In conclusion, it can be stated that satisfactory standard reference materials for Py-MS analysis of complex biomaterials do not yet seem to exist. An ideal biomaterial for calibration ("tuning") of the Py-MS systems should exhibit long-term chemical stability and should preferably be completely soluble in a simple solvent with good coating characteristics. Further, it should be sensitive to changes in analytical conditions and it should provide a pyrolysate with components representative of a wide range of chemical classes and molecular sizes. An example of a potentially good standard reference material for Py-MS is lignin. Some lignins are completely soluble in methanol or ethanol (ref. 85); lignins are chemically very stable and provide a wide range of pyrolysis products. Nitrogen-containing products are usually absent, however, and lignin is not highly sensitive to changes in pyrolysis or mass spectrometry conditions. Instant milk powder might be another interesting candidate, as it also is readily soluble and contains a wide range of biological compounds, e.g. nitrogen-containing compounds and lipids. However, the chemical stability of milk powder might prove to be inadequate.
There is no doubt that the field of analytical pyrolysis would benefit greatly if NBS or other internationally recognised organisations would develop a set of carefully homogenised and stored biomaterials, 10 or 100 mg aliquots of which could be made available to the scientific community as standards for analytical pyrolysis.
Our present knowledge of pyrolysis mechanisms in biomaterials is at best very sketchy and does not compare with the level of knowledge of and insight into fragmentation mechanisms occurring during electron impact ionisation. Although Posthumus et at. (refs. 57, 58) have presented strong evidence for the basic simplicity and straightforwardness of pyrolysis mechanisms in selected organic model compounds, which compare favourably with electron impact fragmentation mechanisms in these molecules, the extreme complexity of many biomaterials will undoubtedly prevent the achievement of a satisfactory degree of insight into the precise pyrolysis mechanisms involved. A more detailed overview of our present knowledge of pyrolysis mechanisms in biomaterials is presented in Chapter 3.
A very difficult task is the qualitative interpretation of a single significant mass peak in a pyrolysis mass spectrum. With the exception of a few peaks in the lower mass range, e.g. at m/z 34 (hydrogen sulphide) or m/z 17 (ammonia), the chemical identity of such a peak cannot be established with certainty, let alone its molecular origin. High-resolution mass spectrometry can be used to establish the elemental composition of a peak in a pyrolysis mass spectrum (refs. 53, 71), provided that the pyrolysis experiment can be repeated closely under these conditions. Photoplate registration may be the method of choice if the original spectrum was obtained with a fast-scanning mass spectrometer. Alternatively, a few selected multiplets may be rapidly scanned by varying the electrical potentials on a highresolution magnetic sector instrument and by recording the signals with a signal averager, as described by Freudenthal and Gramberg (ref. 86). High resolution mass spectrometry, of course, does not distinguish between isomeric structures. Application of Py-GC/MS techniques to establish the chemical identity of a Py-MS peak is severely handicapped by differences in pyrolysis conditions and transmission characteristics of the systems.
A promising approach, though requiring extremely specialised instrumentation, is the use of MS/MS for the identification of selected nominal mass peaks. Using a collisional induced dissociation (CID) technique Levsen and Schulten (ref. 75) were able to identify several molecular ions in the low-energy EI mass spectrum of a DNA pyrolysate. Another MS/MS system, especially designed for measuring very short-lasting pyrolysis processes has been described by Tuithof et al. (ref. 76), using simultaneous electro-optic ion detection. With this instrument, a preliminary study of a component in the pyrolysates of Myeobaaterium cells, important for sub-species identification has been described (ref. 80). In this study it was demonstrated that isomeric molecular ions such as those of trimethylamine and propylamine show significantly different collision-induced fragment patterns.
McLafferty (ref. 77) and Todd (ref. 81) constructed a high resolution instrument consisting of two double-focussing mass spectrometers in tandem for the analysis of multicomponent mixtures. A highly interesting development for structure analysis is the tandem mass spectrometer consisting of three quadrupole assemblies recently described by Yost and Enke (ref. 87), and of a double quadrupole system, reported by Siegel (ref. 88). These developments may bring the use of MS/MS techniques within the reach of a much larger number of laboratories during the next few years since commercial versions are already available (refs. 88, 89).
Other promising approaches to a more detailed qualitative analysis of mixtures of pyrolysis products are the use of selective reagent gases in CI-MS systems or the differentiation of pyrolysis products with the same nominal mass but derived from different parent compounds by comparing time-resolved pyrolysis patterns such as those obtained by Risby and Yergey (refs. 19, 20).
Was this article helpful?