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A fluorescence labeling approach to assess the deterioration state of aged papers

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A fluorescence labeling approach to assess the deterioration state of aged papers
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  A fluorescence labeling approach to assess the deterioration stateof aged papers Ute Henniges 1 , Thomas Prohaska 1 , Gerhard Banik 2 and Antje Potthast 1, * 1 Department of Chemistry, University of Natural Resources and Applied Life Sciences (BOKU), Muthgasse18, A-1190, Wien, Austria;  2 Staatliche Akademie der Bildenden Ku ¨ nste Stuttgart, Ho ¨ henstraße 16, D-70736,Fellbach, Germany; *Author for correspondence (e-mail: antje.potthast@boku.ac.at; phone: +43-1-36006-6071; fax: +43-1-36006-6059) Received 14 October 2005; accepted in revised form 28 November 2005 Key words:  Carbonyl and carboxyl groups, Cellulose degradation, Fluorescence labeling, Migration of copper ions, Molecular weight distribution Abstract Deterioration of historical papers is caused by several processes, such as acid hydrolysis or autoxidationdue to the presence of metal ions contained in inks or pigments. Both processes can be studied by fluo-rescence labeling of carbonyl and carboxyl groups in combination with GPC-MALLS. This techniqueallows to determine not only the extent of hydrolysis, but also the concentration of oxidized functionalitieswithin very low sample amounts.The thermally induced aging of rag papers with lines of copper pigment has been investigated, simulatinggreen or blue copper pigments in historic wall papers. The cellulose parts with pigment coverage andadjacent pigment-free regions were analyzed separately and compared to paper parts not affected by metalions. The cellulose underneath and close to the applied pigment strokes was severely affected. Althoughthere was no difference in the molecular weight distribution, distinct differences in the carbonyl andcarboxyl content were observed. Copper ion migration is suggested to be one possible explanation for thisobservation as a strong correlation between distribution of copper ions and carbonyl groups was found.For the first time, a detailed examination of cellulose damage in spatial proximity to metal-containingpigment lines is thus presented. Introduction Migration of transition metal ions from pigmentsor inks into surrounding or underlying papermaterial is known for a long time to be a seriousthreat to the preservation of cultural heritage onpaper and parchment (Williams et al. 1977;Shahani and Hengemihle 1986). Under certainconditions, such as elevated humidity, transitionmetal ions, especially iron and copper, are releasedfrom painting or printing pigments, provokingthe catalytic and thus accelerated oxidation of cellulose. Frequently used writing and paintingmedia, such as the ubiquitous iron gall ink and thebrilliantly colored copper acetate, most often causethis damage on works of art. During this process,cellulose is chemically altered and deterioratedor even fully destroyed (Lewin and Mark 1997).Visually, this process can be perceived in browningof pigment and surrounding paper, as well as inloss of mechanical strength. Although parallels doexist between copper corrosion and degradationby ferrous and ferric ions (the so-called ink decay)these two degradation types are not following an Cellulose (2006) 13:421–428    Springer 2006DOI 10.1007/s10570-005-9030-3  identical degradation path (Banik 1982; Baniket al. 1982; Daniels 2002). The influence of tran-sition metal ions on paper can be shown byapplying a droplet of water on aged copper cor-rosion paper samples. Humidification will nolonger occur homogenously, but there are differ-ences between pigment layer, its immediate sur-roundings, and more distant areas. These changesinto hydrophobic and hydrophilic zones can alsobe observed in other degradation processes trig-gered by transition metals (Reißland 1999).This observation is eventually attributed tomigration of metal ions from writing or paintingmedia into the surrounding paper material. Thewater required for migration processes of transi-tion metal ions might be provided from paper it-self, from ambient humidity or accidental wetting.In historical preparations for painting purposes,copper acetate is a mixture between neutral andbasic copper acetates, which are in part watersoluble right from the beginning. During naturalageing and accelerated degradation of the bindingagent itself, mostly glue or gelatine, more andmore ions are released into surrounding paper.As metal ion migration is considered to be themain cause for negative changes in paper, visibleby different behavior towards water and discol-oration of paper and pigment, oxidative pro-cesses are the likely causes. Thus, investigation of molecular weight distribution as well as oxidativechanges (carbonyl and carboxyl content) – ascommunicated within this study – are expected todocument the observations. Another aspect is thedetermination of the metal ion distribution in therespective cellulose samples. A close correlationbetween the damage of cellulose molecules and theamount of metal ions is expected. Methods and material Carbazole-9-Carbonyl-Oxy-Amine (CCOA) label-ing of carbonyl groups was performed as describedearlier (Potthast et al. 2003; Ro ¨hrling et al. 2003a, b).Fluorenyl-Di-Azo-Methan (FDAM) labeling of carboxyl groups was performed as described byBohrn (2005). The labeling approach is shown inFigure 1. Figure 1.  Approach to fluorescence labeling of oxidized cellulose with CCOA and FDAM. 422  General analytics Gel permeation chromatography (GPC) measure-ments used the following components: onlinedegasser, Dionex DG-2410; Kontron 420 pump,pulse damper; autosampler, HP 1100 columnoven, Gynkotek STH 585, fluorescence detectorTSP FL2000 (CCOA) and Agilent FLD G1321A(FDAM); multiple-angle laser light scattering(MALLS) detector, Wyatt Dawn DSP with argonion laser ( k 0  ¼ 488 nm); refractive index (RI)detector, Shodex RI-71; Data evaluation wasperformed with standard Chromeleon, Astra andGRAMS/32 software. GPC method  The following parameters were used in the GPCmeasurements: flow, 1.00 ml min ) 1 ; columns, fourPL gel mixedA LS, 20 l m, 7.5  300 mm 2 ; fluo-rescence detection,  k ex  ¼ 290 nm,  k em  ¼ 340 nm(CCOA),  k ex  ¼ 252 nm,  k em  ¼ 323 nm (FDAM);injection volume, 100 l l; run time, 45 min. N,N  -dimethylacetamide/lithium chloride (0.9% w/v), filtered through a 0 : 02 l  m filter, was used asthe eluent. Laser ablation Paper analysis was performed by means of directlaser ablation inductively coupled plasma massspectrometry (LA-ICP-MS). A Nd:YAG 213 nmlaser system (ablascope, bioptic, Germany) wascoupled to a ICP-MS (Perkin Elmer DRC II, Per-kin Elmer, Canada). The ablated material wastransported by a 0.7 l min ) 1 He gas flow directlyinto the ICP-plasma after passing a glass wool filterof approximately 5 mm length which was directlyplaced into the tubing after the filter. An additionalflow of 0.8 l min ) 1 of Ar was added after the filter.This setup corresponds to our standard setup asdescribed in previous work (Prohaska et al. 2002).Analysis was performed on line scans using thelaser parameter as shown in Table 1. Every mea-surement cycle started with the acquisition of ablank value which was subtracted from the sub-sequent transient laser ablation signal.ICP-MS analysis of Cu was performed by ana-lyzing the  63 Cu/ 12 C signal.  12 C was used as internalstandard. The optimized ICP-MS parameters aregiven in Table 2. Test papers Test papers were prepared of modern handmadepaper composed of linen and flax fibers withoutadditional sizing or fillers. Copper acetate (90%basic and 10% neutral copper acetate) was printedon unsized paper according to historical protocols.The pigment was bound in skin glue media. Cop-per corrosion was simulated with accelerated age-ing at 55   C and fluctuating humidity from 35 to80% relative humidity (RH) every 6 h to enforcewater migration within the paper web. This ageingtreatment was performed on all paper samplesanalyzed in this study.Some paper samples were additionally subjectedto artificial ageing for seven days at 80   C and65% RH. To get a detailed picture of oxidativechanges, measured as carbonyl and carboxylgroup distributions as well as changes in themolecular weight distribution, the sample paperwith simulated copper corrosion was cut into pie-ces, which were investigated separately.The cutting scheme is given in Figure 2: socalled lines (dashed line rectangles) and blankspaces (black squares) were studied, in addition a Table 1.  Laser ablation parameter (ablascope, bioptic, Ger-many).Parameter ValueWavelength 213 nmEnergy (fluence) 8 J/cm 2 Spot size 50 l mScan speed 16 l m/s Table 2.  Optimized ICP-DRC-MS parameter (Elan DRC II,Perkin Elmer, Canada).Parameter ValueNebulizer PFASpray chamber CyclonNebulizer gas flow 0.8 l min ) 1 Auxiliary gas flow 1.275 l min ) 1 Plasma gas flow 15 l min ) 1 ICP RF power 1200 W 423  mixture of blank spaces and lines, as well as paperfrom the edge of the paper, which had not been incontact with copper pigment, and therefore shouldnot have been influenced by any migration process(Henniges et al. 2005). Results and discussion Visual analysis In Figure 3a, the damage of paper samples withsimulated copper corrosion is shown. As opposed Figure 2.  Cutting scheme for copper corrosion paper samples.Black dashed line rectangles depict pigment lines, black squaresshow blank spaces (lines are about 0.5 mm in srcinal, seeFigure 3). Figure 3.  (a) After applying a droplet of water on test samples with a simulation of copper corrosion a difference in penetration can beobserved. (b) Areas that cannot be wetted by water anymore correlate with areas of enhanced fluorescence after labeling with a markerspecific for carbonyl groups (dansyl hydrazine). (c) Removal of copper pigment before labeling with dansyl hydrazine was controlledby LA-ICP-MS. For visual control, a paper with copper lines still on it was taken, before analysis copper lines were removed byEDTA. Copper content is still detectable in paper, but to a very low extent. 0510152025303540 space and lineblank spaceline   c  a  r   b  o  n  y   l  g  r  o  u  p  c  o  n   t  e  n   t   i  n  m  m  o   l   /   k  g 0d7dedge 0510152025303540 space and lineblank spacelineedge   c  a  r   b  o  n  x  y   l  g  r  o  u  p  c  o  n   t  e  n   t   i  n  m  m  o   l   /   k  g 0d7d Figure 4.  Left: development of carbonyl groups without accelerated ageing (0 days), and 7 days of accelerated ageing at 80   C and65% RH. Right: development of carboxyl groups without accelerated ageing (0 days) and 7 days of accelerated ageing at 80   C and65% RH. 424  to non-aged samples, the pigment was no longerwater soluble. After applying a droplet of water atransition from homogeneously wettable materialinto paper with hydrophilic and hydrophobicareas occurred.A detailed analysis revealed that hydrophobicareas occur directly next to the pigment layer,while blank areas between the pigment lines andareas apart from pigment application stay hydro-philic. Labeling of the sample with dansyl hydra-zine, a label selective to carbonyl groups, gavefurther insight into the distribution of carbonylgroups within the damaged paper sample. Asshown in Figure 3b, areas with high fluorescenceintensity correspond with areas of high hydro-phobicity. The removal of copper acetate wascontrolled by LA-ICP-MS (see Figure 3c). Oxidative changes CCOA and FDAM analyses permitted a detailedstudy of oxidized structures in a model paper withsimulated copper corrosion. The development of carbonyl and carboxyl groups in a model paperwas observed during an ageing period of sevendays at elevated temperatures and humidity. It wasevident that carbonyl groups underwent morechanges than carboxyl groups (see Figure 4).Figure 5 helps to illustrate how the distributionof carbonyl groups and the molecular weight dis-tribution (MWD) changed during the ageing per-iod. At the beginning of the ageing period, theMWD looked very similar for all tested areas. Thecarbonyl groups showed differences, according toboth, the location tested in the paper and along the 6.5%55.9%19%18.6% <100 100-200 200-2000 >2000 carbonyl groupssample: sT 0d +Cu 4.8%45.8%14.3%35.1%<100 100-200 200-2000 >2000 carbonyl groupssample: sT 0d blank space 543 6 70.00.20.40.60.81.01.21.4    D   i   f   f .  w  e   i  g   h   t   f  r  a  c   t   i  o  n Log Mw  Blank space Line Whole paper Edge Figure 5.  Different facets of CCOA analysis. Samples without accelerated ageing. Upper figures: distribution of carbonyl groups indifferent DP regions of the molecules, left: below copper pigment; right: in blank spaces. Lower figures: total carbonyl content indifferent regions of the paper sample (right) and molecular weight distributions in these regions (left). 425
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