Resistively Heated Filaments

Vacuum Deposition. Vacuum deposition, sometimes called vacuum evaporation, is a PVD process in which the material is thermally vaporized from a source and reaches the substrate without coUision with gas molecules in the space between the source and substrate (1 3). The trajectory of the vaporized material is therefore line-of-sight. Typically, vacuum deposition takes place in the pressure range of 10 10 Pa (10 10 torr), depending on the level of contamination that can be tolerated in the resulting deposited film. Figure 3 depicts a simple vacuum deposition chamber using a resistively heated filament vaporization source. 

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...

Figure 11.10 Comparison of extracted ion pyrograms of fragment ion m/z 217 of two samples collected from the paint surfaces of Universal Judgement and Stories of Holy Fathers (Monumental Cemetery of Pisa, Italy, painted by Buffalmacco, fourteenth century) with nitro cellulose, starch and arabic gum. 1, Unidentified compound 2, 1,2,3,5 tetrakis (O TMS) xylo furanose 3, tri (O TMS) levoglucosane 4, isomer of 1,2,3,5 tetrakis (O TMS) xylofuranose. Pyrogram obtained with a resistively heated filament at 60CPC in the presence of HMDS [30]...

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

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. 

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 ... 

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). 

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]. 

Some MS systems have, besides a temperature controlled probe, a heated-filament probe. This type of probe allows a more direct heating of the sample when it is deposited directly on the filament [48]. However, most of these probes still operate within common values for the current intensity and voltage and have a TRT that is longer than those used for flash pyrolysis. True flash pyrolysis using a resistively heated filament requires boosted current or boosted voltage for achieving a rapid heating (see Section 4.3), and such systems are commercially available. 

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. 

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

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]. 

Fig. 30. Variation of temperature over resistively heated filament. Tungsten sample 15.6 cm long, 2.52 X 10 a cm diameter leads at 77°K. Potential for temperature flash 10 volts. Distance between probes 1—2,1.19 cm 2—3, 2.19 cm 3-4, 2.23 cm 4—5, 3.04 cm 5-6, 6.95 cm.

Vapors of high melting metals and inorganic materials can be produced in situ by evaporation, either from resistively heated filaments of the material itself or else from bulk crucibles. These are all standard procedures 122). In a variant of this approach, permanent gases have been evolved by high temperature decomposition of a suitable solid, as for example H2 from ZrH2, 02 from CuO 44d), or from oxygen dissolved in silver 123), and CO from Mo(CO)a 124). In all these it is... 

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. 

Samples may be applied to resistively heated filaments in the same manner used for Curie-point wires. Soluble materials may be deposited from a solvent, which is then dried before pyrolysis. However, the solution is generally applied to the filament from a syringe, instead of dipping, since the filament is attached to a probe or housing. Insoluble materials may be melted in place to secure them before pyrolysis. Since the filament may be a flat ribbon or contain a grooved surface, placement of some solid materials may be simpler than when using a Curie-point wire. 

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. 

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