Direct Probe Pyms

The first application of this technique to biopolymers may well have been the study of DNA pyrolysed directly in the source of a high resolution magnetic sector instrument reported by Boettger and Kelly (ref. 13) in 1969. This study was primarily motivated by the search for extraterrestrial life, and was published as an extended abstract only. Among the pyrolysis products identified were intact bases. One year later, Charnock and Loo (ref. 14) independently reported an almost identical experiment. A curious aspect of this publication is that the authors completely avoided the use of the term pyrolysis - or equivalent terms - thus creating the misleading impression that the DNA sample was somehow evaporated from the direct probe rather than pyrolysed. This ambiguity has persisted in some of the later literature reports on direct probe Py-MS by other authors, reflecting a basic difficulty in accurately defining the boundaries between evaporation and pyrolysis in practical experimental conditions. As a result, the literature on direct probe mass spectrometry abounds with vaporisation experiments where the larger, less volatile molecules were inadvertently pyrolysed, and with pyrolysis experiments in which the more volatile molecules were undoubtedly evaporated intact.

After 1970, further direct probe Py-MS studies on biomaterials were reported by Wiebers et al. (refs. 15 - 17), Anhalt and Fenselau (ref. 18), Risby and Yergey (refs. 19, 20), Buchhorn et al. (ref. 21) and Luderwald (ref. 22). The work of Wiebers et al. on nucleic acids appears to represent a direct continuation of Charnock and Loo's experiments. The report by Anhalt and Fenselau deals with the differentiation of microorganisms on the basis of lipid patterns in the higher mass range, illustrating the above-mentioned problems of defining the boundaries between pyrolysis and vaporisation. Risby and Yergey's studies opened a new dimension in direct probe Py-MS through the introduction of time-resolved recording of pyrolysis patterns referred to by the authors as linear programmed thermal degradation mass spectrometry (LPTDMS) and applied to the analysis of microorganisms and leukaemic white blood cells. In order to detect large evaporated or pyrolysed molecules -most probably of a lipid nature - they used chemical ionisation rather than electron impact ionisation techniques. Finally, Luderwald and Buchhorn described the analysis of connective tissue samples for the presence of poly(ethyleneterephthalate) particles originating from prosthetic implant devices.

Characteristic features of the above described direct probe Py-MS experiments are the slow heating rates (typically less than l°C/s), the low pyrolysis temperatures (generally below 400°C) and the relatively long residence time of the products in the pyrolysis zone. The slow heating rate technique has the advantage of allowing pyrolysis to occur directly in the ion source, thus avoiding loss of products through adsorption on transfer lines. This enables the technique to be used with slow scanning magnetic sector instruments while still affording time-resolved registration of the pyrolysis patterns. Also, minimal specialised instrumentation is required since direct probe inlets are available with most mass spectrometers. Disadvantages of the approach are considerable contamination of the ion source (which may have an adverse influence on long-term reproducibility), excessive charring because of the slow heating rate (ref. 23) and the occurrence of secondary pyrolysis reactions because of the long residence time of the products in the pyrolysis zone.

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