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ToF-SIMS and XPS study of ancient papers
Applied Surface Science 257 (2011) 2142–2147
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
ToF-SIMS and XPS study of ancient papers Francesca Benetti, Nadia Marchettini, Andrea Atrei ∗ Dipartimento di Chimica, Università di Siena, Via A. Moro, 2, 53100 Siena, Italy
a r t i c l e
i n f o
Article history: Received 15 July 2010 Received in revised form 17 September 2010 Accepted 17 September 2010 Available online 29 September 2010 Keywords: ToF-SIMS XPS Ancient paper Surface analysis
a b s t r a c t The surface composition of 18th century papers was investigated by means of ToF-SIMS and XPS. The aim of the present study was to explore the possibility of using these surface sensitive methods to obtain information which can help to determine the manufacturing process, provenance and state of conservation of ancient papers. The ToF-SIMS results indicate that the analyzed papers were sized by gelatin and that alum was added as hardening agent. The paper sheets produced in near geographical areas but in different paper mills exhibit a similar surface composition and morphology of the fibers as shown by the ToF-SIMS measurements. The ToF-SIMS and the XPS results indicate that a significant fraction of the cellulose fibers is not covered by the gelatin layer. This was observed for the ancient papers and for a modern handmade paper manufactured according to the old recipes. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The surface analysis of ancient papers is relevant from several points of view [1]. The determination of the surface composition is important to understand the aging and deterioration mechanisms of papers and to develop the appropriate restoration procedures. A recent study carried out by means of bulk analytical techniques highlights the effects of pH, gelatin content and the presence of residual metals on the state of conservation of naturally aged papers [2]. Gelatin, which was used in the past in the sizing process (that is the coating of the cellulose fibers to prevent the bleeding of inks and to improve the strength and the abrasion resistance of paper sheets) seems to play an important role in determining the long-term paper stability [3–6]. It was shown that samples of ancient papers with a higher gelatin content were in better conditions than those with a lower content [2,6]. On the other hand, sizing is a surface process and in addition to the total gelatin content also its distribution over the cellulose fibers is expected to be important for the long-term stability of paper. Since the paper manufacturing process (in particular the sizing materials and techniques) changed from the 14th to 19th century, the determination of the surface composition should allow one to determine the historical origin of the examined papers. In the present work, we used Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS) techniques to investigate the surface composition of papers
∗ Corresponding author. E-mail address: [email protected] (A. Atrei). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.09.063
dating back to the 18th century and manufactured in various paper mills located in Tuscany (Italy). The aim of this study was to explore the possibility of using these surface sensitive techniques to obtain information about the manufacturing process and the production period of the examined papers looking in particular at the composition of the sizing layer. We exploited the capability of ToF-SIMS to determine the nature and spatial distribution of organic and inorganic components, simultaneously, in a single sample and in one analytical step [7]. Moreover, due to the high sensitivity of ToFSIMS [8], we were able to detect trace elements which could affect the long-tem stability of the paper. The XPS technique was used to obtain quantitative results about the surface composition of the examined paper samples.
2. Experimental details The specimens were cut from paper sheets dating back to the 18th century and manufacturing in the following paper mills: “La Stella”, “Al Masso”, “Al Buonvisi” and “Vorno”. They were located in Lucca and Pistoia areas (Tuscany, Italy) and were distant less than 100 km. The paper sheets carried no printing and were apparently in good conditions. In particular, on the paper sheets foxing effects were not significant. The samples coming from the different paper mills were indicated with the letters A, B, C and D. The following reference materials were used: cis-4-hydroxy-Lproline (purity ≥ 99.0%, Fluka), Whatman Paper N.1 filter paper and the Fabriano paper sample (Perusia type). The latter was manufactured recently and was made of 22% cotton and 78% linters of cotton with gelatin sizing. Thereafter this sample is indicated with the letter F. The cis-4-hydroxy-l-proline (an aminoacid characteristic
F. Benetti et al. / Applied Surface Science 257 (2011) 2142–2147
of gelatin) reference sample was dissolved in water and dropped onto a silicon wafer with a pipette to form a thick film. Before the ToF-SIMS analysis it was dried at room temperature. ToF-SIMS analyses were carried out by using a TRIFT III apparatus (Physical Electronics, Chanhassen, MN, USA) equipped with a gold liquid metal ion source utilizing Au+ ions with an energy of 22 keV and a beam current of 600 pA. Before the measurements, the samples were kept inside the vacuum chamber overnight for degassing. Positive and negative spectra were acquired in high mass resolution mode (up to 5000 m/m) in the range of 0–2000 m/z over an area of 100 m × 100 m. The primary ion dose was kept below 1012 ions/cm2 to ensure the static SIMS conditions [9]. In all measurements, a flooding electron beam was used for charge compensation. Positive mass spectra were calibrated by using the m/z values of the following ions: CH3 + (m/z 15.023), C2 H3 + (m/z 27.023) and C3 H5 + (m/z 41.039). For the calibration of the negative mass spectra we used the signals of the following ions: CH− (m/z 13.008), OH− (m/z 17.003) and C2 H− (m/z 25.008). The chemical maps showing the spatial distribution of the most relevant ion fragments were acquired over 300 m × 300 m areas. XPS measurements were performed in an ultra high vacuum chamber using a hemispherical electron energy analyzer (HA100, VSW Ltd., UK) and a non-monochromatized Al X-ray source. The spectra were collected at a take off angle of 90◦ . Survey spectra were acquired with a pass energy of 90 eV. A pass energy of 44 eV was used to measure the higher resolution energy regions around the peaks of interest. The binding energy (BE) scale was calibrated setting the aliphatic component of the C 1s peak to 285.0 eV. For the quantification of the XPS data, we used the atomic sensitivity factors reported in Ref. [10].
3. Results and discussion 3.1. ToF-SIMS measurements The ToF-SIMS spectra were measured for the samples without any treatment and after scratching by a cutter blade. This treatment should remove the thickest parts of the contamination layer. The ToF-SIMS images were acquired for the as-introduced samples. The ToF-SIMS spectra of the samples after scratching show the presence of the same inorganic and organic components observed in the spectra measured for the non- scratched samples. This observation suggests that the surface composition determined by ToF-SIMS is not significantly altered by the contamination layer accumulated at the surface of the paper sheets. The ToF-SIMS spectra measured for the A, B, C and D samples are very similar. Thus, we report here only the spectra measured for the sample A (Fig. 1) and for sample B (Fig. 2). In the mass region between m/z 20 and 80, the positive ToFSIMS spectra of sample A (Fig. 1a) and sample B (Fig. 2a) indicate the presence of aluminum (m/z 27), potassium (m/z 39), sodium (m/z 23), calcium (m/z 40), iron (m/z 56) and copper (m/z 63). In the mass region between 80 and 160 m/z, the peaks of the ion fragment (C4 H8 NO+ ) of 4-hydroxyproline at m/z 86 and two ion fragments of proline, i.e. C4 H6 N+ at m/z 68 and C4 H8 N+ at m/z 70, as well as three characteristic fragments of cellulose, i.e. C5 H5 O2 + (m/z 97), C6 H7 O3 + (m/z 127), C6 H9 O4 + (m/z 145), are detected in sample A (Fig. 1b) and in sample B (Fig. 2b). Since 4-hydroproline and proline are aminoacids characteristic of gelatin [11], the presence of the molecular fragments coming from these molecules indicates that gelatin was used as sizing material of the paper. In the spectra of all samples, peaks which can be attributed to fragments of hydrocarbons (C3 H5 + , C4 H7 + , etc.) and other organic molecules are detectable. These species are probably due to contaminants coming
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from a variety of sources and the observation of these fragments is scarcely useful for diagnostic purposes. In the negative ToF-SIMS spectra (not shown) we observed the mass peak of sulfur at m/z 32 and those of other sulfur-containing ions, like SH− (m/z 33), SO− (m/z 48), HSO− (m/z 49), SO2 − (m/z 64), HSO2 − (m/z 65), SO3 − (m/z 80), HSO3 − (m/z 81). The presence of potassium, aluminum and sulfur is due to the addition of alum during the papermaking process. Since the 17th century the papermakers added alum to gelatin in order to improve the sizing process. Alum reduces the solubility of gelatin in water and acts as a coagulant, inducing its precipitation on the cellulose fibers. The presence of calcium may be attributed to several reasons, such as the hardness of the water used in the manufacturing process of the paper and the hide, bones and other animal parts used in the gelatin preparation, as well as the slaked lime [12]. The presence of iron and copper may be attributed to contamination coming from the tools used in the papermaking process [12]. Iron and copper play an important role as catalysts in the oxidative reactions of the cellulose, causing foxing and discoloration effects on the paper surface. The total ion images obtained for sample A and sample F are shown in Fig. 3. Similar images were obtained for the other samples of ancient papers.The morphology of the cellulose fibers appears very similar in the images collected for the ancient and recently manufactured papers. In particular, the cellulose fibers of the ancient papers are not frayed or fibrillated. It has been shown that dark, physically damaged papers are characterized by more fibrillated fibers as observed by optical microscopy [2]. Hence, by comparing the appearance of the cellulose fibers in the ToF-SIMS images collected for the ancient and the recently manufactured papers, we can conclude that the ancient papers are in a good conservation state. In Fig. 4 we reported the ToF-SIMS images showing the spatial distributions of Al+ , K+ , Ca+ , C4 H8 NO+ signals and the sum of the intensities of three fragments of cellulose, for the sample A. The distributions of Al+ and of the ion fragment of 4-hydroxyproline exhibit a uniform distribution over the analyzed area, whereas calcium and potassium appear to be located in small aggregates with an average size of 10 m. The other samples show chemical maps similar to that of sample A. Since the depth probed by ToF-SIMS (5–10 nm) is much smaller than the average thickness of the sizing layer of gelatin (estimated to be of the order of a few microns considering the sizing methods), the detection of the cellulose signals suggests that the cellulose fibers are not completely covered by the gelatin. The overlay image of the spatial distributions of C4 H8 NO+ ions and of the cellulose fragments reveals that these signals do not come from the same points of the surface since distinct red and green clusters of pixels are visible instead of uniform yellow regions (Fig. 5). However, there are not large patches of cellulose fibers not coated by the gelatin layer and the average size of these areas appears to be slightly larger than the lateral resolution of the ToF-SIMS images (i.e. of the order of 2–5 m). Both the ancient paper and the paper recently manufactured have similar spatial distributions of the gelatin and cellulose signals (Fig. 5). 3.2. XPS measurements The XPS measurements were performed for samples A and F. C 1s, O 1s and N 1s peaks are the main signals in the survey spectra of both samples (Fig. 6). The survey spectrum of sample A reveals the presence of aluminum and sulfur, in agreement with the ToFSIMS data. The amount of alum is rather low (below 1 mol%) so that the K 2p peaks (falling on the tail of the intense C 1s peak) are not detectable in the XPS spectra. The surface composition
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F. Benetti et al. / Applied Surface Science 257 (2011) 2142–2147
Fig. 1. Positive ToF-SIMS spectrum of sample A in the mass region 20–80 m/z (a) and in the mass region 80–160 m/z (b). The peaks corresponding to polydimethylsiloxane (PDMS), a typical surface contaminant accumulating during the handling of the sample, fragments are marked by asterisks
of these two kinds of paper are similar (see Table 1). The N/C atomic ratio determined for sample A is within the range of values reported in Ref. [13] for ancient papers in a good state of conservation. As evidenced by ToF-SIMS data, the observation of the ion fragments of cellulose suggests that the cellulose fibers are not completely covered by the gelatin layer. As discussed above, the average thickness of the gelatin layer is expected to be two order of magnitude larger than the depth
Table 1 Elemental composition of the ancient paper (sample A) and of the modern paper “Perusia” type (sample F) in atomic % determined by XPS. The accuracy of the reported values is ±10%.For the sample A the amount of sulfur and aluminum is below 1 at.%.
Sample A Sample F
C (at.%)
O (at.%)
N (at.%)
60 59
33 35
6 6
probed by the photoelectrons so that the contribution to the C 1s peak from the cellulose would be completely attenuated by a continuous film of gelatin. Hence, we can assume that the peak areas of the C 1s components of cellulose are proportional to the fraction of cellulose fibers not covered by the gelatin layer. In order to determine this fraction, we performed a curve fitting analysis of the C 1s spectra. The C 1s components of the cellulose are expected to have BEs very close to those of gelatin [14,15]. Hence, the fitting parameters for the components of cellulose in the C 1s spectrum were determined by analyzing the C 1s spectrum measured for a sample of pure cellulose. The curve fitting of the C 1s spectrum measured for the sample A is shown in Fig. 7. The curve fitting analysis shows that for the sample A the cellulose contribution is about 30% of the total C 1s peak area. A similar contribution of the cellulose components to the C 1s spectrum was determined also for sample F. Therefore, the partial coverage of the cellulose fibers by the gelatin layer cannot be due entirely to the aging and deterioration of the paper sheet. The observation of the cellulose signals in the ToF-SIMS and XPS spectra can be tentatively attributed to a partial
F. Benetti et al. / Applied Surface Science 257 (2011) 2142–2147
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Fig. 2. Positive ToF-SIMS spectrum of sample B in the mass region 20–80 m/z (a) and in the mass region 80–160 m/z (b). The peaks corresponding to polydimethylsiloxane (PDMS), a typical surface contaminant accumulating during the handling of the sample, fragments are marked by asterisks.
Fig. 3. ToF-SIMS total ion images of sample A and sample F over a 300 m × 300 m area.
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Fig. 4. ToF-SIMS images of total ions, Al+ , K+ , Ca+ , the ion fragment of hydroxyproline (C4 H8 NO+ ) and the sum of the cellulose fragments (97, 127, 145 m/z) measured for the sample A over a 300 m × 300 m area.
Fig. 5. Overlay images of the distribution the ion fragment of hydroxyproline (C4 H8 NO+ ) and the sum of the cellulose fragments over a 300 m × 300 m area of sample A and sample F, respectively.
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wetting of the cellulose fibers by the sizing solution or to a differential shrinkage of the gelatin layer and of the cellulose fibers during the drying the paper sheets. 4. Conclusions
Fig. 6. Survey XPS spectra of sample A and sample F.
A set of 18th century paper samples coming from paper mills in Tuscany (Italy) were investigated by means of the ToF-SIMS and the XPS techniques. The results of this study reveal minor differences in their surface composition. The presence of elements like calcium, potassium, aluminium, iron and copper seems to be related to the manufacturing process and to the materials (alum) used in the fabrication of the studied papers rather than to the surface contamination caused by the environment and by the sheet handling. Although the sample set studied in this work is rather small, on the basis of the present results we can conclude that papers produced in neighboring areas and in the same historical period have very similar surface compositions as determined by ToF-SIMS and XPS. We found that the sizing layer of gelatin covers only in part the cellulose fibers of the ancient papers. This result should be taken into account in order to understand the effect of gelatin on the long-term conservation of paper. Acknowledgements The authors kindly thank Curzio Bastianoni and Renzo Sabbatini (Dipartimento Studi Storico-Sociali e Filosofici, University of Siena, Italy) for providing us with the ancient papers and for the helpful discussions. Flavia Pinzari (Istituto centrale per il restauro e la conservazione del patrimonio archivistico e librario, Rome, Italy) is also gratefully acknowledged for providing us with the Fabriano paper, Perusia type. References
Fig. 7. Results of the curve fitting analysis of the C 1s spectrum measured for sample A. The component at 285 eV corresponding to aliphatic carbon atoms is due to the gelatin layer and to contamination. The components located at BEs values around of 286.5 eV correspond to carbon atoms singly bonded to oxygen or nitrogen atoms. The components at BEs around 288.5 eV can be attributed to carbon atoms in –O–C–O–, O C–NH and O C–OH groups. The parameters (peak position, width, line shape and relative intensities) of the components characteristic of cellulose were determined by fitting the C 1s spectrum of a pure cellulose sample and used to fit the spectrum of sample A.
[1] T.E. Conners, S. Banerjee (Eds.), Surface Analysis of Paper, CRC-Press, Boca Raton, FL, 1995. [2] C.H. Stephens, T. Barrett, P.M. Whitmore, J.A. Wade, J. Mazurek, M. Schilling, J. Am. Inst. Conserv. 47 (2008) 201. [3] A.-L. Dupont, J. Chromatogr. A 950 (2002) 113. [4] M. Missori, M. Righini, S. Selci, Opt. Commun. 231 (2004) 99. [5] M. Missori, M. Righini, A.-L. Dupont, Opt. Commun. 263 (2006) 289. [6] T. Barrett, C. Mosier, J. Am. Inst. Conserv. 34 (1995) 173. [7] M. Dowsett, A. Adriaens, Nucl. Instrum. Methods Phys. Res. Sect. B 226 (2004) 38. [8] P. Fardim, B. Holmbom, Appl. Surf. Sci. 249 (2005) 393. [9] G. Marletta, S.M. Catalano, S. Pignataro, Surf. Interface Anal. 16 (1990) 407. [10] D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis, Wiley, New York, 1985. [11] G.N. Batts, A.J. Paul, Langmuir 10 (1994) 218. [12] M. Manso, M. Costa, M. Carvalho, Spectrochim. Acta Part B 63 (2008) 1320. [13] G. Righini, A.L. Segre, G. Mattogno, C. Federici, P. Mufanò, Naturwissenschaften 85 (1998) 171. [14] G. Polzonetti, C. Battocchio, M. Dettin, R. Gambaretto, C. Di Bello, V. Carravetta, S. Monti, G. Iucci, Mater. Sci. Eng. C 28 (2008) 309. [15] J.S. Stevens, S.L.M. Schroeder, Surf. Interface Anal. 41 (2009) 453.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
ToF-SIMS and XPS study of ancient papers Francesca Benetti, Nadia Marchettini, Andrea Atrei ∗ Dipartimento di Chimica, Università di Siena, Via A. Moro, 2, 53100 Siena, Italy
a r t i c l e
i n f o
Article history: Received 15 July 2010 Received in revised form 17 September 2010 Accepted 17 September 2010 Available online 29 September 2010 Keywords: ToF-SIMS XPS Ancient paper Surface analysis
a b s t r a c t The surface composition of 18th century papers was investigated by means of ToF-SIMS and XPS. The aim of the present study was to explore the possibility of using these surface sensitive methods to obtain information which can help to determine the manufacturing process, provenance and state of conservation of ancient papers. The ToF-SIMS results indicate that the analyzed papers were sized by gelatin and that alum was added as hardening agent. The paper sheets produced in near geographical areas but in different paper mills exhibit a similar surface composition and morphology of the fibers as shown by the ToF-SIMS measurements. The ToF-SIMS and the XPS results indicate that a significant fraction of the cellulose fibers is not covered by the gelatin layer. This was observed for the ancient papers and for a modern handmade paper manufactured according to the old recipes. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The surface analysis of ancient papers is relevant from several points of view [1]. The determination of the surface composition is important to understand the aging and deterioration mechanisms of papers and to develop the appropriate restoration procedures. A recent study carried out by means of bulk analytical techniques highlights the effects of pH, gelatin content and the presence of residual metals on the state of conservation of naturally aged papers [2]. Gelatin, which was used in the past in the sizing process (that is the coating of the cellulose fibers to prevent the bleeding of inks and to improve the strength and the abrasion resistance of paper sheets) seems to play an important role in determining the long-term paper stability [3–6]. It was shown that samples of ancient papers with a higher gelatin content were in better conditions than those with a lower content [2,6]. On the other hand, sizing is a surface process and in addition to the total gelatin content also its distribution over the cellulose fibers is expected to be important for the long-term stability of paper. Since the paper manufacturing process (in particular the sizing materials and techniques) changed from the 14th to 19th century, the determination of the surface composition should allow one to determine the historical origin of the examined papers. In the present work, we used Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS) techniques to investigate the surface composition of papers
∗ Corresponding author. E-mail address: [email protected] (A. Atrei). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.09.063
dating back to the 18th century and manufactured in various paper mills located in Tuscany (Italy). The aim of this study was to explore the possibility of using these surface sensitive techniques to obtain information about the manufacturing process and the production period of the examined papers looking in particular at the composition of the sizing layer. We exploited the capability of ToF-SIMS to determine the nature and spatial distribution of organic and inorganic components, simultaneously, in a single sample and in one analytical step [7]. Moreover, due to the high sensitivity of ToFSIMS [8], we were able to detect trace elements which could affect the long-tem stability of the paper. The XPS technique was used to obtain quantitative results about the surface composition of the examined paper samples.
2. Experimental details The specimens were cut from paper sheets dating back to the 18th century and manufacturing in the following paper mills: “La Stella”, “Al Masso”, “Al Buonvisi” and “Vorno”. They were located in Lucca and Pistoia areas (Tuscany, Italy) and were distant less than 100 km. The paper sheets carried no printing and were apparently in good conditions. In particular, on the paper sheets foxing effects were not significant. The samples coming from the different paper mills were indicated with the letters A, B, C and D. The following reference materials were used: cis-4-hydroxy-Lproline (purity ≥ 99.0%, Fluka), Whatman Paper N.1 filter paper and the Fabriano paper sample (Perusia type). The latter was manufactured recently and was made of 22% cotton and 78% linters of cotton with gelatin sizing. Thereafter this sample is indicated with the letter F. The cis-4-hydroxy-l-proline (an aminoacid characteristic
F. Benetti et al. / Applied Surface Science 257 (2011) 2142–2147
of gelatin) reference sample was dissolved in water and dropped onto a silicon wafer with a pipette to form a thick film. Before the ToF-SIMS analysis it was dried at room temperature. ToF-SIMS analyses were carried out by using a TRIFT III apparatus (Physical Electronics, Chanhassen, MN, USA) equipped with a gold liquid metal ion source utilizing Au+ ions with an energy of 22 keV and a beam current of 600 pA. Before the measurements, the samples were kept inside the vacuum chamber overnight for degassing. Positive and negative spectra were acquired in high mass resolution mode (up to 5000 m/m) in the range of 0–2000 m/z over an area of 100 m × 100 m. The primary ion dose was kept below 1012 ions/cm2 to ensure the static SIMS conditions [9]. In all measurements, a flooding electron beam was used for charge compensation. Positive mass spectra were calibrated by using the m/z values of the following ions: CH3 + (m/z 15.023), C2 H3 + (m/z 27.023) and C3 H5 + (m/z 41.039). For the calibration of the negative mass spectra we used the signals of the following ions: CH− (m/z 13.008), OH− (m/z 17.003) and C2 H− (m/z 25.008). The chemical maps showing the spatial distribution of the most relevant ion fragments were acquired over 300 m × 300 m areas. XPS measurements were performed in an ultra high vacuum chamber using a hemispherical electron energy analyzer (HA100, VSW Ltd., UK) and a non-monochromatized Al X-ray source. The spectra were collected at a take off angle of 90◦ . Survey spectra were acquired with a pass energy of 90 eV. A pass energy of 44 eV was used to measure the higher resolution energy regions around the peaks of interest. The binding energy (BE) scale was calibrated setting the aliphatic component of the C 1s peak to 285.0 eV. For the quantification of the XPS data, we used the atomic sensitivity factors reported in Ref. [10].
3. Results and discussion 3.1. ToF-SIMS measurements The ToF-SIMS spectra were measured for the samples without any treatment and after scratching by a cutter blade. This treatment should remove the thickest parts of the contamination layer. The ToF-SIMS images were acquired for the as-introduced samples. The ToF-SIMS spectra of the samples after scratching show the presence of the same inorganic and organic components observed in the spectra measured for the non- scratched samples. This observation suggests that the surface composition determined by ToF-SIMS is not significantly altered by the contamination layer accumulated at the surface of the paper sheets. The ToF-SIMS spectra measured for the A, B, C and D samples are very similar. Thus, we report here only the spectra measured for the sample A (Fig. 1) and for sample B (Fig. 2). In the mass region between m/z 20 and 80, the positive ToFSIMS spectra of sample A (Fig. 1a) and sample B (Fig. 2a) indicate the presence of aluminum (m/z 27), potassium (m/z 39), sodium (m/z 23), calcium (m/z 40), iron (m/z 56) and copper (m/z 63). In the mass region between 80 and 160 m/z, the peaks of the ion fragment (C4 H8 NO+ ) of 4-hydroxyproline at m/z 86 and two ion fragments of proline, i.e. C4 H6 N+ at m/z 68 and C4 H8 N+ at m/z 70, as well as three characteristic fragments of cellulose, i.e. C5 H5 O2 + (m/z 97), C6 H7 O3 + (m/z 127), C6 H9 O4 + (m/z 145), are detected in sample A (Fig. 1b) and in sample B (Fig. 2b). Since 4-hydroproline and proline are aminoacids characteristic of gelatin [11], the presence of the molecular fragments coming from these molecules indicates that gelatin was used as sizing material of the paper. In the spectra of all samples, peaks which can be attributed to fragments of hydrocarbons (C3 H5 + , C4 H7 + , etc.) and other organic molecules are detectable. These species are probably due to contaminants coming
2143
from a variety of sources and the observation of these fragments is scarcely useful for diagnostic purposes. In the negative ToF-SIMS spectra (not shown) we observed the mass peak of sulfur at m/z 32 and those of other sulfur-containing ions, like SH− (m/z 33), SO− (m/z 48), HSO− (m/z 49), SO2 − (m/z 64), HSO2 − (m/z 65), SO3 − (m/z 80), HSO3 − (m/z 81). The presence of potassium, aluminum and sulfur is due to the addition of alum during the papermaking process. Since the 17th century the papermakers added alum to gelatin in order to improve the sizing process. Alum reduces the solubility of gelatin in water and acts as a coagulant, inducing its precipitation on the cellulose fibers. The presence of calcium may be attributed to several reasons, such as the hardness of the water used in the manufacturing process of the paper and the hide, bones and other animal parts used in the gelatin preparation, as well as the slaked lime [12]. The presence of iron and copper may be attributed to contamination coming from the tools used in the papermaking process [12]. Iron and copper play an important role as catalysts in the oxidative reactions of the cellulose, causing foxing and discoloration effects on the paper surface. The total ion images obtained for sample A and sample F are shown in Fig. 3. Similar images were obtained for the other samples of ancient papers.The morphology of the cellulose fibers appears very similar in the images collected for the ancient and recently manufactured papers. In particular, the cellulose fibers of the ancient papers are not frayed or fibrillated. It has been shown that dark, physically damaged papers are characterized by more fibrillated fibers as observed by optical microscopy [2]. Hence, by comparing the appearance of the cellulose fibers in the ToF-SIMS images collected for the ancient and the recently manufactured papers, we can conclude that the ancient papers are in a good conservation state. In Fig. 4 we reported the ToF-SIMS images showing the spatial distributions of Al+ , K+ , Ca+ , C4 H8 NO+ signals and the sum of the intensities of three fragments of cellulose, for the sample A. The distributions of Al+ and of the ion fragment of 4-hydroxyproline exhibit a uniform distribution over the analyzed area, whereas calcium and potassium appear to be located in small aggregates with an average size of 10 m. The other samples show chemical maps similar to that of sample A. Since the depth probed by ToF-SIMS (5–10 nm) is much smaller than the average thickness of the sizing layer of gelatin (estimated to be of the order of a few microns considering the sizing methods), the detection of the cellulose signals suggests that the cellulose fibers are not completely covered by the gelatin. The overlay image of the spatial distributions of C4 H8 NO+ ions and of the cellulose fragments reveals that these signals do not come from the same points of the surface since distinct red and green clusters of pixels are visible instead of uniform yellow regions (Fig. 5). However, there are not large patches of cellulose fibers not coated by the gelatin layer and the average size of these areas appears to be slightly larger than the lateral resolution of the ToF-SIMS images (i.e. of the order of 2–5 m). Both the ancient paper and the paper recently manufactured have similar spatial distributions of the gelatin and cellulose signals (Fig. 5). 3.2. XPS measurements The XPS measurements were performed for samples A and F. C 1s, O 1s and N 1s peaks are the main signals in the survey spectra of both samples (Fig. 6). The survey spectrum of sample A reveals the presence of aluminum and sulfur, in agreement with the ToFSIMS data. The amount of alum is rather low (below 1 mol%) so that the K 2p peaks (falling on the tail of the intense C 1s peak) are not detectable in the XPS spectra. The surface composition
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F. Benetti et al. / Applied Surface Science 257 (2011) 2142–2147
Fig. 1. Positive ToF-SIMS spectrum of sample A in the mass region 20–80 m/z (a) and in the mass region 80–160 m/z (b). The peaks corresponding to polydimethylsiloxane (PDMS), a typical surface contaminant accumulating during the handling of the sample, fragments are marked by asterisks
of these two kinds of paper are similar (see Table 1). The N/C atomic ratio determined for sample A is within the range of values reported in Ref. [13] for ancient papers in a good state of conservation. As evidenced by ToF-SIMS data, the observation of the ion fragments of cellulose suggests that the cellulose fibers are not completely covered by the gelatin layer. As discussed above, the average thickness of the gelatin layer is expected to be two order of magnitude larger than the depth
Table 1 Elemental composition of the ancient paper (sample A) and of the modern paper “Perusia” type (sample F) in atomic % determined by XPS. The accuracy of the reported values is ±10%.For the sample A the amount of sulfur and aluminum is below 1 at.%.
Sample A Sample F
C (at.%)
O (at.%)
N (at.%)
60 59
33 35
6 6
probed by the photoelectrons so that the contribution to the C 1s peak from the cellulose would be completely attenuated by a continuous film of gelatin. Hence, we can assume that the peak areas of the C 1s components of cellulose are proportional to the fraction of cellulose fibers not covered by the gelatin layer. In order to determine this fraction, we performed a curve fitting analysis of the C 1s spectra. The C 1s components of the cellulose are expected to have BEs very close to those of gelatin [14,15]. Hence, the fitting parameters for the components of cellulose in the C 1s spectrum were determined by analyzing the C 1s spectrum measured for a sample of pure cellulose. The curve fitting of the C 1s spectrum measured for the sample A is shown in Fig. 7. The curve fitting analysis shows that for the sample A the cellulose contribution is about 30% of the total C 1s peak area. A similar contribution of the cellulose components to the C 1s spectrum was determined also for sample F. Therefore, the partial coverage of the cellulose fibers by the gelatin layer cannot be due entirely to the aging and deterioration of the paper sheet. The observation of the cellulose signals in the ToF-SIMS and XPS spectra can be tentatively attributed to a partial
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Fig. 2. Positive ToF-SIMS spectrum of sample B in the mass region 20–80 m/z (a) and in the mass region 80–160 m/z (b). The peaks corresponding to polydimethylsiloxane (PDMS), a typical surface contaminant accumulating during the handling of the sample, fragments are marked by asterisks.
Fig. 3. ToF-SIMS total ion images of sample A and sample F over a 300 m × 300 m area.
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Fig. 4. ToF-SIMS images of total ions, Al+ , K+ , Ca+ , the ion fragment of hydroxyproline (C4 H8 NO+ ) and the sum of the cellulose fragments (97, 127, 145 m/z) measured for the sample A over a 300 m × 300 m area.
Fig. 5. Overlay images of the distribution the ion fragment of hydroxyproline (C4 H8 NO+ ) and the sum of the cellulose fragments over a 300 m × 300 m area of sample A and sample F, respectively.
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wetting of the cellulose fibers by the sizing solution or to a differential shrinkage of the gelatin layer and of the cellulose fibers during the drying the paper sheets. 4. Conclusions
Fig. 6. Survey XPS spectra of sample A and sample F.
A set of 18th century paper samples coming from paper mills in Tuscany (Italy) were investigated by means of the ToF-SIMS and the XPS techniques. The results of this study reveal minor differences in their surface composition. The presence of elements like calcium, potassium, aluminium, iron and copper seems to be related to the manufacturing process and to the materials (alum) used in the fabrication of the studied papers rather than to the surface contamination caused by the environment and by the sheet handling. Although the sample set studied in this work is rather small, on the basis of the present results we can conclude that papers produced in neighboring areas and in the same historical period have very similar surface compositions as determined by ToF-SIMS and XPS. We found that the sizing layer of gelatin covers only in part the cellulose fibers of the ancient papers. This result should be taken into account in order to understand the effect of gelatin on the long-term conservation of paper. Acknowledgements The authors kindly thank Curzio Bastianoni and Renzo Sabbatini (Dipartimento Studi Storico-Sociali e Filosofici, University of Siena, Italy) for providing us with the ancient papers and for the helpful discussions. Flavia Pinzari (Istituto centrale per il restauro e la conservazione del patrimonio archivistico e librario, Rome, Italy) is also gratefully acknowledged for providing us with the Fabriano paper, Perusia type. References
Fig. 7. Results of the curve fitting analysis of the C 1s spectrum measured for sample A. The component at 285 eV corresponding to aliphatic carbon atoms is due to the gelatin layer and to contamination. The components located at BEs values around of 286.5 eV correspond to carbon atoms singly bonded to oxygen or nitrogen atoms. The components at BEs around 288.5 eV can be attributed to carbon atoms in –O–C–O–, O C–NH and O C–OH groups. The parameters (peak position, width, line shape and relative intensities) of the components characteristic of cellulose were determined by fitting the C 1s spectrum of a pure cellulose sample and used to fit the spectrum of sample A.
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