Pyrograms

The average spectrum is defined, without background subtraction, by integrating across the pyrogram peak from the time it first rises above baseline until it returns to within 10% of the baseline. This covers a period of around 45 seconds during a typical acquisition, from scans 75 to 300 in Figure 5.4. 

Figure 25 Total ion chromatogram (pyrogram) of PVC. Reproduced with permission from Wampler [71]. 2005, Perkin Elmer, Inc.

One of the main problems of the pyrolysis technique is related to the low volatility of pyrolysis products arising from natural and some synthetic macromolecules. In fact, the polar acidic, alcoholic and aminic moieties are not really suitable for gas chromatographic analysis. Their poor volatility and their polarity cause a rather low reproducibility of the pyrograms, low sensitivity for specific compounds, and strong memory effects. Memory effects need to be borne in mind when the pyrolysis of polar molecules is performed. Polar pyolysis products may not be completely eluted by the gas chromatographic column, and... 

When natural macromolecules are pyrolysed, a very high fragmentation of the original molecules occurs. This gives rise to pyrograms-which are quite difficult to interpret due to the high amount of unspecific compounds formed. 

To demonstrate these aspects, the different pyrograms obtained from the pyrolysis of aged linseed oil samples using different derivatising agents and different pyrolysers are shown. 

Figure 11.1 shows the pyrogram of lead white pigmented linseed oil paint obtained at 610 °C with a Curie-point pyrolyser, with on-line methylation using 2.5% methanolic TMAH. The pyrolyser was a Curie-point pyrolysis system FOM 5-LX, specifically developed at FOM Amolf Institute (Amsterdam, the Netherlands), to reduce cold spots to a minimum. This means that the sample can be flushed before pyrolysis in a cold zone, and it also ensures optimum pressure condition within the pyrolysis chamber, thus guaranteeing an efficient transport to the GC injection system [12]. 

In the pyrogram obtained in the presence of TMAH, a-methylated fatty and di acids are observed, due to the strong alkalinity of the reagent (as explained above). 

Figure 11.2 shows the pyrograms obtained from an aged linseed oil paint layer obtained with different pyrolysers in the presence of HMDS. Figure 11.2a was obtained with a continuous mode microfurnace pyrolysing injection system Pyrojector (SGE, Austin, Texas, USA) and Figure 11.2b with a CDS Pyroprobe 5000 series (CDS Analytical Inc., Oxford, USA). 

Figure 11.2 Py/silylation GC/MS chromatograms of aged linseed oil pyrolysed in the pre sence of HMDS, (a) Pyrogram obtained with a microfurnace pyrolyser pyrolysis temperature 600 °C furnace pressure 14 psi purge flow 0.5 ml min (b) Pyrogram obtained with a resistively heated filament pyrolyser pyrolyser interface I80°C transfer line 300°C valve oven 290°C. 1, Hexenoic acid, trimethylsilyl ester 2, hexanoic acid, trimethylsilyl ester 3, heptenoic acid, trimethylsilyl ester 4, heptanoic acid, trimethylsilyl ester 5, octenoic acid, trimethylsilyl ester 6, octanoic acid, trimethylsilyl ester 7, nonenoic acid, trimethylsilyl ester 8, nonanoic acid, trimethylsilyl ester 9, decanoic acid, trimethylsilyl ester 10, lauric acid, trimethylsilyl ester 11, suberic acid, trimethylsilyl diester 12, azelaic acid, trimethylsilyl diester 13, myristic acid, trimethylsilyl ester 14, sebacic acid, trimethylsilyl diester 15, palmitic acid, trimethylsilyl ester 16, stearic acid, trimethylsilyl ester...

Pyrolysis in the presence of tetramethylammonium hydroxide (THM)-GC/MS allowed the identification of high- and low-molecular weight components in manila Copal and sandarac fresh and artificially aged samples. The pyrograms showed signals due to the polymer fraction and to free diterpenoids [43]. THM-GC/MS has also been used to determine the molecular composition of Pinaceae resins, allowing the study of fresh, naturally and artificially aged samples [16, 44 46]. 

Figure 11.6 Total ion current pyrogram obtained from 30 jig of bleached beeswax with a resistively heatedpyrolyser at 800°C. FA, fatty acid F C, hydrocarbon X Y, chain length X with Y double bonds. Reprinted from J. Anal. Appl. Pyrol., 50, Asperger et al., 2, 13, Copyright 1999 with permission from Elsevier...

Figure 11.7 Pyrogram of a beeswax sample obtained with a microfurnace pyrolyser at 600°C, in the presence of HMDS. TIC total ion current m/z 117 profile of carboxylic acid trimethylsilyl esters, showing a maximum with palmitic acid m/z 57 profile of hydrocarbons, showing a maximum with heneicosane m/z 103 profile of alcohol trimethylsilyl ethers, showing a max imum with docosanol. For the identification of all peaks, see Bonaduce and Colombini [70]...

Figure 11.9 shows the pyrogram of a paint sample collected from the sixteenth century wall paintings in the Messer Filippo cell of the tower in Spilamberto, Italy. Pyrolysis was performed with a microfurnace pyrolyser, at 600°C, in the presence of HMDS. On the basis of the pyrolysis products found, egg and a carbohydrate material were identified. 

The mural paintings in the Monumental Cemetery of Pisa were detached in a restoration after the Second World War, and were later restored on several occasions. They showed an advanced state of degradation. In Figure 11.10 the pyrograms of two samples collected from the paint surfaces (CSG 13bis from Universal Judgement and CSA 5b from Stories of Holy Fathers ) are compared with the pyrograms of nitrocellulose, starch and arabic gum. 

Figure 11.10 Comparison of extracted ion pyrograms of fragment ion m/z 217 of two samples collected from the paint surfaces of Universal Judgement and Stories of Holy Fathers (Monumental Cemetery of Pisa, Italy, painted by Buffalmacco, fourteenth century) with nitro cellulose, starch and arabic gum. 1, Unidentified compound 2, 1,2,3,5 tetrakis (O TMS) xylo furanose 3, tri (O TMS) levoglucosane 4, isomer of 1,2,3,5 tetrakis (O TMS) xylofuranose. Pyrogram obtained with a resistively heated filament at 60CPC in the presence of HMDS [30]...

Figure 11.11 Pyrogram of a paint sample collected from a decorative frame of the Universal Judgement by Bonamico Buffalmacco (fourteenth century, Monumental Cemetery of Pisa, Italy). Pyrolysis was performed with a microfurnace pyrolyser, at 600°C, in the presence of HMDS. 1, Benzene 2, ethyl acrylate 3, methyl methacrylate 4, acetic acid, trimethyl silyl ester 5, pyrrole 6, toluene 7, 2 methylpyrrole 8, 3 methylpyrrole 9, crotonic acid 10, ben zaldehyde 11, phenol 12, 2 methylphenol 13, 4 methylphenol 14, 2,4 dimethyl phenol 15, benzyl nitrile 16, 3 phenylpropionitrile 17, indole 18, phthalate 19, phthalate 20, ben zyl benzoate HMDS pyrolysis products [27]...

Figure 12.5 Pyrogram of Mowilith 30, a vinyl acetate polymer used in conservation. Peak assignments 1, acetic acid 2, benzene 3, styrene 4, indene 5, 1,2 dihydro naphthalene 6, naphthalene 7, 2 methyl naphthalene 8, 1 methyl naphthalene 9, biphenyl 10, fluorene 11, anthracene...

Figure 12.7 Pyrogram of Laropal K80. Peak assignments 1, cyclohexene 2, cyclohexanol 3, cyclohexanone 4, 2 methyl cyclohexanone 5, 2 methylene cyclohexanone 6, 2 methyl 6 methylene cyclohexanone 7, 2,6 dimethylene cyclohexanone 8, cyclohexenyl cyclohex anol 9, cyclohexenylmethyl cyclohexanone 10, cyclohexanolmethyl cyclohexanone 11,2 methylene 6 cyclohexenylmethyl cyclohexanone 12,13, 2,2 methylenebiscyclohexa none 14,15, 2,2 methylenebiscyclohexanol. Peaks labelled with letters, and having mass spectra with molecular ions m/z—302 and m/z—304, have been assigned to tricyclic cyclohexanone based molecules...

Figure 12.8 Pyrogram of Regalrez 1094. Peak assignments 1, cyclohexane 2, cyclohexene 3, ethenyl cyclohexane 4, ethyl cyclohexane 5, ethylidene cyclohexane 6, 1 methylethyl cyclohexane 7, 1 methylethyl cyclohexene 8, 1 methylethylidene cyclohexane 9, 1,1 dimethylethyl cyclohexane 10, 1,2 propadienyl cyclohexane...

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