Pyrolysis filament pyrolyzer

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). 

Ericsson I (1980) Determination of the temperature time profile of filament pyrolyzers J Anal Appl Pyrolysis 2 187-194... 

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 ... 

The true temperature of a sample heated using a filament pyrolyzer can be quite different from the above profile temperature, significantly lower temperatures being recorded inside the samples [4]. In order to obtain a correct Teq, modern equipment uses a feedback controlled temperature system (see e g. [5] for a more detailed description of this type of pyrolyzer). Several other procedures for a precise temperature control of the filament are available, such as the use of optical pyrometry or thermocouples [6, 7], Special pyrolysis systems that allow programmed heated rates at different time intervals also are available [8]. 

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). 

The shape and size of joining elements between the pyrolyzer and the GC influence the dead volumes encountered by the analytes affecting the efficiency of separation. These dead volumes should be kept as small as possible, and for Curie point and filament pyrolyzers this task is readily achieved. However, for microfurnace pyrolyzers, the gas flow and dead volumes may raise some problems [2]. Some special transfer capabilities for the pyrolysate were reported with improved results regarding the transfer, for example using a system similar to that of an "on-column" injector employed to separate high-boiling compounds [3]. In-column pyrolysis [4] also avoids any additional dead volumes in pyrolysate transfer. 

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. 

Because of their simple construction and operation, furnace pyrolyzers are frequently inexpensive and relatively easy to use. Since they are operated isothermally, there are no controls for heating ramp rate or pyrolysis time. The analyst simply sets the desired temperature and, when the furnace is at equilibrium, inserts the sample. Although this simplicity may lose its attractiveness as soon as the analyst requires control over heating rate or time, there are some experiments and sample types that capitalize on the design of a furnace. Liquid, especially gaseous samples, are pyro-lyzed much more easily in a furnace than by a filament-type pyrolyzer. Because filament pyrolyzers depend on applying a cold sample to the filament and then... 

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. 

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. ... 

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. 

Curie-point pyrolyzers are generally not used in this stepwise fashion, since they are limited to one temperature per sample because of the way heating is controlled. Microfumaces, however, have been designed with a separate desorption zone, so that a sample may be manually lowered into a low-temperature zone for a first run, retrieved, and then lowered into the pyrolysis zone for a second run. Filament pyrolyzers are now available with a low-mass, programmable interface zone along... 

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. 

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. 

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. 

Figure 8.45 Apparatus for pyrolysis gas chromatography. A, filament or ribbon-type pyrolyzer and B, Curie-point pyrolyzer. (Reproduced with perm.i ion from ref. 848. Copyright American Chemical society).

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 underlying concept of this method for the synthesis of filamentous carbonaceous nanomaterials is fairly simple. As the temperature rises above a certain limit, which depends on the thermodynamic and kinetic para meters of carbon containing compounds, such as hydrocarbons, such compounds tend to pyrolyze in the air free conditions to form free carbon. For example, the noncatalytic pyrolysis of methane can be achieved at ambient pressure and at temperatures above 900—1000 K to produce soot (near spherical nanosized carbon particles) and hydrogen ... 

Isothermal (flash pyrolysis). The temperature of the sample is suddenly increased (10-100 ms) to reach the thermal decomposition level (500 - 800°C). This process can be carried out by means of a platinum or platinum-rhodium filament heated by an electrical current directly coupled to the injector port of the GC. Some pyrolysis fragments are obtained in a very short time and can be directly sent to the column and detector. In spite of this short time for the pyrolysis, it is possible to indicate three different phases (a) heating (10 -10 s), (b) stabilization of the maximum temperature, and (c) cooling. However, the main drawback of this technique is the lack of equilibrium between temperatures with the pyrolyzer. 

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]. 

A factor that must be considered with furnace pyrolyzers as well as with the other types of pyrolyzers is the achieving of short TRT values. A slow sample introduction in the hot zone of the furnace will end in a long TRT. A poor contact between the sample and the hot source may also lead to long TRT, most of the heat being transferred by radiation and convection and not by conduction. However, fairly short TRTs in furnace pyrolyzers were reported in literature [14, 15]. Also, in furnace pyrolyzers it is more common to see differences in the temperature between the furnace and the sample. Due to the poor contact between the sample and the hot source, the sample may reach a lower actual temperature than the temperature of the furnace wall. This may be the explanation why there were reported variations in the pyrolysis products in microfurnace systems as compared to the results obtained in inductively or filament heated pyrolyzers [16,17]. 

Other techniques utilize lasers for sample evaporation/pyrolysis and excitation such as laser induced desorption (LID) or laser microprobe mass analysis (LAMMA) (see e g. [1]). Some of the sample introduction procedures in Py-MS enhance the information obtained from Py-MS by the use of time-resolved, temperature-resolved, or modulated molecular beams techniques [10]. In time-resolved procedures, the signal of the MS is recorded in time, and the continuous formation of fragments can be recorded. Temperature-resolved Py-MS allows a separation and ionization of the sample from a platinum/rhodium filament inside the ionization chamber of the mass spectrometer based on a gradual temperature increase [11]. The technique can be used either for polymer or for additives analysis. Attempts to improve selectivity in Py-MS also were done by using a membrane interface between the pyrolyzer and MS [12]. 

The term pyrolytic carbon can be applied to carbon filaments, carbon blacks, and carbon films, as well as to the more massive deposits which are the subject of this section. Pyrocarbon materials, made by chemical vapor deposition (CVD), vary in density, properties, and structure as much as the bulk materials discussed in 17.3.4.1. A heated hydrocarbon gas decomposes into an entire series of molecular species with a wide spectrum of carbon contents and molecular weights Within this pyrolyzing atmosphere, droplets form that pyrolyze and condense on a nearby surface, or large carbonaceous complexes may condense directly on the surface of the chamber. The former condition produces a fluffy, sooty, soft carbon, not far removed from carbon black, while the latter produces a hard solid carbon. The second of these materials is of primary interest here. The structure of the carbon produced by the CVD process has been shown to depend on the type of hydrocabon and its concentration, the pyrolysis temperature, the contact time, and the geometry of the pyrolyzing chamber. Of these, the pyrolysis temperature is perhaps the most important, but it is the nature of the chamber that conveniently divides the carbons produced into two distinct types. 

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