Pyrolysis fragmentation patterns

The use of PyGC fingerprinting to correlate pyrolysis fragmentation patterns to structural information may be conducted at several levels of sophistication [596,661]. Forensic applications are largely concerned with fingerprint identification of samples (paints, fibres, and adhesives). Other fields of application are in art (identification of forgeries), identification of paints, lacquers and varnishes. In all cases emphasis is placed upon the reliability of comparisons. 

High-vacuum pyrolysis of 2,5-dimercapto-l,3,4-thiadiazole 34 and 2-mercapto-5-methyl-l,3,4-thiadiazole 9 performed between ambient and 800 °C gave products that were trapped by matrix-isolation techniques and characterized by IR spectroscopy. Pyrolysis of the dimercaptothiadiazole 34 gave HNCS, CS2, and HCN (Equation 2), whereas the thiadiazolethione 9 showed a more complex fragmentation pattern forming HNCS, CH3NCS, HCN, and CS2 (Equation 3) <2002J(P2)1620>. 

Fig. 5.2(A) presents the pyrolysis mass spectrum for the soil extract. In previous work (ref. 358,359,365) it was shown that complex organic materials like polysaccharides, proteins, lignins, and soil humic fractions have characteristic peaks yielding a typical pattern, which give preliminary information about the composition of the pyrolysis fragments. Thus, characteristic peaks for polysaccharides were observed at 60, 68, 82, 84, 96, 98, 110, 112, and 126 m/z, which were also present in the soil extract. They were shown to be related to acetic acid, furan, methylfuran, hydroxyfuran, furfural, furfuryl alcohol, methylfurfural, methoxy-methylfuran, and a typical pyrolysis fragment of polysaccharides with hexose and/or deoxyhexose units, respectively.

As part of a study of the pyrolysis and photolysis of some 4//-thiopyrans, mass spectrometry was used to evaluate fragmentation pathways. The fragmentation pattern of 2,6-diamino-3,5-dicyano-4//-thiopyran is representative of the range of compounds evaluated which included some 4-aryl analogues (Scheme 8) <1998RRC163>. 

There was one further problem, namely the 1-0-ethyl-ll-hydroxy-A9-THC was susceptible to pyrolysis at the elevated temperatures of the ion source. This resulted in irreproducible mass spectra. Silylation of the allylic alcohol functionality overcame this difficulty and the resulting electron impact fragmentation pattern was quite simple showing only one major peak, base peak at m/e = 327. The trimethylsilyl ion appeared at m/e =73 (7). 

Mass spectral fragmentation patterns in the spectra of these compounds are in accord with the formation of alkynes. The first step in the fragmentation of 1,2,3-selenadiazoles is the loss of N2 followed by extrusion of selenium and formation of the corresponding alkyne. The abundance of the alkynic ion in the mass spectrum appears to be dependent on the nature of the substituent group present in the selenadiazole. When the alkynic ion cannot be stabilized by the formation of a cation on the adjacent carbon atoms, the abundance of the alkynic ion decreases (10% in the parent compound and zero for 4-f-butyl-l,2,3-selenadiazole). On the basis of the mass spectral pattern it is possible to predict the yield of the alkynic compound formed through pyrolysis or photolysis of a given... 

Wilson, Barnes and Goldsack have reported the mass spectra and pyrolysis products of a series of nitraminopyridines. The dominant fragmentation patterns of 2-nitraminopyridine under electron impact and thermolysis are shown in Eq. (20) and (21). It appears that the two pathways are directly parallel with the exception of the fact that the radicals that result from the thermolysis pick up a hydrogen atom before they are analyzed. 

The mass spectral fragmentation pattern of l,10-diethylbenzo[c]cinnoline (20), (Scheme 2) has been used in an explanation for the dilference in flash vacuum pyrolysis of this compound, which gives a complex mixture containing phenanthrene, and its dimethyl analogue which gives 1,8-dimethylbiphenylene as a major product. The base peak of (20) is that due to phenanthrene, mjz 178 <88JOC4333>. 

Pyrolysis of poly(methyl methacrylate) at low temperature produces monomer, whereas other acrylics fragment with loss of side chains, scission of the chain backbone, elimination or rearrangement of the products. Knowledge of the degradation pathways for particular polymer sequences is required to interpret the fragmentation patterns obtained from pyrolysis (65-70). 

Matsuoka et al. [363] have reported an LPyGC system. Advantages claimed for laser pyrolysis-gas chromatography include relatively simple fragmentation patterns, less secondary reactions, high sensitivity, time saving and good reproducibility. 

Folmer [15, 16] studied the effects of different operating conditions and methods of sample preparation on fragmentation patterns. Clear or translucent samples give reproducible results if mixed with carbon. This laser pyrolysis - gas chromatographic technique is used to identify unknown polymers from the pattern of the breakdown products of their pyrolysis products. 

The combination of the flash vacuum pyrolysis (FVP) technique169 with mass spectrometry proved to be particularly useful in identification and characterization of both the fragmentation/rearrangement patterns, intermediates and/or final products formed (see Section IV.E.l). Usually, no structures are indicated in the mass spectra, although ionization and appearance potential can, in principle, provide structural information. 

---