Analysis of Anthraquinoid and Indigoid Dyes Used in Ancient Artistic Works by Thermally Assisted...

J. Anal. Appl. Pyrolysis 56 (2000) 167–178

Analysis of anthraquinoid and indigoid dyesused in ancient artistic works by thermallyassisted hydrolysis and methylation in the

presence of tetramethylammonium hydroxide

Daniele Fabbri a,*, Giuseppe Chiavari a, He Ling b

a Laboratorio di Chimica Ambientale, c/o ITIS, Uni6ersita di Bologna, 6ia Marconi 2,I-48100 Ra6enna, Italy

b Xi’an Center for the Conser6ation and Restoration of Cultural Property,Xian, People’s Republic of China

Received 22 November 1999; received in revised form 17 March 2000; accepted 25 April 2000

Abstract

Thermally assisted hydrolysis and methylation (THM) in the presence of tetramethylam-monium hydroxide (TMAH) was applied to the analysis of red dyes based on the 9,10-an-thracenedione skeleton (anthraquinone), namely alizarin, quinizarin, purpurin, carminic acid,and the blue pigment indigo. Alizarin and purpurin are the principal coloured compounds ofmadder, and carminic acid of cochineal red. Synthetic dyes were dissolved in aqueousTMAH, pyrolysed at 700°C for 10 s and the evolved thermochemolytic products wereanalysed by gas chromatography/mass spectrometry (GC/MS). The resulting chromatogramsof all the madder dyes were characterised by the presence of the methyl esters of 1,2-ben-zenedicarboxylic acid (dimethyl-o-phthalate) and benzoic acid. In addition, thermochemoly-sis of alizarin and quinizarin afforded 1,2-dimethoxyanthraquinone and1,4-dimethoxyanthraquinone, respectively. Carminic acid produced a series of methylatedfragments deriving from both the substituted anthraquinoid moiety and the glycosidically-C-linked glucose. Finally, the principal thermochemolysis products of indigo were the N-methylated derivatives of 2-amino benzoic acid methyl ester followed by methylated anilineand indoles. The potential of THM as a rapid technique for the identification of dyes in realsamples was illustrated by the analysis of madder lake, cochineal lake, and natural indigodispersed in a binder. © 2000 Elsevier Science B.V. All rights reserved.

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* Corresponding author. Tel.: +39-0544-400870; fax: +39-0544-484001.E-mail address: [email protected] (D. Fabbri).

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168 D. Fabbri et al. / J. Anal. Appl. Pyrolysis 56 (2000) 168–178

Keywords: Tetramethylammonium hydroxide; Thermally assisted hydrolysis and methylation; Ther-mochemolysis; Pyrolysis; Gas chromatography/mass spectrometry; Madder; Cochineal; Indigo;Hydroxyanthraquinones

1. Introduction

A great variety of organic substances have been used by artists to dye, protectand bind their works [1]. Pyrolysis (Py) coupled with gas chromatography (GC)and/or mass spectrometry (MS) is among the numerous analytical techniquesemployed for the characterisation and identification of organic materials used inartistic objects [2]. The potential of analytical pyrolysis in the field of culturalheritage, has increased in recent years with the use of the derivatising agenttetramethylammonium hydroxide (TMAH) in combination with pyrolysis [3], in theso-called TMAH thermochemolysis or thermally assisted hydrolysis and methyla-tion (THM), or more simply pyrolysis-methylation. In the analysis of paintingsamples, pyrolysis-methylation has mostly been applied to the identification ofcomplex high molecular weight organic materials used as binders and varnishes byartists, such as drying oils [4], proteinaceous materials [5], natural [6] and artificialresins [7].

Considering the benefits of analytical pyrolysis as a screening methodology, it isworth assessing its applicability to the analyses of pigments and colorants. A fewexamples have appeared in the literature showing that conventional Py-GC/MS iscapable of revealing the presence of inorganic pigments which produce volatilethermal degradation products, such as orpiment (formation of As4, [8]) andcinnabar (formation of elemental Hg, [9]).

The study here presented reports on the potential of analytical pyrolysis for thedetection of organic dyes used in the past. We focused our attention on madder,cochineal and indigo, which are amongst the most important and oldest organicdyestuffs used in paintings and textiles [1]. The substances responsible for thecolour of madder and cochineal are hydroxyanthraquinones and thin layer chro-matography [10], high-pressure liquid chromatography [11] and visible or fluores-cence spectroscopy [12] have been used for their identification.Hydroxyanthraquinones and indigo contain polar groups, which could be methy-lated improving gas chromatographic separation, therefore we applied Py-GC/MSin combination with TMAH for their analysis.

2. Experimental

2.1. Materials

The synthetic dyes alizarin (1,2-dihydroxyanthraquinone, C.I 58000), quinizarin(1,4-dihydroxyanthraquinone, C.I. 58050), purpurin (1,2,4-trihydroxyan-

169D. Fabbri et al. / J. Anal. Appl. Pyrolysis 56 (2000) 168–178

thraquinone, C.I. 58205), carminic acid (7-C-a-D-glucopyranosyl-3,5,6,8-tetrahy-droxy-1-methyl-2-anthraquinone carboxylic acid, C.I. 75470), indigo (2-(1,3-dihy-dro-3-oxo-2H-indol-2-ylidene)-1,2-dihydro-3H-indol-3-one, C.I. 73000) werepurchased from Aldrich. Standard samples were kindly prepared by Merlini Gi-anluca of the Cultural Association ‘Quelli del Ponte’ (Ravenna, Italy) usingnatural sources (plants and insects purchased from Zecchi, Florence, Italy). Thevegetal indigo was extracted from Indigofera tinctoria with basic water andmixed with arabic gum as binder. The madder lake was prepared from the driedroot of Rubia tinctorum by extraction of the dyes with hot water and precipita-tion of the lake with alum cake (as mordant). The lake obtained by decantingsupernatant, was air dried and mixed with arabic gum as binder. The cochineallake was obtained in a similar way from the insect Coccus cacti. Materials andprocedures adopted for the preparation of the lakes were similar to those em-ployed in middle age for miniatures [13].

2.2. Methods

Each synthetic dye (1 mg) was dissolved in 100 ml aqueous solution of TMAH(25% w/w, Aldrich). An aliquot of the solution (corresponding to 10–50 mg ofdye) was placed onto the central part of a quartz tube and pyrolysed at 700°Cfor 10 s at the maximum heating rate. Standard samples (less than 1 mg) weremixed with aqueous TMAH (3 ml) and pyrolysed as described above.

Analysis of madder lake was also performed on the dyes isolated by means ofthe stripping procedure developed by Schweppe [10]. Briefly, the original samplewas heated at 100°C for 1 min with 10% aqueous H2SO4, extracted with ethylacetate, the organic layer was washed with water and the solvent evaporated.The residue was dissolved in methanol and analysed by absorption spectroscopyor in aqueous TMAH for pyrolysis-methylation experiments.

2.3. Instrumentation

The pyrolyser was a CDS 1000 pyroprobe interfaced to a Varian 3400 GCcoupled to a Saturn II ion trap mass spectrometer. The pyrolysis interface andGC injector were kept at 250°C. The column flow was 1.5 ml min−1 (at initialtemperature) and split flow was 60 ml min−1. The products were separated on aSupelco SPB5 capillary column (30 m×0.25 mm i.d., 0.25 mm film thickness)programmed from 50 (2 min hold) to 300°C at a heating rate of 5°C min−1.Mass spectra were recorded at 1 scan s−1 under electron impact at 70 eV, scanrange 45–450 m/z. The ion source was kept at 170°C. These are standardisedconditions routinely employed in our laboratory. The assignment of most of theproducts was based on match with both the NIST 1992 and Wiley 1996 massspectrum libraries.

Absorption measurements were performed with a Perkin–Elmer Lambda 16spectrophotometer.


3. Results and discussion

3.1. Madder

Madder is a red colouring material extracted from the roots of the madder plantR. tinctorum which contains a mixture of hydroxyanthraquinones, free or in theform of glycosides. The principal dyes occurring in the dried root are alizarin andpurpurin, while quinizarin is present in small amount [10]. Numerous colouringsubstances belonging to the class of hydroxyanthraquinones are present in the rootsof other plants of the type of madder (family Rubiaceae) and used as sources of reddyes in the past [10].

3.1.1. Alizarin, quinizarin and purpurinChromatograms obtained from the THM reaction of synthetic hydroxyan-

thraquinones are shown in Fig. 1. The proposed structural assignments for theprincipal peaks are reported in Table 1.

Even though their relative abundances were not reproducible, the characteristicproducts of alizarin and quinizarin were the methylated dye itself, the dimethylesterof 1,2-benzenedicarboxylic acid (dimethylphthalate) and the methyl ester of benzoicacid. Several compounds having the same molecular weight of dimethoxyan-thraquinone (molecular ion at m/z 268 in their mass spectra) were detected in thechromatograms of both alizarin and quinizarin. These compounds were interpreted

Fig. 1. Reconstructed ion chromatograms obtained from THM–GC/MS of madder dyes. (a) Alizarin;(b) quinizarin; (c) purpurin. Peak numbers are those of Table 1.


Table 1Principal THM products of madder dyesa

cm/z Tentative structural assignmentDye

77, 105, 136 Benzoic acid, methyl ester 1Alizarin3Dimethyl-o-phthalate77, 133, 163

Dimethoxyanthracenedione238, 253, 268 6Dimethoxyanthracenedione238, 253, 268 8

10Dimethoxyanthracenedione210, 253, 2681177, 165, 300 Unknown131,2-Dimethoxyanthraquinone225, 239, 268

Benzoic acid, methyl ester77, 105, 136 1Quinizarin21,4-Dimethoxybenzene95, 123, 138

Dimethyl-o-phthalate77, 133, 163 3Unknown163, 211, 300 7

91-Hydroxy-4-methoxyanthraquinone127, 225, 25412225, 253, 268 9,10-Dimethoxyanthracene-1,4-dione (?)141,4-Dimethoxyanthraquinone221, 239, 268

Benzoic acid, methyl ester77, 105, 136 1Purpurin3Dimethyl-o-phthalate77, 133, 163

Unknown175, 203, 218 45217, 245, 260 Unknown

a Peak numbers, numbered in order of increasing retention time, are those reported in Figs. 1 and 3.

as isomers occurring as contaminants in the commercial product and/or methylatedtautomers of the dye, which formed in the basic TMAH solution.

Dimethyl-ortho-phthalate is by far the most abundant product released from theTMAH thermochemolysis of purpurin (Fig. 1c). Low levels of other methylatedfragments were observed in the chromatogram of purpurin, among which themethyl ester of benzoic acid. The fully or partially methylated purpurin was notdetected.

A possible mechanism of formation of dimethyl-o-phthalate from hydroxyan-thraquinones is proposed in Fig. 2, exemplified for quinizarin. Even though inorganic solvents quinizarin appears to exist solely in the meso form (1,4-dihydroxy-9,10-anthraquinone) [14], in the basic medium it could tautomerise into more or lessstable keto–enol forms. Among these, b-diketones, even though less stable, un-dergo rapid alkaline cleavage forming carboxylic acid derivatives. As shown inscheme of Fig. 2, when hydroxide ions attack on carbonyl carbons 9 and 10, thecleavage of the bonds between the carbonyl carbon and the carbon of thedihydroxybenzene ring is expected, with formation of ortho-phthalate and 1,4-dihy-droxybenzene, which are subsequently methylated. If the attack of hydroxide ionsinvolve carbonyl carbons 1 or 4, aliphatic structures (ketoacids) are produced whichare rapidly degraded under basic/pyrolytic conditions.

Quinizarin released 1,4-dimethoxybenzene in high yield, but only trace levels of1,2-dimethoxybenzene and 1,2,4-trimethoxybenzene were observed in the chro-matograms of alizarin and purpurin, respectively. The occurrence of hydroxygroups in ortho position may be responsible for the faster degradation of the


hydroxybenzene ring observed for alizarin and purpurin with respect to quinizarin,whose hydroxyls are in para configuration. No methylated hydroxy o-phthalic acidswere detected in the chromatograms of madder dyes, in agreement with theproposed scheme.

3.1.2. Madder lakeThe chromatogram obtained from the THM reaction of madder lake, shown in

Fig. 3a, was dominated by peaks derived from the binder which was present in

Fig. 2. Proposed mechanism for the formation of the THM products of quinizarin.


Fig. 3. Chromatograms obtained from THM–GC/MS of madder lake. (a) Reconstructed ion chro-matogram; and (b) mass chromatogram at m/z 105 and 163 of the original madder lake; (c) recon-structed ion chromatogram of the extracted dyes. Peak numbers are those of Table 1.

much greater proportions than the colorants. The occurrence of dimethylphthalateand the methyl ester of benzoic acid, markers for both alizarin and purpurin, weredetected by displaying the chromatograms using the intensity of characteristic ions(m/z 105, and 163, see Fig. 3b), whereas the methylated alizarin was not revealed.In order to evaluate whether the absence of methylated alizarin was caused bymatrix effects (mordant and binder), madder dyes were isolated from the lake andpyrolysed with TMAH (see Section 2). The resulting chromatogram, given in Fig.3c, showed two major peaks resulting from methylated phthalic and benzoic acids,but did not exhibit signals associated with methylated alizarin. Therefore, it has tobe assumed that dimethylphthalate was derived principally from purpurin. Thisfinding was confirmed by the absorption spectrum of the extract, which was almostcoincident with that of standard purpurin, in agreement with the results of Milianiet al. [1] who reported that purpurin was present at higher levels (85%) than alizarinin madder extracts analysed by spectrophotometry. Even though alizarin is knownto be the main anthraquinone of madder root, purpurin may prevail in the finalsample as a consequence of extraction procedures, drying, ageing and chemicaltreatments in the analytical procedure [10,11].

3.2. Cochineal red

Cochineal red is a dyestuff obtained from the insect Dactylopius coccus Costa(Coccus cacti ). Several red dyestuffs from scale insects were used in the past,


among them Kermes and Lac dye, and cochineal red, which became increasinglypopular after its discovery in Mexico owing to its superiority to other insect dyes[1]. Insect dyes are composed of hydroxyanthraquinones, such as laccaic acids,kermesic acid and carminic acid. Carminic acid is the most abundant anthraquinoiddye found in cochineal red.

3.2.1. Carminic acidThe chromatogram obtained from TMH of carminic acid is shown in Fig. 4 and

the tentative structural assignments of the main peaks are reported in Table 2.The most intense peak was derived from the glucose unit and was identified as a

methylated inositol. Glucose is known to produce principally methylated deoxyglu-curonic acids (saccharinic acids) when subjected to pyrolysis in the presence ofTMAH [15], but their presence was not detected in the chromatogram of carminic

Fig. 4. Reconstructed ion chromatogram obtained from THM–GC/MS of carminic acid. Peak numbersare those of Table 2.

Table 2Principal THM products of carminic acid (cochineal)a

Tentative structural assignmentm/z c

12-Methoxypropanoic acid, methyl ester59, 75, 1032,4-Dimethoxybutanoic acid, methyl ester45, 71, 103 2Unknown71, 115, 131 3

42-Methyl-6-methoxybenzoic acid, methyl ester (?)121, 149, 180Hexa-O-methyl-myo-inositole75, 101, 144 5

6Unknown190, 203, 2187206, 207, 238 3-Methyl-5-methoxy-1,2-benzenedicarboxylic acid, dimethyl ester (?)875, 101, 111 Methylated glucose derivative

3-Methyl-5-methoxy-1,2,4-benzenetricarboxylic acid, trimethylester (?)178, 265, 296 9

a Peak numbers are those reported in Figs. 4 and 5.


acid. This finding was explained by the fact that glucose is linked to the an-thraquinone ring through a C-linkage instead of the more common O-linkage, thusthe aldehydic functionality necessary for the saccharinic acid formation is lacking.

The thermochemolytic compounds released from the anthraquinoid moiety ofcarminic acid gave mass spectra not included in the libraries, thus their identifica-tion is only tentative. Differing from the madder dyes discussed above, carminicacid did not yield dimethyl-o-phthalate, in accordance with the fact that bothbenzene rings bear substituents. Adopting the fragmentation pattern proposed forquinizarin (Fig. 2), methylated derivatives of 3-methyl-5-hydroxy-1,2,4-benzenetri-carboxylic acid should be expected in place of the methylated 1,2-benzenedicar-boxylic acid typical of madder dyes. Consistently with this hypothesis, a compound(peak 9) was detected at high retention times which was tentatively identified as thefully methylated 3-methyl-5-hydroxy-1,2,4-benzenetricarboxylic acid, its mass spec-trum being characterised by a molecular ion at m/z 296 and a base peak at 265 (lossof a methoxy group). Compounds 7 and 4 were thought to be virtually derivedfrom compound 9 following the loss of one and two carboxymethyl groups,respectively. Thus, peak 4 and 7 were tentatively identified as the methyl esters of2-methyl-6-methoxybenzoic acid and 3-methyl-5-methoxy-1,2-benzenedicarboxylicacid, respectively, but other isomeric structures differing for the position of thecarboxymethyl groups could be assigned as well.

Compound 9 and its derivatives may be considered characteristic markers ofscale insect dyes, since the structural unit of the anthraquinoid molecule from whichthey are thought to be derived is common to other insect dyes, such as kermesicacid (3,5,6,8-tetrahydroxy-1-methyl-2-anthraquinone carboxylic acid) and flavoker-mesic acid (3,6,8-trihydroxy-1-methyl-2-anthraquinone carboxylic acid, laccaic acidD).

3.2.2. Cochineal lakeAs in the case of madder lake, the mass chromatogram obtained from the THM

reaction of the cochineal lake was dominated by peaks originated from the matrix(Fig. 5a). The occurrence of markers of carminic acid was revealed in the masschromatograms using the principal ions of the mass spectra of compounds 4 and 9of Table 2, as depicted in Fig. 5b and c. Because carminic acid is stronglyfragmented upon thermochemolysis, the intensity of markers can be very low andinterference may become important. For example, the characteristic ions at m/z 265and 296 of compound 9, are also present in the mass spectrum of oleic acid methylester which has a similar retention time. In this case, the identification of compound9 was confirmed by the detection of the more specific ion at m/z 178 in the masschromatogram (Fig. 5c).

3.3. Indigo

3.3.1. Synthetic dyeTHM of synthetic indigo afforded the methyl esters of 2-(dimethylamino)-ben-

zoic acid and 2-(methylamino)-benzoic acid (Fig. 6a and Table 3) as principal


Fig. 5. Chromatograms obtained from THM–GC/MS of cochineal red. (a) Total ion current, masschromatograms; (b) at m/z 149 and 180; and (c) at m/z 178.

Fig. 6. Chromatograms obtained from THM–GC/MS of (a) synthetic indigo; and (b)–(c) natural indigomixed with arabic gum. Peak numbers are those of Table 3.

products. The methylated products arising from the decarboxylation (i.e. benze-neamine and its methylated derivatives) and deamination (i.e. benzoic acid methylester) of 2-aminobenzoic acid were detected at lower levels. The chromatogram wasalso characterised by the presence of methylated indole derivatives. As in the caseof hydroxyanthraquinones, the formation of the carboxylic group can be explainedas occurring from the attack of the hydroxide ion on carbonyl carbons of indigo.


3.3.2. Natural dyeThe mass chromatogram obtained from the pyrolysis-methylation of natural

indigo dispersed in arabic gum is presented in Fig. 6b. Even though the pyrogramwas again dominated by peaks deriving from the binder, signals associated withindigo markers (peaks 6 and 8) could be clearly distinguished. The detection ofmarkers arising from the dye can be made more evident by plotting masschromatograms using characteristic ions as shown in Fig. 6c.

4. Conclusions

This study shows that THM in connection with GC/MS can be effectively usedfor the identification of anthraquinoid and indigoid dyes dispersed in complexmixtures. Through the analysis of synthetic model compounds, a series of methy-lated benzenecarboxylic acid derivatives were tentatively identified as key ther-mochemolysis products whose formation was supposedly promoted by the attack ofhydroxide ions on carbonyl carbons of the dye.

Characteristic markers of indigo (e.g. 2-(methylamino)-benzoic acid, methylester), madder (e.g. dimethyl-o-phthalate) and cochineal (e.g. methyl-methoxyben-zoic acid, methyl ester) were detected in the chromatograms of the correspondingstandard samples, containing the natural dye dispersed in a matrix (mordant andbinder). However, the specificity of the markers should be carefully examined for aproper interpretation in real cases. For example, dimethyl-o-phthalate may beconsidered a good marker for madder-like pigments, readily distinguishable frominsect dyes, which do not release dimethylphthalate upon pyrolysis with TMAH.On the other hand, materials obtained from phthalic acid, like alkyd resins, are verycommon in modern applications, e.g. as varnishes in restorations, and theirpresence may interfere with the analysis of madder. Interference problems areincreased by the paucity of the sample, a typical situation encountered in the

Table 3Principal THM products of indigoa

cm/z Tentative structural assignment

65, 66, 93 Benzeneamine 1250, 76, 103 Benzenenitrile

N-methyl-benzenamine77, 106, 107 377, 120, 121 4N,N-dimethylbenzenamine

103, 130, 131 1-Methylindole 592, 119, 151 2-Amino-benzoic acid, methyl ester 6

72-(Dimethylamino)-benzoic acid, methyl ester148, 164, 179104, 132, 165 82-(Methylamino)-benzoic acid, methyl ester

Dimethylindole77, 144, 145 93-Methoxy-1-methylindole (?) 10118, 146, 1611,2-Dimethylindol-3-one (?)118, 146, 161 11

a Peak numbers are those reported in Fig. 6.


analysis of museum objects with destructive techniques. Data resulting frompyrolysis may be complemented by the use of conventional and time-consumingtechniques for a more reliable and accurate identification of the pigments.

In conclusion, THM in combination with GC/MS proved its potential as asimple and rapid approach to obtain preliminary information on the nature of theprincipal classes of organic materials used by artists, that is binders, protectives andpigments.

Acknowledgements

Thanks are given to Gianluca Merlini for kindly providing the standard samples.DF and GC thank the University of Bologna and the National Council of Research(Progetto finalizzato Beni Culturali contract number 96.01097.PF36) for financialsupport. The work was partly done within the People’s Republic of China–Italycollaboration (project of the Italian Institute for Africa and Orient).

References

[1] J.S. Mills, R.W. White, The Organic Chemistry of Museum Objects, Butterworths, London, UK,1996.

[2] A. Shedrinsky, N.S. Baer, The application of analytical pyrolysis to the study of cultural materials,in: T.P. Wampler (Ed.), Applied Pyrolysis Handbook, Marcel Dekker, New York, 1995.

[3] J.M. Challinor, J. Anal. Appl. Pyrolysis 16 (1989) 323.[4] G. Chiavari, G.C. Galletti, P. Bocchini, P. Russo, N. Gandini, D. Fabbri, Sci. Technol. Cultural

Heritage 7 (1998) 19.[5] G. Chiavari, N. Gandini, P. Russo, D. Fabbri, Chromatographia 47 (1998) 420.[6] I. Pastorova, K.J. van der Berg, J.J. Boon, J.W. Verhoeven, J. Anal. Appl. Pyrolysis 43 (1997) 41.[7] J.D.J. van der Berg, J.J. Boon, K.J. ven den Berg, I. Fiedler, M.A. Miller, Anal. Chem. 70 (1998)

1823.[8] G. Chiavari, D. Fabbri, G.C. Galletti, Rapid Commun. Mass Spectrometry 9 (1995) 559.[9] G. Chiavari, D. Fabbri, S. Prati, R. Mazzeo, D. Bikiaris, S. Daniila, Y. Chryssoulakis, Sixth

International Conference on ‘Non-Destructive Testing and Microanalysis for the Diagnostics andConservation of the Cultural and Environmental Heritage, Rome, vol. 2, 1999, pp. 1147–1162.

[10] H. Schweppe, in: S.H. Zeronian, H.L. Needles (Eds.), Historic Textile and Paper Materials II:Conservation and Characterization, American Chemical Society, Washington, DC, 1989, pp.188–219.

[11] J. Wouters, Stud. Conserv. 30 (1985) 119.[12] C. Miliani, A. Romani, G. Favaro, Spectrochim. Acta Part A 54 (1998) 581.[13] F. Brunello, In: N. Pozza (Ed.), De Arte Illuminandi e altri trattati sulla tecnica della miniatura

medievale, Padova, Italy, 1992.[14] Y. Berger, A. Castonguay, Org. Magn. Reson. 11 (1978) 375.[15] D. Fabbri, R. Helleur, J. Anal. Appl. Pyrolysis 49 (1999) 277.

Analysis of Anthraquinoid and Indigoid Dyes Used in Ancient Artistic Works by Thermally Assisted...

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