Resistively heated Filament pyrolyzers

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. 

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... 

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. 

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). 

There are several construction principles for pyrolyzers, such as with resistively heated filaments, inductively heated, furnace type, and radiatively heated. Detailed descriptions for instrument construction can be found in literature [1] or obtained from instrument manufacturers. The pyrolysis unit usually consists of a controller and the pyrolyzer itself. The controller provides the appropriate energy needed for heating. A simplified scheme of a pyrolyzer based on the design of a flash heated filament system (made by CDS Inc.) is shown in Figure 3.1.1. 

Different practical constructions of a Curie point pyrolyzer are commercially available. In these systems, the sample is put in direct contact with the ferromagnetic alloy, which is usually in the shape of a ribbon that can be folded over the sample forming a sample holder. The sample and its holder are maintained in a stream of inert gas in a similar way as for resistively heated filaments. The housing where the sample and its ferromagnetic holder are introduced is also heated to avoid the condensation of the pyrolysate but without decomposing the sample before pyrolysis. Autosample capabilities for Curie point pyrolyzers are also commercially available (e.g. DyChrom modelJPS-330) [11, 12]. 

The main disadvantage of a resistively heated pyrolyzer results from the fact that the filament must be physically connected to the controller. The temperature control of a resistively heated filament is based on the resistance of the entire filament loop, including the filament and its connecting wires. Anything that damages or alters the resistance of any part of the loop will have an effect on the actual temperature produced by the controller. 

Pyrolyzers have been adapted to provide automatic, imattended control of Py-GC. An early system used precoated pyrolysis wires held in quartz tubes on a turntable. These were sequentially loaded, accurately positioned in the induction coil, pyrolyzed, analyzed by capillary GC, and ejected. An alternative has used an automatic solids injector for samples enclosed in iron foil, and a furnace system has enabled sampling of the Martian surface. Autosampling systems based upon conventional pyrolyzers are now commercially available for resistively heated filaments, microfurnaces, and Curie-point pyrolyzers. One such system... 

Pulse-mode pyrolyzers include resistively-heated electrical filaments or ribbons and radio frequency induction-heated wires [841,842,846,848,849]. The filament or ribbon-type pyrolyzers are simple to construct. Figure 8.45, and typically consist of an inert wire or ribbon (Pt or Pt-Rh alloy) connected to a high-current power supply. Samples soluble in a volatile solvent are applied to the fileutent as a thin film. Insoluble materials are placed in a crucible or quartz tube, heated by a basket-lilce shaped or helical wound filiunent. The coated filament is contained within a low dead volume chamber through which the carrier gas flows, sweeping the pyrolysis products onto the column. The surface temperatui of the filament is raised rapidly from ambient temperature to He equilibrium pyrolysis temperature. This... 

The filament shape commonly used in resistively heated pyrolyzers is either a ribbon or a coil. The sample can be put directly on the filament or in a silica tube that fits in the... 

As with Curie-point systems, the filament of a resistively heated pyrolyzer must be housed in a heated chamber that is interfaced to the analytical device. This interface chamber is generally connected directly to the injection port of a gas chromatograph, with column carrier gas flowing through it. The sample for pyrolysis is placed onto the pyrolysis filament, which is then inserted into the interface housing and sealed to ensure flow to the column (Figure 2.3). When current is supplied to the filament, it heats rapidly to pyrolysis temperatures and the pyrolysate is quickly swept into the analytical instrument. 

Current versions of resistively heated pyrolyzers incorporate small computers to control and monitor the filament temperature. These computers may be used to control the voltage used, adjust for changes in resistance as the filament heats, and compensate for differences when broken filaments are replaced. In addition, instruments have been designed that include photodiodes, which are used by the computer to measure the actual temperature of the filament during a run. Other instruments include a small thermocouple welded directly to the filament for temperature readout, or use the computer to measure the resistance of the filament itself and make adjustments as needed during a program. 

The central advantage of a resistively heated pyrolyzer is that the filament may be heated to any temperature over its usable range, at a variety of rates. This permits the examination of a sample material over a range of temperamres without the need to change filaments for each temperature. A sample may be placed onto the filament, heated to a set point temperature and the products examined, then heated to higher temperatures in a stepwise fashion without ranoving the sample or filament from the analytical device. This ability also allows pyrolysis at tanperatures between the discrete values permitted by Curie-point filaments, and frequently at tanperatures higher than those permitted by furnaces. 

Since the temperature and rate of heating of a resistive filament pyrolyzer are functions of the current, these instruments are frequently used to provide slow... 

Long-term reproducibility is affected by eveutual deterioration of resistive filaments or sample wires. All components exposed to sample during pyrolysis (GC injection port liners and quartz sample tubes if used) often require acid cleaning, solvent washing, and oven drying. Active pyrolyzer elements (coils and ribbons in filament pyrolyzers) can be heated without sample to ranove contamination (lOOO C for 2 sec is usually adequate). Curie-poiut wires are inexpensive enough to be discarded after use. 

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