In the previous steps, sample preparation and sample pyrolysis, the original sample was transformed into a multicomponent mixture of pyrolysis products (the pyrolysate). Reproducible mass spectrometric analysis of this multicomponent mixture requires careful control of several factors, including the transfer conditions between the pyrolysis zone and the .ion source, the ionisation conditions and the subsequent mass analysis and ion detection conditions.
Ideal pyrolysate transfer conditions should allow the pyrolysis products to reach the ionisation zone without any loss, degradation or recombination of products during transfer; in practice these conditions are almost never fulfilled. First, some pyrolysis products tend to remain on the filament in the form of nonvolatile chars and thus are completely inaccessible to further analysis. Presently available evidence indicates that the amount of char formed is inversely proportional to the heating rate (ref. 23) and thus can be minimised by avoiding excessively slow heating rates. Some products volatile enough to escape from the pyrolysis zone may be difficult to transfer to the ionisation zone. These products may have a tendency to condense on the walls of the reaction chamber and/or subsequent transfer lines. Heating these walls to a high enough temperature to avoid condensation may result in further thermal degradation of the products. Obviously, the ideal solution would be to achieve a wall-less transfer of pyrolysis products, that is, by pyrolysing directly in front of the ion source, or by analysing only those molecules which reach the ionisation region without previous wall collisions. However, in practice it is difficult to avoid strong contamination of the ion source under these conditions. If the beam of pyrolysis products is sufficiently collimated to avoid wall contact in the ion source, then contamination may be minimised but the signal intensity will be strongly reduced. Therefore, the glass reaction tube shown in Figure 14 serves two purposes, namely to trap the relatively involatile pyrolysis products which might contaminate the ion source and to obtain maximum signal intensity by producing a forward oriented beam of volatile pyrolysis products which may directly enter the expansion chamber or the ion source.
The expansion chamber shown in Figure 18 serves to broaden the pressure/time profile in the ion source, in order to enable a sufficient number of mass scans to be made to obtain a representative averaged ma,ss spectrum of the pyrolysate. The expansion chamber should have chemically inert walls, e.g. quartz or gold-coated, in order to avoid degradation of pyrolysis products. Moreover, these walls should be z o
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