Journal of Chromatography A, 1149 (2007) 30–37 Derivatisation using m-(trifluoromethyl)phenyltrimethylammonium hydroxide of organic materials in artworks for analysis by gas chromatography–mass spectrometry: Unusual reaction products with alcohols Ken Sutherland ∗ Philadelphia Museum of Art, Conservation Department, Box 7646, Philadelphia, PA 19101, USA Available online 26 December 2006 Abstract The quaternary ammonium reagent m-(trifluoromethyl)phenyltrimethylammonium hydroxide (TMTFTH) has gained widespread use for the derivatisation for GCMS analysis of organic materials found in artists’ materials, such as oils, plant resins and waxes. This paper discusses products formed from reactions of TMTFTH with alcohols – including fatty alcohols, hydroxyacids, terpenes and glycerol – that have diagnostic value in the analysis of samples from artworks. In addition to methyl ethers, (trifluoromethyl)phenyl ethers were formed to variable degrees from the different types of alcohol. The products were identified from their EI mass spectra. An understanding of these multiple reaction products is important for the interpretation of GCMS data from complex samples, especially in the case of polyfunctional alcohols such as glycerol, which forms a number of methyl, (trifluoromethyl)phenyl and mixed ethers. The significance of the reaction products, and the relative advantages of TMTFTH as compared to alternative ammonium and sulfonium methylating reagents, are discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: Quaternary ammonium reagents; TMTFTH; Alcohols; Methylation; (Trifluoromethyl)phenyl ethers; Artists’ materials 1. Introduction The study of the organic materials of artworks has been greatly developed and refined in recent decades for the investigation of artists’ techniques, and for providing an understanding of the conservation history and deterioration of the materials. GCMS in particular has become a powerful technique for the characterisation of natural materials encountered in artworks such as drying oils, waxes and plant resins [1–7]. These materials may be found, often in combination, in paint binding media, coatings (varnishes) or adhesives [8,9]. A variety of reagents have been employed in the chemical preparation of samples to improve the chromatographic behaviour of characteristic components for GCMS analysis. Among these, quaternary ammonium reagents including tetramethylammonium hydroxide (TMAH) and m-(trifluoromethyl) ∗ Tel.: +1 215 684 7559; fax: +1 215 684 7550. E-mail address: [email protected]. 0021-9673/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2006.12.015 phenyltrimethylammonium hydroxide (TMTFTH) have gained popularity for their ability to perform efficient methylation of organic acids without complicated sample pretreatment or extraction steps—an important consideration for samples from artworks which are often very small and complex. In particular, these reagents are used to produce methyl esters of compounds such as fatty acids, which are diagnostic components of drying oils and waxes; and acidic di- and triterpenes in plant resins [4–6,10–15]. Fatty acids bound in the form of triglycerides or wax esters are also converted to their methyl esters [5,10,16]. The reactions are generally agreed to occur via the formation of quaternary ammonium salts of the organic acids, which then decompose in the heated GC inlet (or pyrolysis interface) to give the corresponding methyl esters, along with the tertiary amine (trimethylamine for TMAH, or N,N-dimethyl (mtrifluoromethyl)aniline for TMTFTH) as a byproduct [16–18]. Compared with TMAH, TMTFTH has the advantage that methylation can be carried out at lower temperatures (220–300 ◦ C; TMAH is typically used with a pyrolysis interface at 360 ◦ C or higher). This means that methylation can K. Sutherland / J. Chromatogr. A 1149 (2007) 30–37 be conveniently performed by injection of the sample solution (usually in methanol, or a mixture of methanol with benzene or toluene) into a conventional GC inlet. Furthermore, using TMTFTH, unwanted reactions such as isomerisation of polyunsaturated fatty acids and methylation of multiple sites in organic acids, reported for TMAH [13,15,19–22], are largely avoided [15,16,19]. Evaluations of TMTFTH for the analysis of art materials, and comparisons with other reagents, have focussed on the methylation of organic (fatty and terpene) acids [10,12–15,19,23], and the reactions of alcohols have received less attention. This is despite the fact that numerous types of alcohol – including fatty alcohols, hydroxyacids, terpenes and glycerol – have value as chemical markers in the study of natural organic materials. This paper reports multiple reaction products obtained from the reaction of TMTFTH with different alcohols of these types. The significance of the products with respect to the interpretation of data from complex samples is discussed, as well as the relative advantages of TMTFTH as compared to alternative ammonium and sulfonium methylating reagents: trimethylphenylammonium hydroxide (TMPAH) and trimethylsulfonium hydroxide (TMSH). 2. Experimental 2.1. Materials and reagents Ethanol, 2-propanol, 1-pentanol, 1-decanol, 1-octadecanol, 1-eicosanol, 1-docosanol, 1-tetracosanol, 1-hexacosanol, 1octacosanol, 1-triacontanol, glycerol, ricinoleic acid, menthol and stearyl stearate; all having purity 97% or greater, were purchased from Sigma–Aldrich, St. Louis, MO 63178, USA. Bleached beeswax was obtained from the Gettens Collection of painting materials (#100-C2), Straus Center, Harvard University Art Museums, Cambridge, MA 02138, USA. A sample of linseed oil-based varnish was taken from Moritz Berendt, Elijah in the Desert, Philadelphia Museum of Art, #1978-8-1, Philadelphia, PA 19101, USA. Meth-Prep II (TMTFTH: m-(trifluoromethyl)phenyltrimethylammonium hydroxide, 0.2 M in methanol) was purchased from Alltech Associates, Deerfield, IL 60015, USA; MethElute (trimethylphenylammonium hydroxide, 0.2 M in methanol) from Pierce, Rockford, IL 61105, USA; TMPAH (trimethylphenylammonium hydroxide, 0.2 M in methanol) from UCT, Bristol, PA 19007, USA; TMSH (trimethylsulfonium hydroxide, 0.2 M in methanol) from Macherey-Nagel, D-52355 Düren, Germany. Benzene, EMD OmniSolv HPLC grade, 99.7%, was supplied by VWR, West Chester, PA 19380, USA. 31 bilisation of non-polar analytes [16]. Although this length of pretreatment is more than is needed for the standard materials, the procedure was used for consistency with analysis of samples from artworks, which often require such treatment for solubilisation and efficient reaction. Standards were prepared to give concentrations of approximately 50 ppm (mass/mass) in the reagent solution. The wax and varnish sample sizes were judged empirically to give approximately equal peak responses in the TIC to those for the standards. For complex samples containing insoluble material, the reaction vial was centrifuged at ca. 500 × g for 1 min; 1 l of the sample solution was injected into the GC. 2.3. GCMS analysis An Agilent 6890N GC with 7683 autoinjector was used with a HP-5MS capillary column ((5%-phenyl)-methylpolysiloxane; 30 m, 0.25 mm i.d., 0.25 m df ), interfaced to a 5973N MS (Agilent, Palo Alto, CA 94306, USA). The GC inlet temperature was 300 ◦ C, MS interface 300 ◦ C. The oven was programmed from 50 ◦ C, with a 2 min hold, then increased at 10 ◦ C/min to 300 ◦ C, and held isothermally for 3 min; total run time 30 min. For the beeswax sample, the final temperature was 310 ◦ C, hold time 12 min; total run time 40 min. The inlet was operated in splitless mode, with 0.7 min purge-on time. Helium was the carrier gas (≥99.9995%), with a linear velocity of 45 cm/s. The MS was run in scan mode (m/z 50–600; or a smaller scan range for lower molecular weight compounds) with the source at 230 ◦ C and the quad at 150 ◦ C. Electron ionisation energy was 70 eV. Data were processed using Agilent Chemstation software (v. D.00.00.38). 2.4. Blank analyses of reagents Blank runs of reagent solutions made prior to sample analyses demonstrated a wide variability in quality of the commercially available reagents. To illustrate this variability, Fig. 1 shows total ion chromatograms (TICs) for four reagents tested: TMTFTH, TMPAH (UCT and Pierce), and TMSH. All of these were analysed as supplied, undiluted, as 0.2 M solutions in methanol; chromatograms are displayed to the same scale for comparison. High levels of impurities – including reagent degradation products, as well as various chemical contaminants – are present in several of the reagents. These were found to be highly problematic, interfering with data interpretation in some cases. Although not intended as a rigorous measure of purity (which will also vary from batch to batch), the data highlight a significant practical consideration in selecting suitable commercial reagents. Aside from several reagent peaks eluting early in the temperature programme, the TMTFTH supplied by Alltech (Meth-Prep II) showed the least interfering components of the reagents tested. 2.2. Derivatisation 3. Results and discussion Standard samples, reference materials, and the varnish sample from the Berendt painting were treated by adding a 1:1 mixture of the TMTFTH or TMPAH methanolic solution with benzene; and left to react overnight at room temperature. The benzene is added to the reagent solution to facilitate solu- 3.1. Reactions with n-alcohols Long chain n-alcohols (fatty alcohols) are significant in the study of art materials, due to their occurrence in natural (plant 32 K. Sutherland / J. Chromatogr. A 1149 (2007) 30–37 Fig. 1. Total ion chromatograms from blank analyses of: (a) TMTFTH (Meth-Prep II, Alltech), (b) TMPAH (UCT), (c) TMPAH (MethElute, Pierce), and (d) TMSH (Macherey-Nagel); all reagents 0.2 M in methanol. Chromatograms are displayed to the same scale for comparison. or insect) waxes, and bitumens. To investigate their possible reactions with TMTFTH, sample solutions of C18–30 fatty alcohols were prepared for GCMS analysis. Fig. 2 shows the TIC obtained for 1-octadecanol. The chromatogram exhibits three major peaks: the first two eluting compounds were identified on the basis of their tR and EI mass spectra as the underivatised alcohol (tR 20.3 min) and its methyl ether (tR 19.8 min). This partial methylation is consistent with results previously reported using TMSH [24,25] and TMAH [26–28], with which alcohols were found to convert to their methyl ethers to some degree. The reaction is presumed to be analogous to that of organic acids, via the formation of a quaternary ammonium (or sulfonium) salt with the deprotonated alcohol, and its decomposition in the GC inlet to produce the methyl ether [27]. The compound eluting at tR 24.4 min was identified as the (trifluoromethyl)phenyl (TFMP) ether of the alcohol based on its EI mass spectrum (Fig. 3). Characteristic ions in the spectrum are M+ (m/z 414), [CF3 C6 H4 ]+ (m/z 145), and a prominent peak at m/z 162 corresponding to [CF3 C6 H4 OH]+ , produced by loss of C18 H36 . The 162 ion is formed by a rearrangement characteristic of aromatic ethers [29], and hence is diagnostic for this type of derivative. Fig. 2. Total ion chromatogram for 1-octadecanol treated with TMTFTH: (a) methyl ether, (b) underivatised alcohol, and (c) (trifluoromethyl)phenyl ether. Fig. 3. EI mass spectrum for peak (c) in Fig. 2: 1-octadecanol, (trifluoromethyl)phenyl ether. K. Sutherland / J. Chromatogr. A 1149 (2007) 30–37 33 Fig. 4. Scheme illustrating possible reaction mechanism to form (trifluoromethyl)phenyl ethers. Table 1 Relative proportions of derivatives detected in GCMS analysis of fatty alcohols treated with TMTFTH Alcohol 1-Octadecanol (C18) 1-Eicosanol (C20) 1-Docosanol (C22) 1-Tetracosanol (C24) 1-Hexacosanol (C26) 1-Octacosanol (C28) 1-Triacontanol (C30) M+ of TFMP ether % of reaction producta Alcohol Me ether TFMP ether 414 442 470 498 526 554 582 32.8 23.2 19.0 12.6 8.6 5.1 5.4 32.8 33.1 34.7 36.8 38.2 40.4 42.9 34.4 43.7 46.3 50.6 53.2 54.5 51.7 a Proportions of reaction products, expressed as percentages of the combined quantities of alcohol, methyl ether, and TFMP ether; estimated from peak areas in the TIC. The TFMP moiety of the reagent is intended to enhance the efficiency of methylation, by improving the properties of the leaving group [10,17], and so its substitution to form chemical derivatives is somewhat unexpected. To the author’s knowledge, TFMP derivatives have not been described previously in the GC literature. The reaction to produce the TFMP ether likely occurs by a nucleophilic aromatic substitution, with transfer of the TFMP group to the alcohol, and loss of trimethylamine, see Fig. 4. The strongly electron-withdrawing CF3 group will make the aromatic ring more susceptible to such a nucleophilic attack. Support for this mechanism comes from comparison with results from similar analyses performed with trimethylphenylammonium hydroxide (TMPAH), which has a similar chemical structure to TMTFTH, but lacking the CF3 group on the aromatic ring. From the same series of fatty alcohols, TMPAH yielded predominantly methyl ethers, with only very small quantities of phenyl ethers detected (characterised by a prominent peak at m/z 94 corresponding to [C6 H5 OH]+ ), suggesting that the CF3 group of TMTFTH does play a significant role in the formation of the aromatic ethers. Similar results were obtained for the other fatty alcohols analysed, with variations in the extent of derivatisation observed corresponding to chain length: the formation of both methyl and TFMP ethers increased with chain length, and the largest quantities of unreacted alcohol were found for the shorter chain alcohols. The proportions of the different reaction products are listed in Table 1. The differences in reactivity presumably relate to the chemical properties of the alcohols such as nucleophilicity [30] and acidity [31]. Shorter chain alcohols analysed – 1-decanol, 1-pentanol, 2-propanol and ethanol – formed only trace amounts of TFMP ethers, in relation to the underivatised alcohol (data not shown). The methyl TFMP ether (Mr 176) is consistently present in analyses using TMTFTH, presumably resulting from reaction of TMTFTH with the methanol solvent in which the reagent is supplied, but is often overlooked because of its incorporation into the solvent delay. A similar reaction with the reagent solvent has been reported for TMPAH, to form anisole (methyl phenyl ether) [18,32]. Variations in the proportions of reaction products with TMTFTH were also observed with the use of different GC inlet temperatures, and different ratios of methanol/benzene in the reaction mixture. A discussion of these factors is beyond the scope of this paper, but the results indicate that the reactions will be instrument and method dependent (additional factors may include inlet geometry, nature of sample introduction, etc.). For this reason, the data presented may not be representative for all GCMS set-ups. Alcohols combined with fatty acids in the form of wax esters were found to react with TMTFTH to give similar products to those formed with the free alcohols. A TIC obtained from the analysis of stearyl stearate, the ester of stearic acid and 1octadecanol, is shown in Fig. 5. Together with a peak for methyl stearate, three peaks for 1-octadecanol and its methyl and TFMP ethers are detected, in roughly similar proportions to those found with the free alcohol (Fig. 2). 3.2. Reaction with other types of alcohol An alcohol routinely encountered in the analysis of art materials is glycerol, which is found combined with fatty acids in the form of triglycerides, the major constituents of oils and fats such as the drying oils used as paint binding media. Fig. 5. Total ion chromatogram for stearyl stearate treated with TMTFTH: (a) octadecanol methyl ether, (b) octadecanol, (c) octadecanol (trifluoromethyl)phenyl ether; S, methyl stearate. 34 K. Sutherland / J. Chromatogr. A 1149 (2007) 30–37 Table 2 Compounds identified in GCMS analysis of glycerol Label tR (min) Glycerol derivativea M+ [M–F]+ m/z values of characteristic fragment ions (rel. int., %) 1 2 3 4 5 6 7 8 22.6 19.3 19.1 18.8 18.2 13.6 13.1 12.9 TFMP triether 1,2-TFMP diether 1,3-TFMP diether 1,2-TFMP diether, 3-methyl ether 1,3-TFMP diether, 2-methyl ether Methyl monoether, TFMP monoetherb 1-Methyl ether, 2-TFMP ether 1,2-Methyl diether, 3-TFMP ether 524 380 380 394 394 250 250 264 505 361 361 375 375 231 231 245 362 (2), 201 (100), 187 (32), 175 (92), 145 (55) 218 (25), 187 (40), 175 (68), 162 (96), 145 (100) 218 (11), 189 (28), 187 (27), 162 (100), 145 (35) 232 (7), 187 (100), 175 (19), 145 (37) 201 (61), 187 (59), 175 (92), 145 (65), 71 (100) 187 (51), 159 (28), 145 (22), 75 (100) 187 (27), 162 (100), 145 (41), 89 (45), 75 (56) 187 (85), 159 (45), 145 (30), 89 (100) a The positions of substituents on the glycerol molecule have been tentatively assigned based on chromatographic retention times and relative ion intensities in the mass spectra. b Structural interpretation problematic due to coelution of compound with reagent contaminant. Glycerol is also a component of certain types of alkyd resin, and is sometimes used as a plasticiser in watercolour paints. A TIC obtained from the analysis of a standard sample of glycerol, derivatised using TMTFTH, is shown in Fig. 6. The complexity of the reaction is indicated by the numerous peaks observed in the chromatogram: the various reaction products were identified from their EI mass spectra as methyl, TFMP and mixed ethers of glycerol, in addition to derivatives containing unreacted –OH. The major products and their characteristic ions are listed in Table 2, with numbers corresponding to the peak labels in Fig. 6. To illustrate a typical fragmentation pattern the mass spectrum for the TFMP trisubstituted ether, tR 22.6 min (1), is shown in Fig. 7. Among the characteristic ions are M+ (m/z 524), [M–F]+ (m/z 505), [M–CF3 C6 H4 OH]+ (m/z 362), [CF3 C6 H4 ]+ (m/z 145), and a series of peaks at m/z 175, 187 and 201 corresponding, respectively, to [CF3 C6 H4 OCH2 ]+ , [CF3 C6 H4 OC2 H2 ]+ and [CF3 C6 H4 OC3 H4 ]+ . The multiple products obtained from this single compound using TMTFTH illustrate how data interpretation can become complicated when polyfunctional alcohols such as glycerol are present. However, despite this obvious drawback, the TFMP ethers do have the serendipitous advantage of being easily recognisable from the prominent molecular and characteristic fragment ions in their mass spectra. Hence, although they are essentially byproducts of the intended methylation reaction, they can act as very distinctive markers in complex samples. Fig. 7. EI mass spectrum for peak 1 in Fig. 6: glycerol, (trifluoromethyl)phenyl triether. Other types of alcohol encountered in the analysis of materials of artworks include hydroxyacids – compounds containing both carboxylic acid and hydroxyl groups – which occur in materials such as plant and insect waxes, castor oil, shellac, and as oxidation intermediates in drying oils. Certain types of alcoholic terpene are also found in plant resins and essential oils. Although these types of compound have not been examined in detail in this study, two examples were analysed using TMTFTH: ricinoleic acid, a characteristic hydroxyacid component of castor oil, and menthol, a monoterpene found in essential oils. Both compounds underwent partial reaction to form TFMP ethers, as for the other alcohols studied. Mass spectra of the TFMP ethers are shown in Fig. 8. 3.3. Significance of multiple reaction products Fig. 6. Total ion chromatogram for glycerol treated with TMTFTH: peak labels refer to Table 2. In general, reagents that produce multiple derivatives from individual compounds are undesirable, as they complicate data interpretation, including quantitation; and increase detection limits. For these reasons, and if alcohols are of specific interest in a sample, alternative reagents such as TMPAH and TMSH might be considered in place of TMTFTH. The properties of these reagents have been assessed in various comparative studies [14,33–36]. TMSH in particular has gained popularity in recent years for the analysis of lipids (especially those K. Sutherland / J. Chromatogr. A 1149 (2007) 30–37 35 Fig. 9. Total ion chromatogram for an oil-based varnish sample from a painting by Moritz Berendt, treated with TMTFTH. P, methyl palmitate, S, methyl stearate, di-C8, suberic (octanedioic) acid dimethyl ester, di-C9, azelaic (nonanedioic) acid dimethyl ester, di-C10, sebacic (decanedioic) acid dimethyl ester. Numbered peaks correspond to glycerol derivatives, see Fig. 6 and Table 2. Fig. 8. EI mass spectra for: (a) ricinoleic acid, methyl ester, (trifluoromethyl)phenyl ether and (b) menthol, (trifluoromethyl)phenyl ether. containing polyunsaturated fatty acids) [24,25,33,37–41], including applications in the study of artworks [42]. Despite their disadvantages, a potentially useful aspect of the additional derivatives, as mentioned above, is that the TFMP ethers formed from alcohols using TMTFTH have a number of properties that make them distinctive and diagnostic markers. The presence in their mass spectra of prominent molecular ions and characteristic fragment ions can assist in the identification of unknowns, and in the detection of alcohol derivatives in complex data sets, for example using extracted ion chromatograms. Analyses of complex natural samples can be used to illustrate the drawbacks and advantages of the multiple derivatives formed from alcohols. Fig. 9 shows a TIC obtained from analysis of a TMTFTH treated sample of linseed oil-based varnish, taken from a 19th century painting by the German artist Moritz Berendt, in the collection of the Philadelphia Museum of Art (see Section 2 for details) [43]. The chromatogram shows the characteristic peaks for fatty acids found in drying oils: monocarboxylic acids including palmitic and stearic acids, along with dicarboxylic acids, primarily azelaic (nonanedioic), produced from oxidation of the unsaturated fatty acids in the linseed oil. Fig. 10. Total ion chromatogram for bleached beeswax treated with TMTFTH. FAME, fatty acid methyl ester, HC, straight chain hydrocarbon, FAlc, fatty alcohol, FAlc ME, fatty alcohol methyl ether, FAlc TFMP, fatty alcohol (trifluoromethyl)phenyl ether. 36 K. Sutherland / J. Chromatogr. A 1149 (2007) 30–37 In addition to these peaks, compounds deriving from glycerol are also observed, which are reported in Fig. 9 with labels corresponding to those used for the glycerol reference data in Fig. 6. Under the analysis conditions used, the peaks for glycerol are clearly resolved from those for the fatty acids, and hence do not interfere with their identification and quantitation. The ability to detect glycerol in addition to fatty acids is advantageous, since its presence can potentially give additional information on the type and condition of the oil [44,45]. However, further study is needed to determine whether the formation of glycerol TFMP ethers is reproducible, and has the potential to be used reliably for quantitative comparisons. Fig. 10 shows a portion of the TIC for a sample of bleached beeswax, also treated with TMTFTH. A complicated pattern of peaks is obtained, with different classes of compound present in characteristic series. Fatty acids are detected, as their methyl esters (C16–36, even carbon numbers), together with a sequence of straight chain saturated hydrocarbons (23–33, odd carbon numbers). Fatty alcohols are detected in two series, as their methyl ethers and TFMP ethers (C24–32, even carbon numbers). Underivatised alcohols are also detected at low levels, although with difficulty in some cases due to overlap with other peaks in the chromatogram. Clearly these multiple products further complicate the interpretation of what is already a complex sample. However, the mass spectral properties of the TFMP ethers can be exploited to discern clearly the alcohols present, for example by observation (using extracted ion chromatograms, or selected ion monitoring) of characteristic ions such as m/z 162. This cannot be done as effectively for the underivatised alcohols or their methyl ethers, whose mass spectra do not contain such prominent and characteristic peaks. EI mass spectra for 1-octadecanol and its methyl ether are reported in Fig. 11, to illustrate problems in the discrimination of these compounds, as discussed elsewhere [46]. Neither the alcohol (Fig. 11a) nor its methyl ether (Fig. 11b) gives a significant M+ ion, and both exhibit peaks at m/z 252 and 224. These correspond in the alcohol to [M–H2 O]+ and [M–(H2 O + C2 H4 )]+ , and in the methyl ether to [M–CH3 OH]+ and [M–(CH3 OH + C2 H4 )]+ [24]. Furthermore, the spectra for the alcohol and ether also show similarity to those for other organic compound classes, particularly alkenes [47]. The similarity of the mass spectra of these very different compounds may lead to misinterpretation of data from complex samples, particularly if the components are present at low levels. In contrast, the mass spectrum for the TFMP ether of 1-octadecanol (Fig. 3) is very characteristic and unambiguous. 4. Conclusions The quaternary ammonium reagent m-(trifluoromethyl) phenyltrimethylammonium hydroxide (TMTFTH) was found to undergo partial reaction with a number of different types of alcohol to produce (trifluoromethyl)phenyl (TFMP) ethers, in addition to methyl ethers. The recognition of these additional reaction products is important when interpreting GCMS data from complex samples. Although the formation of multiple reaction products is generally undesirable – with regard to interpretation of complex data sets, quantitation, and detection limits – the TFMP ethers have the advantage of giving very characteristic fragmentation patterns with EI MS, which can make them useful chemical markers for the detection and identification of alcoholic components. The additional products are hence more problematic if GC is used without MS, in which case an alternative reagent such as TMPAH or TMSH may be preferable. The extent to which the different reactions occur is likely to be instrument and method dependent, and further work is needed to understand more fully the mechanisms of the competing reactions leading to the formation of methyl and TFMP ethers, and to evaluate possible ways in which these can be controlled or minimised. Further evaluation and comparative study of ammonium and sulfonium reagents for the study of artists’ materials – including those not yet commercially available [34,35] – will also be valuable. Acknowledgements Fig. 11. EI mass spectra for: (a) 1-octadecanol, and (b) 1-octadecanol, methyl ether. The author is very grateful to Beth Price and Andrew Lins (Philadelphia Museum of Art) for their support and valuable input during this research. Dr. Tianlan Zhang and Dr. Abraham Benderly (Rohm and Haas Company, Springhouse, PA 19477, USA) provided helpful advice with interpretation of experimental data. 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