Figure 18. Schematic diagram of a Curie-point Py-MS system. Typical operating conditions:buffer volume (expansion chamber) temperature 150-175°C; Eei 13-15 eV; ion energy 5-10 eV; mass range m/z 15 to between m/z 130 and m/z 200; scanning speed 8-10 spectra/s; total scanning time 15-30 s.
heated to a temperature high enough to prevent condensation of pyrolysis products from leaving the reaction tube, yet low enough to avoid secondary pyrolysis reactions. A good compromise temperature range appears to be 150-200°C. The temperature should be kept fairly constant in order to avoid variations in ionisation fragmentation patterns due to different initial temperatures of the molecules entering the ionisation region (ref. 101, 108, 109). In the configuration shown in Figure 18, the walls of the reaction tube are heated by radiation from the pyrolysis filament only and are the coldest location in the transfer line. As a consequence, most pyrolysis products with low volatility and also large evaporated compounds, e.g. lipids, will condense on the wall of the reaction tube directly around the pyrolysis zone. Although the loss of such substances leads to a loss of valuable information, the advantage of this situation is that contamination of expansion chamber and ion source is minimal. In fact, a system such as that shown in Figure 18 can be used for over 1 year, analysing more than 1,000 samples, without cleaning the ion source (ref. 100, 101).
Figure 19. Selected ion profiles from time-resolved Py-f1S analysis of an oil-shale kerogen sample. The profiles were obtained by removing the modular expansion chamber, prolonging tT to approximately 6 s. The profiles at m/z 56, believed to represent mainly C4-aikenes, shows at least four overlapping kinetic events. The ion profiles at m/z 34 (H2S+ •) and m/z 64 (S02+- and/or Sg"1") are even more complex. Especially the m/z 64 profile shows many overlapping events, the first of which appears to represent early distillation of elemental sulphur from the sample, thus producing an Sg+" molecular ion signal at m/z 256 as well as a suite of fragment ions, among which the S2+" ion contributes to the ion profile at m/z 64. Conditions: Sample 10 yg (from methanol suspension); Tc 610°C; tj 6 s (estimated); ts 10 s; Eel 20 eV.
A newly designed version of the Py-MS inlet system (ref. 110) incorporates a modular design which allows easy removal of the expansion chamber, thus positioning the reaction tube directly in front of the ion source. In addition, the reaction tube is designed in such a way that the walls around the pyrolysis zone may be preheated to several hundred degrees without heating the sample, which is temporarily kept in a cool portion of the reaction tube. The potentially stronger contamination of the ion source can be controlled by operating the ion source at a slightly higher temperature than the reaction tube, e.g. at 200 and 175°C, respectively. The pressure/time profile of pyrolysis products entering the ion source can be broadened sufficiently by slowing the heating rate of the wire to 100°C/s. As demonstrated in Figure 19 this allows for efficient scanning of the pressure/time profile by the quadrupole, but is still orders of magnitude faster than the heating rates of direct probe systems, thus minimizing excessive char formation. In principle, this configuration combines the advantages of Curie-point pyrolysis (batch processing, easy automation) and fast filament heating (minimal char formation, minimal secondary reactions) with those of direct probe techniques (minimal loss of low-volati1ity pyrolysis products, possibility of obtaining time-resolved pyrolysis profiles).
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