Resistively Heated Filament Pyrolysers

This type of pyrolyser commonly uses a platinum filament that has a precisely determined electrical resistance R1. This filament is incorporated in a Wheatstone electrical bridge. This bridge is balanced (V1 = V2) when the values of the electrical resistances in the bridge fulfill the relation  

To start the pyrolysis, the operational amplifier A1 through the power amplifier A2 switches on the power transistor Q1, and the power supply provides full current to the 

Several other procedures for a precise temperature control of the filament are also available, such as the use of optical pyrometry or thermocouples [13, 14]. 

The filament shape commonly used in resistively heated pyrolysers 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 piatinum coil. A silica (quartz) tube used as a sample container can be extremely useful in accommodating for pyrolysis of a wide variety of samples. Flowever, when a silica tube is used, the TRT times are increased due to the larger mass that needs to be heated. The filament, the silica tube (if present), and the sample are maintained in a stream of inert gas and inside a heated housing as described in Section 4.1. This secondary heating is necessary as mentioned before to avoid the condensation of the pyrolysate. The stream of gas inside the heated chamber can be used further as a 

There are several advantages of the resistively heated filament pyrolysers 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). 

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Figure 11.2 Py/silylation GC/MS chromatograms of aged linseed oil pyrolysed in the pre sence of HMDS, (a) Pyrogram obtained with a microfurnace pyrolyser pyrolysis temperature 600 °C furnace pressure 14 psi purge flow 0.5 ml min (b) Pyrogram obtained with a resistively heated filament pyrolyser pyrolyser interface I80°C transfer line 300°C valve oven 290°C. 1, Hexenoic acid, trimethylsilyl ester 2, hexanoic acid, trimethylsilyl ester 3, heptenoic acid, trimethylsilyl ester 4, heptanoic acid, trimethylsilyl ester 5, octenoic acid, trimethylsilyl ester 6, octanoic acid, trimethylsilyl ester 7, nonenoic acid, trimethylsilyl ester 8, nonanoic acid, trimethylsilyl ester 9, decanoic acid, trimethylsilyl ester 10, lauric acid, trimethylsilyl ester 11, suberic acid, trimethylsilyl diester 12, azelaic acid, trimethylsilyl diester 13, myristic acid, trimethylsilyl ester 14, sebacic acid, trimethylsilyl diester 15, palmitic acid, trimethylsilyl ester 16, stearic acid, trimethylsilyl ester...

Figure 11.3 Chromatograms of linseed oil mature films some of which contain pigments, obtained after (a) pyrolysis/methylation and (b) pyrolysis/silylation, at 600°C with a resistively heated filament pyrolyser. Reprinted from j. Anal. Appl. Pyrol., 74, Chiavari et at., 6, Copyright 2005 with permission from Elsevier...

Figure 11.4 Chromatogram relative to a mature linseed oil paint sample containing a high amount of sulfates, obtained by pyrolysis/silylation with a resistively heated filament pyrolyser at 600° C...

Figure 11.8 THM GC trace of bleached beeswax. FAME, fatty acid methyl ester obtained with a resistively heated filament pyrolyser at 550°C MeO FAME, methyl ester of methoxy fatty acid ME, alkyl methyl ether DiME, dimethoxyalkane EtC, hydrocarbon X Y, carbon chain length number of double bonds. Reprinted from J. Anal. Appl. Pyrol., 52, Asperger et al., 1, 13, Copyright 1999 with permission from Elsevier...

TABLE 4.1.1. The isoprene/dipentene ratio as a function of temperature for the pyrolysis of Kraton 1107 in an inductively heated or a resistively heated filament pyrolyser. 

Resistively heated filament pyrolysers were used for a long time in polymer pyrolysis [9], A schematic drawing of a common filament pyrolyser is shown in Figure 4.1.1. The principle of this type of pyrolyser is that an electric current passing through a resistive conductor generates heat in accordance with Joule s law ... 

Another problem with the filament pyrolysers is the possibility that the filament may be non-uniformly heated over its length. This may determine different Teq 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 pyrolysers are among the most common ones, and very good reproducibility has been reported frequently [12]. 

Disadvantages of resistively heated filament pyrolysers are difficult automation and the fact that the pyrolysis temperature is difficult to control, as the thermal properties of the sample and filament vary with sample size and filament ageing. Consequently, in spite of constant energy supply to the filament, the temperature attained by the sample during the transient period of pyrolysis is not accurately fixed. The temperature of the surface, which may act catalyti-cally, is difficult to measure. 

There are several construction principles for pyrolysers, such as inductively heated, resistively heated filament, furnace type, and radiative heated. The principles of construction for the main types of pyrolysers will be discussed in Section 4.2 to Section 4.6. 

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]. 

From the above, some important features of pyrolysis GC-MS emerge, as given in Table 2.33. On-line flash pyrolysis GC-MS, with Curie-point, resistively-heated filament or furnace pyrolysers, is very widely utilised for identification of pyrolysis products from synthetic polymers. The main characteristics of PyGC-MS of polymers, as given by Schulten et al. [692], are shown in Table 2.34. PyGC-MS is an excellent tool for fast product quality control for R D purposes fiiU control of the (many) experimental parameters is needed. Polymer standards e.g. SEC standards) can be used to determine sensitivity and precision of PyGC-MS. 

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. 

A third type of pyrolyser sometimes utilizes a filament heated by its own electrical resistance. The most effective pyrolysers of this type use an initial pulse of heating at a high voltage to produce a high current and rapid heating to the pyrolysis temperature, i.e. 700°C in 12 ms, followed by reduction to an accurately controlled maintenance voltage to maintain the pyrolysis temperature. 

Filament (ribbon) pyrolyser A pyrolyser in which the sample is placed on a metal filament (ribbon) that is resistively or inductively heated to induce pyrolysis. 

The system applied in the study mentioned above consisted of a CDS model 122 Pyroprobe with a ribbon filament as the heating surface (see Chapter 3 and Appendix 1). This pyrolyser heats by varying the resistance of the platinum element. Temperature rise times for flash pyrolysis are typically of the order of milliseconds. IR spectra were obtained with an FT-IR bench system equipped with a CDS pyrolysis/FT-IR interface. The data were collected at 8 cm" with a deuterated triglycine sulfate (DTGS) detector. The interface is cylindrical in shape with two potassium bromide windows for the IR beam to pass through. 

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