20m × 0.18mm × 0.18µm.

[0030] In some embodiments of the present invention, the temperature program of the gas chromatograph oven is as follows: initial temperature 40~50℃, temperature increase at 4~6℃ / min to 190~210℃, hold for 6~8min, then temperature increase at 8~12℃ / min to 290~320℃, hold for 1~3min.

[0031] In some embodiments of the present invention, the temperature program of the gas chromatograph oven is as follows: initial temperature 45°C, temperature increase to 200°C at 5°C / min, hold for 7 min, then temperature increase to 300°C at 10°C / min, hold for 2 min.

[0032] In some embodiments of the present invention, the parameter settings of the mass spectrometer include at least one of the following: a) The mass spectrometer transfer line is maintained at 240~260℃; b) The ion source is maintained at 210~230℃; c) The quadrupole is maintained at 140~160℃; d) The scanning range of the mass spectrometer is 50~550 m / z.

[0033] In some embodiments of the present invention, the parameter settings of the mass spectrometer include at least one of the following: a) The mass spectrometer transfer line is maintained at 250°C; b) The ion source is maintained at 220°C; c) The quadrupole is kept at 150°C.

[0034] The present invention also provides a thermal pyrolysis fingerprint library constructed by the above construction method.

[0035] This invention also provides a method for identifying organic pigments in murals based on pyrolysis fingerprinting, comprising the following steps: S1. Establish a thermal decomposition fingerprint library for organic pigment standard samples: Construct a thermal decomposition fingerprint library using the above method; S2. Obtain the thermal decomposition fingerprint spectrum of organic pigments in the mural sample to be tested: Use the same detection conditions as the thermal decomposition fingerprint spectrum of the organic pigment standard sample in step S1 to obtain the thermal decomposition fingerprint spectrum of organic pigments in the mural sample to be tested; S3. Identification of organic pigments: The spectrum obtained in step S2 is compared with the spectrum in the thermal decomposition fingerprint spectrum library established in step S1, and the types of organic pigments in the mural sample are determined based on the comparison results.

[0036] In this invention, firstly, the optimized THM-Py-GC / MS method is used to analyze standard organic pigment samples to obtain the total ion current chromatogram (TIC chromatogram) of the samples; then, the thermal decomposition path of each pigment is analyzed in detail; next, based on the thermal decomposition path, fingerprint decomposition fragments and characteristic decomposition fragment combinations that identify organic pigments are determined, and a framework for the analysis of organic pigments in (ancient) murals based on THM-Py-GC / MS is established; finally, the fingerprint decomposition fragments and characteristic decomposition fragment combinations determined by the aforementioned framework are applied to the determination of organic pigments in (ancient) mural samples to verify the feasibility of the analytical framework.

[0037] In some embodiments of the present invention, the identification method further includes the step of analyzing the content of organic pigments after determining the types of organic pigments in the mural sample.

[0038] This invention also proposes the application of the above-mentioned construction method, the above-mentioned pyrolysis fingerprint library, or the above-mentioned identification method in mural restoration.

[0039] In some embodiments of the present invention, the mural is an ancient mural.

[0040] Compared with the prior art, the above-mentioned technical solution of the present invention has the following advantages: The present invention selects six representative organic pigment standard samples from ancient murals and analyzes them using thermally assisted hydrolysis-methylation-gas chromatography / mass spectrometry (THM-Py-GC / MS) to obtain their thermal degradation fingerprint spectra. By studying the pyrolysis and methylation behavior of the samples under specific thermal degradation conditions, the characteristic thermal degradation pathways and their corresponding fingerprint degradation products are clarified. Applying the fingerprint spectrum library obtained by this method to the analysis of actual mural samples can accurately analyze the types of pigments used in the murals, demonstrating good feasibility and reliability. Attached Figure Description

[0041] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0042] Figure 1 This is the total ion flow chromatogram (TIC) of six organic pigment standard samples in the embodiments of the present invention.

[0043] Figure 2 This is a magnified view of the sampling locations of the mural samples in an embodiment of the present invention.

[0044] Figure 3 This is the total ion flow chromatogram of the mural sample in an embodiment of the present invention. Detailed Implementation

[0045] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments are not intended to limit the present invention. Unless otherwise specified, the experimental methods used in the embodiments are conventional methods; the materials and reagents used, unless otherwise specified, are commercially available. Unless otherwise specified, the same parameter value is the same in each embodiment. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0046] In the description of this invention, references to terms such as "some embodiments" indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0047] The indigo, madder, cochineal, shellac, gamboge, or turmeric used in the following examples were purchased from Kermer, but can also be extracted manually using existing techniques.

[0048] This example provides a method for constructing a thermal decomposition fingerprint library of organic pigments. The specific steps are as follows: Organic pigment standard samples (approximately 0.1 mg each of indigo, madder, cochineal, shellac, gamboge, or turmeric, purchased from Kermer) were placed in 50 µL stainless steel Eco-cup containers. Then, 3 µL of 25 wt% tetramethylammonium hydroxide aqueous solution (TMAH, Aldrich, USA) was added to derivatize the samples, converting unstable compounds into more volatile products. This technique utilizes high-temperature pyrolysis under an inert atmosphere to decompose large organic molecules in the sample into more volatile smaller molecules. The products are then separated by gas chromatography and detected by mass spectrometry, enabling precise analysis of the organic components and their degradation products in the material.

[0049] Pyrolysis was performed using a Frontier Lab PY-3030D dual-gun pyrolysis apparatus at 550°C for 50 min, with an interface temperature of 300°C. The pyrolyzer was coupled to an Agilent Technologies 7890B gas chromatograph / 5977B inert mass selective detector (MSD). The split / non-split injector temperature was 290°C, with a split ratio of 50:1 and no solvent delay. Separation was performed using an Agilent J&W Ultra-Inert DB-5MS capillary column (20 m × 0.18 mm × 0.18 µm) at a flow rate of 1.0 mL / min using helium as the carrier gas. The GC oven temperature program was as follows: initial temperature 45°C, ramped to 200°C at 5°C / min (hold for 7 min), then ramped to 300°C at 10°C / min (hold for 2 min). The mass spectrometer transfer line was maintained at 250°C, the ion source at 220°C, and the quadrupole at 150°C. The mass spectrometer has a scanning range of m / z 50-550.

[0050] The identification of the compounds was based on the AMDIS program developed by the National Institute of Standards and Technology (NIST) and the RADICAL ESCAPE workbook developed by Michael Schilling of the Getty Conservation Institute in collaboration with conservationists at the J. Paul Getty Museum. The naming of all compounds in this invention scheme is based on the RADICAL database.

[0051] The total ion chromatograms of six organic pigment standard samples are as follows: Figure 1 As shown. From Figure 1As can be seen, under the same THM-Py-GC / MS conditions, all six organic pigment standard samples yielded total ion current chromatograms (TICs) with good reproducibility, indicating that this method is applicable to organic pigments from different sources and with different structural types. Although there are significant differences in the chromogenic molecular skeleton and functional group composition among the pigments, their cleavage products are mainly concentrated in aromatic compounds, heterocyclic compounds, and their methylated derivatives, reflecting a close correlation between the cleavage process and the core structure of the chromogenic molecule. Therefore, subsequent analysis based on the core structure of the chromogenic molecule will further... Figure 1 The pyrolysis behavior of six organic pigments was classified and discussed to highlight the key role of pyrolysis mechanism analysis in the identification of organic pigments.

[0052] 1) Indigo-based organic pigments: Indigo Indigo is mainly produced by fermenting the leaves of plants containing indole acid, such as Polygonum tinctorium, Isatis indigotica, Indigofera tinctoria, and Strobilanthes cusia. It is a typical water-soluble non-azo organic pigment. Its molecular structure consists of two indole ring units connected by a central C=C double bond, forming a highly conjugated indole ketone system.

[0053] The common pyrolysis pathway in the pyrolysis of indigo is as follows: .

[0054] Under THM-Py-GC / MS conditions, indigo molecules undergo methylation under the catalysis of a TMAH base. Due to the stabilizing effect of the intramolecular -C=CC=O conjugated system and the intermolecular hydrogen bonding, trimethylindigo (peak 67) is preferentially formed. Simultaneously, under high temperature, some indigo molecules undergo central double bond rotation and configuration rearrangement, resulting in tetramethylindorubin (peak 66) after isomerization and methylation.

[0055] As the cleavage reaction proceeds, the central double bond, the most chemically reactive component in the indigo molecule, breaks further, producing an indole ketone intermediate. Part of this intermediate undergoes methylation to form indole derivatives (peaks 41, 45), while the other part undergoes oxidative rearrangement, intramolecular condensation, dehydrogenation, and aromatization to further generate tryptophan ketone (peak 68). The indole ketone intermediate undergoes secondary cleavage at high temperatures, forming aminobenzoic acid ester derivatives (peaks 18, 29, 33, 44, 48, 49). In the later stages of cleavage, these intermediate fragments, under the combined influence of high temperature and TMAH, undergo a series of reactions including carbon chain rearrangement, oxidation, dehydrogenation, and decarboxylation, ultimately generating various fatty acid methyl esters and other byproducts.

[0056] Based on the correspondence between the pyrolysis products and the structure of the indigo parent material, characteristic pyrolysis products were selected as shown in Table 1.

[0057] Table 1. Fingerprint fragments / characteristic lysis products for identifying indigo

[0058] As shown in Table 1, methylated derivatives retaining the indigo skeleton or indole ketone structure, including peaks 44, 49, 66, 67, and 68, directly derived from the indole-type parent nucleus structure of indigo, can serve as fingerprint cleavage products for identifying indigo in ancient murals, and are the most specific diagnostic markers for indigo identification. Furthermore, products generated by further cleavage of indole ketone intermediates (peaks 18, 29, 33, 41, 45, and 48) can serve as characteristic cleavage products for identifying indigo in ancient murals. Although these products do not completely retain the indigo parent nucleus structure, their formation pathway is highly correlated with the indigo indole structure.

[0059] It should be noted that the above-mentioned characteristic lysis products overlap to some extent with the lysis products of protein materials (such as egg white) under THM-Py-GC / MS conditions. Due to their limited specificity, they cannot be used as the sole diagnostic basis for determining the presence of indigo in ancient murals. A comprehensive analysis combining indigo fingerprint lysis products is required.

[0060] 2) Anthraquinone organic pigments: Madder, Cochineal, Schellac The main pigments in madder, carmine, and shellac all possess anthraquinone structures. However, due to the differences in the molecular structures of their main components, they exhibit different cleavage pathways under THM-Py-GC / MS conditions. Alizarin, shikonin, and alizarin in madder are typical anthraquinone compounds, and their cleavage pathways are primarily controlled by the anthraquinone core structure. The core component of carmine is glycosylated anthraquinone carboxylic acid (carmine acid), and its cleavage reaction preferentially occurs at the C-glycosidic bond between the anthraquinone skeleton and the glycosyl group. Shellac, on the other hand, is mainly composed of resinous components made up of hydroxy fatty acids (clavic acid) and hydroxysesquiterpene acids, with a lower pigment content (only 1-3%). Its cleavage pathway is primarily dominated by fatty acid and sesquiterpene structures. These structural differences lead to significant variations in the composition of cleavage products and characteristic markers among the three, providing a basis for distinguishing red organic pigments in ancient murals.

[0061] 2.1 Madder Under the action of the TMAH online methylating agent, the general pyrolysis pathway of Rubia cordifolia during pyrolysis is as follows: .

[0062] From its general dead-end pathway, it can be seen that the hydroxyl groups on the benzene ring of the anthraquinone pigment molecules in madder first undergo methylation, generating a series of methylated derivatives that retain the anthraquinone skeleton (peaks 64-67), whose molecular structures correspond to alizarin, flavonoid alizarin and shikonin.

[0063] During the subsequent high-temperature pyrolysis stage, the C / C bond at the ortho position of the carbonyl group in the anthraquinone molecule preferentially undergoes α-cleavage, accompanied by decarboxylation and molecular rearrangement, leading to partial destructive of the anthraquinone core and the formation of phthalate ester products (peak 47). Simultaneously, some anthraquinone structures undergo oxidative ring-opening and intramolecular rearrangement, generating benzoate and phthalate ester intermediates (peaks 43, 50, 51, 53), which are further cleaved and methylated under the action of TMAH, generating various methoxy-substituted benzoate ester derivatives (peaks 34, 35, 36, 40). In the later stages of thermal pyrolysis, the anthraquinone core undergoes deep decomposition, further generating small molecule products such as benzenes, phenols, and fatty acid methyl esters.

[0064] Based on the correspondence between the pyrolysis products and the madder parent structure, characteristic pyrolysis products were selected as shown in Table 2.

[0065] Table 2 Fingerprint fragments / characteristic lysis products for identifying madder.

[0066] As can be seen from Table 2, methylated derivatives that retain the anthraquinone core structure (peaks 64-67) and primary and secondary cleavage products with a clear anthraquinone origin (peaks 47 and 34) can be used as key fingerprint cleavage products for identifying madder pigment. Other cleavage fragments closely related to the anthraquinone structure (peaks 43, 50, 51, 53 and 35, 36, 40) can be used as auxiliary characteristic markers.

[0067] 2.2 Cochineal The common pyrolysis pathway of cochineal during pyrolysis is as follows: .

[0068] Under THM-Py-GC / MS conditions, the initial cleavage of carmine acid preferentially involves the breaking of C-glycosidic bonds between the anthraquinone backbone and the glycosyl group, generating glycosyl fragments and anthraquinone intermediates. Further cleavage of the glycosyl intermediates forms typical polysaccharide thermal cleavage products (peaks 44 and 46), while the anthraquinone intermediates, through further cleavage and methylation, generate benzoate derivatives (peaks 31 and 41).

[0069] It should be noted that this cleavage pathway differs significantly from that of plant-derived non-glycosylated anthraquinone pigments (such as madder). The cleavage products mainly originate from differences in the C-glycoside substitution position and conjugation system within the carmine acid molecule. Therefore, although both contain anthraquinone structural units, the composition and relative abundance of their thermal cleavage products differ significantly. This difference can serve as an important basis for distinguishing between insect-derived and plant-derived anthraquinone dyes.

[0070] As the pyrolysis reaction proceeds, a large number of fatty acid methyl esters, long-chain alkanes, and alkenes can also be detected in the spectrum. Their formation mechanism is highly consistent with the thermochemical transformation process of residual lipids and waxes in the insect body at high temperature. Characteristic pyrolysis products are shown in Table 3.

[0071] Table 3. Fingerprint fragments / characteristic lysis products for identifying cochineal red.

[0072] As can be seen from Table 3, under THM-Py-GC / MS conditions, the combined characteristics of polysaccharide derivatives, benzoic acid esters, and long-chain lipid cleavage products can be used as fingerprint cleavage products for determining the presence of cochineal pigment in ancient murals.

[0073] 2.3 Shellac The common pyrolysis pathways in the shellac pyrolysis process are as follows: .

[0074] Under TMAH-Py-GC / MS conditions, shellac resin initially undergoes ester bond cleavage and base-catalyzed methylation during the initial pyrolysis phase, generating methyl ester derivatives of hydroxy fatty acids and sesquiterpene acids. Among these, the methylation products of aleuritic acid (peaks 23 and 32) and the methylation pyrolysis products of various sesquiterpene acids (peaks 24, 27-30) retain the structural characteristics of the carboxylic acid components of shellac resin and can serve as key fingerprint pyrolysis products for identifying shellac pigments.

[0075] As the pyrolysis reaction continues, the above products undergo further breakage and rearrangement, generating a series of long-chain fatty acid methyl esters and a small amount of hydrocarbon pyrolysis products. In addition, a small amount of anthraquinone pigments (such as laccaic acids) in shellac also generate anthraquinone-related fragments (peaks 17, 19, and 31) during thermal pyrolysis, but their relative abundance is low. Characteristic pyrolysis products are shown in Table 4.

[0076] Table 4. Fingerprint fragments / characteristic pyrolysis products for identifying shellac

[0077] Overall, as shown in Table 4, shellac exhibits a characteristic combination of cleavage products centered on methyl hydroxy fatty acids and methyl sesquiterpene acids under THM-Py-GC / MS conditions. This combination has clear structural indicative significance and can effectively distinguish shellac from other resin-based or anthraquinone-based organic pigments.

[0078] 3) Benzo[a]pyrene]uranium organic pigments: Gamboge The common pyrolysis pathways during the pyrolysis of Gamboge are as follows: .

[0079] Under THM-Py-GC / MS conditions, gambogeylic acid, rich in chemically active functional groups such as benzene rings, phenolic hydroxyl groups, and carbonyl groups, is difficult to maintain its original skeleton under the strong alkaline environment and high-temperature thermal decomposition conditions of TMAH. It is prone to thermochemical reactions such as decarboxylation, dehydroxylation, aromatic ring cleavage, and molecular rearrangement, which lead to the deconstruction of its anthrone / flavonoid skeleton and further generate a series of stable small molecule aromatic compounds, mainly including naphthalenes, indenes, benzene and its derivatives, as well as phenolic compounds.

[0080] Besides the cleavage of the aromatic skeleton, the polyisoprene segments enriched in the gamboge acid structure preferentially undergo isoprene thermal decomposition at high temperatures. The C / C bonds between these segments break, generating isoprene characteristic fragments with high volatility and relative stability. Peak 39 directly reflects the cleavage behavior of the isoprene structural units and can serve as a key fingerprint cleavage product for identifying gamboge pigments in ancient murals. Furthermore, during the cleavage and depolymerization process, the polyisoprene segments can also undergo Diels-Alder cyclization, forming cyclic dimers with a terpene skeleton (peak 30). The co-occurrence of peaks 30 and 39 can significantly improve the reliability of gamboge pigment identification.

[0081] The characteristic pyrolysis products are shown in Table 5.

[0082] Table 5. Fingerprint fragments / characteristic lysis products for identifying gamboge.

[0083] It should be noted that the free isoprene units generated during the pyrolysis process exhibit high reactivity at high temperatures, readily participating in radical addition, olefin addition, secondary polymerization, β-pyrolysis, and other reactions to transform into benzene derivatives, cyclic olefins, and longer-chain alkane / olefin products. These pyrolysis products are closely related to the isoprene structure in gamboge acid and can serve as characteristic pyrolysis products for identifying gamboge.

[0084] Furthermore, the fatty acid methyl ester byproducts detected in the later stages of the pyrolysis reaction mainly originate from common degradation pathways of resinous organic matter under high-temperature pyrolysis and TMAH methylation conditions, and are considered non-specific background signals. Given the similarity between the pyrolysis characteristics of some long-chain alkanes and olefins and those of waxy materials (such as beeswax), in the actual interpretation of ancient mural samples, a comprehensive judgment should be made primarily relying on fingerprint pyrolysis products with clear structural indicative significance, such as peaks 30 and 39, to avoid potential misinterpretations.

[0085] 4) Flavonoid organic pigments: Turmeric The common pyrolysis pathways during the pyrolysis of turmeric are as follows: .

[0086] Curcumin compounds possess an aryl heptane skeleton with an α,β-unsaturated β-diketone central chain. Under high temperature and TMAH alkaline catalysis, they readily undergo β-cleavage, ortho-carbonyl cleavage, decarboxylation, and molecular rearrangement, leading to the breakage of the central heptane chain and generating characteristic cleavage product peaks 57, 58, and 62. Simultaneously, the volatile oil components in turmeric (mainly turmerone-type sesquiterpenes) undergo dehydrogenation aromatization and cleavage reactions under the reaction conditions, generating fragment peaks 53 and 55. These five cleavage products all originate directly from the core structure of the main components of turmeric and can serve as fingerprint cleavage products for identifying turmeric pigments in murals.

[0087] The characteristic pyrolysis products are shown in Table 6.

[0088] Table 6 Fingerprint fragments / characteristic lysis products for identifying turmeric

[0089] As the cleavage reaction proceeds further, methylation, secondary cleavage, and aromatization reactions occur, forming fragments with peaks 25, 32, 34, 36, 49, and 56. These fragments retain structural information about the aromatic ring of curcumin. Although their specificity is relatively low, their formation pathway is highly correlated with curcumin-like compounds. Therefore, they can serve as important auxiliary characteristic cleavage products for identifying turmeric pigments, thereby improving the reliability of turmeric pigment identification.

[0090] In deeper cleavage stages, aromatic structures undergo further condensation, dehydrogenation, and esterification reactions, generating byproducts such as benzene derivatives, naphthalene derivatives, and fatty acid methyl esters. Furthermore, the polysaccharide cleavage products detected in the spectrum (such as peaks 3, 10, 50, 52, and 54) mainly originate from plant cell wall polysaccharides and associated sugars through dehydration, cyclization, and cleavage reactions. These products can reflect the background of plant material but lack turmeric specificity; therefore, they can only serve as supplementary background information and are not suitable as diagnostic markers.

[0091] This example also provides the application of the fingerprint database obtained by the above method for the identification and restoration of organic pigments in murals. Specifically, a method for identifying organic pigments in murals based on pyrolysis fingerprint spectra is provided, and the specific operation is as follows: Samples of the murals in Cave 254 of Mogao Grottoes were obtained using the same processing method as the standard samples of the aforementioned organic pigments. The sampling locations are as follows: Figure 2 As shown. The obtained thermal decomposition fingerprint spectrum of organic pigments in the mural sample is as follows. Figure 3 As shown.

[0092] To verify the applicability of the pyrolysis path and fingerprint identification strategy established based on standard reference samples in ancient mural samples, this invention selected painted layer samples from the west wall mural of Cave 254 at Mogao Grottoes for THM-Py-GC / MS analysis. By comparing the pyrolysis products of the mural samples with the fingerprint pyrolysis products and characteristic pyrolysis products of different color-developing parent organic pigments in the organic pigment standard samples, the identification ability and reliability of this method in complex, aged mural systems were evaluated.

[0093] like Figure 3 As shown, fingerprint fragments consistent with the indigo standard sample were detected in sample 254-4 in the blue-black area of ​​the Thousand Buddha garment texture, including methyl anthranilate and its methylated derivatives, as well as fragments related to indigo and indirubin with different degrees of methyl substitution. Simultaneously, several small-molecule heterocyclic compounds derived from further cleavage of the indole skeleton were also detected as characteristic fragments of indigo. In sample 254-1 in the red area of ​​the Thousand Buddha backlight, a group of fragments highly correlated with shellac were detected in the fragmentation spectrum of sample 254-1. These fragments showed good consistency with the fingerprint fragments of the shellac standard sample mentioned above, and were accompanied by several characteristic products derived from the cleavage of associated resin structures. Notably, the THM-Py-GC / MS analysis results of the two mural samples also detected m / z Pyrrole and m / z Diketo-dipyrrole at retention times of 2.944 min and 26.504 min, respectively, indicating that the binder used in the mural painting layer was animal glue. This result is consistent with the common choice of binders in traditional mural painting techniques. The analysis results of sample 254-1 also detected Sulfuric acid and dimethyl ester at a retention time of 4.577 min, indicating the presence of sulfur-containing mineral pigments such as cinnabar (HgS), reflecting the characteristic of the combined use of organic and inorganic mineral pigments in the Mogao Grottoes mural painting layer.

[0094] Based on the preceding analysis of the pyrolysis pathways of organic pigments, the synergistic appearance of fingerprint pyrolysis fragments and characteristic pyrolysis products in the mural samples indicates that organic pigments such as indigo and shellac were indeed used in this painted layer. This result not only verifies the effectiveness of the identification strategy based on the colorimetric nucleus and pyrolysis pathways in actual cultural relic samples, but also further illustrates the application of indigo and shellac in the Mogao Grottoes mural painting system.

[0095] The murals in Cave 254 of the Mogao Grottoes, a central pagoda-style cave carved during the Northern Wei Dynasty over 1500 years ago, are a typical representative of similar early caves in the Mogao Grottoes. The reliable identification of organic pigments such as indigo and shellac in the mural samples is of great significance in terms of materials history and painting technique history. Traditional research generally holds that early Dunhuang murals primarily used inorganic mineral pigments, with relatively limited use of organic pigments, and few examples surviving to this day. This invention, using the THM-Py-GC / MS method, and supported by strict lysis pathway and fingerprint criteria, clearly detected indigo and shellac in the painted layer of Cave 254, providing direct and reliable chemical evidence for the application of organic pigments in early Dunhuang murals.

[0096] From a painting technique perspective, neither indigo nor shellac are natural dyes that can be used directly in painting. They require certain chemical processing steps, such as mordant or lake treatment, to transform them into painting pigments with stable coloring capabilities. This indicates that, at least by the Northern Wei Dynasty (5th-6th centuries AD), Dunhuang painters had mastered a relatively mature system of organic pigment processing and color mixing techniques, enabling them to transform and stably apply natural dyes from complex sources to mural creation. This discovery helps to correct the traditional understanding that early murals were "dominated solely by mineral pigments," revealing the complexity and maturity of the painting material system at that time at the technical level.

[0097] From the perspective of material sources and cultural exchange, the main historical production areas of shellac were distributed in India, Myanmar, and Southeast Asia. Its discovery in the murals of Cave 254 at Mogao Grottoes provides important empirical evidence for the long-distance material and technological exchange between Dunhuang and the Indian subcontinent via the Silk Road. In contrast, indigo may have originated from the traditional dyeing system of the Central Plains region or been introduced to Dunhuang via the Western Regions trade network. The simultaneous identification of "southern shellac" and "eastern indigo" in murals from the same cave and the same era vividly demonstrates, from a material culture perspective, Dunhuang's pivotal position as a key node on the Silk Road in the 5th century AD and the diverse cultural and technological convergence it facilitated.

[0098] Therefore, this study not only expands the application of THM-Py-GC / MS in the identification of organic pigments in ancient murals in terms of analytical methods, but also provides a new material evidence basis for understanding the material selection, painting techniques and the cross-regional exchange network behind the early Dunhuang murals through the analysis of specific cultural relic samples.

[0099] This invention systematically studies the fragmentation behavior and characteristic product composition of various common organic pigments in ancient murals under tetramethylammonium hydroxide-assisted pyrolysis-gas chromatography / mass spectrometry (THM-Py-GC / MS) conditions. Through comparative analysis of organic pigments with different chemical structures and origins, such as indigo, turmeric, gamboge, madder, cochineal, and shellac, typical fragmentation modes and diagnostic fingerprint compounds of various organic pigments were established, providing a reliable basis for the scientific identification of organic pigments in complex cultural relic systems.

[0100] The results show that THM-Py-GC / MS can effectively overcome the analytical challenges of low content, complex structure, easy aging and degradation, and coexistence with inorganic pigments and cementing materials in organic pigments. This method significantly improves the volatility and detectability of pyrolysis products through in-situ methylation, allowing for the clear presentation of key structural indicator fragments formed during the thermal pyrolysis of different organic pigments. In particular, for plant-based dyes, resin-based pigments, and animal-derived organic compounds, the pyrolysis products exhibit significant differences in compositional characteristics and relative abundance, providing a foundation for establishing stable and reliable identification markers.

[0101] Based on this, the fingerprint lysis information established in this invention was successfully applied to the actual analysis of mural samples from Cave 254 of the Mogao Grottoes, detecting and identifying the presence of organic materials such as indigo and shellac. The results not only verified the feasibility and reliability of this method in complex mural samples, but also provided new empirical evidence for understanding the color characteristics, material selection, and painting techniques of the murals in this cave.

[0102] Overall, this study demonstrates that THM-Py-GC / MS is an effective tool for analyzing organic pigments in ancient murals under micro-sampling conditions, providing an important analytical method for the systematic identification and comparative study of organic pigments. The organic pigment pyrolysis characteristics and diagnostic fingerprint system established in this invention is expected to provide methodological reference and data support for the identification of organic components in future research and conservation and restoration practices of ancient mural materials.

[0103] In summary, this invention systematically studies the reaction pathways and fingerprint pyrolysis product formation mechanisms of organic pigments with different structural types under THM-Py-GC / MS conditions from a chemical mechanism perspective. Six representative organic pigment standard samples from ancient murals were selected, and the compositional characteristics and formation patterns of their thermally assisted hydrolysis-methylation pyrolysis products were analyzed. The behavioral differences of different pigments during the thermal pyrolysis process were compared, and their pyrolysis pathways were summarized. The research results provide a theoretical basis for the accurate identification of organic pigments in complex mural samples and systematically establish analytical methods and identification criteria applicable to the identification of organic pigments in ancient murals. This invention provides new analytical ideas and methodological references for the scientific identification of organic pigments in ancient murals and lays a materials science foundation for related conservation and restoration research.

[0104] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for constructing a pyrolysis fingerprint library of organic pigments, characterized in that: Includes the following steps: Organic pigment standard samples and aqueous solutions of methylation derivatization reagents were placed together in an inert atmosphere for thermal pyrolysis. The pyrolysis products were introduced into a gas chromatography column with a carrier gas for decomposition. The fingerprint spectrum of the organic pigment standard samples was obtained by gas chromatography-mass spectrometry. Based on the thermal pyrolysis fingerprint spectrum of the standard samples, a fingerprint spectrum database and a characteristic peak database were established. The thermal pyrolysis included the following conditions: pyrolysis temperature of 450-650℃ and pyrolysis time of 45-55 min.

2. The method for constructing a thermal decomposition fingerprint library of organic pigments according to claim 1, characterized in that: The standard samples of organic pigments include at least one of indigo organic pigments, anthraquinone organic pigments, flavonoid organic pigments, or benzopyran organic pigments.

3. The method for constructing a thermal decomposition fingerprint library of organic pigments according to claim 1 or 2, characterized in that: The standard samples of organic pigments include at least one of indigo, madder, cochineal, shellac, turmeric, or gamboge.

4. The method for constructing a thermal decomposition fingerprint library of organic pigments according to claim 3, characterized in that: The pyrolysis fingerprint library satisfies at least one of the following conditions: If the organic pigment standard sample includes indigo, then the fingerprint database contains the following fingerprint cleavage product peaks: methyl anthranilate, N-methyl anthranilate, tetramethylindirubin, trimethylindigo, tryptamine ketone, and the characteristic peak database contains the following characteristic cleavage product peaks: aniline, 3-methylaniline, methyl benzoate, indole, 1,3-dimethyl-1H-indole, methyl 2-(dimethylamino)benzoate; If the organic pigment standard sample includes madder, then the fingerprint database contains the following fingerprint cleavage product peaks: veratrine ether, dimethyl phthalate, 3-O-methyl erythrin, 2-O-methyl alizarin, alizarin dimethyl ether, and 1,3-di-O-methyl erythrin; and the characteristic peak database contains the following characteristic cleavage product peaks: 1,4-dimethoxybenzene, 2,3-dimethoxytoluene, 1,2,3-trimethoxybenzene, methyl 3-methoxybenzoate, methyl 3,5-dimethoxybenzoate, methyl 3,4-dimethoxybenzoate, and methyl 3,4,5-dimethoxybenzoate. If the organic pigment standard sample includes carmine, then the fingerprint database contains the following fingerprint cleavage product peaks: methyl benzoate, methyl 4-methoxybenzoate, 5,6-dimethoxy-2-methyltetrahydro-2H-pyran-3-ylacetate and 2,3-dimethoxy-6-methyltetrahydro-2H-pyran-4-ylacetate; If the organic pigment standard sample includes shellac, then the fingerprint database contains the following fingerprint cleavage product peaks: methyl 9,10-dimethoxyhexadecanoate, trimethyl shellac derivative, tetramethyl shellac derivative, tetramethyl laxoic acid derivative, tetramethyl shellac derivative, tetramethyl shellac derivative, and trimethyl shellac ketone acid isomer. If the organic pigment standard sample includes gamboge, then the fingerprint database contains the following fingerprint pyrolysis product peaks: borneolene and isoprene dimer, and the characteristic peak database contains the following characteristic pyrolysis product peaks: 3-methylenecyclopentene, 2-heptene, 4-methylcyclohexene, 1-methylcyclohexene, 2-methyl-1-heptene, 5,6-dimethyl-1,3-cyclohexadiene, 3-methyl-2-cyclopenten-1-one, 1-dodecene, 1-tetrideene, 1-tetradecene, 1-pentadecanene, 1-heptadecene, 1-octadecene, 1-nonadecanene, hexacosane, heptadecane, octadecane, and nonadecanene; If the organic pigment standard sample includes turmeric, then the fingerprint database contains the following fingerprint cleavage product peaks: β-eucalyptol, γ-juniperene, aromatic curcuminone, methyl cis-p-methoxycinnamate, and methyl 3-(3,4-dimethoxyphenyl)-2-acrylate. The characteristic peak database contains the following characteristic cleavage product peaks: 2-methyl anisole, 2-methoxyphenol, 1,2-dimethoxybenzene, p-methoxystyrene, 1,2,3-trimethoxybenzene, 1,2,4-trimethoxybenzene, and methyl 3,4-dimethoxybenzoate.

5. The method for constructing a thermal decomposition fingerprint library of organic pigments according to claim 1 or 2, characterized in that: The analysis includes at least one of the following conditions: 1) The pyrolysis is performed using a Frontier Lab PY-3030D dual-gun pyrolysis apparatus; 2) The pyrolysis interface temperature is 290~310℃; 3) The gas chromatography-mass spectrometry includes the following device: a pyrolyzer coupled to an Agilent Technologies 7890B gas chromatograph / 5977B inert mass selective detector; 4) The injection temperature of the organic pigment sample solution is 285~300℃; 5) The gas chromatographic column is an Agilent J&W Ultra-Inert DB-5MS capillary column; 6) The dimensions of the gas chromatographic column are 20m × 0.18mm × 0.18µm; 7) The temperature program of the gas chromatograph oven is as follows: initial temperature 40~50℃, increase temperature to 190~210℃ at 4~6℃ / min, hold for 6~8min, then increase temperature to 290~320℃ at 8~12℃ / min, hold for 1~3min; 8) The parameter settings for the mass spectrometer include at least one of the following items: a) The mass spectrometer transfer line is maintained at 240~260℃; b) The ion source is maintained at 210~230℃; c) The quadrupole is maintained at 140~160℃; d) The scanning range of the mass spectrometer is 50~550 m / z.

6. The thermal decomposition fingerprint library constructed by the construction method according to any one of claims 1 to 5.

7. A method for identifying organic pigments in murals based on pyrolysis fingerprinting, characterized in that: Includes the following steps: S1. Establish a thermal decomposition fingerprint library for organic pigment standard samples: Construct a thermal decomposition fingerprint library using the method described in any one of claims 1 to 5; S2. Obtain the thermal decomposition fingerprint spectrum of organic pigments in the mural sample to be tested: Use the same detection conditions as the thermal decomposition fingerprint spectrum of the organic pigment standard sample in step S1 to obtain the thermal decomposition fingerprint spectrum of organic pigments in the mural sample to be tested; S3. Identification of organic pigments: The spectrum obtained in step S2 is compared with the spectrum in the thermal decomposition fingerprint spectrum library established in step S1, and the types of organic pigments in the mural sample are determined based on the comparison results.

8. The method for identifying organic pigments in murals based on pyrolysis fingerprinting according to claim 7, characterized in that: The identification method also includes the step of analyzing the content of organic pigments after determining the types of organic pigments in the mural sample.

9. The application of the construction method as described in any one of claims 1 to 5, the pyrolysis fingerprint library as described in claim 6, or the identification method as described in claim 7 or 8 in mural restoration.

10. The application according to claim 9, characterized in that: The murals in question are ancient murals.

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