Pyrolysis Curie-point pyrolyzer

Heating should be instantaneous to prevent drawn out transfer of the pyrolyzates through the injection port. Heated filament and Curie-point pyrolysis result in less secondary pyrolysis products compared to furnace pyrolysis. Curie-point pyrolyzers accurately reproduce pyrolysis conditions with a rapid temperature rise time, yet the choice of different pyrolysis temperatures is limited. Very little sample preparation or pretreatment is required for laser pyrolysis however, a specific laser wavelength may not be appropriate for aU types of 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).

Curie-point pyrolysis employs high-frequency (HF) inductive heating of a ferromagnetic wire (d 1 mm). A diagram of a Curie-point pyrolyzer designed for Py-GC is shown in Fig. 4.7.2. 

Fig. 4.7.2. Schematic diagram of a Curie-point pyrolyzer (Fischer, Germany). Note the possible modifications of the wire tip (a, b, and c) for solid samples. Pyrolysis glass injector (/), ferromagnetic wire (2), carrier gas inlet (3), impulse cable from power generator (4), induction coil (5), aluminum box (6), adapter for GC injector (7), GC inlet (8), GC septum (9), GC oven (10)...

Pyrolysis involves the thermal decomposition, degradation, or cracking of a large molecule into smaller fragments. Pyrolysis GC is an excellent technique for identifying certain types of compounds which cannot be analyzed by derivatization, e.g., polymers. The pyrolysis temperamre is typically between 400°C and 1000°C. A number of analytical pyrolyzers have been introduced and are commercially available. The devices consist of platinum resistively heated and Curie point pyrolyzers. The carrier gas is directed through the system, and the platinum wire is heated to a certain temperature. The material decomposes, and the fragmentation products are analyzed. ... 

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 humic substances and soils were pyrolyzed in a type 0316 Curie-point pyrolyzer (Fischer, 53340 Meckenheim, Germany). The samples were not pretreated except drying and milling. The final pyrolysis temperatures employed were 573 K, 773 K and 973 K, respectively. The total heating time was varied between 3 and 9. 9 s. 

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. 

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... 

Among the definite advantages of the Pyroprobe over Curie-point pyrolyzers are the absence of solvent and grinding for sample introduction, ease in weighing the sample, and freedom of temperature choice. It is also hard to overestimate the possibilities provided by CDS Pyroprobe to carry out so-called sequential pyrolyses i.e., the pyrolysis temperature and time are chosen in a way that each pyrolysis affords only fractional decomposition of the sample. This additional capability of the CDS instrumentation was successfully used to identify the provenance of amber artifact from Hasanlu. ... 

Analysis was performed by desorbing the organics from the traps with a Curie-point pyrolyzer unit in series with a quadrupole mass spectrometer. The data produced were similar to Py-MS data, although, quite likely, thermal desorption was taking place rather than pyrolysis. The typical mass spectrum obtained from the contaminated areas was dominated by the major ions of PCE. Table 7.5 shows the various compounds that were identified in spectra obtained from the 25 samples spaced around the contaminated area. Table 7.6 shows the compounds identified by static trapping from a particular location and by purge-and-trap GC/MS analysis of water from an adjacent well. 

Curie-point pyrolysis involves placing the sample wire into a radio frequency field that induces eddy currents in the ferromagnetic material and causes a temperature rise. When the wire reaches the Curie-point temperature, it becomes paramagnetic and stops inducting power. The temperature at which the wire stabilizes (the Curie point) is a function of the type of metal. For example, the Curie points of cobalt, iron, and nickel are 1128, 770, and 358°C, respectively. Wires made from alloys of these metals produce intermediate temperatures. For example, the commonly used nickel-iron wire has a Curie point of 510°C. Differences between filament and Curie-point pyrolyzers depend on the pyrolysates examined and may be obscured by other instrumental differences, including the design of the transmission system to the detector. 

Figure 3 High-powered Curie-point pyrolyzer 1, glass pyrolysis injector with stainless steel needle 2, ferromagnetic wire 3, Teflon carrier gas tubing 4, impulse cable 5, induction coil 6, aluminum housing 7, adaptor for fastening 8, GC inlet 9, GC septum 10, GC column 11, carrier gas changeover valve. (Reproduced from Fischer Labor- und Verfahrenstechnik, Meckenbeim bei Bonn, Germany.)...

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... 

The first meaningful study involving Curie-point PyMS was published in 1970, and dealt with the analysis of dyes, fatty acid derivatives, and substituted benzoic acids. The apparatus used in this study consisted of a Curie-point pyrolyzer coupled to a magnetic sector mass spectrometer by means of a short length of capillary tubing. The tubing led from the pyrolyzer to a conventional molecular separator positioned in front of the ionization chamber. This instrument became the benchmark for future developments in PyMS and led directly to the introduction of the first automated Curie-point pyrolysis mass spectrometer in 1973. 

Pyrolysis-gas chromatography (Py-GC) Thermal decomposition is one of the oldest methods for studying the composition of polymers, and is a valuable tool in the industrial analysis of plastics. The analytical use of Py-GC is based on the fact that the polymer structure determines its reactivity and thus also the qualitative and quantitative composition of the pyrolysis products. The technique combines the advantages of a highly efficient separation method with a directly connected pyrolysis unit, so that the degradation products can be analyzed immediately after their formation. Curie-point pyrolyzers yield the most reproducible results due to short temperature... 

The Curie-point pyrolyzer uses the Curie points of ferromagnetic sample holders to achieve precisely controlled temperatures when the holder containing the sample is subjected to high-frequency induction heating. Foils of various ferromagnetic materials enable the analyst to select pyrolysis temperatures from 150 to 1000°C. 

Most of the pyrolysis experiments in the field of natural polymer characterization involve the use of Curie-point instruments or resistance-heating apparatus. Both methods are described in the following sections. Laser pyrolyzers are not yet common in biopolymer analysis and will not be discussed here. 

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. 

In pyrolysis-mass spectrometry (Py-MS) the pyrolysate is directly transferred to a mass spectrometer and analyzed, generating a complex spectrum. The sample introduction can be done using various techniques. One simple technique is the direct insertion probe (DIP) where the sample is deposited on an insert that has the capability of heating the sample and of introducing the pyrolysate directly into the ion source of the mass spectrometer (see e.g. [1]). Another technique is the Curie point Py-MS where an attachment to the mass spectrometer allows the sample to be placed in a radio frequency (RF) region continued by an expansion chamber connected to the ion source. The sample is pyrolyzed and the pyrolysate ionized and analyzed in the MS instrument. A schematic diagram of a Curie point Py-MS system is shown in Figure 3.3.2. 

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. 

In the early stage of Py-GC, significant interlaboratory discrepancies between pyrolysis data (pyrograms) were reported even for the same polymer types. This was mainly because of a diversity of pyrolysis devices operated under varied conditions. Owing to continued improvement of pyrolyzers and fundamental studies to control the operating conditions and obtain reproducible and characteristic degradation of the studied materials, most of the commercially available pyrolyzers now have made the interlaboratory discrepancies a minor problem. Now, various flash filament-, furnace-, and Curie-point type pyrolyzers are utilized for both Py-MS and Py-GC. 

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. ... 

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