Alkyd paints in art: Characterization using integrated mass spectrometry

Analytica Chimica Acta 797 (2013) 64–80

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Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca

Alkyd paints in art: Characterization using integrated mass spectrometry Jacopo La Nasa, Ilaria Degano, Francesca Modugno ∗ , Maria Perla Colombini University of Pisa, Department of Chemistry and Industrial Chemistry, Via Risorgimento, 35, I-56126 Pisa, Italy

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• We characterized commercial alkyd paints used in contemporary art.

• We used a multi-analytical approach based on mass spectrometry.

• We present the first application of TAGs profiling by HPLC–ESI-Q-ToF to alkyd paints. • FIA-ESI-Q-ToF was tested for the first time for the analysis of alkyd paints.

a r t i c l e

i n f o

Article history: Received 23 April 2013 Received in revised form 29 July 2013 Accepted 10 August 2013 Available online 20 August 2013 Keywords: Alkyd resins Gas chromatography/mass spectrometry High performance liquid chromatography Electrospray ionization-quadrupole-time of flight mass spectrometry Flow injection analysis

a b s t r a c t Alkyd resins have been commonly used as binders in artist paints since the 1940s. The characterization of alkyds in samples from artworks can help to solve attribution and dating issues, investigate decay processes, and contribute to the planning of conservation strategies. Being able to assess the components of industrially formulated paint materials and to differentiate between different trademarks and producers is extremely interesting and requires multi-analytical approaches. In this paper we describe the characterization of commercial alkyd paint materials using a multianalytical approach based on the integration of three different mass spectrometric techniques: gas chromatography–mass spectrometry (GC/MS), high performance liquid chromatography coupled with electrospray ionization mass spectrometry with a tandem quadrupole-time of flight mass spectrometer (HPLC–ESI-Q-ToF), and flow injection analysis (FIA) in the ESI-Q-ToF mass spectrometer. GC/MS was successful in determining the fatty acid and aromatic fractions of the resins after hydrolysis; HPLC–ESI-Q-ToF analysis enabled us to identify the triglycerides (TAGs) and diglycerides (DAGs) profile of each resin, and FIA analysis was used as a rapid method to evaluate the presence of possible additives such as synthetic polymers. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Alkyd resins have been diffused as commercial paint binders since the 1940s, as an industrial evolution of the classical oil paint media. One of the first alkyd paint products for artists was the DuLux series which DuPont began marketing in 1931. The adoption of these oil-based industrial polymers by artists is one of the

∗ Corresponding author. Tel.: +39 050 2219303; fax: +39 050 2219260. E-mail addresses: [email protected] (J. La Nasa), [email protected] (I. Degano), [email protected] (F. Modugno), [email protected] (M.P. Colombini). 0003-2670/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aca.2013.08.021

milestones in the evolution of painting techniques in the 20th century art scene. Traditional natural binders such as proteinaceous media and drying oils were gradually replaced by a variety of organic synthetic paint materials. Artists such as Frank Stella, Jackson Pollock and Picasso experimented with alkyd-based paints [1]. The characterization of alkyd resins in paint samples from artworks represents an important contribution to the study of the painting techniques used in contemporary art. In addition, the possibility to assess the components of industrially formulated paint materials and to differentiate between different trademarks and producers is extremely interesting, in order to solve possible attribution and dating issues, as well as to investigate decay processes

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and interactions with other materials. Consequently, conservation scientists, restorers and conservators have increasingly focused on developing and evaluating analytical tools able to achieve these goals [1]. It has been recently demonstrated that the presence of additives in the formulation of industrial tube paints may presents a risk for the stability of artworks. Hydrophilic additives increase the water sensitivity of paint films, and, as a consequence, in some cases the cleaning of modern unvarnished modern oil paintings is extremely delicate and requires specific procedures [2]. Identifying the formulation of industrial paint products for artists is thus also important for planning conservation strategies. FTIR spectroscopy, thermal analysis and analytical pyrolysis coupled with gas chromatography/mass spectrometry (Py-GC/MS) are the current approaches used for the chemical characterization of synthetic polymers. These techniques have been used to study alkyd resins in paint samples and their curing processes [3–6]. In particular, Py-GC/MS offers the possibility to characterize alkyd resins in short time and without any sample pretreatment. Thermogravimetric analysis (TGA) has also been applied to investigate the reticulation processes of alkyds [5]. However, the complex chemical composition of alkyd resins requires a multi-analytical approach for a detailed molecular analysis. Alkyds are oil-modified polyesters manufactured from polyols (typically glycerol or pentaerythritol), aromatic polybasic acids (phthalic anhydride and phthalic acids are the most common) and a source of fatty acids, usually a vegetable oil. Drying and semidrying oils, such as linseed, soybean and castor oil are used for the production of alkyd resins. Modification of alkyd paints with additives is common in industrial production: fatty acids, acrylic, styrene and silicone compounds, inorganic fillers and driers, and dispersing agents have been reported as possible modifiers [1,7]. The fatty acid portion plays a fundamental role in regulating the properties of alkyd paints, such as cross-linking potential, yellowing tendency, and compatibility with different solvents. The amount and type of oil(s) used in their synthesis is extremely important in order to define the tendency of the paint to dry and its drying speed. Triglycerides, with a high content of polyunsaturated acyl chains, undergo radical-oxidative cross-linking processes and are responsible for the curing of the paint film [1,2,4,5,7–13]. The extent of the curing and oxidation of glycerolipids can be investigated by determining their fatty acid profile by gas chromatography/mass spectrometry (GC/MS) after hydrolysis and derivatization. This approach has been widely applied in the literature to the characterization of traditional lipid paint binders used in art since ancient times, such as egg yolk and drying oils. It has proved a fundamental tool for investigating painting techniques and assessing the state of conservation of paint layers. Two applications of GC/MS methods for the characterization of the fatty acid profile of alkyd resins are described in the literature [6,7]. The first [7] is based on hydrolysis in presence of n-butylamine followed by derivatization with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) and hexamethyldisilazane (HMDS) before the GC/MS injection. The results suggest that GC/MS can be successfully applied to the study of the fatty acid composition of alkyd resins, although the proposed procedure is time consuming due to the hydrolysis step (5 h for the hydrolysis and 30 min for the derivatization). The second method [6] based on GC/MS reported in the literature entails a single step for hydrolysis and derivatization using Meth-Prep II. The results reported in the paper refer to qualitative analysis only, in absence of calibration curves, internal standard and method validation. An alternative approach to gain detailed information on lipid composition is to determine the profile of the triglycerides (TAGs). TAGs are generally profiled by analytical techniques based on HPLC

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coupled with mass spectrometry [14–21]. Such techniques have never been applied to alkyd paint materials. In this paper we describe the full characterization of commercial alkyd paint materials using a multi-analytical approach based on the integration of three different mass spectrometric techniques. We exploited gas chromatography–mass spectrometry (GC/MS), high performance liquid chromatography coupled with electrospray ionization mass spectrometry with a tandem quadrupole-time of flight mass spectrometer (HPLC–ESI-Q-ToF), and flow injection analysis (FIA) in the ESI-Q-ToF mass spectrometer. The developed analytical approach is an useful complement to analysis by Py-GC/MS. On one hand pyrolysis achieves a qualitative characterization of the resins and offers the possibility to identify the polyols used in the synthesis [4,6]. On the other hand, Py-GC/MS cannot be performed as quantitative analysis, and in additions it produces relatively complex chromatograms. Moreover, pyrolysis alters the molecular profile introducing thermal induced reactions as isomerizations, loss of functional groups as hydroxyl functions, chain shortening, etc., leading to the formation of a variety of products not originally present in the resin. This aspect makes analytical pyrolysis not optimal for quantitative analysis and aging studies of lipid materials. Thus, GC/MS after off line hydrolysis is a precious complement to Py-GC/MS in the study of oxidation/degradation state of glycerolipids in general and of alkyds in particular. We used a fully validated GC/MS method to determine the fatty acid profile after a sample treatment entailing fast microwaveassisted saponification. This was followed by a derivatization step with BSTFA. GC/MS analysis enabled us to obtain quantitative information on the fatty acid content and on the presence of oxidation products of acylic chains. The low limits of detection and quantitation of the method allow for identifying minimal amount of fatty acids [22–25]. The aromatic polybasic acids are also characterized by GC/MS in the same chromatographic run. HPLC–ESI-Q-ToF was used to determine the triglyceride profile of the lipid fraction of alkyd paints. TAG profiling enabled us to identify the specific oils used to produce the resins, and to gain information on the strategies used in their synthesis. We were also able to identify individual triglycerides, thus complementing the information on the fatty acid profile obtained by GC/MS. In addition, we tested the applicability of mass spectrometric analysis by flow injection-ESI-Q-ToF. FIA-ESI-Q-ToF fingerprint enabled us to obtain the profile of the glyceride fraction and possible additives in just 2 min. The integrated analytical approach was applied to three commercial oil-based artistic paints: Ferrario Alkyd paint containing three different pigments, Winsor & Newton Griffin alkyd paint (“fast drying oil color”) containing three different pigments, and Kremer alkyd resin oil (no pigment). The detailed molecular characterization of these painting materials is reported. This study is the first step toward the understanding of the molecular features of these materials and represents the basis for future research on aging and curing of alkyd paints in artworks.

2. Materials and methods 2.1. Alkyd paint samples Commercial artistic tubes of Ferrario “Alkyd” and Winsor & Newton “Griffin Fast Drying Oil Colour” paints were analyzed. For Ferrario, three paints were considered, containing, according to the supplier, the pigments iron oxide yellow (PY42), burnt Sienna (PBr7) and ivory black (PBK9). The Winsor & Newton paints

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contained yellow ochre (PY43), cobalt blue (PB28) and ivory black. All the paint tubes were purchased from Zecchi (Florence, Italy). We also analyzed Kremer “alkyd resin oil” (pure resin, no pigment), directly purchased from Kremer Pigmente, GmbH & Co. (Aichstetten, Germany). To test the performance of the analytical method on casestudies, a paint sample (0.8 mg) was collected from the artwork “Salto di qualità”, by Patrizia Zara (Milano, Italy), 2008. According to the author, the artwork was painted using Winsor & Newton Griffin alkyd “Griffin Fast Drying Oil Colour” commercial tubes. 2.2. Materials and reagents The solvents used for the GC/MS analysis were: diethyl ether, n-hexane and isooctane (HPLC/MS grade; Sigma–Aldrich, U.S.A.). N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) containing 1% trimethylchlorosilane used for fatty acid derivatization was purchased from Sigma–Aldrich, U.S.A. Fatty acid solutions were prepared in acetone, and contained lauric acid (4.10 ␮g g−1 ), suberic acid (4.27 ␮g g−1 ), azelaic acid (3.95 ␮g g−1 ), myristic acid (4.11 ␮g g−1 ), sebacic acid (3.85 ␮g g−1 ), palmitic acid (4.39 ␮g g−1 ), linolenic acid (6.50 ␮g g−1 ), linoleic acid (5.20 ␮g g−1 ), oleic acid (6.32 ␮g g−1 ), stearic acid (6.62 ␮g g−1 ), and ricinoleic acid (5.47 ␮g g−1 ). All standard solutions were used to derive calibration curves. The acids were purchased from Sigma–Aldrich, U.S.A. (purity >99%). Tridecanoic acid (purity 99%; Sigma–Aldrich, U.S.A.) solution in isooctane, 139.91 ␮g g−1 , was used as an internal standard for derivatization; hexadecane (purity 99%; Sigma–Aldrich, U.S.A.), solution in isooctane, 142.00 ␮g g−1 , was used as an internal standard for injection. The solvents used for the HPLC and FIA analysis were: isopropanol, n-hexane and methanol (HPLC/MS grade; Fluka, U.S.A.). 2.3. Instruments and methods 2.3.1. Gas chromatography/mass spectrometry GC/MS instrumentation consisted of a Trace GC 2000 chromatographic system equipped with a PTV injection port and coupled to an ITQ 900 ion trap (Thermo Quest, U.S.A.). Samples were injected in splitless mode at 280 ◦ C. GC separation was performed on a fused silica capillary column HP-5MS (J&W Scientific, Agilent Technologies, US, stationary phase 5% diphenyl–95% dimethyl-polysiloxane, 30 m length, 0.25 mm i.d., 0.25 ␮m film thickness). Chromatographic conditions were adapted from [22,23]: initial temperature 80 ◦ C, 2 min isothermal, 10 ◦ C min−1 up to 200 ◦ C, 4 min isothermal for the separation of unsaturated C18 fatty acids and their isomers, 6 ◦ C min−1 up to 280 ◦ C, 40 min isothermal. The helium (purity 99.9995%) gas flow was set in constant flow mode at 1.2 mL min−1 . MS parameters: electron impact ionization (EI, 70 eV) in positive mode; ion source temperature 230 ◦ C; scan range 50–700 m/z; interface temperature 280 ◦ C. The injection volume was 2 ␮L. Peak assignment was based on a comparison with library mass spectra (NIST 1.7, WILEY275). 2.3.2. High performance liquid chromatography/mass spectrometry HPLC–ESI-Q-ToF analyses were carried out using a 1200 Infinity HPLC, coupled with a Quadrupole-Time of Flight tandem mass spectrometer 6530 Infinity Q-ToF detector by a Jet Stream ESI interface (Agilent Technologies, U.S.A.). The HPLC conditions were: Poroshell 120 EC-C18 column (3.0 mm × 50 mm, 2.7 ␮m particle size) with a Zorbax eclipse plus C-18 guard column (4.6 mm × 12.5 mm, 5 ␮m particle size); a flow rate of 0.3 mL min−1 , an injection volume of 1 ␮L and a column

temperature of 45 ◦ C. Separation was achieved using a gradient of methanol (eluent A) and iso-propanol (eluent B). The elution gradient was programmed as follows: 90% A for 5 min, followed by a linear gradient to 90% B in 25 min, then held for 5 min. Reequilibration time for each analysis was 10 min. The ESI operating conditions were: drying gas (N2 , purity >98%): 350 ◦ C and 10 L min−1 ; capillary voltage 4.5 kV; nebulizer gas 35 psig; sheath gas (N2 , purity >98%): 375 ◦ C and 11 L min−1 . High resolution MS and MS/MS spectra were acquired in positive mode in the range 100–1700 m/z. The fragmentor was kept at 200 V, nozzle voltage 1000 V, skimmer 65 V, octapole RF 750 V, and the collision energy for the MS/MS experiments was set at 50 V. The collision gas was nitrogen (purity 99.999%). The data were collected by auto MS/MS acquisition with an MS scan rate of 1.03 spectra s−1 and an MS/MS scan rate of 1.05 spectra s−1 ; only one precursor was acquired per cycle (relative threshold 0.010%). The structures of individual glycerides were identified by interpreting their tandem mass spectra, where the most abundant fragments derive from the combined elimination of two adjacent fatty acid residues. Structural assignation was based on the assumption, made on the basis of literature data, that the formation of sn-1 and sn-3 diacylglycerol ions in the tandem mass spectra is energetically more favored than the loss of the acyl chain in sn-2 position [11,18,26–28]. Fragmentation patterns for the identified triglycerides are reported in Table 1. The following abbreviations were used to indicate acyl substituents in TAGs and DAGs: Rn: ricinoleyl (C18:1,OH ); Ln: linolenyl (C18:3 ); L: linoleyl (C18:2 ); O: oleyl (C18:1 ); S: stearyl (C18:0 ); Po: palmitoleyl (C16:1 ); P: palmityl (C16:0 ); U: undecyl (C11:0 ). The mass axis was calibrated using the Agilent tuning mix HP0321 (Agilent Technologies U.S.A.) prepared in acetonitrile and was also corrected online by tuning with the m/z 121.0509 and 922.0098 from a reference solution in acetonitrile/water of purine and HP0921 reference solution (Agilent Technologies, U.S.A.). MassHunter® Workstation Software (B.04.00) was used to carry out mass spectrometer control, data acquisition, and data analysis [21]. 2.3.3. Flow injection analysis The applicability of FIA-ESI-Q-ToF was tested as a fast fingerprinting technique, in order to evaluate its capacity to obtain information on the TAG profiles directly in hexane extracts without chromatographic separation. FIA-ESI-Q-ToF flow injection analysis (FIA) was carried out using a 1200 Infinity HPLC coupled to a Jet Stream ESI interface with a Quadrupole-Time of Flight tandem mass spectrometer 6530 Infinity Q-TOF (Agilent Technologies, U.S.A.). The eluents were methanol and iso-propanol (90:10); the flow rate was 0.2 mL min−1 and the injection volume was1 ␮L. The ESI and MS operating conditions were the same as for the HPLC [21,29]. 2.4. Sample treatment The analytical procedure for characterizing the paint materials is reported in Fig. 1. In detail, 3–5 mg of each fresh resin were submitted to extraction in an ultrasonic bath at 60 ◦ C with hexane (600 ␮L) for 5 min. Then, • For the GC/MS analysis an analytical procedure successfully applied for the characterization of drying oil was used [22–25]: 200 ␮L of the extract were subjected to saponification assisted by Milestone (U.S.A.) microwaves Ethos One (power 200 W) with 300 ␮L of KOHETOH 10% wt at 80 ◦ C for 60 min. In order to maximize the extraction yield, two solvents were used: the neutral compounds were extracted with n-hexane; the

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Table 1 Observed DAGs and TAGs in the HPLC–ESI-Q-ToF chromatograms of the alkyd resins. Acyl substituent abbreviations: Rn: ricinoleyl (C18:1,OH ); Ln: linolenyl (C18:3 ); L: linoleyl (C18:2 ); O: oleyl (C18:1 ); S: stearyl (C18:0 ); Po: palmitoleyl (C16:1 ); P: palmityl (C16:0 ); U: undecyl (C11:0 ). DAG/TAG

MS1

Assigned structure

Precursor ion

MS2 Formula

Daughter ions

+

[M+Na]+ [M−C18 H33 O3 ]+ [M−C18 H39 O4 +Na]+ [C18 H33 O3 ]+

RnL

657.4506

[C39 H70 O6 +Na]

657.4506 337.2701 377.2615 321.2365

LnL

637.4779

[C39 H66 O5 +Na]+

637.4779 337.2560 335.2560

[M+Na]+ [M−C18 H29 O2 ]+ [M−C18 H31 O2 ]+

LL

639.5032

[C39 H68 O5 +Na]+

639.5032 337.2701 303.2146

[M+Na]+ [M−C18 H31 O2 ]+ [C18 H31 O2 ]+

LO

641.5123

[C39 H70 O5 +Na]+

641.5123 339.2827 337.2670

[M+Na]+ [M−C18 H31 O2 ]+ [M−C18 H33 O2 ]+

823.6515

+

[C50 H88 O7 +Na]

823.6515 639.4877 525.3947 541.3793

[M+Na]+ [M−C11 H21 O2 +Na]+ [M−C18 H33 O3 ]+ [M−C18 H29 O2 +Na]+

[C57 H100 O7 +Na]+

919.8047 639.4935 621.4813 599.4984 321.2374

[M+Na]+ [M−C18 H31 O2 +Na]+ [M−C18 H33 O3 +Na]+ [M−C18 H33 O3 ]+ [C18 H33 O3 +Na]+

[C57 H102 O7 +Na]+

921.8008 643.5197 641.5023 623.5112 321.2362

[M+Na]+ [M−C18 H31 O2 +Na]+ [M−C18 H33 O2 +Na]+ [M−C18 H33 O3 +Na]+ [C18 H33 O3 +Na]+

[C57 H104 O7 +Na]+

923.8025 643.5197 641.5094 603.5273 321.2365

[M+Na]+ [M−C18 H31 O2 +Na]+ [M−C18 H35 O2 +Na]+ [M−C18 H33 O3 ]+ [C18 H33 O3 +Na]+

[C50 H88 O6 +Na]+

805.6313 621.4784 599.5118 525.3947 503.4111

[M+Na]+ [M−C11 H21 O2 +Na]+ [M−C11 H21 O2 ]+ [M−C18 H29 O2 +Na]+ [M−C18 H29 O2 ]+

[C57 H92 O6 +Na]+

895.6777 617.4496 595.4673 301.2104

[M+Na]+ [M−C18 H29 O2 +Na]+ [M−C18 H29 O2 ]+ [C18 H29 O2 ]+

897.6922

[C57 H94 O6 +Na]+

897.6922 619.4641 617.4487 597.4823 595.4663

[M+Na]+ [M−C18 H29 O2 +Na]+ [M−C18 H31 O2 +Na]+ [M−C18 H29 O2 ]+ [M−C18 H31 O2 ]+

873.6940

+

[C55 H94 O6 +Na]

873.6940 617.4607 595.4775 573.4937

[M+Na]+ [M−C16 H31 O2 +Na]+ [M−C16 H31 O2 ]+ [M−C18 H29 O2 +Na]+ [M+Na]+ [M−C18 H29 O2 +Na]+ [M−C18 H31 O2 +Na]+ [M−C18 H29 O2 ]+ [M−C18 H31 O2 ]+

ULnRn

RnLL

RnLO

RnLS

ULnLn

LnLnLn

LLnLn

LnLnP

919.8047

921.8008

923.8025

805.6313

895.6777

LnLL

899.7129

[C57 H96 O6 +Na]+

899.7129 621.4910 617,4479 599.4974 597.4661

LLL

901.7281

[C57 H98 O6 +Na]+

901.7281 621.4826 599.4881

[M+Na]+ [M−C18 H31 O2 +Na]+ [M−C18 H31 O2 ]+

877.7246

[C55 H98 O6 +Na]+

877.7246 621.4775 599.4776 575.4961

[M+Na]+ [M−C16 H31 O2 +Na]+ [M−C16 H31 O2 ]+ [M−C18 H31 O2 ]+

PLL

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Table 1 (Continued) DAG/TAG

MS1

Assigned structure

Precursor ion

LLO

LOP

LLS

OOP

OLS

ORnLL

ORnLO

ORnLS

ORnRnLL

903.7451

879.7449

905.7613

881.7548

907.7764

1182.0142

1184.0146

1186.0187

1463.3857

MS2 Formula

Daughter ions

[C57 H100 O6 +Na]+

903.7451 623.4954 621.4798 601.5169 599.4986

[M+Na]+ [M−C18 H31 O2 +Na]+ [M−C18 H33 O2 +Na]+ [M−C18 H31 O2 ]+ [M−C18 H33 O2 ]+

[C55 H100 O6 +Na]+

879.7449 623.4957 601.5116 599.4796 597.4796 577.5127 575.4982

[M+Na]+ [M−C16 H31 O2 +Na]+ [M−C16 H31 O2 ]+ [M−C18 H31 O2 +Na]+ [M−C18 H33 O2 +Na]+ [M−C18 H31 O2 ]+ [M−C18 H33 O2 ]+

[C57 H102 O6 +Na]+

905.7613 625.5103 621.4778 603.5286 599.4967

[M+Na]+ [M−C18 H31 O2 +Na]+ [M−C18 H35 O2 +Na]+ [M−C18 H31 O2 ]+ [M−C18 H35 O2 ]+

[C55 H98 O6 +Na]+

881.7548 621.4981 597.4981 599.4856 575.5157

[M+Na]+ [M−C16 H31 O2 +Na]+ [M−C18 H31 O2 +Na]+ [M−C16 H31 O2 ]+ [M−C18 H31 O2 ]+

[C57 H104 O6 +Na]+

907.7764 627.5499 625.5229 623.5070 605.5520 603.5404 601.5239

[M+Na]+ [M−C18 H31 O2 +Na]+ [M−C18 H33 O2 +Na]+ [M−C18 H35 O2 +Na]+ [M−C18 H31 O2 ]+ [M−C18 H33 O2 ]+ [M−C18 H35 O2 ]+

[C74 H130 O8 +Na]+

1182.0142 901.7192 621.4788 599.4965 583.4646

[M+Na]+ [M−C18 H29 O2 +Na]+ [C39 H67 O4 +Na]+ [C39 H67 O4 ]+ [C36 H65 O4 +Na]+

[C74 H132 O8 +Na]+

1184.0146 903.7318 623.4787 601.5113 583.4634

[M+Na]+ [M−C18 H31 O2 +Na]+ [C39 H69 O4 +Na]+ [C39 H69 O4 ]+ [C36 H65 O4 +Na]+

[C74 H134 O8 +Na]+

1186.0187 905.7456 625.5081 603.5265 583.4626

[M+Na]+ [M−C18 H33 O2 +Na]+ [C39 H71 O4 +Na]+ [C39 H71 O4 ]+ [C36 H65 O4 +Na]+

[C93 H168 O10 +Na]+

1463.3857 1184.9003 1182.9558 901.7217 864.7070 621.4818 599.5001 583.4669

[M+Na]+ [M−C18 H29 O2 +Na]+ [M−C18 H33 O2 +Na]+ [C57 H99 O6 +Na]+ − [C54 H97 O6 +Na]+ [C39 H67 O4 +Na]+ [C39 H67 O4 ]+ [C36 H65 O4 +Na]+

residual solution was acidified with hydrochloric acid (6 M) and then carboxylic acids were extracted with diethyl ether (200 ␮L, three times). The two extracts (neutral + acid fraction) were combined for GC/MS analysis in order to analyze them in a single chromatographic run evaporated to dryness under nitrogen stream and subjected to derivatization with 20 ␮L of N,O-bistrimethylsilyltrifluoroacetamide (BSTFA), 200 ␮L of isooctane and 5 ␮L of tridecanoic acid solution at 60 ◦ C for 30 min. 5 ␮L of hexadecane solution were added just before injection. • For the HPLC–MS analysis, 200 ␮L of the extract were purified with a water/ethanol 1:2 mixture (50 ␮L) three times (adapted from [20]), dried under a nitrogen stream, diluted with the elution mixture and filtered on a 0.45 ␮m PTFE filter (Grace Davison Discovery Sciences, U.S.A.) just before injection.

• For the FIA analysis, 200 ␮L of the extract were dried under a nitrogen stream, diluted with the elution mixture and filtered on a 0.45 ␮m PTFE filter (Grace Davison Discovery Sciences, U.S.A.) just before injection. 2.5. Quantitative analysis A GC/MS quantitative analysis was performed using calibration curves calculated on the basis of the extract ion chromatograms (extracted m/z: lauric acid 117–257, suberic acid 169–303, azelaic acid 149–317, myristic acid 117–285, sebaci acid 149–331, palmtic acid 117–3131, linolenic acid 117–335, linoleic acid 117–337, oleic acid 117–339, stearic acid 117–341, ricinoleic acid 187–328). The quantitative analysis of undecanoic acid was performed on the basis

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Sample (3-5 mg) Hexane extraction (600 µL)

GC/MS analysis

HPLC-ESI-Q-ToF analysis

FIA-ESI-Q-ToF analysis

Saponification assisted with microwave

Purification with ethanol/water mixture (2:1)

Dried under N 2

Diluted with methanol/iso-propanol (90:10)

Extraction with hexane Dried under N2 Acidification with HCl Diluted with methanol/iso-propanol (90:10)

Extraction with diethyl ether

Synthetic polymers used as additives

Glyceride profile High molecular weight synthesis products

Derivatization with BSTFA

Fatty acid profil e Neutral fraction Aromatic fraction Additives Fig. 1. Analytical procedure for the characterization of alkyd resins.

Table 2 Fatty acid profiles (%) of the Ferrario, Winsor & Newton and Kremer alkyd resins based on GC/MS. The profiles of the reference material are compared to the one of the Patrizia Zara paint sample from the artwork “Salto di Qualità” (2008). The relative standard deviation (RSD%) for all the fatty acids was lower than 3%. Compounds

Undecanoic acid Azelaic acid Sebacic acid Palmitic acid Linolenic acid Linoleic acid Oleic acid Stearic acid Ricinoleic acid a

Lipid fraction (% of individual fatty acids)

Paint sample (2008)

Ferrario

Winsor & Newton

Kremer

– – – 18 – 53 21 8 –

– – – 13 8 28 26 8 16

4a – – 7 – 58 10 7 14

– 35 4 22 – 1 3 16 19

Calculated on the basis of the lauric acid calibration curves.

of lauric acid calibration curves. Replicate analyses were performed to ensure the reliability of the data presented. For the HPLC–ESI-Q-ToF quantitation, relative abundances were calculated on the basis of extract ion chromatograms, considering the isotopic distribution of the molecular species, and normalized to 100%.

The results of GC/MS analysis are reported in Tables 2 and 3. The relative abundances of the identified TAG and DAG species are reported in Table 4. The DAG and TAG distributions are reported as normalized histograms in Fig. 2. The results highlight important differences in the formulation of the three sets of materials.

3. Results and discussion

3.1. Ferrario alkyd resin

This section reports and discusses the results obtained by GC/MS, HPLC–ESI-Q-ToF and FIA-ESI-Q-ToF techniques for the analysis of Winsor & Newton, Ferrario and Kremer paint samples.

The GC/MS total ion chromatogram of a Ferrario paint is reported in Fig. 3. The fatty acid profile of Ferrario resin is characterized by the presence of palmitic, stearic, oleic (cis and trans

Table 3 Molecular markers identified in the aromatic fraction of the Ferrario, Winsor & Newton and Kremer alkyd resins by GC/MS analysis. The characteristics of the reference materials are compared to the ones of the Patrizia Zara paint sample from the artwork “Salto di Qualità” (2008). Compounds

Aromatic fraction Ferrario

1,2-Benzendicarboxylic acid 1,3-Benzendicarboxylic acid

– √

Paint sample (2008) Winsor & Newton √ √

Kremer – –

√ √

J. La Nasa et al. / Analytica Chimica Acta 797 (2013) 64–80

Kremer

– – 1.0 1.1 – – – – – 8.5 8.8 – 13.3 14.5 9.2 19.8 8.0 8.3 6.3 1.1 – – – –

– 0.9 0.6 0.9 – – – – – 18.7 14.5 2.7 21.6 7.1 1.6 18.8 2.7 6.6 1.7 1.6 – – – –

1.4 – 1.5 1.0 0.6 8.6 4.2 5.4 5.2 – – – – 33.7 – 6.6 – 4.4 – – 11.9 3.4 5.7 6.4

Paint sample (2008) – – – – – – – – – 21.9 16.5 2.7 22.1 6.9 1.6 17.5 4.1 3.6 1.4 1.7 – – – –

isomers), and linoleic acids (Tables 2 and 3). The aromatic component is represented by 1,3-benzendicarboxylic acid. The fatty acid fraction reveals that the chemical compositions of the three tubes were the same independently from the pigment: the observed differences are in the range of variability of the method. The HPLC chromatogram of Ferrario alkyd extract is reported in Fig. 4. The first portion of the chromatogram is characterized by the presence of diglycerides, such as LL (m/z 639.5032, [M+Na]+ ), and LO (m/z 641.5123, [M+Na]+ ). The second portion of the chromatogram is characterized by the presence of TAGs. Linolenyl, linoleyl and oleyl are the main acyl substituents. As expected from the literature, the elution order follows the equivalent carbon number (defined by the formula: ECN = CN − 2n, where E is equivalent, CN is the acyl carbon number and n is the number of double bonds [14]): LnLnLn (m/z 895.6777, [M+Na]+ ), LnLnL (m/z 897.6922, [M+Na]+ ), LnLL (m/z 899.7129, [M+Na]+ ), LLL (m/z 901.7281, [M+Na]+ ) and LLO (m/z 903.7451, [M+Na]+ ) elute in this order. At higher retention times, TAGs containing saturated substituents elute. The main TAGs in this group are LLP (m/z 877.7246, [M+Na]+ ), LOP (m/z 897.7449, [M+Na]+ ) and LLS (m/z 905.7613, [M+Na]+ ). HPLC analysis highlighted the presence of conformational isomers of unsaturated fatty acids, most probably formed due to the high temperature conditions applied to achieve transesterification during the synthesis of the commercial product [30,31]. This result is consistent with the detection of both cis and trans isomers of octadecadienoic acid (oleic acid) by GC/MS. With regard to the FIA-ESI-Q-ToF analysis, the fingerprint mass spectrum of Ferrario alkyd resin (Fig. 5a) is dominated by the presence of a Gaussian distribution of peaks around m/z 555.4218 characterized by a m/z of 44 units. This profile can be attributed to polyethylene glycol, a known additive used in the production of alkyd resins: since the main polyethylene glycol’s ion cluster is characterized by a double charge ([M]2+ ), the profile of the polymer is compatible with the one of PEG2000 [32,33]. The TAG fraction is visible in the range 800 and 910 m/z (Fig. 5b). The observed triglycerides can be subdivided into two different clusters on the basis of their m/z: in the first cluster, the main m/z are: m/z 877.7205 (LLP,

60 40 20 0

RnL LnL LL LO ULnRn RnLL RnLO RnLS ULnLn LnLnLn LLnLn LnLnP LnLL LLL PLL LLO LOP LLS OOP OLS ORnLL ORnLO ORnLS ORnRnLL

Winsor & Newton

80

Winsor & Newton 100 80 60 40 20 0

RnL LnL LL LO ULnRn RnLL RnLO RnLS ULnLn LnLnLn LLnLn LnLnP LnLL LLL PLL LLO LOP LLS OOP OLS ORnLL ORnLO ORnLS ORnRnLL

Ferrario

100

Kremer 100 80 60 40 20 0

RnL LnL LL LO ULnRn RnLL RnLO RnLS ULnLn LnLnLn LLnLn LnLnP LnLL LLL PLL LLO LOP LLS OOP OLS ORnLL ORnLO ORnLS ORnRnLL

RnL LnL LL LO ULnRn RnLL RnLO RnLS ULnLn LnLnLn LLnLn LnLnP LnLL LLL PLL LLO LOP LLS OOP OLS ORnLL ORnLO ORnLS ORnRnLL

Relative abundances (%)

Relative abundance [%]

DAG/TAG

Ferrario

Relative abundance [%]

Table 4 TAG s and DAGs profiles (%) of the Ferrario, Winsor & Newton and Kremer alkyd resins acquired by HPLC–ESI-Q-ToF. Acyl substituent abbreviations: Rn: ricinoleyl (C18:1,OH ); Ln: linolenyl (C18:3 ); L: linoleyl (C18:2 ); O: oleyl (C18:1 ); S: stearyl (C18:0 ); Po: palmitoleyl (C16:1 ); P: palmityl (C16:0 ); U: undecyl (C11:0 ).

Relative abundance [%]

70

Fig. 2. TAGs distribution in the alkyd resins.

[M+Na]+ ), m/z 879.7250 (LOP, [M+Na]+ ) and m/z 881.7150 (OOP, [M+Na]+ ). The main ions in the second cluster are: m/z 901.7223 (LLL, [M+Na]+ ), m/z 903.7361 (LLO, [M+Na]+ ) and m/z 905.7412 (LLS, [M+Na]+ ). Mass peaks with the lowest intensities are attributed to LnLnLn (m/z 895.6777, [M+Na]+ ), LnLnL (m/z 897.7159, [M+Na]+ ) and LnLL (m/z 899.7159, [M+Na]+ ). At higher m/z values, ion clusters due to the polymer deriving from the transesterification process used for the synthesis were observed [34]. The flow injection analysis shows the absence of residual free polyols and polybasic acids. Comparing the triglyceride profile observed for this resin to a set of reference oils, similarities with linseed oil and soybean are observed [21]. These two plant oils are characterized by a similar composition, and by the presence of consistent amounts of triglycerides containing linolenyl and linoleyl acyl substituents. The relative abundances of the triglycerides in Ferrario resin, and the presence of both LLO, characteristic of soybean oil, and of LnLnLn, characteristic of linseed oil, suggest that the alkyd resins was produced from a mixture of these two raw materials [21].

IS1

IS2

stearic acid

palmic acid

Relave abundance (%)

100

12

71

linoleic acid oleic acid (cis) oleic acid (trans)

1,3-benzenedicarboxylic acid

J. La Nasa et al. / Analytica Chimica Acta 797 (2013) 64–80

14

16

18

20 Time (min)

22

24

26

Fig. 3. GC/MS chromatogram of the extract of Ferrario “Alkyd” paint (PY42); IS1: hexadecane, IS2: tridecanoic acid.

The results of HPLC and FIA analysis of paint containing different pigments do not show significant differences.

and ricinoleic acids (Tables 2 and 3). The three selected Winsor & Newton paint tubes show the same fatty acid composition. The aromatic fraction was made up of 1,2-benzendicarboxilyc and 1,3benzendicarboxilyc acids: our procedure allowed us to characterize two different phthalic acid isomers, not detected in previous studies performed by GC/MS [6,7]. Ricinoleic acid is considered in the literature as a molecular marker of castor oil, while linolenic acid is indicative of linseed or soybean oil. The presence of conformational isomers of unsaturated fatty acids is related to heating processes

3.2. Winsor & Newton

4

3

5

6

7

8

9

OOP OLS

LnLnL

2

LnLnLn

LL LO

1

*

LnLL

LLP LOP

LLS

LLL

LLO

The GC/MS chromatogram of a sample of Winsor & Newton paint is reported in Fig. 6. GC/MS analysis highlighted that the hydrolyzed fraction of Winsor & Newton alkyd resin contains palmitic, stearic, oleic (cis and trans isomers), linoleic, linolenic

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Counts vs. Acquisition Time (min)

Fig. 4. HPLC-ESI-Q-ToF chromatogram of the extract of the Ferrario “Alkyd” paint (PY42); the figure was obtained by overlapping the extract ion chromatograms relative to the 12 identified TAG species; (*) contamination.

72

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* 14

16

18

20 Time (min)

22

ricinoleic acid

stearic acid

linoleic acid 12

linolenic acid

palmic acid

1,3-benzenedicarboxylic acid

IS1

IS2

Relave abundance (%)

1,2-benzenedicarboxylic acid

100

oleic acid (cis) oleic acid (trans)

Fig. 5. FIA-ESI-Q-ToF high resolution mass spectrum of Ferrario alkyd resin (PY42) (a) and zoom of the mass range m/z 870–920 (b).

24

26

28

Fig. 6. GC/MS chromatogram of the extract of Winsor & Newton “Griffin Fast Drying Oil Colour” (PY43); IS1: hexadecane, IS2: tridecanoic acid; (*) contamination.

LLL

LLO

73

ox TAG 2

LLn

ox TAG 1 LL LO

OOP OLS

LLP LOP LLS

LnLnP

LnLnLn

LnLnL

LnLL

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Counts vs . Acquisi tion Time (min)

Fig. 7. HPLC-ESI-Q-ToF chromatogram of the extract of Winsor & Newton “Griffin Fast Drying Oil Colour” (PY43); the figure was obtained by overlapping the extract ion chromatograms of 14 identified TAG species.

Fig. 8. Tandem mass spectra and fragmentation pattern of oxidized TAGs characterized by m/z 961.7948 and 945.807.

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x10

5

3

899.7103

2.8 2.6 2.4 2.2

945.8091

2 1.8 1.6 1.4 1.2

961.7964

1 0.8

155.1215

0.6

285.1663 417.3533

639.4963

0.4

1123.8204

801.5378

1388.0454

0.2 0 200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

Counts vs. Mass-to-Charge (m/z)

Fig. 9. FIA-ESI-Q-ToF high resolution mass spectrum of Winsor & Newton alkyd paint (PY43).

applied during the synthesis [35], as mentioned for the Ferrario resin. The HPLC chromatogram of the Winsor & Newton Griffin alkyd resin is shown in Fig. 7. The TAG profile of this resin is very similar to the one observed for the Ferrario resin, except for the presence of oxidized TAGs (Fig. 8) at short retention times. As expected, the first part of the chromatogram is characterized by DAGs followed by the TAG fraction. The TAG distribution (Fig. 2) of this resin differs from the Ferrario distribution in terms of the higher relative abundance of TAGs containing linolenic acid. Although GC/MS analysis highlighted the presence of ricinoleic acid, it is notable that glycerides containing ricinoleyl units are absent in the HPLC–MS chromatogram. The FIA-MS fingerprint mass spectrum of the Winsor & Newton alkyd resin (Fig. 9) shows the presence of the DAG and TAG fractions. The ions m/z 637.4912 (LnL, [M+Na]+ ), m/z 639.4856 (LL, [M+Na]+ ) and m/z 641.5004 (LO, [M+Na]+ ) are characteristic of DAG fractions. The composition of the TAG fraction is very similar to the one of the Ferrario resin, except for two intense ions (945.8091 and 961.7964), characteristic of the oxidized TAGs In the FIA-ESI mass spectrum. In agreement with the HPLC results, no ions at m/z corresponding to glycerides containing the ricinoleyl group were identified. As observed for the Ferrario, ion clusters due to the polymer deriving from the transesterification process used for the synthesis are observed at higher m/z values [34]. The high relative abundance of the triglyceride fraction suggests that the main component of the commercial product is the drying oil. No free polyols and polybasic acids are identified. Flow injection analysis shows the absence of additives, such as polyethylene glycol identified in the Ferrario resins, and the presence of the ion m/z 110.0490 that can be due to presence of 2-butanone oxime, used as anti-skinning additive in the resin formulation [36]. The glyceride profile derived both from the HPLC chromatogram and the FIA mass spectrum shows the absence of DAGs and TAGs including a ricinoleyl unit. In absence of ricinoleyl-triglycerides, the identification of ricinoleic acid in the GC/MS analysis suggests that this fatty acid was mixed as an additive. The purpose of this formulation could be to improve the siccative proprieties of the resin: the drying speed depends on the amount of unsaturated fatty acids incorporated in the alkyd resin. The use of ricinoleic acid for this purpose is reported in the literature and in a patented formulation

[3,15,37]. The absence of ions due to free ricinoleic acid (and free fatty acids in general) in the flow injection mass spectrum is due to the low ionization yield of this acid in the experimental conditions adopted for TAGs and DAGs analysis. Comparing the obtained triglyceride profile of Winsor & Newton alkyd paint to a database of reference oils, the lipid ingredient used in the preparation of the paint material appears to be a mixture of linseed and soybean oil, on the basis of the same considerations made for Ferrario resin [21]. The three selected Winsor & Newton alkyd paints containing different pigments show the same qualitative and quantitative chemical composition. 3.3. Kremer alkyd resin oil The fatty acid profile of the Kremer alkyd resin oil after saponification is reported in Table 2 and the total ion chromatogram in Fig. 10. It is characterized by palmitic, stearic, oleic (cis and trans isomers), linoleic (Z,Z, Z,E and E,E isomers), ricinoleic (cis and trans isomers), and undecanoic acids. The amount of linoleic acid (58%) is particularly high in the GC/MS chromatogram compared to the other materials and to the values reported in the literature for the majority of plant oils. GC/MS analysis highlighted the absence of aromatic compounds, indicating that this material is not a conventional alkyd resin. The HPLC–MS profile of the Kremer alkyd resin oil is reported in Fig. 11. The DAGs RnL (m/z 657.4506, [M+Na]+ ), LL (m/z 639.5032, [M+Na]+ ) and LO (m/z 641.5123, [M+Na]+ ) are shown in the first portion of the chromatogram. The second portion highlights the predominance of TAGs with a ricinoleyl unit as an acyl substituent, characteristic markers of castor oil: RnLL (m/z 919.8047, [M+Na]+ ), RnLO (m/z 921.8008, [M+Na]+ ) and RnLS (m/z 923.8025, [M+Na]+ ). At higher retention times, 12-acyltriglycerides and 1212 -acyltriglycerides (triglyceride estolides) were detected: ORnLL (m/z 1182.0142, [M+Na]+ ), ORnLO (m/z 1184.0146, [M+Na]+ ), ORnLS (m/z 1186.0187, [M+Na]+ ) and ORnRnLL (m/z 1463.3857, [M+Na]+ ). The spectrum of ORnRnLL is reported and interpreted in Fig. 12. The presence of ricinoleic acid in the GC/MS fraction suggests the use of castor oil in the preparation of the resin and is clearly confirmed by the detection of specific TAGs, including the ricinoleyl chain, in the HPLC chromatogram. The absence of triricinolein (RnRnRn, m/z 955.7587, [M+Na]+ ), which is usually very abundant in raw castor oil, and the high

J. La Nasa et al. / Analytica Chimica Acta 797 (2013) 64–80

75

Fig. 10. GC/MS chromatogram of the extract of Kremer alkyd resin oil; IS1: hexadecane, IS2: tridecanoic acid; (*) contamination.

also explains the occurrence of different conformational isomers of different TAGs. In addition ULnRn (m/z 823.6515, [M+Na]+ ) and ULnLn (m/z 805.6313, [M+Na]+ ), triglycerides containing undecanoic acid, are present in the TAG profile of Kremer alkyd resin oil. TAGs containing this acyl substituent have never been observed in a natural oil, and thus can only be formed in the above-mentioned transesterification process after the addition of undecanoic acid in the reaction mixture. Undecanoic acid has been reported as an antifungal additive [38].

2

3

4

5

6

7

8

ORnRnLL

ORnLO ORnLS

LLS

ORnLL

LLO

RnLS + RnLO (conformational isomer)

RnLO (conformational isomer)

LO ULnRn

ULL

RnL

LL

RnLO

LLL

abundance of LLL can be explained by the strong heating treatment used for increasing the siccative proprieties of the resin through dehydration involving the free hydroxyl groups of ricinoleyl substituents. These types of reactions take place at a high temperature, around 150–260 ◦ C [34,35], depending on the type of catalyst; The heating process also caused the transesterification–condensation reactions involving the hydroxyl group of Rn chains, leading to the formation of 12-acyltriglycerides and 12-12 -acyltriglycerides, and DAGs such as RnL, LL and LO as secondary products [3,9]. Heating

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Counts vs . Acquisition Time (min)

Fig. 11. HPLC-ESI-Q-ToF chromatogram of extract of the Kremer alkyd resin oil; the figure was obtained by overlapping the extract ion chromatograms relative to the 14 identified TAG species.

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Fig. 12. Tandem mass spectrum and fragmentation pattern of ORnRnLL sodiate adduct.

The ESI-Q-ToF fingerprint mass spectrum of the Kremer alkyd oil, shown in Fig. 13, appears less complex than those of the Ferrario and Winsor & Newton resins. FIA analysis confirmed the presence of the TAGs, DAGs, 12-acyltriglycerides and 12-12 acyltriglycerides identified by HPLC. The ions at m/z 639.4963 (LL, [M+Na]+ ) and m/z 657.5071 (RnL, [M+Na]+ ) are specific of DAG fractions. The peaks at m/z 805.6313 and m/z 823.6515 are due to ULnLn, [M+Na]+ and ULnRn, [M+Na]+ , respectively. The main triglyceride ion cluster is centered at m/z 901.7360 (LLL, [M+Na]+ ) and m/z 919.7258 (RnLL, [M+Na]+ ). The two clusters centered on the ions m/z 1182.0142 and m/z 1463.2083 correspond to the four and five acyl chain glycerides deriving from castor oil through the above-mentioned dehydratation–transesterification process. In contrast to the Ferrario and Winsor & Newton resins, the mass spectrum acquired by flow injection analysis shows the absence

x10

of the ion clusters characteristic of the alkyd resin polymer, confirming the oil-nature of the resin. Summarizing, the GC/MS profile of Kremer alkyd oil highlighted the presence of oleic, linoleic and ricinoleic acids, which are molecular markers of castor oil, and undecanoic acid. No aromatic fraction was identified. The high amount of linoleic acid in the GC/MS chromatogram, and of linoleyl-containing triglycerides, in the HPLC chromatogram suggest that the material was dehydrated and transesterified at high temperatures. HPLC–MS analysis showed the presence of DAGs, 12-acyltriglycerides and 12-12 -acyltriglycerides not present in castor oil. These products derive from transesterification reactions involving the hydroxyl group of ricinoleyl chains. In conclusion, this paint material is not an alkyd paint, but an industrially modified oil containing castor oil as its main component. The modifications

6

1.5 1.45 1.4 1.35 1.3 1.25 1.2 1.15 1.1 1.05 1 0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

919.7360 901.7258 1182.0142

1199.9748

937.7483

1463.2083

657.5071 639.4963

823.6515 805.6313

250

300

350

400

450

500

550

600

650

700

750

800

850

900

1085.8694

950

1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650

Counts vs . Mass -to-Charge (m/z)

Fig. 13. FIA-ESI-Q-ToF high resolution mass spectrum of Kremer alkyd resin.

ricinoleic acid Hydroxy acids

stearic acid

linoleic acid oleic acid

2-hydroxyazelaic acid

palmitic acid

sebacic acid

*

IS2 1,3-benzenedicarboxylic acid azelaic acid

3-methyl-hexanedioic acid

Relave abundance (%)

IS1

4-hydroxyhex-2-enoic acid

100

77

1,2-benzenedicarboxylic acid

J. La Nasa et al. / Analytica Chimica Acta 797 (2013) 64–80

0 12

14

16

18

20 Time (min)

22

24

26

Fig. 14. GC/MS chromatogram of the extract of the paint sample collected from “Salto di Qualità” (Patrizia Zara, 2008); IS1: hexadecane, IS2: tridecanoic acid; (*) contamination.

4. Analysis of a paint sample The proposed analytical methods have been applied to a paint sample from an artwork, in order to evaluate the performances of this approach on a case study. The sample (0.8 mg) was collected

1

2

3

4

5

6

8

9

LOP LLS OLS

LnLL

LLL LLP LLO

LnLnL

ox TAG ( [M+Na]+=929.8174) 7

LnLnLn

ox TAG (

[M+Na]+=943.7893)

ox TAG ( [M+Na]+=959.7837)

ox TAG ( [M+Na]+=917.7689)

ox TAG 1

ox TAG 2

included a combination of transesterification and dehydration, which leads to a paint with high siccative proprieties. The presence of TAGs containing undecanoic acid, which are not present in nature, suggest that this acid was added during the synthesis.

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

Counts vs . Acquisition Time (min)

Fig. 15. HPLC–ESI-Q-ToF chromatogram of the extract of the paint sample “Salto di Qualità” (Patrizia Zara, 2008): the chromatogram was obtained by overlapping the extract ion chromatograms of the 15 identified TAG species.

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x10 5 1.8 1.7

a

1.6

945.8293

1.5 1.4 1.3 1.2 1.1 1 0.9 0.8

929.8361

0.7 397.2953

0.6 0.5

961.8058

0.4

285.1691

0.3

465.1743 687.2434

0.2

827.7086

0.1

1135.2864

0 100

200

300

400

x10 4

LLL

1.5 1.45 1.4 1.35 1.3 1.25 1.2 1.15 1.1 1.05 1 0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

901.7869

b

500

600

700

800

Counts vs. Mass -to-Charge (m/z)

900

1000

1100

1200

902.7911

LnLnL LnLL

LLO 903.7681

Oxidized TAGs

897.7323 899.7323

LLS

917.7731

905.7637

LnLnLn

913.8184

895.7063

911.7798

904.7712

898.7288

918.7772 914.8205

910.7897

900.7402

912.7953

915.8070

895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918

Counts vs . Mass -to-Charge (m/z)

` (Patrizia Zara, 2008) (a); zoom of the mass range corresponding to Fig. 16. FIA-ESI-Q-ToF high resolution mass spectrum of the extract of the paint sample “Salto di Qualita” the oxidised TAGs (b).

from the paint on canvas “Salto di Qualità” by Patrizia Zara (Italy, 2008). The GC/MS chromatogram of the paint sample is reported in Fig. 14 and the results are summarized in Tables 2 and 3. GC/MS analysis shows the presence of the aromatic acids 1,2benzendicarboxylic and 1,3-benzendicarboxylic acids, and of a fatty acid profile dominated by palmitic, stearic and ricinoleic acid. Dicarboxylic acids (azelaic acid as the most abundant), unsaturated fatty acids, and hydroxylated long chain fatty acids are also present. The chromatographic profile is in agreement with that

of an oxidized alkyd paint. The comparison with the fatty acid profile obtained in the analysis of the commercial paint tubes (Tables 2 and 3) confirms that a Winsor & Newton alkyd paint was used: the sample from Zara paint contains 1,2-benzendicarboxylic and ricinoleic acid, both found in the Winsor & Newton alkyd paint described in Section 3.2. The HPLC profile of the paint sample is reported in Fig. 15. The glycerides profile is characterized by the presence of oxidized TAGs with m/z 917.7689 (PC18,OH C18,OH , [M+Na]+ ), 929.8361 (SSC18,OH , [M+Na]+ ), m/z 945.8293 (SC18,OH C18,OH , [M+Na]+ ),

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961.8058 (C18,OH C18,OH C18,OH , [M+Na]+ ) deriving from the oxidation process of the alkyd resin. A small amount of free, non-crosslinked triglycerides were identified, mainly LnLnLn, LnLnL, LnLL, LLL, LLP, LLO, LOP, LLS and OLS. Comparing the relative abundances of the non-oxidized TAGs to the three reference materials (Table 4), the profile of Zara paint sample shows striking similarities with the Winsor & Newton alkyd paint. In particular, the presence of LnLnP, a marker detected only in the Winsor & Newton Griffin fast drying oil color, supports our identification of the trademark. Our hypothesis was further confirmed by the artist, who provided the information that the artwork was indeed painted using Winsor & Newton Griffin alkyd “Griffin Fast Drying Oil Colour” paint tubes. The triglyceride profile was also characterized by FIA-ESI-Q-ToF. The mass spectrum (Fig. 16a) shows the presence of m/z 929.8361, m/z 945.8293, m/z 961.8058 as main ions, characteristic of saturated oxidized TAGs. In the mass range m/z 890–920 (Fig. 16b) the main observed ions are: m/z 895.7063 (LnLnLn, [M+Na]+ ), m/z 897.7323 (LLnLn, [M+Na]+ ), m/z 899.7323 (LnLL, [M+Na]+ ), m/z 901.7869 (LLL, [M+Na]+ ), m/z 903.7681 (LLO, [M+Na]+ ) and m/z 905.7637 (LLS, [M+Na]+ ). In the mass range m/z 910–920, ions are attributable to unsaturated oxidized TAGs with different number of hydroxyl groups. 5. Conclusions The proposed analytical approach, based on the integration of GC/MS, HPLC–ESI-Q-ToF and FIA-ESI-Q-ToF analyses, proved to be very powerful in fully characterizing the chemical composition of three different sets of industrial oil-based paint materials. The proposed method also enabled us to understand the processes exploited to produce these materials. GC/MS analysis after saponification assisted by microwaves and derivatization enabled us to identify the fatty acid profile and the aromatic fraction of the paint materials. HPLC–ESI-Q-ToF enabled us to determine the triglyceride profiles of the three sets of materials. FIA-ESI-Q-ToF proved capable to yield information on the TAG profiles directly in hexane extracts without chromatographic separation, and to identify additives, even in an oxidized paint layer from an artwork. The results highlight the variety of the chemical compositions of commercial products of this category. Thus, their characterization in artworks appears fundamental in order to plan preventive conservation and choose specific cleaning protocols. The main features of each alkyd resin can be summarized as follows: • The Ferrario Alkyd resin was produced by transesterification in presence of linseed and soybean oil. GC/MS and HPLC analysis revealed the presence of palmitic, stearic, oleic and linoleic acids, and LL, LO, LnLnLn, LLnLn, LnLL, LLL, PLL, LLO, LOP, LLS, OOP and OLS, respectively. FIA analysis showed the presence of PEG used as an additive. The aromatic component was 1,3benzendicarboxylic acids. The three selected resins with different pigments show the same chemical composition. • The Winsor & Newton resin was found to have a very similar glycerolipid composition to Ferrario alkyd, addressing to linseed and soybean oil. GC/MS analysis highlighted the presence of palmitic, stearic, oleic, linolenic, linoleic and ricinoleic acids. However the absence of TAGs containing a ricinoleyl substituent in the HPLC–MS chromatogram suggested that free ricinoleic acid was added to the alkyd resins to improve their physical proprieties. The aromatic components were 1,2-benzendicarboxilyc and 1,3benzendicarboxylic acids. Neither FIA nor HPLC analysis revealed the presence of other additives. The FIA results suggest that the resin was produced by transesterification and the drying oil was

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the main component of the paint tubes. As for the Ferrario resins, no differences in the composition were identified between the three resins with different pigments. • No aromatic fraction was identified in the Kremer alkyd oil, thus suggesting that this material is not an alkyd resin, but an industrially modified oil. GC/MS analysis revealed the presence of oleic, linoleic, ricinoleic and undecanoic acids. The DAG and TAG profiles, including the presence of 12-acyltriglycerides and 12-12 -acyltriglycerides, enabled us to identify castor oil as the main raw material in the production of the oil, whose components were subjected to dehydration and transesterification at a high temperature. The presence of undecanoic acid in the GC/MS profile and of TAGs containing undecanoic acid in the HPLC chromatogram and FIA-ESI-MS mass spectra suggests that this free acid was added during the synthesis. The results obtained by FIA-ESI-Q-ToF showed a high degree of coherency with the TAG profiles obtained by HPLC–ESI-Q-ToF. These results highlight the suitability of this analytical technique for the rapid characterization of triglycerides and polymeric additives in unknown samples, in a shorter analysis time and a simpler instrumental optimization than HPLC separation. The application of the three combined mass spectrometry techniques on the paint sample collected from the artwork “Salto di qualità” by Patrizia Zara (2008) proved the efficiency of the analytical method for the characterization of alkyd resins in real samples. The procedure is thus suitable for the characterization of aged alkyd resins and for discriminating between different trademarks. In conclusion, the proposed analytical approach can be exploited for evaluating the state of conservation of artworks, for aging studies, and for investigating curing and oxidation processes. Acknowledgment The authors gratefully acknowledge the COPAC project “Preventive Conservation of Contemporary Art”, funded by PAR-FAS Regione Toscana 2011–2013, for financial support. References [1] T. Learner, Analysis of Modern Paints, Getty Publication, Los Angeles, 2004. [2] A. Burnstock, K.J.V.D. Berg, S.D. Groot, L. Wijnberg, in: T. Learner (Ed.), Reprints of the Modern Paints Uncovered Conference, Getty, Los Angeles, 2008, pp. 177–188. [3] A. Spyros, J. Appl. Polym. Sci. 88 (2003) 1881–1888. [4] R. Ploeger, D. Scalarone, O. Chiantore, J. Cult. Herit. 9 (2008) 412–419. [5] R. Ploeger, D. Scalarone, O. Chiantore, Polym. Degrad. Stab. 94 (2009) 2036–2041. [6] S. Wei, V. Pintus, M. Schreiner, J. Anal. Appl. Pyrol. (2013), http://dx.doi.org/ 10.1016/j.jaap.2013.05.028 (in press). [7] M.R. Schilling, J. Mazurek, T.J.S. Learner, Modern Paints Uncovered: Proceedings from the Modern Paints Uncovered Symposium, L.A. Getty Conservation Institute, 2007, pp. 129–139. [8] R. Ploeger, S. Musso, O. Chiantore, Prog. Org. Coat. 65 (2009) 77–83. [9] M. Lazzari, O. Chiantore, Polym. Degrad. Stab. 65 (1999) 3003–3313. [10] S.S. Narine, X. Kong, Vegetable Oils in Production of Polymers and Plastics, John Wiley & Sons, Inc., 2005. [11] R.J. Hamilton, C. Kalu, E. Prisk, F.B. Padley, H. Pierce, Food Chem. 60 (1997) 193–199. [12] H. Mutlu, M. Meier, Eur. J. Lip. Sci. Technol. 112 (2010) 10–30. [13] D. Scalarone, M. Lazzari, O. Chiantore, J. Anal. Appl. Pyrol. 58 (2001) 503–512. [14] M. Holˇcapek, P. Jandera, P. Zderadiˇcka, J. Chromatogr. A 1010 (2003) 195–215. [15] J.T. Lin, A. Arcinas, L.R. Harden, C.K. Fagerquist, J. Agric. Food Chem. 54 (2006) 3498–3504. [16] F.J. Palmer, A.J. Palmer, J. Chromatogr. 465 (1989) 369–377. [17] A. Stolyhwo, H. Colin, G. Guiochon, Anal. Chem. 57 (1985) 1342–1354. [18] A. Zeb, M. Murkovic, Eur. J. Lip. Sci. Technol. 112 (2010) 844–851. [19] H.R. Mottram, S.E. Woodbury, R.P. Evershed, Rapid Commun. Mass Spectrom. 11 (1997) 1240–1251. [20] F. Saliu, F. Modugno, M. Orlandi, M.P. Colombini, Anal. Bioanal. Chem. 401 (2011) 1785–1800.

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