analytical mass spectrometry of artists’ acrylic emulsion paints by direct temperature resolved...

Journal of Analytical and Applied Pyrolysis64 (2002) 327–344 www.elsevier.com/locate/jaap

Analytical mass spectrometry of artists’ acrylicemulsion paints by direct temperature resolved

mass spectrometry and laser desorptionionisation mass spectrometry�

Jaap J. Boon a,*, Tom Learner b

a MOLART, FOM Institute for Atomic and Molecular Physics, Kruislaan 407,1098 SJ Amsterdam, The Netherlandsb Tate Gallery, Millbank, London, UK

Received 1 August 2001; accepted 17 January 2002

Abstract

Direct temperature resolved mass spectrometry (MS) is a microanalytical technique toanalyse modern paints by thermal separation and ionisation of organic pigments andpolymeric fractions from a platinum/rhodium filament inside the ionisation chamber of themass spectrometer. Most organic pigments in modern paints are desorbed at lower tempera-tures. Ethyl acrylate/methylmethacrylate or butyl acrylate/methylmethacrylate copolymersused in acrylic emulsion paints produce mono and oligomeric subunits released from thecopolymers at high temperature by pyrolysis. Characteristic low voltage electron ionisationand ammonia chemical ionisation (CI) mass spectra of these copolymers facilitate theiridentification. DTMS of three different commercial acrylic emulsion paints showed low andhigh temperature events that could be related to the presence of organic pigments and theacrylic copolymers. Polyethylene glycols with molecular weight up to 2000 Da were identifiedas additives under ammonia CI conditions. The azo pigments PY3, PY73 and PY74, and thephthalocyanine pigment PG7 reported show molecular ions and a few characteristic frag-ment ions under direct temperature resolved mass spectrometry (DTMS) analytical condi-tions. Yellow azo pigments were identified under DTMSMS conditions by their high energy

� Part of the results have been presented as oral papers at the ICOM-CC Interim Meeting on ModernMaterials held in Cologne from 12–14 March, 2001 and at the ASMS conference for Mass Spectrometryand Allied Topics held in Chicago from 27–31 May, 2001.

* Corresponding author. Tel.: +31-20-608-1234; fax: +31-20-668-4106E-mail addresses: [email protected] (J.J. Boon), [email protected] (T. Learner).

0165-2370/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.

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328 J.J. Boon, T. Learner / J. Anal. Appl. Pyrolysis 64 (2002) 327–344

collisionally induced fragment patterns of their parent ions. Laser desorption ionisation massspectrometry (LDIMS) using a nitrogen laser (337 nm) of an acrylic emulsion paint with fourdifferent organic pigments produced radical cations, protonated or sodiated ions of thepigments. The amide bond in the azo pigment PY3 was photolytically cleaved and produceda specific fragment ion. The trace additive polyethylene glycol was observed preferentiallywhile the acrylic copolymers were transparent. © 2002 Elsevier Science B.V. All rightsreserved.

Keywords: Acrylic emulsion paint; Yellow azo pigment (PY3, PY73 and PY 74); Phthalocyanine green(PG7); Ethyl acrylate/methylmethacrylate and butyl acrylate/methylmethacrylate copolymers; Directtemperature resolved mass spectrometry; Laser desorption ionisation mass spectrometry

1. Introduction

Many 20th century paints contain a wide variety of synthetic organic pigmentswhich are extremely finely dispersed and difficult to identify without advancedanalytical tools. The pigments have been used in a wide variety of syntheticmedia, but are also present in oil paint and watercolour media. Their utilisationis even more prevalent nowadays because of environmental regulations whichlimit the use of toxic inorganic pigments. The polymers or dyes themselves areproduced by very few companies, but their products are mixed into paint bymany different businesses, each with its own ‘recipe’. Apart from the mediumand colouring matter, a paint will also contain fillers, emulsifiers, antioxidants,plasticisers, light stabilisers, biocides and so on. The resulting great diversity inthe composition of the paint product is a challenge for the analytical chemist [1].The synthesis of the pigments however is well documented as well as theirappearance on the market, so the structural diversity and temporal distributionof colouring dyes in the 20th century have potential for dating and authenticitytesting. The same reasoning may apply to the formulation technique, and theappearance and application of additives.

Paints are formulated according to proprietary methods, so the exact composi-tion of commercially available artists’ paints is often unknown. Painters may haveoften complicated the media even more by mixing of media or addition of othercomponents. With time, these paints will undergo ageing processes leading tounknown compositions. The characterisation and identification of paints presentmany analytical difficulties especially in mixtures of paint and samples frompaintings. For the pure components Fourier transform infra red spectroscopy(FTIR) is a suitable analytical method for identification of the compound class oreven a specific dye [2,3]. FTIR may fail to characterise a dye in the presence ofother materials in the paint because of interfering characteristic absorptions.Appreciable quantities of an organic pigment are required so that its diagnostic

329J.J. Boon, T. Learner / J. Anal. Appl. Pyrolysis 64 (2002) 327–344

peaks are not masked by other components in the paint. FTIR can be used tofingerprint most synthetic polymer systems used as paint, but does not giveinformation about small differences in chemical structure of the monomeric units ina polymer nor about the molecular weight distribution and end groups. Therefore,additional techniques must be applied in a more detailed identification procedure.We are exploring various mass spectrometric techniques for this purpose.

Earlier, Learner explored the usefulness of pyrolysis GC and PyGCMS forcharacterisation and identification of modern media, additives and some dyes. Ingeneral many media can be identified in this way by marker compounds [4] buildingon earlier pyrolysis GC and GCMS studies [5–9]. Some fragments of dyes can alsobe recognised in this way [10], while others fail to produce any information [3]. Thismay be caused by the thermal stability of the compounds or the fact that large orpolar molecules do not pass a GC system. In order to minimize compound transferproblems and to meet the challenges of complexity and small sample size, we arenow exploring the potential of direct mass spectrometric methods for characterisa-tion of 20th C paint media. In this approach, the sample is present inside the massspectrometer (MS) where it is subjected to volatilisation, desorption, ablation andpyrolysis on a thermal probe, by lasers or by ion beams before ionisation. Thedesorbed product ions are directly analysed by MS or MSMS. This paper mainlyfocuses on direct temperature-resolved mass spectrometry (DTMS) of acrylicemulsion paints and presents one case of laser desorption ionisation mass spec-trometry (LDIMS).

DTMS is capable of detecting the molecular weight and mass spectrometricfragments of the organic dyes as well as thermally cleaved substructures of thebinding medium in the paint. In DTMS the polymer fractions, additives andcolouring matter are often physically separated: they appear in different tempera-ture windows, because of their different physical chemical state in the sample.DTMS is performed on a resistively heated platinum/rhodium filament probe thatis inserted inside the ionisation chamber and is positioned near the Z-axis (i.e. theion flight path) of the mass spectrometer. This set-up minimizes secondary reactionsduring desorption and thermal dissociation, and improves the detection ofoligomers released by pyrolysis from polymers [11]. Desorption and pyrolysis eventsare directly reflected in the total ion current trace. DTMS only requires about 10 �gof sample for a full analysis of pigments and media, which makes it attractive forpainting studies. This paper presents data on the structural units in the ethylacrylate/methylmethacrylate (EA/MMA) and butyl acrylate/methylmethacrylate(BA/MMA) copolymers of three different commercial water based acrylic emulsionpaints. The colors detected are various yellow azo pigments and phthalocyaninedyes.

LDIMS samples and ionises the paint from a small spot in the laser focus whichmakes this technique very suitable for spatially resolved studies in cross sections ofpaint samples from paintings. A recent application of this approach is a study of across section from a painting by Patrick Caulfield [1,12]. Here, we are exploring thepotential of this technique for the characterisation of organic pigments in acrylicemulsion paints using a nitrogen laser (337 nm) in combination with time of flight


mass spectrometry (TOFMS). Paint is applied on a cellulose TLC plate surfaceglued to the steel surface of the sample probe. The information obtained iscompared with the DTMS data.

2. Experimental

2.1. Materials

‘Lukascryl Helio Yellow’ acyrlic emulsion paint from Lukas (Germany) andHansa Yellow Medium acrylic emulsion paint from Golden Artist Colors (USA)were obtained in 1993. The PY 3 standard was obtained from Winsor and Newton.The PY 73 standard was obtained from Golden. The Permanent Green Light paintwas a Winsor and Newton Finity concentrated acrylic colour obtained commer-cially in 2001. This paint was specified to be an acrylic copolymer emulsion with thepigments PY74, PY3 and PG7. Cellulose coated on TLC glass plates were fromMerck Inc.

2.2. Direct temperature-resol�ed mass spectrometry (DTMS and DTMSMS)

Aliquots of 1 �l of the sample suspension were deposited on the 0.1 mmdiameter, platinum/rhodium (90:10) filament (Drijfhout, The Netherlands) of theDTMS probe. DTMS analysis was performed on a Jeol SX 102A double focusingmass spectrometer with B/E geometry or on the four sector Jeol SX 102-102A(B/E–B/E). The filament was resistively heated in the ion source by ramping thecurrent at a rate of 0.5 A min−1. Using this ramp the temperature was increasedlinearly from ambient to approximately 800 °C in 2 min. Desorbed and pyrolysedmaterial was ionised by 16 eV electron impact ionisation or by ammonia chemicalionisation (ammonia pressure 5×10−4 Pa) and accelerated to 8 kV. The massspectrometer was scanned over a m/z range of 20–1000 Da using a cycle time of0.96 s or over a mass range of 20–2000 Da using a cycle time of 1.6 s. MSMSspectra were obtained with a helium collision gas pressure of 10−3 Pa with a 8 kVacceleration voltage. The MS2 was scanned over 0–400 Da with a cycle time of0.96 s. The resolution of the MS system was set at 1000. Data were processed usingthe Jeol MSMP9020D software on a HP workstation.

2.3. Laser desorption ionisation MS (LDI-MS)

The LDI-MS spectra were obtained on a Bruker Biflex TOFMS (Bruker-FranzenAnalytik GMBH, Bremen, Germany). The instrument uses a nitrogen laser (OEMVSL-337i) with a wavelength of 337 nm (Laser Science Inc., Newton, MA). Thelaser was used with a repetition rate of 2 Hz, a pulse width of 3 ns with a spotdiameter of about 30 m. The laser power was attenuated to 25% delivering 0.06 Wm−2 per pulse. Spectra were obtained by averaging 50 individual spectra, recordedusing delayed extraction, the reflectron mode and an accelerating voltage of 19.6


kV. Data were processed using the XMASS 5.0 software (Bruker DaltonikGMBH, Bremen, Germany).

3. Results and discussion

3.1. DTMS of acrylic emulsions

Modern water-based acrylic emulsion paints are often formulated with thecopolymers EA/MMA (ethylacrylate/methylmethacrylate) or n-BA/MMA (n-buty-lacrylate/methyl methacrylate), although the production of many EA/MMA co-polymer emulsions has diminished in the last decade in favour of the n-BA/MMAemulsions. The pyrolysis of these copolymers leads to the monomeric acrylate andacrylic acid moieties, and several oligomeric subunits of the copolymers [3,4]. Theseproducts have been tentatively identified as far as they have passed the chromato-graphic column of the GCMS systems utilised. The thermal evolution of thecompounds involves a 1,5-H shift to primary radical sites leading to a rapidunzipping process. Fig. 1 presents the DTMS spectra of an EA/MMA copolymeranalysed under ammonia chemical ionisation and low voltage electron ionisationconditions (16 eV). The CI data in Fig. 1A show a series of peaks with a massincrement of 100 Da corresponding to the molecular weights of the ethylacrylate

Fig. 1. DTMS spectra under ammonia chemical ionisation (A) and low voltage electron ionisation (B)of the acrylic emulsion copolymer ethylacrylate/methylmethacrylate (EA/MMA).


Fig. 2. DTMS spectra under ammonia chemical ionisation (A) and low voltage electron ionisation (B)of the acrylic emulsion copolymer butylacrylate/methylmethacrylate (BA/MMA).

(EA) or methylmethacrylate (MMA) unit. The ions at m/z 218, 318, 418, 518, 618,718 and 818 correspond to ammoniated molecular ions [MNH4

+]of the EA/MMAdimers (218), trimers (318), tetramers (418), pentamers (518), hexamers (618),heptamers (718) and octamers (818). The trimer is present in the highest relativeintensity. The monomeric units at m/z 118 are present in a much lower abundancealthough these moieties have a relatively high abundance in PY-GCMS experi-ments. The oligomeric ions have different chemical compositions depending on thesequence of the monomeric unit. For example, the trimers consist of variouspermutations of the EA (short hand: E) and MMA units (short hand: M): EEE,EEM, EME, MEE, MME, MEM, EMM, MMM. These can be separated by GCinto a not completely resolved multiplet of peaks. Their identification on the basisof their mass spectra is not without difficulties (Learner, 1997) [3]. These trimers arethought to reflect sequence blocks in the copolymer. For DTMS, differences areobservable under EI conditions because loss of the alcoholic moiety of the endgroup leads to relatively dominant fragment ions. For example, the trimers with anethyl-ester end group (E) show a strong loss of an ethoxy radical at m/z 255[M(EEE)−45]+, whereas the methyl ester end group looses a methoxy radicalleading to a m/z 269 [M(EEM)−31]+. This is evident in EI spectrum of theEA/MMA copolymer in Fig. 1B. The spectrum shows a homologous series of ionsat m/z 255, 355, 455, 555 and higher corresponding to oligomers with EA endgroups. Their relative abundance compared to the series m/z 269+n(100D) fromMA end groups is relatively high and suggest that there may be a preference forthermal cleavage at an EA-MMA link. The relative abundance of the parent ionsand fragment ions of the monomeric units at m/z 100, 69, 55 and 45 is strong andcan be used as fingerprinting information confirming the presence of the EA andMMA subunits.

The BA/MMA DTMS spectra in Fig. 2 are more complicated and their interpre-tation has been aided by comparison with the BA homopolymer [3]. The sub-units


in this copolymer have different molecular weights. The MMA adds a massincrement of 100 D, but the BA unit adds 128 D to the polymer chain. DTMSunder ammonia CI (Fig. 2A) shows indeed the ammoniated ions of the monomericmoieties at m/z 118 and 146. The ions at m/z 402, 374 and 346 demonstrate that atrimeric oligomer is also a dominant feature in this copolymer. BBB, MBB (orBMB and BBM), MMB (or MBM and BMM) and MMM blocks would lead toions at m/z 402, 374, 346 and 318, respectively. It is striking that the MMM blockproduces very few ions suggesting that these sub-structures are rare in the copoly-mer, unless there are very large differences in ionisation efficiency between theblocks. Another possibility could be that M-blocks in the copolymer are thermallyless stable and more completely depolymerised. The relative intensity of highermolecular weight oligomeric ions (not shown) is very low compared to theEA/MMA copolymer. BB and MB block moieties give ammoniated ions at m/z 274and 246. The electron ionisation spectrum of the BA/MMA copolymer is arelatively complicated fingerprint. Also in this case, the loss of an butoxy groupexplains the ions at m/z 311 [M(BBB)−73]+, m/z 283 [M(BMB)−73]+ and m/z255 [M(MMB)−73]+. Trimers with an end group of a MMA moiety showing amethoxy group loss appear at m/z 325 [M(BBM)−31]+ and 297 [M(BMM)−31]+. Several even ions at m/z 194, 228, 236 and 250 suggest that rearrangementsare prominent under EI. The origin and significance of these ions will be studiedfurther by MSMS and by comparison with Py-GCMS data.

3.2. DTMS of acrylic emulsion paints

A selection of three acrylic emulsion paints from the companies Lukas, Goldenand Winsor and Newton is presented here as typical examples taken from a verylarge set of analysed paints and pigments. The Lukas and Golden sample wasanalysed as dried paint, which was homogenized in a mini glass mortar with a smallvolume of ethanol. An aliquot of the suspension was applied to the filament. TheWinsor and Newton paint was analysed directly from the tube and diluted withwater before application to the probe.

3.3. ‘Helio Yellow ’ acrylic paint (Lukas)

Fig. 3 shows the DTMS TIC and desorption (A) and pyrolysis (B) mass spectraof Lukas acrylic Helio yellow paint. The TIC shows two events: one desorptionevent between scan 35 and 45 and one pyrolysis event between scan 55 and 70. TheEI summation spectrum of scans 55–70 shown in Fig. 3B closely resembles the EIspectrum of the EA/MMA copolymer (see Fig. 1B). The desorption MS data showa multiple parent ion at m/z 394/396/398, which a ratio that suggests a moleculewith two chlorine atoms. The m/z 359 corresponds to a loss of one chlorine radical.The large fragment ion at m/z 127 with an isotopic peak at m/z 129 suggest thatone chlorine is retained on this fragment. Comparison with standard referencespectra of the yellow azo dye PY3 with two chlorine and one nitro-group sub-stituent on its aromatic rings (see Scheme 1 and Fig. 4) confirms the identity of the


yellow colour in the Lukas paint as the PY3 azo pigment. There are several featuresof interest in the 16 eV EI spectrum of PY3. The amide bond is cleaved undertransfer of a hydrogen to the amide NH leading to an ion at m/z 127 (ring B). Acorresponding cleavage adjacent to the carbonyl group producing an ion at m/z 268[M−126] is very small. Rearrangement of the azo bond leads to cleavage ions atm/z 170/172 and m/z 184, but this is a minor phenomenon under low voltage EIconditions. Our observations by DTMS confirm the Py-GCMS data [3,4]. The azopigment PY3 thermally dissociates to a 2-chlorobenzylamine, which is observed byPy-GCMS.

3.4. ‘Hansa Yellow Medium ’ acrylic paint (Golden)

Two events marked A (desorption) and B (pyrolysis) are present in the TIC ofmedium shown in Fig. 5. The summation spectrum of B is clearly from anEA/MMA medium but there are some differences in the relative intensity, if wecompare this spectrum with the low voltage EI spectra in Figs. 1 and 4, suggestingthat differences exist between the emulsions made by different manufacturers. Thesummation spectrum A shows a molecular ion at m/z 390 with a strong isotope atm/z 392 suggesting one chlorine substituent in the molecule. The strong fragmention at m/z 123 is devoid of chlorine. Comparison with a data base of azo pigment

Fig. 3. DTMS under low voltage electron ionisation conditions of acrylic (Lukas) Helio Yellow paintshowing the total ion current (TIC) and selected summation spectra of the desorped pigment (A) andpyrolysed medium (B).


Scheme 1. The basic structure and substituent patterns of a selection of yellow azo pigments.

DTMS spectra lead to the identification of the compound as PY73 (see Scheme 1)with a methoxy group and nitro group as substituents. Fig. 6 presents the DTMS16 eV MS of this compound also obtained from Golden Artist Colors Cleavage ofthe amide bond by transfer of hydrogen is the dominant feature leading to m/z 123(ring B). The m/z 108 results from loss of a methyl radical from the methoxy groupof this fragment ion.

3.5. ‘Permanent Green Light ’ acrylic paint (Finity by Winsor and Newton)

The greenish paint from the Winsor and Newton Finity series was analysedunder low voltage EI and ammonia CI conditions. The scan speed of thisexperiment is slower by about a factor of two, because spectra were obtained overa larger mass range (20–2000 Da). The corresponding number of scans in the TICplot is therefore about a factor a two lower compared to the other data shown. Thespectra of the medium (spectra not shown) resemble those in Fig. 2 and identifiedthe acrylic emulsion as a BA/MMA copolymer. The summation spectrum summed


over the complete TIC in Fig. 7 shows ions up to a multiplet at m/z 1126 withdominating features at m/z 127, 386 and 394. Scan 18–30 in the TIC (chequeredregion) was summed to obtain the mass spectrum shown in Fig. 7C. Ions at m/z127/129, 170/172, 268, 331, 359 and 394/396/398 can be assigned to the yellow azopigment PY3 (compare Figs. 3 and 4). The remaining m/z 386 and m/z 123 suggestthe presence of another pigment. The m/z 394 as well as 386 were investigatedfurther with DTMSMS. The selected precursor ions were collided with 8 kV onhelium gas in the collision cell of the four sector Jeol SX102-102A. The fragmentsresulting from fragmentation of m/z 394 were m/z 359, 127, 351, 331, 268, 266, 181,154, 198 in descending order of relative intensity confirming the identity of theprecursor molecule as PY 3. Precursor ion m/z 386 produced fragments at m/z 123,343, 167, 108, 205, 150, 264, 262, 371 in descending order of relative intensity (Fig.8A). Comparison of this MS with the low voltage DTMS spectrum of azo pigmentPY74 (Scheme 1) shows many similarities (see Fig. 8A and B). The DTMSMSresults were rationalised as follows. The cleavage of the amide bond by transfer ofhydrogen leads to m/z 123. A small corresponding ion at M−122 is observed atm/z 264. Loss of COCH3 to m/z 343 is more often observed in methoxybenzylcontaining compounds and can also be explained by loss of a methyl radical fromthe parent ion followed by CO loss. Rearrangement of the azo bond leads to m/z167 and 180 (includes ring A), and 205 (includes ring B). The inset D in Fig. 7corresponds to scan 40–45 (cross-hatched region) at temperatures beyond thedissociation temperature of the medium which took place between scan 34 and 38.

Fig. 4. DTMS under low voltage electron ionisation conditions of a PY3 azo pigment standard reference(supplier Winsor and Newton Ltd.).


Fig. 5. DTMS under low voltage electron ionisation conditions of Golden’s ‘Hansa Yellow Medium’acrylic paint showing the total ion current (TIC) and selected summation spectra of the desorpedpigment (A) and pyrolysed medium (B).

Fig. 6. DTMS under low voltage electron ionisation conditions of a PY73 azo pigment standardreference (supplier Golden Artists Colors).


The ion cluster at m/z 1126 corresponds to the molecular ion distribution of theperchlorinated phthalocyanine green (PG7) shown in Scheme 2. This compound isremarkably stable showing only a loss of a chlorine radical under our EI ionisationconditions.

Returning to the summation spectrum in Fig. 7A, we note the high intensity ofthe pigment parent ions compared to the fragment ions of the medium. This suggestthat a relatively large amount of the azo pigments was used in the paint formula-tion in order to obtain the desired green colour shade in the paint, presumably toovercome the strong colour and hiding power of the phthalocyanine pigment.

Most of these observations were confirmed, when the paint was analysed underammonia chemical ionisation conditions, but two new features appeared. Anadditional feature was observed in the scan range from 28 to 33. Fig. 9 shows themass spectral data of the summation spectrum. Homologous series of protonatedions are seen with a mass increment of 44 mass units typical for polyethyleneglycols. One series of PEG ions is observed with n=15–27 maximizing at n=21with a hydroxy end group. Another series starts at higher mass and might continuebeyond mass 2000. The observed ion series ranges from m/z 1502–1986 suggestingammoniated PEG ions from n=34–45 with a methoxy group as end group. PEGsare added to the paint as emulsifiers. We have seen similar additions of PEG andalso polypropylene glycols (PPG) in paints from other manufacturers (unpublished

Fig. 7. DTMS under low voltage electron ionisation conditions of Winsor and Newton Finity‘Permanent Green Light’ acrylic paint showing the total summaton spectrum (A), total ion current (TIC)(B) and selected summation spectra of the desorped pigments (C and D).


Fig. 8. DTMSMS under low voltage electron ionisation conditions of precursor ion m/z 386 from theDTMS data of Winsor and Newton Finity ‘Permanent Green Light’ acrylic emulsion paint (A) andDTMS under low voltage electron ionisation conditions of the PY74 azo pigment standard (B).

Scheme 2. Molecular structure of the blue copper phthalocyanine pigment PB15. The green PG7 copperphthalocyanine pigment is perchlorinated.


results). Fig. 10 summarises a number of the observed features observed byDTMS under ammonia CI in a mass thermogram. This diagram shows veryclearly how well the various desorption and pyrolysis events are separated intime. i.e. in temperature. The pigments are traced by their protonated moleculesat m/z 387 (MH+ of PY74), 412 (MNH4+ of PY3) and 1128 (MH+ of PG7).The m/z 402 is a characteristic ammoniated ion of trimers pyrolytically releasedfrom BA/MMA. A further feature is m/z 242 with a very broad distributionover scan range 17–55. The ion is not very prominent in the TIC. The identityof this material is presently unknown, but it is clear that the material desorbspoorly pointing to a rather polar compound. We infer that this compound ispresent as an additive in the paint.

3.6. Laser desorption ionisation MS of W&N Finity Permanent Green Light paint

Laser desorption ionisation was performed on the Bruker Biflex instrumentusing a specially machined sample stage, which could accommodate small TLCplates (5×5 mm2) coated with cellulose. This surface was found to give verygood ion yields with direct LDI using a nitrogen laser (337 nm). Paint wasapplied directly on the cellulose surface and dried at room temperature. Fig. 11shows the LDIMS spectrum of the paint analysed over a mass range from 20 to3000 Da by addition of about 50 laser shots. Part of the smaller ions aredisplayed in the inset. A striking feature of the paint in this spectrum is the two

Fig. 9. Summation spectrum of the TIC range between the main desorption event and the thermaldissociation event of the medium in Winsor and Newton Finity Permanent Green Light acrylic paint.DTMS was performed under ammonia chemical ionisation conditions.


Fig. 10. DTMS mass thermograms of ions from pigments (m/z 387, 412 and 1128), the BA/MMAmedium (m/z 402) and an unknown compound (presumably a paint additive) with a pseudomolecularion at m/z 242. Mass channels are normalised individually. Multiplication factors relate to the relativeintensities of each channel normalised to m/z 412 (×1.0).

PEG ion series (as sodiated ions), which were also observed by DTMS under CIconditions. Ions at m/z 575 correspond to phthalocyanine blue (PB15) and thecluster at 1126 to phthalocyanine green (PG7) (see Scheme 2). The PB15 mustbe present in a very low concentration as this pigment was not identified in theDTMS data of this paint. The azo pigment PY3 shows dominant MH+ andMNa+ ions at m/z 395/397 and 417/419, respectively. A most striking feature ism/z 268/270 present as a cation which is proposed to correspond to a productarising from the photolytic cleavage of the amide bond of the PY3 azo pigment.The minor yellow azo pigment PY74 in this paint has a poor ionisation yieldwith a M+� at 386 and MH+ at 387, although the pure reference compoundgives a strong parent ion yield. The differences observed in the relative intensityof the pigments suggest that there is a structure related selectivity under LDIconditions. There is a large number of still unidentified fragment (?) ion peaks inthe spectrum at lower mass. Further studies are underway to assign those masspeaks. It is our present conclusion that the acrylic medium is completely trans-parent to LDI but that trace additives such as PEG are ionised with highsensitivity.


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4. Conclusions

DTMS and LDIMS are valuable tools for the microscale analysis of complexpaint samples which consists of organic media and pigments. The physical separa-tion achieved by DTMS makes it possible to analyse the pigments and the bindingmedium directly without pre-treatment. This is important because dried and agedmodern paints become completely insoluble. Experiments with dry paint samplesfrom paintings homogenised to a fine suspension in ethanol and analysed by DTMSare successful. DTMSMS is feasible and valuable as an identification tool. Thecombined data from ammonia and electron ionisation experiments are additionalmeans for fingerprinting and identification. The ionisation efficiency of many of thecompounds analysed is not known, thus hampering a more quantitative analysis.Quantitative DTMS analyses with known amounts of pigments added to emulsionsand paints are therefore underway, but quantitative analysis will be more difficultfor dried and aged paints. Ultra violet-LDI at 337 nm seems a relatively inexpensivemethod for qualitative analysis of colouring matter in complex paints. The analysisof paints from the cellulose surface gives better results than analysis from a stainlesssteel surface. LDIMS of paint chips is feasible and gives results comparable topaints that have been dried momentarily. Further studies of a larger number ofpigments with very high molecular weights have been successful and will appear ina PhD thesis and MOLART Report in late 2002 (Wyplosz, in preparation).Application of a MALDI matrix in some cases improves the ion yields and preventsphotolytic cleavage, but matrix ions complicate the identification of unknowns inthe lower mass ranges.

Acknowledgements

This research is performed as part of the approved program no. 28 of FOM, asubsidiary of the Dutch National Science Foundation (NWO) and of the multidis-ciplinary NWO Priority Project MOLART, a project on Molecular Aspects ofAgeing in painted Art, supported by NWO. We thankfully acknowledge thetechnical assistance of J. van der Horst, M. Duursma and B. Marino. The TateGallery is acknowledged for research support and the Leverhulme Trust is thankedfor a grant to support Learner during his PhD research. O. van den Brink isthanked for critical reading of the manuscript.

References

[1] J. Crook, T.J.S. Learner, The Impact of Modern Paints, Tate Gallery, London, 2000, p. 192pp.[2] T.J.S. Learner, Spectroscopy Europe 8 (4) (1996) 14.[3] T.J.S. Learner, The characterisation of acrylic painting materials and their implications for their

use, conservation and stability, PhD thesis in Chemistry, University of London, 1997.[4] T.J.S. Learner, Stud. Conservation (2001) 46, 225.[5] S. Tsuge, S. Haramitsu, T. Horibe, M. Yamoaka, T. Takeuchi, Macromolecules 8 (1975) 721.


[6] W.J. Irwin, J. Anal. Appl. Pyrol. 1 (1979) 3.[7] S. Paul, J. Coatings Technol. 52 (1980) 47.[8] S. Tsuge, H. Ohtani, H. Matsubara, M. Ohsawa, J. Anal. Appl. Pyrol. 11 (1987) 181.[9] S. Yamaguchi, J. Hirano, Y. Isoda, J. Anal. Appl. Pyrol. 12 (1987) 293.

[10] N. Sonoda, Stud. Conservation 44 (1999) 195.[11] J.J. Boon, Int. J. Mass Spectrom. Ion Process. 118/119 (1992) 755.[12] J.J. Boon, N. Wyplosz, B. Marino, M. Duursma, J van der Horst, T.J.S. Learner, ASMS

Proceedings CD-ROM: PDF file A011183, 2001.

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