Heated Filament Pyrolyzers

In a Curie-point pyrolyzer, an oscillating current is induced into the pyrolysis filament by means of a high-frequency coil. It is essential that this induction coil be powerful enough to permit heating the wire to its specific Curie-point temperature quickly. In such systems, the filament temperature is said to be self-limiting, since the final or pyrolysis temperature is selected by the composition of the wire itself, and not by some selection made in the electronics of the instrument. Properly powered, a Curie-point system can heat a filament to pyrolysis temperature in milliseconds. Providing that wires of the same alloy composition are used each time, the final temperature is well characterized and reproducible. 

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Heated-filament pyrolyzers are often used to analyze lignins (Kratzl et al. 1965, Lindberg et al. 1982, Obst 1983, Gardner et al. 1985, Faix et al. 1987, 1991, Funazukuri et al. 1987, Salo et al. 1989). In this type of analyzer, electric current is passed through a resistance ribbon or coiled wire, both made of platinum. The dissipation of power increases the temperature of the conductor. Heat-up and pyrolysis times are selected from an instrument control. Characteristic parameters of this type of pyrolyzer have been described by Wells et al. (1980) and Wampler and Levy (1987). 

Wells G Voorhees KJ, Futrell JH (1980) Heating profile curves for resistively heated filament pyrolyzers Anal Chem 52 1782-1784... 

INSTRUMENTATION USED FOR PYROLYSIS - Resistively heated filament pyrolyzers... 

Resistively heated filament pyrolyzers have been used for a long time in polymer pyrolysis [1], The principle of this type of pyrolyzer is that an electric current passing through a resistive conductor generates heat in accordance with Joule s law ... 

There are several advantages of the resistively heated filament pyrolyzers compared to other types. They can achieve very short TRT values, the temperature range is large, and Teq can be set at any desired value in this range. Several commercially available instruments are capable of performing programmed pyrolysis, and autosampling capability is also available (such as the CDS AS-2500). 

Another problem with filament pyrolyzers is the possibility that the filament may be non-uniformly heated over its length. This may determine different Tgq s in different points of the filament. If the sample is not always placed in the same point of the filament in repeated experiments, this may introduce a rather drastic reproducibility problem. In spite of these disadvantages, the resistively heated filament pyrolyzers are among the most common ones, and very good reproducibility frequently has been reported. 

The reproducibility of the results for heated filament pyrolyzers and Curie point pyrolyzers as well as the comparison between the two systems was reported for a number of materials [41], The reproducibility of the analysis was evaluated both qualitatively and quantitatively. It was found that for most samples the results are obtained with very good reproducibility for the same instrument. However, differences in the instrumentation may play an important role regarding the dissimilarity of the results, even when they are operated at comparable parameters. These differences are typically less pronounced between filament pyrolyzers and Curie point pyrolyzers. Also, microfurnace pyrolyzers are closer to filament pyrolyzers than large furnace ones. On the other hand, laser micropyrolyzers or sealed vessel furnace pyrolyzers may lead to quite different results. 

TABLE 4.2.1. The isoprene/dipentene ratio as a function of temperature for the pyrolysis of Kraton 1107 in an inductively heated (Curie point) or a resistively heated filament pyrolyzer. 

Like the Curie-point instruments, resistively heated filament pyrolyzers operate by taking a small sample from ambient to pyrolysis temperature in a very short time. The current supplied is connected directly to the filament, however, and not induced. This means that the filament need not be ferromagnetic, but that it must be physically connected to the temperature controller of the instrument. Filaments are generally made of materials of high electrical resistance and wide operating range and include iron, platinum, and nichrome. ... 

FIGURE 2.3 Resistively heated filament pyrolyzer installed on gas chromatograph injection port. 

When a resistively heated filament pyrolyzer (e.g., the Pyroprobe from CDS Analytical, Inc., Oxford, PA) is used, the sample may be placed directly on a platinum filament or in a quartz tube or boat inside a platinum coil. In either case, the placement of sample with respect to the sampling tube or the ribbon should be the same for all samples. For liquid sample suspensions placed on a ribbon or in a coil, the solvent is evaporated prior to pyrolysis. Solid microbial samples can be sandwiched between quartz wool plugs inside the quartz sampling tube so as to reduce extraneous nonvolatile material from leaving the sampling tube during pyrolysis. With quartz, the sample never comes into direct contact with the pyrolyzer filament, as it does when sample is coated directly on a thin ribbon filament. Ribbon filaments sometimes exhibit a memory effect (particularly with polar components), are harder to clean, and typically have a shorter lifetime. Quartz tubes may be reused after cleaning. 

With resistively heated-filament pyrolyzers, the sample is placed on a ribbon that is heated by the passage of an electric current. Fixed voltages provide poor control. With low voltages very long TRTs (10-30 s) result, whereas at higher voltages TRTs are decreased... 

Figure 4 Advanced boosted heated-filament pyrolyzer with thermocouple feedback. The chromel-alumel thermocouple wires d, 25 pm) attached to the nichrome ribbon have a small thermal capacity with a fast response time. When mounted on the GC, the chamber Is surrounded by a heater jacket. (Reproduced with permission from Lehrle RS, Robb JC, and Suggate R (1982) European Polymer Journals 443-461 Elsevier.)...

Resistively heated-filament pyrolyzers offer the most versatility of the available units. They allow a wide range of programmed temperature and time profiles including stepped pyrolysis. This allows the elucidation of the thermal stability profile of the sample it provides data to allow the kinetic analysis of polymer degradation and may facilitate the identification of unknown samples. 

Pine pitch, beeswax, plant oil [46] Adhesive from Egyptian opus sectile Pt-heated filament pyrolyzer HMDS Py temperature 550°C... 

Wine polyphenols [118] Archaeological wine of the Roman period Pt-heated filament pyrolyzer TM AH Py temperature 450°C... 

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