Figure 15. Temperature/time profiles and Curie-point temperatures for pure Ni, Fe, and Co wires (diameter 0.5 mm) when using a 1.5 kW, 1.1 Mhz h.f. power supply.
temperatures the half-life of a degradation reaction may become long compared with the temperature-rise time. In this situation, pyrolysis may be incomplete when T is reached and thus changing t strongly influences the pyrolysis pattern. On the other hand, a high T , e.g. above 700 or 800°C, may also cause changes in the pyrolysis patterns owing to the strong radiant heating of the reaction tube, and may lead to either evaporation or secondary pyrolysis of compounds condensed on the walls of the tube during the initial pyrolysis stages of the sample.
It should be noted that varying T may be used to enhance or reduce the relative contribution of a particular component or moiety in the pyrolysis mass spectrum of a complex mixture or conjugate. For instance, because of the relative thermolability of carbohydrates in comparison with proteins, carbohydrate signals may be specifically enhanced in spectra of carbohydrate-protein mixtures or glycoproteins by selecting lower pyrolysis temperatures. This is shown in Figure 16 for an oligosaccharide-oligopeptide mixture. In the 358°C spectrum (a) the peptide contributes a characteristic series of fragments at m/z 48 (CHgSH, from the Met-residues), 56 (C^Hg isomers, and acrolein), 94 (phenol, from Tyr), 108 (cresol, from Tyr) and 117 (indole, from Trp), whereas contributions of the Phe-residue (m/z 92, toluene, and 104, styrene) are relatively low. Most of the fragment peaks in the spectrum originate from the carbohydrate constituent. In the 610°C spectrum (b) the peptide sub-pattern (arrows) is much more pronounced with respect to the carbohydrate sub-pattern. Moreover, additional information about the peptide constituent is obtained by the increase in relative intensities at m/z 34 (HgS, an additional fragment from the Met residues), 92, 104, 120 (hydroxystyrene, additional fragment from Tyr) and 131 (methylindole, from Trp). The relative intensities of high mass range carbohydrate fragments (m/z 112, 114, 124, 126, 144) have decreased.
The tj is determined by the diameter of the wire, the composition of the alloy, the strength of the high frequency field and the field frequency (ref. 104). Since under practical Py-MS conditions pyrolysis takes place before the wire reaches T , eq the heating rate is generally thought to have a critical influence on the pyrolysis patterns (ref. 105). Unexpectedly, however, varying the heating rate by a factor of 10 (changing ty of a 510°C wire from 0.1s to 1.0s) does not appreciably change the Py-MS patterns of glycogen and albumin (refs. 100, 101). Therefore, the emphasis placed on accurate reproduction of the temperature/time profile in the pyrolysis gas chromatography literature (e.g. refs. 10, 11, 106, 107) does not seem to be as stringent under vacuum pyrolysis conditions, provided that T is neither too low nor too high. Other factors, such as the choice of the filament cleaning technique and of the splvent, and factors that govern transfer of the pyrolysate to the ion source or influence ionisation conditions, appear to determine the reproducibility of Py-MS more strongly than the temperature/time profile, especially with respect to long-term reproducibility (refs. 100, 101).
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